The refractory and supernormal periods of the human median nerve

Abstract

While the excitability changes which follow a propagated nerve impulse have been the subject of numerous studies in animals, there have been few references to this subject in man. An attempt to study the refractory period of the human ulnar nerve was made by Wagman and Flick (1951), but as recordings were made from the hypothenar muscles rather than from the nerve itself, the results are difficult to interpret. Krnjevic, Kilpatrick, and Aungle (1955) applied pairs of shocks to the ulnar nerve at the wrist, and investigated the least interval at which a response to the second shock could be recorded from the nerve at the elbow; only two subjects were examined and no estimate of the relative refractory period was made. Brown (1960) also used stimulation of the ulnar nerve at the wrist with recording at the elbow in four subjects, and noted that the recovery of normal responsiveness was followed by transient supernormality.In the present work we have studied the refractory and supernormal periods of the human median nerve with stimulation at the wrist, action potentials being recorded from the nerve above the elbow by a technique similar to that originally described by Dawson and Scott (1949). A briefpreliminaryaccount of this work has been published elsewhere (Gilliatt and Willison, 1962). METHODSFour healthy subjects were used, their ages ranging from 20 to 40 years. Most of the experiments were carried out on two subjects (R.W.G. and R.G.W.) but the main results were confirmed in the other two subjects as well.Before each experiment the subject sat with both arms in hot water to above the elbows for five or 10 minutes. He then lay covered with blankets on a couch, with the arm under examination wrapped in several layers of cotton wool. When these precautions were taken skin temperature, which was measured by a thermistor close to the stimulating cathode, remained above 35°C. throughout experiments lasting up to one hour; no 'Clinical research fellow, M.R.C. experiments are included in the present series in which skin temperature fell below 35°C.Two stimuli of independently variable intensity were delivered through a single cathode over the median nerve at the wrist. The stimulus interval could be varied in 0-1 msec. steps from 0 to 1 I-0 msec. and in 1 msec. steps for intervals longer than this. Each stimulus was a condenser discharge with a rapid rise-time, the decay having a time constant of 60, 100, or 160 psec. Voltage was continuously variable up to 350V. In most of the experiments a time constant of 160 jtsec. was used for both shocks but when small changes in nerve threshold were to be studied, for example, in plotting the supernormal period, a shorter time constant was used for the test shock. In this situation a brief shock had the advantage that a larger voltage range could be used when the physiological change in nerve threshold was small. Both stimuli were delivered through a single isolating transformer (ratio 1: 1) the output impedance being less than 1 kilohm. To allow mixing of stimuli from two...

Refractory period studies in a human neuromuscular preparation.

“…7 it was found that the RRP of the most excitable motor axon terminals, determined in this way, lasted approximately 5 ms from the end of the ARP. This result is double the value of 2.5 ms found for the motor axons in the forearm by Gilliatt and Willison (1963); the difference would be expected because it is known that small nerve fibres, such as those formed by repeated divisions and subdivisions of the parent motor axon, have long refractory periods, together with low conduction velocities and high thresholds for excitation (Blair and Erlanger, 1933). Apart from the measurements of refractoriness in terminal motor axons and muscle fibres, the present study has enabled for the first time the endplate potentials of human muscle to be recorded in situ.…”

“…Among the latter studies. the investigation of peripheral nerve by Gilliatt and Willison (1963) was noteworthy in that the significance of impulse slowing during the RRP was emphasised; more recent studies of nerve have been undertaken by Lowitzsch and Hopf (1972) and by Tackmann and Lehmann (1974). In all of the studies cited above the main trunks of peripheral nerves were examined and consequently there is no information presently available as to the refractoriness of the distal ramifications of sensory and motor nerve fibres.…”

Mechanically and electrically evoked somatosensory potentials in humans: Effects of stimulus presentation rate

“…The rates of stimulation used here were much slower than those described for the relative refractory period or supernormality following a single stimulus (Uttal 1959;Gilliatt and Willison 1963;Shagass and Schwartz 1964;Betts et al 1976). The amplitude decrease with increasing rate may, however, be explained by a subnormality similar to the post-train subnormality described for human motor axons at similar rates (Bergmans 1969) which is manifested in elevated threshold as well as increased latency of the motor unit potential.…”

Is resistance to ischaemia of motor axons in diabetic subjects due to membrane depolarization?

“…This phenomenon has been called superexcitability. An early description was given by Gilliat and Willison (1963). In axons, which are depolarized dueto ischaemia, no post-spike superexcitability can be seen.…”