Biomedical subjects

N F Skuse

Publications and source records attributed to N F Skuse.

11 recordsLinked to original sources

PubMed1979

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.

Humans↗

PubMed1979

The regularity of muscle spindle discharge in man.

1. The variability of discharge of thirty-nine muscle spindle afferents from the pretibial muscles of normal human subjects was determined for spike train sequences recorded with the ankle joint fixed in 25 degrees plantar flexion, during further stretch and during graded voluntary contractions of the receptor-bearing muscle. 2. In non-contracting muscles with the ankle joint in 25 degrees plantar flexion, a sustained discharge was maintained by twenty-four of the thirty-nine endings. The mean discharge frequency for the active endings was 11.1 Hz (range 4.8--22.1 Hz), the mean coefficient of variation 0.073 (range 0.021--0.183). With further stretch, the discharge of endings maintaining frequencies below 10--12 Hz became more regular. For endings maintaining higher frequencies, changes in the coefficient of variation were small and occurred in either direction. All secondary endings maintained a highly regular discharge, but, at these frequencies, there was no statistically significant difference in the variability of primary and secondary endings. 3. It is considered that these findings are comparable to those of Matthews & Stein (1969) for de-efferented feline spindle endings, and support the view that there is no functionally effective background fusimotor drive to non-contracting muscles of normal human subjects. 4. A voluntary contraction sufficient to accelerate a spindle ending invariably decreased the regularity of its afferent discharge. During voluntary contractions, coefficients of variation up to 0.345 were recorded. However, coefficients as low as 0.1 were not uncommon, and thus the absence of fusimotor drive cannot necessarily be inferred from a regular afferent discharge pattern. 5. With contractions of different strength, the increase in the coefficient of variation did not parallel the increase in discharge frequency. It is concluded that not all fusimotor influences acting on a spindle ending are translated into variability, and that measurements of the variability of discharge do not accurately reflect the level of fusimotor drive. 6. The discharge frequency of some spindle endings decreased slightly in some contractions and this was accompanied by an increase in the variability of discharge. It is suggested that contracting extrafusal muscle fibres can modulate the discharge pattern of spindle endings and contribute to the variability of discharge during a voluntary contraction. 7. In contracting muscles the irregular fusimotor-driven spindle discharge contained a 'hidden' periodicity, but this was not as extensive as has been reported for the cat. No such periodicity could be demonstrated for spindle endings in non-contracting human muscles.

Action Potentials↗

PubMed1978

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 degrees plantar flexion.2. The discharge of eighteen endings accelerated in voluntary contractions when the contraction strength exceeded a threshold level which differed for different endings but was reproducible for the same ending.3. With contractions of slow onset the latency to spindle acceleration varied with the speed of onset of the contraction. Endings with a background discharge were often unloaded by contractions until the contraction strength exceeded the threshold for activation of the ending.4. No correlation was found between the sensitivity of a spindle to external length changes and its ease of activation in a voluntary contraction. For two spindle endings with a background discharge there was no change in either discharge frequency or the regularity of spindle discharge during contractions which were below the threshold for activation of the endings. It is concluded that the threshold for activation of a spindle ending in an isometric voluntary contraction is determined by its fusimotor innervation, and that fusimotor neurones probably have a recruitment order, much as do skeletomotor neurones.5. Once activated, the discharge of spindle endings fluctuated with changes in skeletomotor activity but the relationship for some endings contained non-linearities. Such non-linearities were not as apparent in multi-unit recordings from a number of spindle endings in the contracting muscle. It is concluded that the fusimotor drive to a muscle is proportional to the skeletomotor drive to the muscle, and that skeletomotor and fusimotor neurones are subjected to similar if not identical descending command signals. From the work of Evarts (1968), it seems likely that these command signals are related more to desired muscle force than to desired muscle length.

Action Potentials↗

PubMed1977

A micro-electrode study of peripheral neuropathy in man. Part 1. Responses to single graded stimuli.

Multi-unit micro-electrode recording were obtained from sensory fascicles of the sural and median nerves of 12 control subjects and of 28 patients with peripheral neuropathy. Spontaneous activity and mass responses to mechanical and electrical stimuli were examined. Mechanoreceptor function appeared normal but there was a reduced number of responsive receptors in peripheral neuropathy. The electrical activation threshold of nerve fibres of all conduction velocities was increased in neuropathy and a greater number of fibres needed to be activated for preception to occur. Clinical sensory impairment was associated with a reduction in size of the initial compound action potential of the maximal evoked neurogram and with dispersion of fibre responses. Pathological slowing of fibre conduction velocity was demonstrated in demyelinating neuropathy but in most cases of axomal degeneration the changes in velocity could have been due either to a reduced number of fast conducting fibres, or to conduction block. No changes were observed in C-fibre activity in these patients.

Adult↗

PubMed1977

A micro-electrode study of peripheral neuropathy in man. Part 2. Responses to conditioning stimuli.

Surface, needle and micro-electrode recordings were obtained from sensory nerves of patients with various types of peripheral neuropathy. Changes in amplitude and conduction velocity of nerve action potentials were measured after a single conditioning stimulus and after tetanic stimulation for 2 min. In patients with hereditary forms of axonal degeneration (AD), recovery processes of nerve fibres of all conduction velocities were normal; in acquired forms of AD fibres with conduction velocity less than 30 m/sec had greater and more prolonged post-tetanic depression than control nerves of similar conduction velocity. Where neuropathy was associated with segmental demyelination (SD), fibres of all conduction velocities had prolonged recovery processes after both single and tetanic stimulation. The changes were especially marked at higher skin temperature, and were greater than the changes seen in nerves with acquired forms of AD. Finally, 2 sural nerves were studied during the process of Wallerian degeneration after a biopsy had been obtained proximally, and recovery processes did not change during the period of degeneration. Perceptual abnormalities were similar in AD and SD. It is suggested that changes in recovery processes of nerve fibres with segmental demyelination or regeneration after injury contribute to the perceptual abnormalities which occur in clinically encountered peripheral neuropathies.

Action Potentials↗

PubMed1976

Neurophysiological aspects of peripheral neuropathies.

1. Eighty-eight intrafascicular neural recordings were obtained in 10 normal subjects, 5 patients with axonal degeneration and 11 patients with demyelinating neuropathy. 2. Stimulus levels required for perception and fibre activation were higher in neuropathic subjects. Fibres transmitting touch perception had significantly lower conduction velocities in both patient groups, but were very much lower in the group with demyelinating neuropahty than the group with axonal degeneration. Maximum electrical stimulation evoked dispersed fibre responses in the axonal degeneration group and more dispersed, slowly conducting fibre potentials in the demyelinating group. In patients with hypertrophic Charcot-Marie-Tooth disorder, usually only a small group of slowly conducting low amplitude potentials was recorded. 3. Delivery of a train of supramaximal stimuli caused prolongation of latency and dispersion of fibre potentials in all microneurographic recordings. The changes were significantly greater in the axonal neuropathy group than in normals, and recovery was slower. The demyelinating neuropathies showed significantly greater changes than both the normal and the axonal neuropathy groups, and post-tetanic conduction slowing became even more marked after limb temperature was raised. 4. Surface SAP recordings showed normal refractory period in chronic axonal neuropathy but significant latency prolongation occurred in demyelinating neuropathy. 5. It is concluded that both receptor and nerve fibre abnormalities contribute to sensory dysfunction in degenerative and demyelinating neuropathies.

Action Potentials↗

PubMed1975

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.

Adult↗

PubMed1975

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.

Action Potentials↗

PubMed1975

Fibre function and perception during cutaneous nerve block.

In awake human subjects, neural responses in cutaneous nerves to electrical stimulation were recorded with intrafascicular tungsten micro-electrodes. 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 was mediated by A-beta-gamma fibres, cold and pinprick by A-delta fibres and warmth and dull pain by C fibres.

Action Potentials↗

PubMedDate not supplied

Power spectrum and optimal filtering for visual evoked potentials to pattern reversal.

The optimal bandwidth for recording the visual evoked potential (VEP) to pattern reversal was investigated in 8 normal subjects by re-analyzing off-line data recorded on tape using an open bandwidth of 0.1 Hz-3 kHz. Power spectral analysis of the VEP revealed little energy above 50 Hz. With digital filtering, the amplitude N70-P100 was significantly attenuated only when the low-pass filter was reduced to 50 Hz or when the high-pass filter was raised to 8 Hz. With analogue filtering, there was significant prolongation of latency of P100 when the low-pass filter (12 dB/octave) was reduced to 250 Hz and a significant decrease in latency when the high-pass filter (6 dB/octave) exceeded 3 Hz. However, the effects of analogue filtering were not uniform across subjects: in 2 subjects the latency of P100 was prolonged using a low-pass filter of 600 Hz and in 2 other subjects the latency was shortened when the 1 Hz high-pass filter was introduced. If a restricted bandwidth is used, non-uniform distortion of latency could make a significant contribution to the variability in latency of P100. The optimal bandwidth is one which minimises this contribution to the variability, 0.3 Hz to greater than 600 Hz.

Adult↗

PubMedDate not supplied

Sequence-dependent deterioration in the visual evoked potential in the absence of drowsiness.

When visual evoked potentials (VEPs) are tested patients are often expected to focus on a pattern screen for prolonged periods of time. This may lead to fatigue, failure of concentration and drowsiness, and consequently to a deterioration in the recorded VEP. To determine whether there may be time-dependent changes in the VEP of normal subjects independent of the degree of alertness, attention and altertness were controlled using a reaction time (RT) task in which the subjects were required to re-illuminate the fixation point in the middle of the stimulating screen for the VEP. It was first established that the switching of the fixation point produced little contamination of the background VEP to pattern reversal and that the latency and amplitude of P100 to pattern reversal were identical whether or not the subject was engaged in the RT task. A sequence of 16 averages of the VEP to 256 pattern reversals was recorded, alternately with or without the RT task. The measured RTs decreased during the sequence, presumably due to practice. There was a progressive decrease in the amplitude N70-P100, accompanied by an increase in the variability of the latency of P100. These changes cannot be attributed to lack of alertness, given the improvement in RT. Clinicians should be aware of the possibility of a deterioration in the VEP due to physiological mechanisms when the testing protocol involves multiple averages.

Adult↗