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Apparatus is provided for applying current to a nerve. A cathode is adapted to be placed in a vicinity of a cathodic longitudinal site of the nerve and to apply, a cathodic current to the nerve. A primary inhibiting anode is adapted to be placed in a vicinity of a primary anodal longitudinal site of

Apparatus is provided for applying current to a nerve. A cathode is adapted to be placed in a vicinity of a cathodic longitudinal site of the nerve and to apply, a cathodic current to the nerve. A primary inhibiting anode is adapted to be placed in a vicinity of a primary anodal longitudinal site of the nerve and to apply a primary anodal current to the nerve. A secondary inhibiting anode is adapted to be placed in a vicinity of a secondary anodal longitudinal site of the nerve and to apply a secondary anodal current to the nerve, the secondary anodal longitudinal site being closer to the primary anodal longitudinal site than to the cathodic longitudinal site.

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1. Apparatus for applying current to a nerve having a radius and a longitudinal central axis, comprising:a housing, adapted to be placed in a vicinity of the nerve; first and second electrodes, fixed to the housing; and an insulating element, fixed to the housing between the first and second electro

1. Apparatus for applying current to a nerve having a radius and a longitudinal central axis, comprising:a housing, adapted to be placed in a vicinity of the nerve; first and second electrodes, fixed to the housing; and an insulating element, fixed to the housing between the first and second electrodes so as to define a characteristic closest insulating element distance to the central axis that is at least approximately 1.5 times greater than the radius of the nerve, wherein the first and second electrodes are fixed to the housing so as to define respective first and second closest electrode distances to the axis, when the housing is placed in the vicinity of the nerve, and wherein the first and second closest electrode distances are both greater than the closest insulating element distance. 2. Apparatus according to claim 1 wherein the first electrode comprises a cathode, adapted to be placed in a vicinity of a cathodic longitudinal site of the nerve and to apply a cathodic current to the nerve, wherein the second electrode comprises a primary inhibiting anode, adapted to be placed in a vicinity of a primary anodal longitudinal site of the nerve and to apply a primary anodal current to the nerve, and comprising a secondary inhibiting anode, adapted to be fixed to the housing and placed in a vicinity of a secondary anodal longitudinal site of the nerve and to apply a secondary anodal current to the nerve, the secondary anodal longitudinal site being closer to the primary anodal longitudinal site than to the cathodic longitudinal site.3. Apparatus according to claim 2, wherein the apparatus is adapted to be placed on the nerve such that, relative to the anodal longitudinal sites, the cathodic longitudinal site is proximal to a brain of a subject, the subject including the nerve.4. Apparatus according to claim 2, wherein the apparatus is adapted to be placed on the nerve such that, relative to the anodal longitudinal sites, the cathodic longitudinal site is distal to a brain of a subject, the subject including the nerve.5. Apparatus according to claim 2, wherein the primary inhibiting anode is adapted to apply the primary anodal current to the nerve so as to block propagation of action potentials past the primary anodal longitudinal site.6. Apparatus according to claim 2, wherein the primary inhibiting anode is adapted to apply the primary anodal current to the nerve so as to block propagation past the primary anodal longitudinal site of action potentials in a first set of nerve fibers, and to allow propagation past the primary anodal longitudinal site of action potentials in a second set of nerve fibers, the second set of nerve fibers having characteristic diameters generally smaller than characteristic diameters of the nerve fibers in the first set.7. Apparatus according to claim 2, wherein the cathode comprises a plurality of cathodes, placed in the vicinity of the cathodic longitudinal site of the nerve, at respective positions around an axis of the nerve.8. Apparatus according to claim 7, wherein the plurality of cathodes are adapted to apply the cathodic current at a characteristic frequency greater than 1000 Hz.9. Apparatus according to claim 2, wherein the insulating element is disposed in a position with respect to the cathode and the primary inhibiting anode so as to guide the flow of current between the cathode and the primary inhibiting anode.10. Apparatus according to claim 2, wherein the insulating element includes a primary insulating element, and comprising a secondary insulating element, disposed between the primary inhibiting anode and the secondary inhibiting anode.11. Apparatus according to claim 10, wherein a characteristic size of the secondary insulating element is smaller than a characteristic size of the primary insulating element.12. Apparatus according to claim 10, wherein a characteristic distance of the secondary insulating element to the axis of the nerve is greater than a characteristic distance of the primary insulating element to the axis of the nerve.13. Apparatus according to claim 1, comprising a tertiary inhibiting electrode, adapted to be placed in a vicinity of a tertiary anodal longitudinal site of the nerve and to apply a tertiary anodal current to the nerve, the tertiary anodal longitudinal site being closer to the secondary anodal longitudinal site than to the primary anodal longitudinal site.14. Apparatus according to claim 13, wherein the tertiary inhibiting anode is configured such that a current density of the tertiary anodal current is of lower magnitude than a magnitude of a current density of the secondary anodal current.15. Apparatus according to claim 2, wherein a closest cathode distance to the axis of the nerve, a closest primary inhibiting anode distance to the axis, and a closest secondary inhibiting anode distance to the axis are all at least approximately 1.5 times greater than the radius of the nerve.16. Apparatus according to claim 1, wherein the secondary inhibiting anode is configured such that a secondary anodal current density induced by the secondary anodal current is of lower magnitude than a magnitude of a primary anodal current density induced by the primary anodal current.17. Apparatus according to claim 16, wherein the primary anodal current is substantially of the same magnitude as the secondary anodal current.18. Apparatus according to claim 16, wherein a characteristic surface area of the secondary inhibiting anode is higher than a characteristic surface area of the primary inhibiting anode.19. Apparatus according to claim 18, wherein the characteristic surface area of the secondary inhibiting anode is at least 2 times higher than the characteristic surface area of the primary inhibiting anode.20. Apparatus according to claim 2, wherein the secondary inhibiting anode is configured such that a current density of the secondary anodal current is of lower magnitude than a magnitude of a current density of the primary anodal current.21. Apparatus according to claim 20, wherein a characteristic surface area of the primary inhibiting anode is higher than a characteristic surface area of the secondary inhibiting anode.22. Apparatus according to claim 21, wherein a common voltage is applied to the primary inhibiting anode and to the secondary inhibiting anode.23. Apparatus according to claim 20,wherein the primary inhibiting anode is adapted to have associated therewith a primary level of electrical impedance between the primary inhibiting anode and the nerve, when in the vicinity of the primary anodal longitudinal site, and wherein the secondary inhibiting anode is adapted to have associated therewith a secondary level of electrical impedance between the secondary inhibiting anode and the nerve when in the vicinity of the secondary anodal longitudinal site, the secondary level of impedance having a higher magnitude than the primary level of impedance. 24. Apparatus according to claim 20, wherein the secondary inhibiting anode is adapted to be coupled to the housing so as to define a secondary anode distance to the axis of the nerve, and wherein the primary inhibiting anode is adapted to be coupled to the housing so as to define a primary anode distance to the axis of the nerve that is smaller than the secondary anode distance.25. Apparatus according to claim 24, wherein a ratio of the secondary anode distance to the primary anode distance is greater than approximately 1.5:1.26. Apparatus according to claim 2, comprising a primary fiber-selection anode, adapted to be placed in a vicinity of a primary fiber-selection anodal longitudinal site of the nerve that is closer to the cathodic longitudinal site than to the primary anodal longitudinal site.27. Apparatus according to claim 26 comprising a secondary fiber-selection anode, adapted to be placed in a vicinity of a secondary fiber-selection anodal longitudinal site of the nerve that is closer to the primary fiber-selection anodal longitudinal site than to the cathodic longitudinal site.28. Apparatus according to claim 2, comprising a control unit, adapted to drive the cathode to apply the cathodic current to the nerve, adapted to drive the primary inhibiting anode to apply the primary anodal current to the nerve, and adapted to drive the secondary inhibiting anode to apply the secondary anodal current to the nerve.29. Apparatus according to claim 28, comprising a first resistive element coupled between the control unit and the primary inhibiting anode, and a second resistive element coupled between the control unit and the secondary inhibiting anode, the second resistive element having a resistance higher than a resistance of the first resistive element.30. Apparatus according to claim 28, comprising exactly one lead that leaves the control unit for coupling the control unit to the primary and secondary inhibiting anodes.31. Apparatus according to claim 28, comprising respective leads that leave the control unit and couple the control unit to the primary and secondary inhibiting anodes.32. Apparatus according to claim 28, wherein the control unit is adapted to configure a current density of the secondary anodal current to be of lower magnitude than a current density of the primary anodal current.33. Apparatus according to claim 28, wherein the control unit is adapted to configure an amplitude of a current density of the cathodic current to be between 1.1 and 2 times greater than an amplitude of a current density of the primary anodal current.34. Apparatus according to claim 28, wherein the control unit is adapted to configure an amplitude of a current density of the cathodic current to be between 3 and 6 times greater than an amplitude of a current density of the secondary anodal current.35. Apparatus according to claim 28, wherein the control unit is adapted to configure an amplitude of a current density of the primary anodal current to be at least 2 times greater than an amplitude of a current density of the secondary anodal current.36. Apparatus according to claim 1, wherein the housing is configured such that an arc, defined by an extent that the housing is adapted to surround the nerve, is between about 90 and 270 degrees.37. Apparatus according to claim 1, wherein the housing is configured such that an arc, defined by an extent that the housing is adapted to surround the nerve, is between about 270 and 359 degrees.38. Apparatus according to claim 1, wherein the first and second electrodes comprise a cathode and an anode, respectively, fixed to the housing.39. Apparatus according to claim 38, wherein the closest cathode and anode distances to the axis are both at least approximately 2 times greater than the radius of the nerve.40. Apparatus according to claim 38, wherein the apparatus is adapted to be placed on the nerve such that, relative to the anode, the cathode is in a vicinity of the nerve which is proximal to a brain of a subject, the subject including the nerve.41. Apparatus according to claim 38, wherein the apparatus is adapted to be placed on the nerve such that, relative to the anode, the cathode is in a vicinity of the nerve which is distal to a brain of a subject, the subject including the nerve.42. Apparatus according to claim 38, wherein the cathode comprises a plurality of cathodes, placed in a vicinity of the cathodic longitudinal site of the nerve, at respective positions around the axis of the nerve, each of the respective positions being at a distance from the axis at least 1.5 times greater than the radius of the nerve.43. Apparatus according to claim 38, wherein the anode is adapted to apply anodal current to the nerve so as to block propagation of action potentials past the anode.44. Apparatus according to claim 38 and comprising a control unit, coupled to the anode, and adapted to drive the anode to apply current to the nerve at a level configured so as to block propagation past the anode of action potentials in a first set of nerve fibers, and to allow propagation past the anode of action potentials in a second set of nerve fibers, the second set of nerve fibers having characteristic diameters generally smaller than characteristic diameters of the nerve fibers in the first set.45. Apparatus according to claim 38 wherein a characteristic distance of the anode to the axis is within 30% of the characteristic closest insulating element distance of the insulating element to the axis of the nerve plus a width of the anode.46. Apparatus according to claim 1, wherein the insulating element is adapted to be placed in the vicinity of the nerve at a longitudinal site that is between respective longitudinal sites of the first and second electrodes.47. Apparatus according to claim 1, wherein the insulating element is adapted to be placed in the vicinity of the nerve at a site with respect to the axis of the nerve that is between respective sites of the first and second electrodes, with respect to the axis.48. Apparatus according to claim 1, wherein the first and second closest electrode distances are both at least 30% greater than or equal to the closest insulating element distance.49. Apparatus according to claim 48, wherein the insulating element is adapted to be placed in the vicinity of the nerve at a site with respect to the axis of the nerve that is between respective sites of the first and second electrodes, with respect to the axis.50. Apparatus according to claim 48, wherein the insulating element is disposed in a position with respect to the first and second electrodes so as to guide the flow of current between the first and second electrodes.51. Apparatus according to claim 48,wherein the first electrode comprises a cathode, adapted to be placed in a vicinity of a cathodic longitudinal site of the nerve and to apply a cathodic current to the nerve, wherein the second electrode comprises a primary inhibiting anode, adapted to be placed in a vicinity of a primary anodal longitudinal site of the nerve and to apply a primary anodal current to the nerve, and comprising a secondary inhibiting anode, adapted to be fixed to the housing and placed in a vicinity of a secondary anodal longitudinal site of the nerve and to apply a secondary anodal current to the nerve, the secondary anodal longitudinal site being closer to the primary anodal longitudinal site than to the cathodic longitudinal site. 52. Apparatus according to claim 51, wherein the apparatus is adapted to be placed on the nerve such that, relative to the anodal longitudinal sites, the cathodic longitudinal site is proximal to a brain of a subject, the subject including the nerve.53. Apparatus according to claim 51, wherein the apparatus is adapted to be placed on the nerve such that, relative to the anodal longitudinal sites, the cathodic longitudinal site is distal to a brain of a subject, the subject including the nerve.54. Apparatus according to claim 51, wherein the primary inhibiting anode is adapted to apply the primary anodal current to the nerve so as to block propagation of action potentials past the primary anodal longitudinal site.55. Apparatus according to claim 51, wherein the primary inhibiting anode is adapted to apply the primary anodal current to the nerve so as to block propagation past the primary anodal longitudinal site of action potentials in a first set of nerve fibers, and to allow propagation past the primary anodal longitudinal site of action potentials in a second set of nerve fibers, the second set of nerve fibers having characteristic diameters generally smaller than characteristic diameters of the nerve fibers in the first set.56. Apparatus according to claim 51, wherein the cathode comprises a plurality of cathodes, placed in the vicinity of the cathodic longitudinal site of the nerve, at respective positions around the axis of the nerve.57. Apparatus according to claim 56, wherein the plurality of cathodes are adapted to apply the cathodic current at a characteristic frequency greater than 1000 Hz.58. Apparatus according to claim 51, wherein the insulating element is disposed in a position with respect to the cathode and the primary inhibiting anode so as to guide the flow of current between the cathode and the primary inhibiting anode.59. Apparatus according to claim 51, wherein the insulating element includes a primary insulating element, and comprising a secondary insulating element, disposed between the primary inhibiting anode and the secondary inhibiting anode.60. Apparatus according to claim 59, wherein a characteristic size of the secondary insulating element is smaller than a characteristic size of the primary insulating element.61. Apparatus according to claim 59, wherein a characteristic distance of the secondary insulating element to the axis of the nerve is greater than a characteristic distance of the primary insulating element to the axis of the nerve.62. Apparatus according to claim 51, comprising a tertiary inhibiting electrode, adapted to be placed in a vicinity of a tertiary anodal longitudinal site of the nerve and to apply a tertiary anodal current to the nerve, the tertiary anodal longitudinal site being closer to the secondary anodal longitudinal site than to the primary anodal longitudinal site.63. Apparatus according to claim 62, wherein the tertiary inhibiting anode is configured such that a current density of the tertiary anodal current is of lower magnitude than a magnitude of a current density of the secondary anodal current.64. Apparatus according to claim 51, wherein a closest cathode distance to the axis of the nerve, a closest primary inhibiting anode distance to the axis, and a closest secondary inhibiting anode distance to the axis are all at least approximately 1.5 times greater than the radius of the nerve.65. Apparatus according to claim 51, wherein the secondary inhibiting anode is configured such that a secondary anodal current density induced by the secondary anodal current is of lower magnitude than a magnitude of a primary anodal current density induced by the primary anodal current.66. Apparatus according to claim 65, wherein the primary anodal current is substantially of the same magnitude as the secondary anodal current.67. Apparatus according to claim 65, wherein a characteristic surface area of the secondary inhibiting anode is higher than a characteristic surface area of the primary inhibiting anode.68. Apparatus according to claim 67, wherein the characteristic surface area of the secondary inhibiting anode is at least 2 times higher than the characteristic surface area of the primary inhibiting anode.69. Apparatus according to claim 51, wherein the secondary inhibiting anode is configured such that a current density of the secondary anodal current is of lower magnitude than a magnitude of a current density of the primary anodal current.70. Apparatus according to claim 69, wherein a characteristic surface area of the primary inhibiting anode is higher than a characteristic surface area of the secondary inhibiting anode.71. Apparatus according to claim 70, wherein a common voltage is applied to the primary inhibiting anode and to the secondary inhibiting anode.72. Apparatus according to claim 69,wherein the primary inhibiting anode is adapted to have associated therewith a primary level of electrical impedance between the primary inhibiting anode and the nerve, when in the vicinity of the primary anodal longitudinal site, and wherein the secondary inhibiting anode is adapted to have associated therewith a secondary level of electrical impedance between the secondary inhibiting anode and the nerve when in the vicinity of the secondary anodal longitudinal site, the secondary level of impedance having a higher magnitude than the primary level of impedance. 73. Apparatus according to claim 69, wherein the secondary inhibiting anode is adapted to be coupled to the housing so as to define a secondary anode distance to the axis of the nerve, and wherein the primary inhibiting anode is adapted to be coupled to the housing so as to define a primary anode distance to the axis of the nerve that is smaller than the secondary anode distance.74. Apparatus according to claim 73, wherein a ratio of the secondary anode distance to the primary anode distance is greater than approximately 1.5:1.75. Apparatus according to claim 51, comprising a primary fiber-selection anode, adapted to be placed in a vicinity of a primary fiber-selection anodal longitudinal site of the nerve that is closer to the cathodic longitudinal site than to the primary anodal longitudinal site.76. Apparatus according to claim 75, comprising a secondary fiber-selection anode, adapted to be placed in a vicinity of a secondary fiber-selection anodal longitudinal site of the nerve that is closer to the primary fiber-selection anodal longitudinal site than to the cathodic longitudinal site.77. Apparatus according to claim 51, comprising a control unit, adapted to drive the cathode to apply the cathodic current to the nerve, adapted to drive the primary inhibiting anode to apply the primary anodal current to the nerve, and adapted to drive the secondary inhibiting anode to apply the secondary anodal current to the nerve.78. Apparatus according to claim 77, comprising a first resistive element coupled between the control unit and the primary inhibiting anode, and a second resistive element coupled between the control unit and the secondary inhibiting anode, the second resistive element having a resistance higher than a resistance of the first resistive element.79. Apparatus according to claim 77, comprising exactly one lead that leaves the control unit for coupling the control unit to the primary and secondary inhibiting anodes.80. Apparatus according to claim 77, comprising respective leads that leave the control unit and couple the control unit to the primary and secondary inhibiting anodes.81. Apparatus according to claim 77, wherein the control unit is adapted to configure a current density of the secondary anodal current to be of lower magnitude than a current density of the primary anodal current.82. Apparatus according to claim 77, wherein the control unit is adapted to configure an amplitude of a current density of the cathodic current to be between 1.1 and 2 times greater than an amplitude of a current density of the primary anodal current.83. Apparatus according to claim 77, wherein the control unit is adapted to configure an amplitude of a current density of the cathodic current to be between 3 and 6 times greater than an amplitude of a current density of the secondary anodal current.84. Apparatus according to claim 77, wherein the control unit is adapted to configure an amplitude of a current density of the primary anodal current to be at least 2 times greater than an amplitude of a current density of the secondary anodal current.85. Apparatus according to claim 48, wherein the first and second electrodes comprise a cathode and an anode, respectively, fixed to the housing.86. Apparatus according to claim 85, wherein the closest cathode and anode distances to the axis are both at least approximately 2 times greater than the radius of the nerve.87. Apparatus according to claim 85, wherein the apparatus is adapted to be placed on the nerve such that, relative to the anode, the cathode is in a vicinity of the nerve which is proximal to a brain of a subject, the subject including the nerve.88. Apparatus according to claim 85, wherein the apparatus is adapted to be placed on the nerve such that, relative to the anode, the cathode is in a vicinity of the nerve which is distal to a brain of a subject, the subject including the nerve.89. Apparatus according to claim 85, wherein the cathode comprises a plurality of cathodes, placed in a vicinity of the cathodic longitudinal site of the nerve, at respective positions around the axis of the nerve, each of the respective positions being at a distance from the axis at least 1.5 times greater than the radius of the nerve.90. Apparatus according to claim 85, wherein the anode is adapted to apply anodal current to the nerve so as to block propagation of action potentials past the anode.91. Apparatus according to claim 85, wherein and comprising a control unit, coupled to the anode, and adapted to drive the anode to apply current to the nerve at a level configured so as to block propagation past the anode of action potentials in a first set of nerve fibers, and to allow propagation past the anode of action potentials in a second set of nerve fibers, the second set of nerve fibers having characteristic diameters generally smaller than characteristic diameters of the nerve fibers in the first set.92. Apparatus according to claim 85, wherein a characteristic distance of the anode to the axis is within 30% of the characteristic closest insulating element distance of the insulating element to the axis of the nerve plus a width of the anode.93. Apparatus according to claim 48,wherein the first electrode comprises a cathode, adapted to be placed in a vicinity of a cathodic site of the nerve, and comprising a plurality of anodes, adapted to be placed in a vicinity of respective anodal longitudinal sites of the nerve and to apply respective anodal currents to the nerve, that define, in combination, an anodal activation function having: (a) a hyperpolarizing portion thereof having a maximum hyperpolarizing amplitude, and (b) a depolarizing portion thereof, having a maximum depolarizing amplitude corresponding to a depolarizing site on the nerve distal with respect to the cathode to a site corresponding to the hyperpolarizing portion, wherein the maximum hyperpolarizing amplitude is at least five times greater than the maximum depolarizing amplitude, wherein the second electrode includes one of the plurality of anodes. 94. Apparatus according to claim 93, wherein the plurality of anodes are coupled to the housing, and wherein a distance of a first one of the anodes to the axis of the nerve is less than a distance of a second one of the anodes to the axis, the first one of the anodes being coupled to the housing closer to the cathode than the second one of the anodes.95. Apparatus according to claim 93, wherein the plurality of anodes are coupled to the housing, and wherein a surface area of a first one of the anodes is less than a surface area of a second one of the anodes, the first one of the anodes being coupled to the housing closer to the cathode than the second one of the anodes.96. Apparatus according to claim 93, wherein the plurality of anodes are coupled to the housing, and wherein one of the anodes is positioned within the housing so as to reduce a virtual cathode effect induced by another one of the anodes.97. Apparatus according to claim 93, wherein the cathode and anodes are disposed such that a first one of the anodal longitudinal sites is between the cathodic site and a second one of the anodal longitudinal sites.98. Apparatus according to claim 97, wherein the anodes are disposed such that the second one of the anodal longitudinal sites is between the first one of the anodal longitudinal sites and a third one of the anodal longitudinal sites.99. Apparatus according to claim 97, wherein the anodes are adapted such that a current density of the anodal current applied at the second one of the anodal longitudinal sites has-a lower magnitude than a magnitude of a current density of the anodal current applied at the first one of the anodal longitudinal sites.100. Apparatus according to claim 99, wherein the anodes are adapted such that a ratio of the current density of the anodal current applied at the first site to the current density of the anodal current applied at the second site is at least 2:1.101. Apparatus according to claim 99, wherein the anodes are adapted such that a ratio of the current density of the anodal current applied at the first site to the current density of the anodal current applied at the second site is at least 5:1.102. Apparatus according to claim 48,wherein the first electrode comprises a cathode, adapted to be placed in a vicinity of a first longitudinal site of the nerve, and wherein the second electrode comprises an elongated anode, adapted to be placed in a vicinity of a second longitudinal site of the nerve, and, when so placed, to have associated therewith: (a) a first level of electrical impedance between the nerve and a location on the elongated anode proximal to the cathode, and (b) a second level of electrical impedance, greater than the first level, between the nerve and a location on the elongated anode distal to the cathode. 103. Apparatus according to claim 102, comprising a coating disposed on a surface of the elongated anode, configured to provide the first and second levels of impedance.104. Apparatus according to claim 103, wherein the coating is disposed on the surface in different respective thicknesses at the two locations on the elongated anode.105. Apparatus according to claim 103, wherein the coating comprises a coating that has undergone a surface treatment, and wherein the coating is configured to provide the first and second levels of impedance responsive to having undergone the surface treatment.106. Apparatus according to claim 103 wherein the coating comprises iridium oxide.107. Apparatus according to claim 103, wherein the coating comprises titanium nitrite.108. Apparatus according to claim 103, wherein the coating comprises platinum iridium.109. Apparatus according to claim 48,wherein the first electrode comprises two or more surrounding electrodes, adapted to be placed in a vicinity of a longitudinal site of the nerve, at respective positions around the axis, and comprising a control unit, adapted to: (a) drive current between two of the surrounding electrodes, thereby defining a first pair of the surrounding electrodes and a first direction of current flow, and, less than one millisecond later, (b) drive current between two of the surrounding electrodes, thereby defining a second pair of the surrounding electrodes and a second direction of current flow, and (c) cycle between steps (a) and (b) at a rate greater than 1000 Hz, wherein at least either the first pair of surrounding electrodes is different from the second pair of surrounding electrodes or the first direction of current flow is different from the second direction of current flow. 110. Apparatus according to claim 109, wherein the two or more surrounding electrodes comprise three or more surrounding electrodes.111. Apparatus according to claim 109, wherein the two or more surrounding electrodes comprise four or more surrounding electrodes.112. Apparatus according to claim 109, wherein the control unit is adapted to set the rate to be greater than 4000 Hz.113. Apparatus according to claim 48,wherein the first electrode comprises a set of two or more cathodes, adapted to be placed in a vicinity of a cathodic longitudinal site of the nerve, at respective positions around the axis, and wherein the second electrode comprises a set of two or more anodes, adapted to be placed in a vicinity of an anodal longitudinal site of the nerve, at respective positions around the axis. 114. Apparatus according to claim 113, wherein the two or more cathodes comprise six or more cathodes.115. Apparatus according to claim 113, wherein the two or more cathodes comprise twelve or more cathodes.116. Apparatus according to claim 113, comprising a control unit, adapted to drive current between respective ones of the cathodes and respective ones of the anodes.117. Apparatus according to claim 116, wherein the control unit is adapted to cycle the current driving at a rate greater than 1000 Hz.118. Apparatus according to claim 116, wherein the control unit is adapted to complete a sweep of driving the current through substantially all of the cathodes in less than 1000 microseconds.119. Apparatus according to claim 116, wherein the control unit is adapted to complete a sweep of driving the current through substantially all of the cathodes in less than 100 microseconds.120. Apparatus according to claim 48, wherein the housing is configured such that an arc, defined by an extent that the housing is adapted to surround the nerve, is between about 90 and 270 degrees.121. Apparatus according to claim 48, wherein the housing is configured such that an arc, defined by an extent that the housing is adapted to surround the nerve, is between about 270 and 359 degrees.122. Apparatus according to claim 48, wherein the insulating element is adapted to be placed in the vicinity of the nerve at a longitudinal site that is between respective longitudinal sites of the first and second electrodes.123. Apparatus according to claim 1, wherein the first electrode comprises a cathode, adapted to be placed in a vicinity of a cathodic site of the nerve, and comprising a plurality of anodes, adapted to be placed in a vicinity of respective anodal longitudinal sites of the nerve and to apply respective anodal currents to the nerve, that define, in combination, an anodal activation function having: (a) a hyperpolarizing portion thereof having a maximum hyperpolarizing amplitude, and (b) a depolarizing portion thereof, having a maximum depolarizing amplitude corresponding to a depolarizing site on the nerve distal with respect to the cathode to a site corresponding to the hyperpolarizing portion, wherein the maximum hyperpolarizing amplitude is at least five times greater than the maximum depolarizing amplitude, wherein the second electrode includes one of the plurality of anodes.124. Apparatus according to claim 123, wherein the plurality of anodes are coupled to the housing, and wherein a distance of a first one of the anodes to the axis of the nerve is less than a distance of a second one of the anodes to the axis, the first one of the anodes being coupled to the housing closer to the cathode than the second one of the anodes.125. Apparatus according to claim 123, wherein the plurality of anodes are coupled to the housing, and wherein a surface area of a first one of the anodes is less than a surface area of a second one of the anodes, the first one of the anodes being coupled to the housing closer to the cathode than the second one of the anodes.126. Apparatus according to claim 123, wherein the plurality of anodes are coupled to the housing, and wherein one of the anodes is positioned within the housing so as to reduce a virtual cathode effect induced by another one of the anodes.127. Apparatus according to claim 123, wherein the cathode and anodes are disposed such that a first one of the anodal longitudinal sites is between the cathodic site and a second one of the anodal longitudinal sites.128. Apparatus according to claim 127, wherein the anodes are disposed such that the second one of the anodal longitudinal sites is between the first one of the anodal longitudinal sites and a third one of the anodal longitudinal sites.129. Apparatus according to claim 127, wherein the anodes are adapted such that a current density of the anodal current applied at the second one of the anodal longitudinal sites has a lower magnitude than a magnitude of a current density of the anodal current applied at the first one of the anodal longitudinal sites.130. Apparatus according to claim 129, wherein the anodes are adapted such that a ratio of the current density of the anodal current applied at the first site to the current density of the anodal current applied at the second site is at least 2:1.131. Apparatus according to claim 129, wherein the anodes are adapted such that a ratio of the current density of the anodal current applied at the first site to the current density of the anodal current applied at the second site is at least 5:1.132. Apparatus according to claim 1,wherein the first electrode comprises a cathode, adapted to be placed in a vicinity of a first longitudinal site of the nerve, and wherein the second electrode comprises an elongated anode, adapted to be placed in a vicinity of a second longitudinal site of the nerve, and, when so placed, to have associated therewith: (a) a first level of electrical impedance between the nerve and a location on the elongated anode proximal to the cathode, and (b) a second level of electrical impedance, greater than the first level, between the nerve and a location on the elongated anode distal to the cathode. 133. Apparatus according to claim 132, comprising a coating disposed on a surface of the elongated anode, configured to provide the first and second levels of impedance.134. Apparatus according to claim 133, wherein the coating is disposed on the surface in different respective thicknesses at the two locations on the elongated anode.135. Apparatus according to claim 133, wherein the coating comprises a coating that has undergone a surface treatment, and wherein the coating is configured to provide the first and second levels of impedance responsive to having undergone the surface treatment.136. Apparatus according to claim 133, wherein the coating comprises iridium oxide.137. Apparatus according to claim 133, wherein the coating comprises titanium nitrite.138. Apparatus according to claim 133, wherein the coating comprises platinum iridium.139. Apparatus according to claim 1,wherein the first electrode comprises two or more surrounding electrodes, adapted to be placed in a vicinity of a longitudinal site of the nerve, at respective positions around the axis, and comprising a control unit, adapted to: (a) drive current between two of the surrounding electrodes, thereby defining a first pair of the surrounding electrodes and a first direction of current flow, and, less than one millisecond later, (b) drive current between two of the surrounding electrodes, thereby defining a second pair of the surrounding electrodes and a second direction of current flow, and (c) cycle between steps (a) and (b) at a rate greater than 1000 Hz, wherein at least either the first pair of surrounding electrodes is different from the second pair of surrounding electrodes or the first direction of current flow is different from the second direction of current flow. 140. Apparatus according to claim 139, wherein the two or more surrounding electrodes comprise three or more surrounding electrodes.141. Apparatus according to claim 139, wherein the two or more surrounding electrodes comprise four or more surrounding electrodes.142. Apparatus according to claim 139, wherein the control unit is adapted to set the rate to be greater than 4000 Hz.143. Apparatus according to claim 1,wherein the first electrode comprises a set of two or more cathodes, adapted to be placed in a vicinity of a cathodic longitudinal site of the nerve, at respective positions around the axis, and wherein the second electrode comprises a set of two or more anodes, adapted to be placed in a vicinity of an anodal longitudinal site of the nerve, at respective positions around the axis. 144. Apparatus according to claim 143, wherein the two or more cathodes comprise six or more cathodes.145. Apparatus according to claim 143, wherein the two or more cathodes comprise twelve or more cathodes.146. Apparatus according to claim 143, comprising a control unit, adapted to drive current between respective ones of the cathodes and respective ones of the anodes.147. Apparatus according to claim 146, wherein the control unit is adapted to cycle the current driving at a rate greater than 1000 Hz.148. Apparatus according to claim 146, wherein the control unit is adapted to complete a sweep of driving the current through substantially all of the cathodes in less than 1000 microseconds.149. Apparatus according to claim 146, wherein the control unit is adapted to complete a sweep of driving the current through substantially all of the cathodes in less than 100 microseconds.150. A method for applying current to a nerve having a radius and a longitudinal central axis, comprising:applying current to the nerve from first and second current-application sites that are located at respective first and second closest current-application distances from the axis; and providing insulation from an insulation site that is located between the first and second current-application sites, at a characteristic closest insulation distance to the central axis that is at least approximately 1.5 times greater than the radius of the nerve, wherein the first and second closest current-application distances are both greater than the closest insulation distance. 151. Apparatus according to claim 1, wherein the insulating element is disposed in a position with respect to the first and second electrodes so as to guide the flow of current between the first and second electrodes.152. A method according to claim 150,wherein the first current-application site includes a cathodic longitudinal site of the nerve, wherein the second current-application site includes a primary anodal longitudinal site of the nerve, and wherein applying the current comprises: applying cathodic current in a vicinity of the cathodic longitudinal site; and applying primary anodal current to the nerve in a vicinity of the primary anodal longitudinal site, and comprising applying secondary anodal current to the nerve in a vicinity of a secondary anodal longitudinal site of the nerve that is closer to the primary anodal longitudinal site than to the cathodic longitudinal site. 153. A method according to claim 152, wherein applying the cathodic current comprises applying the cathodic current such that, relative to the anodal longitudinal sites, the cathodic longitudinal site is proximal to a brain of a subject, the subject including the nerve.154. A method according to claim 152, wherein applying the cathodic current comprises applying the cathodic current such that, relative to the anodal longitudinal sites, the cathodic longitudinal site is distal to a brain of a subject, the subject including the nerve.155. A method according to claim 152, wherein applying the primary anodal current comprises configuring the primary anodal current so as to block propagation of action potentials past the primary anodal longitudinal site.156. A method according to claim 152, wherein applying the primary anodal current comprises configuring the primary anodal current so as to block propagation past the primary anodal longitudinal site of action potentials in a first set of nerve fibers, and to allow propagation past the primary anodal longitudinal site of action potentials in a second set of nerve fibers, the second set of nerve fibers having characteristic diameters generally smaller than characteristic diameters of the nerve fibers in the first set.157. A method according to claim 152, wherein applying the cathodic current comprises applying the cathodic current at a plurality of cathodic sites in the vicinity of the cathodic longitudinal site of the nerve, at respective positions around the axis of the nerve.158. A method according to claim 157, wherein applying the cathodic current comprises applying the cathodic current at a characteristic frequency greater than 1000 Hz.159. A method according to claim 152, wherein providing the insulation comprises providing the insulation at a position with respect to the cathodic longitudinal site and the primary anodal longitudinal site so as to guide the flow of current between the cathodic longitudinal site and the primary anodal longitudinal site.160. A method according to claim 152, wherein providing the insulation from the insulation site comprises providing primary insulation from a primary insulation site, and comprising providing secondary insulation from a secondary insulation site that is located between the primary anodal longitudinal site and the secondary anodal longitudinal site.161. A method according to claim 160, wherein a characteristic distance of the secondary insulation site to the axis of the nerve is greater than a characteristic distance of the primary insulation site to the axis of the nerve.162. A method according to claim 152, comprising applying tertiary anodal current to the nerve in a vicinity of a tertiary anodal longitudinal site of the nerve that is closer to the secondary anodal longitudinal site than to the primary anodal longitudinal site.163. A method according to claim 162, wherein applying the tertiary anodal current comprises configuring a current density of the tertiary anodal current to be of lower magnitude than a magnitude of a current density of the secondary anodal current.164. A method according to claim 152, wherein a closest cathodic longitudinal site distance to the axis, a closest primary anodal longitudinal site distance to the axis, and a closest secondary anodal longitudinal site distance to the axis are all at least approximately 1.5 times greater than the radius of the nerve.165. A method according to claim 152, wherein applying the primary and secondary anodal currents comprises configuring a current density of the secondary anodal current to be of lower magnitude than a magnitude of a density of the primary anodal current.166. A method according to claim 165, wherein applying the primary and secondary anodal currents comprises configuring the primary anodal current to be substantially of the same magnitude as the secondary anodal current.167. A method according to claim 165,wherein applying the primary anodal current comprises driving the primary anodal current through a primary electrical impedance associated with the primary anodal longitudinal site, the primary impedance having a primary level of impedance, and wherein applying the secondary anodal current comprises driving the secondary anodal current through a secondary electrical impedance associated with the secondary anodal longitudinal site, the secondary impedance having a secondary level of impedance having a higher magnitude than the primary level of impedance. 168. A method according to claim 165, wherein the secondary anodal longitudinal site is at a secondary anodal distance from the axis of the nerve, and wherein the primary longitudinal site is at a primary anodal distance from the axis of the nerve that is smaller than the secondary anodal distance.169. A method according to claim 168, wherein a ratio of the secondary anodal distance to the primary anodal distance is greater than approximately 1.5:1.170. A method according to claim 152, comprising applying primary fiber-selection anodal current to the nerve in a vicinity of a primary fiber-selection anodal longitudinal site of the nerve that is closer to the cathodic longitudinal site than to the primary anodal longitudinal site.171. A method according to claim 170, comprising applying secondary fiber-selection anodal to the nerve in a vicinity of a secondary fiber-selection anodal longitudinal site of the nerve that is closer to the primary fiber-selection anodal longitudinal site than to the cathodic longitudinal site.172. The method according to claim 152, wherein applying the primary anodal current comprises driving the primary anodal current through a resistance associated with the primary anodal longitudinal site, and wherein applying the secondary anodal current comprises driving the secondary anodal current through a resistance associated with the secondary anodal longitudinal site, the resistance associated with the secondary anodal longitudinal site being higher than the resistance associated with the primary anodal longitudinal site.173. A method according to claim 152, wherein applying the cathodic current and the primary anodal current comprises configuring an amplitude of a current density of the cathodic current to be between 1.1 and 2 times greater than an amplitude of a current density of the primary anodal current.174. A method according to claim 152, wherein applying the cathodic current and the secondary anodal current comprises configuring an amplitude of a current density of the cathodic current to be between 3 and 6 times greater than an amplitude of a current density of the secondary anodal current.175. A method according to claim 152, wherein applying the primary and second anodal currents comprises configuring an amplitude of a current density of the primary anodal current to be at least 2 times greater than an amplitude of a current density of the secondary anodal current.176. A method according to claim 150,wherein the first and second current-application sites include a cathodic current-application site and an anodal current-application site, respectively, wherein the cathodic and anodal current-application sites are located at respective closest cathodic and anodal distances to the axis, and wherein applying the current comprises applying cathodic current and anodal current from the cathodic and anodal current-application sites, respectively. 177. A method according to claim 176, wherein the closest cathodic and anodal distances to the axis are both at least approximately 2 times greater than the radius of the nerve.178. A method according to claim 176, wherein the cathodic longitudinal site, relative to the anodal longitudinal site, is in a vicinity of the nerve which is proximal to a brain of a subject, the subject including the nerve.179. A method according to claim 176, wherein the cathodic longitudinal site of the nerve, relative to the anodal longitudinal site, is in a vicinity of the nerve which is distal to a brain of a subject, the subject including the nerve.180. A method according to claim 176, wherein applying the cathodic current comprises applying the cathodic current at a plurality of cathodic sites in a vicinity of the cathodic longitudinal site of the nerve, at respective positions around the axis of the nerve.181. A method according to claim 176, wherein applying the anodal current comprises configuring the anodal current so as to block propagation of action potentials past the anodal current-application site.182. A method according to claim 176, wherein applying the anodal current comprises setting the anodal current to be at a level configured so as to block propagation past the anodal current-application site of action potentials in a first set of nerve fibers, and to allow propagation past the anodal current-application site of action potentials in a second set of nerve fibers, the second set of nerve fibers having characteristic diameters generally smaller than characteristic diameters of the nerve fibers in the first set.183. A method according to claim 176, wherein a characteristic distance of the anodal current-application site to the axis is within 30% of the characteristic closest insulation distance plus a width of the anodal current-application site.184. A method according to claim 150,wherein the first and second current-application sites include respective first and second longitudinal current-application sites, wherein the insulation site includes a longitudinal insulation site that is between the first and second longitudinal current-application sites, and wherein providing the insulation comprises providing the insulation at the longitudinal insulation site. 185. A method according to claim 150, wherein the insulation site is between the first and second current-applications sites, with respect to the axis.186. A method according to claim 150,wherein the first current-application site includes a cathodic current-application site, wherein applying the current comprises: applying cathodic current from the cathodic current-application site; and applying respective anodal currents from a plurality of anodal longitudinal current-application sites, which anodal currents define, in combination, an anodal activation function having: (a) a hyperpolarizing portion thereof having a maximum hyperpolarizing amplitude, and (b) a depolarizing portion thereof, having a maximum depolarizing amplitude corresponding to a depolarizing site on the nerve distal with respect to the cathodic current-application site to a site corresponding to the hyperpolarizing portion, wherein the maximum hyperpolarizing amplitude is at least five times greater than the maximum depolarizing amplitude, and wherein the second current-application site includes one of the plurality of anodal longitudinal current-application sites. 187. A method according to claim 186, wherein a distance of a first one of the anodal longitudinal current-application sites to the axis is less than a distance of a second one of the anodal longitudinal current-application sites to the axis, the first one of the anodal longitudinal current-application sites being closer to the cathodic current-application site than the second one of the anodal longitudinal current-application sites.188. A method according to claim 186, wherein one of the anodal longitudinal current-application sites is positioned so as to reduce a virtual cathode effect induced by one of the anodal currents applied at another one of the anodal longitudinal current-application sites.189. A method according to claim 186, wherein the cathodic current-application site and the anodal longitudinal current-application sites are disposed such that a first one of the anodal longitudinal current-application sites is between the cathodic current-application site and a second one of the anodal longitudinal current-application sites.190. A method according to claim 189, wherein the anodal longitudinal current-application sites are disposed such that the second one of the anodal longitudinal current-application sites is between the first one of the anodal longitudinal current-application sites and a third one of the anodal longitudinal current-application sites.191. A method according to claim 189, wherein applying the respective anodal currents comprises configuring a current density of the anodal current applied at the second one of the anodal longitudinal current-application sites to have a lower magnitude than a magnitude of a current density of the anodal current applied at the first one of the anodal longitudinal current-application sites.192. A method according to claim 191, wherein applying the respective anodal currents comprises configuring a ratio of the current density of the anodal current applied at the first anodal longitudinal current-application site to the current density of the anodal current applied at the second anodal longitudinal current-application site to be at least 2:1.193. A method according to claim 191, wherein applying the respective anodal currents comprises configuring a ratio of the current density of the anodal current applied at the first anodal longitudinal current-application site to the current density of the anodal current applied at the second anodal longitudinal current-application site to be at least 5:1.194. A method according to claim 150,wherein the first current-application site includes two or more surrounding current-application sites in a vicinity of a longitudinal site of the nerve, at respective positions around the axis, and wherein applying the current to the first current-application site comprises: (a) driving current between two of the surrounding current-application sites, thereby defining a first pair of the surrounding current-application sites and a first direction of current flow, and, less than one millisecond later, (b) driving current between two of the surrounding current-application sites, thereby defining a second pair of the surrounding current-application sites and a second direction of current flow, and (c) cycling between steps (a) and (b) at a rate greater than 1000 Hz, wherein at least either the first pair of surrounding current-application sites is different from the second pair of surrounding current-application sites or the first direction of current flow is different from the second direction of current flow. 195. A method according to claim 194, wherein the two or more surrounding current-application sites include three or more surrounding current-application sites.196. A method according to claim 194, wherein the two or more surrounding current-application sites include four or more surrounding current-application sites.197. A method according to claim 194, wherein cycling between steps (a) and (b) comprises cycling between steps (a) and (b) at a rate greater than 4000 Hz.198. A method according to claim 150,wherein the first current-application site includes a set of two or more cathodic current-application sites in a vicinity of a cathodic longitudinal site of the nerve, at respective positions around the axis, wherein the second current includes a set of two or more anodal current-application sites in a vicinity of an anodal longitudinal site of the nerve, at respective positions around the axis, and wherein applying the current comprises: applying cathodic current from the set of cathodic current-application sites; and applying anodal current from the set of anodal current-application sites. 199. A method according to claim 198, wherein the two or more cathodic current-application sites include six or more cathodic current-application sites.200. A method according to claim 198, wherein the two or more cathodic current-application sites include twelve or more cathodic current-application sites.201. A method according to claim 198, wherein applying the current comprises driving current between respective ones of the cathodic current-application sites and respective ones of the anodal current-application sites.202. A method according to claim 201, wherein applying the current comprises cycling the current application at a rate greater than 1000 Hz.203. A method according to claim 201, wherein applying the current comprises completing a sweep of application of the current through substantially all of the cathodic current-application sites in less than 1000 microseconds.204. A method according to claim 201, wherein applying the current comprises completing a sweep of application of the current through substantially all of the cathodic current-application sites in less than 100 microseconds.205. A method according to claim 150, wherein providing the insulation comprises providing the insulation in a position with respect to the first and second current-application sites so as to guide the flow of current between the first and second current-application sites.206. A method according to claim 150, wherein the first and second closest current-application distances are both at least 30% greater than the closest insulation distance.207. A method according to claim 206,wherein the first and second current-application sites include respective first and second longitudinal current-application sites, wherein the insulation site includes a longitudinal insulation site that is between the first and second longitudinal current-application sites, and wherein providing the insulation comprises providing the insulation at the longitudinal insulation site. 208. A method according to claim 206, wherein the insulation site is between the first and second current-applications sites, with respect to the axis.209. A method according to claim 206, wherein providing the insulation comprises providing the insulation in a position with respect to the first and second current-application sites so as to guide the flow of current between the first and second current-application sites.210. A method according to claim 206,wherein the first current-application site includes a cathodic longitudinal site of the nerve, wherein the second current-application site includes a primary anodal longitudinal site of the nerve, and wherein applying the current comprises: applying cathodic current in a vicinity of the cathodic longitudinal site; and applying primary anodal current to the nerve in a vicinity of the primary anodal longitudinal site, and comprising applying secondary anodal current to the nerve in a vicinity of a secondary anodal longitudinal site of the nerve that is closer to the primary anodal longitudinal site than to the cathodic longitudinal site. 211. A method according to claim 210, wherein applying the cathodic current comprises applying the cathodic current such that, relative to the anodal longitudinal sites, the cathodic longitudinal site is proximal to a brain of a subject, the subject including the nerve.212. A method according to claim 210, wherein applying the cathodic current comprises applying the cathodic current such that, relative to the anodal longitudinal sites, the cathodic longitudinal site is distal to a brain of a subject, the subject including the nerve.213. A method according to claim 210, wherein applying the primary anodal current comprises configuring the primary anodal current so as to block propagation of action potentials past the primary anodal longitudinal site.214. A method according to claim 210, wherein applying the primary anodal current comprises configuring the primary anodal current so as to block propagation past the primary anodal longitudinal site of action potentials in a first set of nerve fibers, and to allow propagation past the primary anodal longitudinal site of action potentials in a second set of nerve fibers, the second set of nerve fibers having characteristic diameters generally smaller than characteristic diameters of the nerve fibers in the first set.215. A method according to claim 210, wherein applying the cathodic current comprises applying the cathodic current at a plurality of cathodic sites in the vicinity of the cathodic longitudinal site of the nerve, at respective positions around the axis of the nerve.216. A method according to claim 215, wherein applying the cathodic current comprises applying the cathodic current at a characteristic frequency greater than 1000 Hz.217. A method according to claim 210, wherein providing the insulation comprises providing the insulation at a position with respect to the cathodic longitudinal site and the primary anodal longitudinal site so as to guide the flow of current between the cathodic longitudinal site and the primary anodal longitudinal site.218. A method according to claim 210, wherein providing the insulation from the insulation site comprises providing primary insulation from a primary insulation site, and comprising providing secondary insulation from a secondary insulation site that is located between the primary anodal longitudinal site and the secondary anodal longitudinal site.219. A method according to claim 218, wherein a characteristic distance of the secondary insulation site to the axis of the nerve is greater than a characteristic distance of the primary insulation site to the axis of the nerve.220. A method according to claim 210, comprising applying tertiary anodal current to the nerve in a vicinity of a tertiary anodal longitudinal site of the nerve that is closer to the secondary anodal longitudinal site than to the primary anodal longitudinal site.221. A method according to claim 220, wherein applying the tertiary anodal current comprises configuring a current density of the tertiary anodal current to be of lower magnitude than a magnitude of a current density of the secondary anodal current.222. A method according to claim 210, wherein a closest cathodic longitudinal site distance to the axis, a closest primary anodal longitudinal site distance to the axis, and a closest secondary anodal longitudinal site distance to the axis are all at least approximately 1.5 times greater than the radius of the nerve.223. A method according to claim 210, wherein applying the primary and secondary anodal currents comprises configuring a current density of the secondary anodal current to be of lower magnitude than a magnitude of a density of the primary anodal current.224. A method according to claim 223, wherein applying the primary and secondary anodal currents comprises configuring the primary anodal current to be substantially of the same magnitude as the secondary anodal current.225. A method according to claim 223,wherein applying the primary anodal current comprises driving the primary anodal current through a primary electrical impedance associated with the primary anodal longitudinal site, the primary impedance having a primary level of impedance, and wherein applying the secondary anodal current comprises driving the secondary anodal current through a secondary electrical impedance associated with the secondary anodal longitudinal site, the secondary impedance having a secondary level of impedance having a higher magnitude than the primary level of impedance. 226. A method according to claim 223, wherein the secondary anodal longitudinal site is at a secondary anodal distance from the axis of the nerve, and wherein the primary longitudinal site is at a primary anodal distance from the axis of the nerve that is smaller than the secondary anodal distance.227. A method according to claim 226, wherein a ratio of the secondary anodal distance to the primary anodal distance is greater than approximately 1.5:1.228. A method according to claim 210, comprising applying primary fiber-selection anodal current to the nerve in a vicinity of a primary fiber-selection anodal longitudinal site of the nerve that is closer to the cathodic longitudinal site than to the primary anodal longitudinal site.229. A method according to claim 228, comprising applying secondary fiber-selection anodal to the nerve in a vicinity of a secondary fiber-selection anodal longitudinal site of the nerve that is closer to the primary fiber-selection anodal longitudinal site than to the cathodic longitudinal site.230. The method according to claim 210, wherein applying the primary anodal current comprises driving the primary anodal current through a resistance associated with the primary anodal longitudinal site, and wherein applying the secondary anodal current comprises driving the secondary anodal current through a resistance associated with the secondary anodal longitudinal site, the resistance associated with the secondary anodal longitudinal site being higher than the resistance associated with the primary anodal longitudinal site.231. A method according to claim 210, wherein applying the cathodic current and the primary anodal current comprises configuring an amplitude of a current density of the cathodic current to be between 1.1 and 2 times greater than an amplitude of a current density of the primary anodal current.232. A method according to claim 210, wherein applying the cathodic current and the secondary anodal current comprises configuring an amplitude of a current density of the cathodic current to be between 3 and 6 times greater than an amplitude of a current density of the secondary anodal current.233. A method according to claim 210, wherein applying the primary and second anodal currents comprises configuring an amplitude of a current density of the primary anodal current to be at least 2 times greater than an amplitude of a current density of the secondary anodal current.234. A method according to claim 206,wherein the first and second current-application sites include a cathodic current-application site and an anodal current-application site, respectively, wherein the cathodic and anodal current-application sites are located at respective closest cathodic and anodal distances to the axis, and wherein applying the current comprises applying cathodic current and anodal current from the cathodic and anodal current-application sites, respectively. 235. A method according to claim 234, wherein the closest cathodic and anodal distances to the axis are both at least approximately 2 times greater than the radius of the nerve.236. A method according to claim 234, wherein the cathodic longitudinal site, relative to the anodal longitudinal site, is in a vicinity of the nerve which is proximal to a brain of a subject, the subject including the nerve.237. A method according to claim 234, wherein the cathodic longitudinal site of the nerve, relative to the anodal longitudinal site, is in a vicinity of the nerve which is distal to a brain of a subject, the subject including the nerve.238. A method according to claim 234, wherein applying the cathodic current comprises applying the cathodic current at a plurality of cathodic sites in a vicinity of the cathodic longitudinal site of the nerve, at respective positions around the axis of the nerve.239. A method according to claim 234, wherein applying the anodal current comprises configuring the anodal current so as to block propagation of action potentials past the anodal current-application site.240. A method according to claim 234, wherein applying the anodal current comprises configuring the anodal current so as to block propagation past the anodal current-application site of action potentials in a first set of nerve fibers, and to allow propagation past the anodal current-application site of action potentials in a second set of nerve fibers, the second set of nerve fibers having characteristic diameters generally smaller than characteristic diameters of the nerve fibers in the first set.241. A method according to claim 234, wherein a characteristic distance of the anodal current-application site to the axis is within 30% of the characteristic closest insulation distance plus a width of the anodal current-application site.242. A method according to claim 206,wherein the first current-application site includes a cathodic current-application site, wherein applying the current comprises: applying cathodic current from the cathodic current-application site; and applying respective anodal currents from a plurality of anodal longitudinal current-application sites, which anodal currents define, in combination, an anodal activation function having: (a) a hyperpolarizing portion thereof having a maximum hyperpolarizing amplitude, and (b) a depolarizing portion thereof, having a maximum depolarizing amplitude corresponding to a depolarizing site on the nerve distal with respect to the cathodic current-application site to a site corresponding to the hyperpolarizing portion, wherein the maximum hyperpolarizing amplitude is at least five times greater than the maximum depolarizing amplitude, and wherein the second current-application site includes one of the plurality of anodal longitudinal current-application sites. 243. A method according to claim 242, wherein a distance of a first one of the anodal longitudinal current-application sites to the axis is less than a distance of a second one of the anodal longitudinal current-application sites to the axis, the first one of the anodal longitudinal current-application sites being closer to the cathodic current-application site than the second one of the anodal longitudinal current-application sites.244. A method according to claim 242, wherein one of the anodal longitudinal current-application sites is positioned so as to reduce a virtual cathode effect induced by one of the anodal currents applied at another one of the anodal longitudinal current-application sites.245. A method according to claim 242, wherein the cathodic current-application site and the anodal longitudinal current-application sites are disposed such that a first one of the anodal longitudinal current-application sites is between the cathodic current-application site and a second one of the anodal longitudinal current-application sites.246. A method according to claim 245, wherein the anodal longitudinal current-application sites are disposed such that the second one of the anodal longitudinal current-application sites is between the first one of the anodal longitudinal current-application sites and a third one of the anodal longitudinal current-application sites.247. A method according to claim 245, wherein applying the respective anodal currents comprises configuring a current density of the anodal current applied at the second one of the anodal longitudinal current-application sites to have a lower magnitude than a magnitude of a current density of the anodal current applied at the first one of the anodal longitudinal current-application sites.248. A method according to claim 247, wherein applying the respective anodal currents comprises configuring a ratio of the current density of the anodal current applied at the first anodal longitudinal current-application site to the current density of the anodal current applied at the second anodal longitudinal current-application site to be at least 2:1.249. A method according to claim 247, wherein applying the respective anodal currents comprises configuring a ratio of the current density of the anodal current applied at the first anodal longitudinal current-application site to the current density of the anodal current applied at the second anodal longitudinal current-application site to be at least 5:1.250. A method according to claim 206,wherein the first current-application site includes two or more surrounding current-application sites in a vicinity of a longitudinal site of the nerve, at respective positions around the axis, and wherein applying the current to the first current-application site comprises: (a) driving current between two of the surrounding current-application sites, thereby defining a first pair of the surrounding current-application sites and a first direction of current flow, and, less than one millisecond later, (b) driving current between two of the surrounding current-application sites, thereby defining a second pair of the surrounding current-application sites and a second direction of current flow, and (c) cycling between steps (a) and (b) at a rate greater than 1000 Hz, wherein at least either the first pair of surrounding current-application sites is different from the second pair of surrounding current-application sites or the first direction of current flow is different from the second direction of current flow. 251. A method according to claim 250, wherein the two or more surrounding current-application sites include three or more surrounding current-application sites.252. A method according to claim 250, wherein the two or more surrounding current-application sites include four or more surrounding current-application sites.253. A method according to claim 250, wherein cycling between steps (a) and (b) comprises cycling between steps (a) and (b) at a rate greater than 4000 Hz.254. A method according to claim 206,wherein the first current-application site includes a set of two or more cathodic current-application sites in a vicinity of a cathodic longitudinal site of the nerve, at respective positions around the axis, wherein the second current includes a set of two or more anodal current-application sites in a vicinity of an anodal longitudinal site of the nerve, at respective positions around the axis, and wherein applying the current comprises: applying cathodic current from the set of cathodic current-application sites; and applying anodal current from the set of anodal current-application sites. 255. A method according to claim 254, wherein the two or more cathodic current-application sites include six or more cathodic current-application sites.256. A method according to claim 254, wherein the two or more cathodic current-application sites include twelve or more cathodic current-application sites.257. A method according to claim 254, wherein applying the current comprises driving current between respective ones of the cathodic current-application sites and respective ones of the anodal current-application sites.258. A method according to claim 257, wherein applying the current comprises cycling the current application at a rate greater than 1000 Hz.259. A method according to claim 257, wherein applying the current comprises completing a sweep of application of the current through substantially all of the cathodic current-application sites in less than 1000 microseconds.260. A method according to claim 257, wherein applying the current comprises completing a sweep of application of the current through substantially all of the cathodic current-application sites in less than 100 microseconds.