Distribution of voltage-dependent ion channels.

Excitability measures for sensory and motor axons. (A) Extended excitability data for motor (●) and sensory (○) axons (n = 10; mean ± SEM [dashed lines]), both recorded using 1 ms test stimuli. (A) threshold electrotonus for conditioning levels of ± 40%, −70% and −100% of control threshold. (B) current–threshold (I–V) relationship for 100 ms and 200 ms conditioning stimuli. The 100 ms conditioning stimuli resulted in a larger decrease in excitability at −100% as less accommodation to hyperpolarization developed over the shorter time span. Note the greater accommodation of sensory axons in A and B. (C) Strength–duration time constant for motor (filled symbols) and sensory (open symbols) studies. Means (continuous lines) ± SEM (dotted lines). Data from published normal control studies are presented on the left (squares). Data from the 10 subjects in the present study (circles; lines link the same subject). The data on the right are the strength–duration time constants as estimated by the mathematical model for sensory and motor axons (triangles). Modified from reference¹⁵ with permission. SEM, standard error of the mean.

Individual motor nerve recordings and motor model. Individual motor nerve recordings (n = 10) of extended threshold electrotonus (A) (conditioning levels of ± 20%, ± 40%, −70% and −100% of unconditioned threshold), and I–V (C) (for clarity, only the 200 ms conditioning stimulus data is dis-played). Threshold electrotonus (B) and I–V (D) as generated by the motor axon model with variation of the voltage for half-activation of I _h from −87.3 to −127.3 mV in 5 mV steps. Reused from reference¹⁵ with permission.

Activity-dependent hyperpolarisation of motor axons due to voluntary contraction. The median nerve was stimulated at the wrist and a 70% CMAP was tracked over the thenar muscles, using increments and decrements in stimulus intensity of 2%. In each panel, maximal voluntary contractions were performed 5 min after the onset of the traces. The increase in the normalised threshold represents the increase in current required to produce the control CMAP, and this reflects the axonal hyperpolarisation. The extent of hyperpolarisation and its duration depend on the duration of contraction (i.e., the impulse load). In A, B and C, the contractions lasted 15, 30 and 60 s, respectively. Each trace represents mean data for six subjects. Reused from reference²³ with permission. CMAP, compound muscle action potential; MVC, maximal voluntary contraction.

Activity-dependent changes in threshold for motor and sensory axons. Mean changes in threshold (± SEM) recorded for 9 subjects following repetitive stimulation of the median nerve at the wrist at 8 Hz for 10 min. Changes are shown for motor (A) and sensory axons (B) using test stimuli of 0.1 and 1 ms duration. Immediately following cessation of impulse trains, axons became less excitable, with a prominent increase in threshold, significantly greater for motor axons when compared to sensory. Reused from reference²⁴ with permission. SEM, standard error of the mean.

The development of conduction block in a human muscle spindle afferent. Raster display of action potentials of a muscle spindle afferent from extensor pollicis longus developing and recovering from conduction block. The muscle spindle afferent discharged irregularly at ~3 Hz when mild pressure was applied to the receptor. The action potential had two peaks generated at nodes of Ranvier on either side of the site of impaled internode.²⁸ The separation of the peaks therefore reflects internodal conduction time, and its pro-longation indicates the security of transmission. When conduction across the impaled internode was blocked the recorded potential consists of only a single peak generated proximal to the site of im-palement. When the pressure was increased (Pressure on), 10 min after the onset of the recording, the discharge rate of the afferent increased to ~20 Hz, and the second positive peak became unstable and disappeared. When pressure was relaxed (Pressure off), the second peak reappeared, only to disappear 2 min later when pressure was again increased. The longest interpeak interval was 975 μs in the first episode and 1.02 ms immediately after the second episode. The illustrated sequence contains 891 consecutive action potentials. Modified from reference²⁷ with permission.