Showing papers by "David Burke published in 1989"

The effects of voluntary contraction on the H reflex of human limb muscles.

Abstract: The effects of a voluntary contraction on the H reflexes of various muscles were quantified to determine whether the reflex responses were sufficiently reproducible to be used in diagnostic studies. During a voluntary contraction, H reflexes could be recorded reliably from tibialis anterior and abductor pollicis brevis, but accurate identification of the onset of the H wave from the on-going background EMG required duplicate averages of multiple responses. During a contraction the H reflex could be obtained at lower stimulus intensities in the forearm flexor muscles than when relaxed, and a more clear separation of the H wave from the M wave was possible. The background contraction abolished the attenuation of reflex amplitude with increasing stimulus repetition rates, such that repetition rates of up to 4 Hz could be used without significant loss of reflex amplitude. There were only small and usually insignificant differences in the latency of the H reflex or its variability when elicited with the forearm muscles relaxed and when flexor carpi radialis was contracting steadily. The reflex latencies of abductor pollicis brevis, tibialis anterior and soleus were compared with F wave latencies for these muscles. The minimal F wave latencies were shorter than the H reflex latencies for abductor pollicis brevis (mean 2.2 ms) and tibialis anterior (mean 1.0 ms) but not for soleus. Comparison of the spread of F wave latencies (F max-F min) suggests that, for soleus, F waves are recorded only from the faster conducting motor units in the pool, presumably those less readily recruited in the H reflex. It was calculated that the distribution of motor conduction velocities responsible for the F waves of abductor pollicis brevis was 8.8 m.s-1. This value underestimates the likely distribution of motor conduction velocities for the thenar muscle by as much as 50%, consistent with the view that F waves rarely occur in slowly conducting motor units, the units of lowest threshold in reflex studies. It is concluded that, for many motoneuron pools, the H reflex and the F wave appear preferentially in different motoneurons, low and high threshold, respectively, and that reflex studies can provide information not available from somatosensory evoked potentials or F wave studies.

Conduction velocities of muscle and cutaneous afferents in the upper and lower limbs of human subjects.

Abstract: In the cat and monkey the fastest axons in the peripheral nerve are group I afferents from muscle, but there are no definitive data on conduction velocity for these afferents in human subjects Knowledge of the relative conduction velocities of muscle and cutaneous afferents is important for the interpretation of reflex studies, evoked potentials and other aspects of motor control To rectify this deficiency, the conduction velocities of the fastest muscle and cutaneous afferents were determined for the median, ulnar and tibial nerves of normal subjects Low-threshold muscle afferents innervating abductor pollicis brevis, abductor digiti minimi and abductor hallucis were stimulated selectively through a microelectrode inserted percutaneously at the motor point Low-threshold cutaneous afferents were stimulated with ring electrodes around the proximal phalanx of digits II or V for the upper limb and digit II for the lower limb Compound action potentials were recorded with bipolar near-nerve electrodes at two sites in the proximal limb segment and conduction velocities of the fastest afferents in the neural volley calculated The mean conduction velocities of the muscle and cutaneous afferents were, respectively, 747±65 ms-1 and 803±67 ms-1 for the median nerve, 675±102 ms-1 and 675±105 ms-1 for the ulnar nerve, and 547±34 ms-1 and 528±32 ms-1 for the tibial nerve For upper and lower limb nerves the conduction velocities of low-threshold muscle and cutaneous afferents were not significantly different when measured over the same proximal segment

Changes in excitability of human cutaneous afferents following prolonged high-frequency stimulation

Abstract: Prolonged high-frequency stimulation of cutaneous nerves can result in paraesthesiae that begin 20 to 30 s after the end of the train and last for 5 to 10 min. In the present experiments the effects of such stimulation on the excitability of human cutaneous afferents and on their refractory and supernormal periods were measured to determine whether these changes could explain the postactivation paraesthesiae. Attention was focused on the axons of lowest threshold (1.0-1.5 T) in the compound sensory action potential evoked by stimulating the digital nerves of the index or middle fingers. Repetitive activation produced two opposing effects on the excitability of low-threshold cutaneous afferents. Following stimulus trains of short duration (1-5 min) the dominant effect was a long-lasting decrease in excitability, such that the amplitude of a test afferent volley was always less than before stimulation. With these trains, no subject experienced paraesthesiae. For 10 min after stimulus trains lasting longer than 7 to 12 min the dominant effect was an increase in excitability such that the amplitude of the test volley was greater than before stimulation. Within this interval, following such trains, subjects experienced paraesthesiae. The extent and duration of supernormality induced by a supramaximal conditioning stimulus were greatly increased by stimulation for 1 min. Following stimulation for 10 min, the degree of supernormality of the enhanced test volley was much the same as before stimulation, but was inappropriately high for the size of the test volley. The sum total of the excitability change and the change in supernormality resulted in a larger potential after stimulation, whether the train lasted 1 min or 10 min. It is concluded that the postactivation changes in axonal excitability could predispose the most excitable axons to generate ectopic impulses and, thereby, to produce paraesthesiae.

Paraesthesiae and hypaesthesia following prolonged high-frequency stimulation of cutaneous afferents

Abstract: The activity of cutaneous afferents was recorded in human subjects using microelectrodes inserted into individual fascicles of the median nerve at the wrist before and after a 10 min train of electrical stimuli at 200 Hz delivered to the appropriate digital nerve (via ring electrodes) or to individual afferent axons (via the microelectrode). Changes in neural activity produced by the stimulation were correlated with the time course of paraesthesiae and with changes in the ability to detect cutaneous stimuli. From approximately 20 s after the end of the stimulus train, there was a progressive increase in neural activity, and individual afferents became spontaneously active and discharged in high-frequency bursts. At this time the subjects began to experience paraesthesiae. Repetitive stimulation proximal to a complete digital nerve block induced paraesthesiae that were felt distal to the block in the insensate digit, indicating that they did not arise from the unmyelinated terminal segment of the axon or from a stimulus-induced disorder of receptor function. Recordings of the compound action potential evoked by submaximal test stimuli were made after the 10 min stimulus train and revealed evidence of an early transient increase in excitability superimposed on a long-lasting decrease in excitability, reaching a nadir approximately 30-40 min after the end of the repetitive stimulation. In parallel recordings, there was no detectable change in the cutaneous afferent volley evoked by mechanical stimulation, paraesthesiae, can be attributed directly to a disturbance in peripheral afferent fibres, while the poststimulation negative symptoms such as hypaesthesia arise from stimulation-induced refractoriness at central synaptic relays.