Nevell F. Skuse

Myogenic potentials generated by a click-evoked vestibulocollic reflex.

Electromyograms (EMGs) were recorded from surface electrodes over the sternomastoid muscles and averaged in response to brief (0.1 ms) clicks played through headphones. In normal subjects, clicks 85 to 100 dB above our reference (45 dB SPL: close to perceptual threshold for normal subjects for such clicks) evoked reproducible changes in the averaged EMG beginning at a mean latency of 8.2 ms. The earliest potential change, a biphasic positive-negativity (p13-n23), occurred in all subjects and the response recorded from over the muscle on each side was predominantly generated by afferents originating from the ipsilateral ear. Later potentials (n34, p44), present in most but not all subjects, were generated bilaterally after unilateral ear stimulation. The amplitude of the averaged responses increased in direct proportion to the mean level of tonic muscle activation during the recording period. The p13-n23 response was abolished in patients who had undergone selective section of the vestibular nerve but was preserved in subjects with severe sensorineural hearing loss. It is proposed that the p13-n23 response is generated by activation of vestibular afferents, possibly those arising from the saccule, and transmitted via a rapidly conducting oligosynaptic pathway to anterior neck muscles. Conversely, the n34 and p44 potentials do not depend on the integrity of the vestibular nerve and probably originate from cochlear afferents.

Fibre function and perception during cutaneous nerve block.

In awake human subjects, neural responses in radial nerves to electrical stimulation were recorded with intrafascicular tungsten microelectrodes. Changes in the activity of individual fibre groups during blocking procedures were recorded and correlated with simultaneous alterations in the perception of standardized stimuli. Light touch sensibility in hairy skin appeared to depend on the integrity of A-beta-gamma fibres, cold and pinprick on A-delta fibres, and warmth and dull pain on C fibres.

Task-dependent changes in the responses to low-threshold cutaneous afferent volleys in the human lower limb

1. In seven human subjects who were standing without support the sural nerves were stimulated electrically using trains of non‐painful stimuli (five pulses at 300 Hz), designed to activate afferents from cutaneous mechanoreceptors. The reflex effects of the stimulus train on different muscles of the ipsilateral and contralateral legs were sought in post‐stimulus averages of rectified EMG. Changes in the pattern of reflex influence were investigated when the subjects maintained different postures. 2. Clear reflex responses were seen in ipsilateral tibialis anterior, soleus, biceps femoris and vastus lateralis, but only when the muscles were actively contracting. In each muscle, inhibition was the dominant reflex response within the first 100 ms. In four of the seven subjects, reflex changes were detectable in the contralateral tibialis anterior and soleus, the peak‐to‐peak modulation within the first 200 ms being 25‐50% of that for the homologous ipsilateral muscle. 3. When subjects attempted to stand on a tilted platform, an unstable platform or on one leg with the other flexed, different combinations of muscles were active, involving both flexors and extensors or predominantly flexors or predominantly extensors. In each posture the reflex effects were demonstrable only in the active muscles. 4. With ipsilateral tibialis anterior, there were task‐dependent changes in the short‐latency components of the EMG response, approximately 60 ms and 80 ms after the stimulus. When seated performing voluntary contractions these components were difficult to define, and when standing on a platform tilted toe‐up they were small. When the ipsilateral leg was flexed or when standing on an unstable base, these early components were more prominent in each subject. With contralateral tibialis anterior, the dominant reflex pattern was inhibition when seated and contracting voluntarily, and facilitation during bipedal stance tilted toe‐up. These changes in reflex pattern could not be explained by different levels of background contraction. 5. It is concluded that cutaneous mechanoreceptors of the foot have widespread reflex actions on muscles throughout both limbs, particularly the ipsilateral limb, and that the reflex pattern in different muscles and within a single muscle may change dependent on the task that the subject is undertaking. These task‐dependent changes indicate plasticity in the expression of cutaneous reflex activity, affecting both short‐latency spinal as well as long‐latency pathways.

Interactions between cutaneous and muscle afferent projections to cerebral cortex in man

In order to demonstrate interactions between cutaneous and muscle afferent volleys in the ascending somatosensory pathways, different nerves of the lower limb were stimulated together in a conditioning-test paradigm, the changes in the earliest component of the cerebral potential evoked by the test stimulus being taken to indicate such an interaction. It was first confirmed that the cerebral potential evoked by stimulation of the posterior tibial nerve at the ankle is derived from muscle afferents in the mixed nerve and has shorter latencies than the cerebral potential evoked by purely cutaneous volleys in the sural nerve (see Burke et al. 1981). Complete suppression of the cerebral potential evoked by stimulation of muscle or cutaneous afferents was produced by conditioning volleys in a different nerve or in a different fascicle of the same nerve. The major factors determining the degree of suppression were found to be the relative sizes of the conditioning and test volleys and their timing, rather than whether the volleys were of cutaneous or muscular origin. It is concluded that the transmission of cutaneous or muscle afferent volleys to cortex can be profoundly altered in normal subjects by conditioning activity. The possibility that normal background afferent activity can similarly modify afferent transmission has implications for diagnostic studies, particularly when they are performed under non-standard conditions, such as in the operating theatre or intensive care unit. It is also concluded that, although a subject may perceive cutaneous paraesthesiae when the posterior tibial nerve is stimulated at the ankle, there may be no cutaneous component to the evoked cerebral potential.

Cutaneous and muscle afferent components of the cerebral potential evoked by electrical stimulation of human peripheral nerves

Abstract The cerebral potentials evoked by stimulation of the posterior tibial and sural nerves at ankle level were compared in normal human subjects. The latency of onset and latency to peak of the first positive wave were shorter by 5.6 msec and 6.7 msec respectively with posterior tibial stimulation. Stimulation of purely cutaneous posterior tibial afferents using ring electrodes around the hallux evoked a potential of longer latency than could be accounted for by the additional conduction time from hallux to ankle, indicating that the latencies of the cerebral potential evoked by posterior tibial stimulation at the ankle cannot be explained by the cutaneous afferents coursing in the nerve. Peripheral nerve conduction velocities were calculated for the ankle-popliteal fossa segment: in all subjects, the conduction velocity of the afferent potential due to posterior tibial stimulation at ankle level was 5–6 m/sec faster than the afferent potential due to purely cutaneous afferents in the sural or posterior tibial nerves. In one subject, muscle afferents in the posterior tibial nerve at the ankle were stimulated selectively using a microelectrode inserted into pure muscle nerve fascicles innervating the intrinsic muscles of the foot. The resulting cerebral potential had the same latency as the potential evoked by surface stimulation of the posterior tibial nerve trunk. It is concluded that in man group I muscle afferents from the lower limbs project to cerebral level and are responsible for the shorter latency of the cerebral potential evoked by stimulation of the posterior tibial nerve at the ankle. Appropriate calculations suggest that most of the discrepancy in the latencies of the cerebral potentials evoked by posterior tibial and sural stimulation may be attributed to the differences in conduction velocity of the primary afferent fibres.

Sensory conduction of the sural nerve in polyneuropathy

Using surface electrodes, sensory nerve action potentials (SAP) have been recorded in the proximal segment (mid-calf to lateral malleolus) and the distal segment (lateral malleolus to toe 5) of the sural nerve and in the median nerve in 79 control subjects. The values obtained for the distal segment of the sural nerve varied widely and in seven apparently normal subjects no SAP could be distinguished. In the proximal segment conduction velocities were over 40 m/s and there was no significant change with age, unlike the median nerve in which a highly significant slowing occurred with age. Comparison of the results of sural and median sensory conduction studies in 300 consecutive patients screened for sensory polyneuropathy confirms the value of sural nerve sensory studies as a routine screening test, and confirms the belief that the changes in polyneuropathy are usually more prominent in lower limb nerves. It is therefore suggested that studies of sural sensory conduction form the single most useful test in the diagnosis of sensory polyneuropathy.

Anticipation and fusimotor activity in preparation for a voluntary contraction.

1. Multi‐unit and single‐unit recordings were made of muscle spindle afferent activity from the pretibial muscles of human subjects who were initially relaxed. The muscles were subjected to a stretching perturbation of 1 s duration, occurring irregularly, on average once every 5 s. In test sequences, an auditory or visual warning was provided 1 . 06 s before some of the perturbations. Subjects were required to oppose every perturbation by contracting the receptor‐bearing muscle as rapidly as possible. 2. Following the warning all subjects sometimes tensed the receptor‐bearing muscle unintentionally in preparation for the perturbation. In these contractions, the discharge of a spindle ending accelerated only if the contraction strength exceeded the endings threshold for activation, established in control voluntary contractions performed under isometric conditions. 3. When the receptor‐bearing muscle did not contract in the interval between warning and perturbation, there was no detectable change in the multi‐unit recordings of spindle activity or in recordings from twelve of thirteen single spindle afferents. The thirteenth spindle afferent discharged prior to the perturbation in the absence of detectable e.m.g. in response to (only) three of twenty‐three warning stimuli. However, this ending had been so responsive during isometric voluntary contractions that a contraction level at which it did not respond could not be established, and it is suggested that the findings with this ending resulted from its low threshold rather than from selective activation of the fusimotor system. 4. When subjects were warned of the perturbations, the dynamic response of spindle endings to the perturbations was not increased in size or altered in latency. 5. The motor response to perturbations without warning generally contained only long‐latency (volitional) e.m.g. activity occurring 107‐‐200 ms after the onset of the perturbation. When a warning was given, short‐latency (reflex) e.m.g. activity was also recorded, beginning 46‐‐76 ms after the onset of the perturbation. 6. It is concluded that anticipation of the need to contract a muscle does not result in selective activation of fusimotor neurones in preparation for the contraction. The change in stretch reflex gain that occurs as a result of ‘anticipation’ occurs through a central process which does not involve the fusimotor system.

Voluntary activation of spindle endings in human muscles temporarily paralysed by nerve pressure.

1. In normal human subjects, the afferent activity from muscle spindle endings in the pretibial muscles was recorded while a pressure block was applied to the peroneal nerve proximally in the popliteal fossa. 2. In five of ten blocks, spindle activity increased in attempted isometric voluntary contractions when the receptor‐bearing muscles were completely paralysed. In the remaining five blocks, voluntary effort still increased spindle activity when maximum voluntary power was reduced by more than 90%, but the ability to activate spindles voluntarily was lost with or slightly before block of the last motor units. When the ability to activate spindle endings in an attempted voluntary contraction was lost sympathetic efferent fibres remained unblocked. 3. It is concluded that the fusimotor effects seen during a voluntary contraction are mediated by myelinated fibres of small calibre which probably innervate intrafusal structures exclusively (gamma fusimotor fibres). There is no necessity to postulate that skeleto‐fusimotor (beta) fibres are responsible for the tight alpha‐gamma co‐activation seen in man during voluntary contractions.

Cutaneous afferent activity in median and radial nerve fascicles: a microelectrode study.

Cutaneous afferent activity was recorded in fascicles of the median and radial nerves of normal subjects using percutaneous microelectrodes. Multi-unit fascicular responses were dominated by activity in large myelinated fibres. Easily tolerated electrical stimuli evoked the full spectrum of fast and slow myelinated fibre activity but more intense levels were required to activate unmyelinated fibres. Correlation of the evoked potentials and the sensations reported by the subject suggested that fast myelinated fibres mediate tactile sensations. Pricking pain appeared with the activation of slow myelinated fibres. The only sensations induced by electrical stimuli were tactile or painful.

Recruitment order of human spindle endings in isometric voluntary contractions

1. The responses of twenty‐two spindle endings in the anterior tibial and toe extensor muscles of human subjects were studied during isometric voluntary contractions of the receptor‐bearing muscle with the ankle joint fixed in 25° plantar flexion.