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Threshold and conduction velocity in human median nerve sensory fibers
718
INTERNATIONAL FEDERATION - 7TH CONGRESS
lying pathology is discussed.
16.
Motor unit frequency control in facial neuropathy.-J. H. Petajan (College, Alaska, U.S.A.).
Using audiovisual feedback of motor unit action potentials, the ability of 11 patients with facial neuropathy to control firing rate was examined. The following mean interspike intervals were determined when firing rate was: (1) at its lowest most stable level, o n s e t interval and (2) increased to a level at which recruitment of a second motor unit occurred, r e c r u i t m e n t interval. Mean onset interval was decreased while mean recruitment interval was unchanged. In 20% of orbicularis oris and 12% of frontalis motor units studied a second motor unit was not recruited despite high firing rates while in normal subjects recruitment always occurs. The findings can be explained by a loss of larger motor units as would be produced by ischemia or pressure block. Regardless of the number of motor units lost in any territory, if recruitment can occur, then it occurs when the primary unit reaches a critical firing rate, the normal recruitment interval.
17. Threshold and conduction velocity in human median nerve sensory fibers.-W. C. Wiederholt (Columbus, Ohio, U.S.A.). Fast conducting nerve fibers have a lower threshold than slow conducting fibers. Contrary to this generally accepted fact, Hodes et al. concluded from study of human motor nerves that fibers of low threshold conduct more slowly than those of high threshold. Recently, Wiederholt demonstrated in isolated mixed mammalian nerves that increasing stimulus intensities produce progressive shortening of latency, no change of conduction velocity, but movement of the effective point of excitation away from the stimulating cathode. Conduction velocity in median nerve sensory fibers and its relation to stimulus intensity was studied in 30 normal subjects. Sensory fibers were stimulated with square pulses (0.1 msec, 30 to 110V) at the base of the third digit. Evoked nerve action potentials were recorded simultaneously from electrodes distal and proximal to the carpal tunnel. The distance from the distal electrode to the first electrode proximal to the carpal tunnel averaged 3.8 cm and between electrodes proximal to the carpal tunnel 1.94 cm. Conduction times between electrodes were used to calculate conduction velocities. With just supramaximal stimulation conduction velocity across the carpal tunnel was 54.4+10.3 (S.D.) m/sec and proximal to the carpal tunnel 64.3-+9.3 m/sec. When stimulation was increased from minimal (30 to 40 V) to just
supramaximal (80 to 110 V), latency from shock artifact to beginning of the evoked potential (proximal to carpal tunnel), decreased from 2.24:ff).24 msec to 2.085:0.23 msec but conduction velocity remained the same. This could result from movement of the effective point of excitation 1.04:b0.32 cm away from the stimulating cathode. The results suggest that (1) the fastest conducting sensory fibers in the human median nerve have a lower threshold than slower conducting fibers, and (2) the effective point of excitation moves away from the stimulating cathode as intensity of stimulation is increased.
18. Effect of local temperature on resting membrane potential in rat musele.-T. Nakanishi and F. H. Norris, Jr. (San Francisco, Calif., U.S.A.). The resting membrane potential (RMP) of human muscle has been measured as a diagnostic tool, but large variations have made determination of the normal range difficult. As o n e of the technical causes of such variation, the effect of local temperature on the RMP was investigated in rat muscle in vivo. The longissimus dorsi medialis muscle was irrigated with Liley's solution at various temperatures and RMPs measured within a depth of 1 mm from the surface using 3 M KCl-filled micropipette electrodes. The muscle RMP increased and decreased as the local temperature was raised and lowered between 25 and 44°C. Two experiments were performed. In one, fibers were studied for long periods of time while serial RMP measurements were made. The coefficient of the RMP was 1.3-+0.6 (S.D.)/aV per degree with a Q I 0 of 1.215:0.03 for increasing temperatures; the coefficient was 1.4:b0.7/aV per degree with a Q10 of 1.25:!:0.05 for decreasing temperatures. In the second experiment, 20 different fibers were impaled successfully at each of several temperatures. The coefficient of the RMP was 1.0-+0.4 /zV per degree with a Q10 of 1.155:0.08 for increasing temperatures; the coefficient was 1.1-+0.3/2V per degree with a Q I 0 of 1.16-+0.07 for decreasing temperatures. These linear relationships of RMP and temperature might account for some of the reported variations of the normal RMP.
19.
Subclinical lead neuropathy in man.-P. M. Fullerton and M. J. G. Harrison (London, G.B.).
Guinea pigs given lead acetate orally for several months often develop peripheral neuropathy, associated in some instances with considerable slowing of conduction. Pathological changes of segmental demyelination have been demonstrated in such animals.
INTERNATIONAL FEDERATION - 7TH CONGRESS
lying pathology is discussed.
16.
Motor unit frequency control in facial neuropathy.-J. H. Petajan (College, Alaska, U.S.A.).
Using audiovisual feedback of motor unit action potentials, the ability of 11 patients with facial neuropathy to control firing rate was examined. The following mean interspike intervals were determined when firing rate was: (1) at its lowest most stable level, o n s e t interval and (2) increased to a level at which recruitment of a second motor unit occurred, r e c r u i t m e n t interval. Mean onset interval was decreased while mean recruitment interval was unchanged. In 20% of orbicularis oris and 12% of frontalis motor units studied a second motor unit was not recruited despite high firing rates while in normal subjects recruitment always occurs. The findings can be explained by a loss of larger motor units as would be produced by ischemia or pressure block. Regardless of the number of motor units lost in any territory, if recruitment can occur, then it occurs when the primary unit reaches a critical firing rate, the normal recruitment interval.
17. Threshold and conduction velocity in human median nerve sensory fibers.-W. C. Wiederholt (Columbus, Ohio, U.S.A.). Fast conducting nerve fibers have a lower threshold than slow conducting fibers. Contrary to this generally accepted fact, Hodes et al. concluded from study of human motor nerves that fibers of low threshold conduct more slowly than those of high threshold. Recently, Wiederholt demonstrated in isolated mixed mammalian nerves that increasing stimulus intensities produce progressive shortening of latency, no change of conduction velocity, but movement of the effective point of excitation away from the stimulating cathode. Conduction velocity in median nerve sensory fibers and its relation to stimulus intensity was studied in 30 normal subjects. Sensory fibers were stimulated with square pulses (0.1 msec, 30 to 110V) at the base of the third digit. Evoked nerve action potentials were recorded simultaneously from electrodes distal and proximal to the carpal tunnel. The distance from the distal electrode to the first electrode proximal to the carpal tunnel averaged 3.8 cm and between electrodes proximal to the carpal tunnel 1.94 cm. Conduction times between electrodes were used to calculate conduction velocities. With just supramaximal stimulation conduction velocity across the carpal tunnel was 54.4+10.3 (S.D.) m/sec and proximal to the carpal tunnel 64.3-+9.3 m/sec. When stimulation was increased from minimal (30 to 40 V) to just
supramaximal (80 to 110 V), latency from shock artifact to beginning of the evoked potential (proximal to carpal tunnel), decreased from 2.24:ff).24 msec to 2.085:0.23 msec but conduction velocity remained the same. This could result from movement of the effective point of excitation 1.04:b0.32 cm away from the stimulating cathode. The results suggest that (1) the fastest conducting sensory fibers in the human median nerve have a lower threshold than slower conducting fibers, and (2) the effective point of excitation moves away from the stimulating cathode as intensity of stimulation is increased.
18. Effect of local temperature on resting membrane potential in rat musele.-T. Nakanishi and F. H. Norris, Jr. (San Francisco, Calif., U.S.A.). The resting membrane potential (RMP) of human muscle has been measured as a diagnostic tool, but large variations have made determination of the normal range difficult. As o n e of the technical causes of such variation, the effect of local temperature on the RMP was investigated in rat muscle in vivo. The longissimus dorsi medialis muscle was irrigated with Liley's solution at various temperatures and RMPs measured within a depth of 1 mm from the surface using 3 M KCl-filled micropipette electrodes. The muscle RMP increased and decreased as the local temperature was raised and lowered between 25 and 44°C. Two experiments were performed. In one, fibers were studied for long periods of time while serial RMP measurements were made. The coefficient of the RMP was 1.3-+0.6 (S.D.)/aV per degree with a Q I 0 of 1.215:0.03 for increasing temperatures; the coefficient was 1.4:b0.7/aV per degree with a Q10 of 1.25:!:0.05 for decreasing temperatures. In the second experiment, 20 different fibers were impaled successfully at each of several temperatures. The coefficient of the RMP was 1.0-+0.4 /zV per degree with a Q10 of 1.155:0.08 for increasing temperatures; the coefficient was 1.1-+0.3/2V per degree with a Q I 0 of 1.16-+0.07 for decreasing temperatures. These linear relationships of RMP and temperature might account for some of the reported variations of the normal RMP.
19.
Subclinical lead neuropathy in man.-P. M. Fullerton and M. J. G. Harrison (London, G.B.).
Guinea pigs given lead acetate orally for several months often develop peripheral neuropathy, associated in some instances with considerable slowing of conduction. Pathological changes of segmental demyelination have been demonstrated in such animals.