Search PubMed⌕ Search

Biomedical subjects

J Noth

Publications and source records attributed to J Noth.

At least 109 records · Page 6Linked to original sources
PubMed1984

Long latency reflex force of human finger muscles in response to imposed sinusoidal movements.

Reflex stiffness of the flexing human index finger was studied using sinusoidal movements at 3-16 Hz. The Nyquist stiffness diagram indicates the presence of a 'presonance' at around 4 Hz, its 'C' shape after correction for the mechanical properties of the relaxed finger is consistent with the involvement of a stretch reflex in its generation. This contention was supported by the presence of negative friction around 4 Hz and the disappearance of the modulation of the stiffness curve after afferent ischaemic block. Correction for the mechanical properties of active muscle, measured after afferent block, permitted the isolation of the reflex component of stiffness. The circular form of the Nyquist diagram indicates a relatively flat frequency response for the reflex over the range tested, and its radius gives a measure of reflex gain. The low value of the frequency at which the frictional force is minimal, suggests the involvement of a reflex of longer than spinal latency. This is discussed in relation to mechanisms of tremor genesis and the interaction of spinal and long latency reflexes in distal hand muscles.

Biomechanical Phenomena↗
PubMed1984

Somatosensory evoked potentials to mechanical disturbances of positioning movements in man: gating of middle-range components.

Somatosensory evoked potentials (SEPs) and EMG responses to mechanical disturbances of positioning movements of the index finger were recorded from normal subjects who performed smooth flexions against the constant load of a torque motor in order to reach a specified target zone. According to a random series 60% of the movements were disturbed by step increases or decreases of the load which subjects had to compensate for (movement condition, MC). As control procedures (a) the same stimuli were applied while the subject had to maintain a constant finger position against the same load (hold condition, HC), and (b) comparable mechanical stimuli were administered to the stationary relaxed finger (resting condition, RC). In MC and HC, the EMG responses consisted of a long-latency (53-61 msec) reflex component, and a 'voluntary' component (98-134 msec). In all 3 conditions, the sequential SEP deflections measured over the sensorimotor cortex included peaks P85 (ipsilateral), N150 (bilateral) and P220 (bilateral). A contralateral N90 component (onset latency 55-60 msec), present in HC and RC, was markedly reduced or absent in MC. Neither in the preceding nor in the consecutive SEP components were there any significant condition-dependent differences, though the N150 showed a trend to diminished amplitude in MC as well. The suppression of SEP components in MC is discussed in terms of efferent and afferent 'gating' actions exerted on the somatosensory input during movement.

Adult↗
PubMed1984

Evoked potentials in patients with Huntington's disease and their offspring. I. Somatosensory evoked potentials.

Spinal and cortical somatosensory evoked potentials (SEPs) were recorded in a large sample of patients with Huntington's disease (n = 37) and subjects at risk (first order offspring, n = 43). The SEPs were elicited by stimulation of the median and tibial nerves and recorded at Erb's point, the cervical level (C2) and at the corresponding scalp areas. The most striking finding in patients with Huntington's disease was a drastic diminution of the amplitude of the early cortical components, especially N20/P25 for the median nerve and N33/P40 for the tibial nerve. The latencies (Erb's point, C2, cortical) were only slightly prolonged in comparison to the normal values. Forty-three per cent of the persons at risk exhibited pathological results with a clear reduction in amplitude of the early cortical responses or with a pathological side difference between the amplitudes. Fifty-three per cent of the persons at risk exhibited a normal result. Two persons at risk (= 6%) could not be classified unambiguously. Some patients with benign and symptomatic chorea were investigated. These showed normal results with one exception. The diagnostic and predictive value of the investigation of SEPs in Huntington's disease is discussed.

Adolescent↗
PubMed1983

Absence of long latency reflexes to imposed finger displacements in patients with Huntington's disease.

Long latency reflexes in the electromyogram (EMG) of the first dorsal interosseus muscle were elicited by short finger displacements under isometric conditions. In all healthy subjects tested the spinal response was followed by a second involuntary component. Patients with Huntington's disease lacked the late EMG response almost completely, but exhibited a spinal component indistinguishable from that of the control group. A spinal mechanism responsible for this result is unlikely, since double stretches evoked two distinct EMG responses in these patients. Moreover, drastically reduced cortical somatosensory evoked potentials in all patients support the notion that the second EMG response seen in our motor paradigm is of supraspinal origin.

Adolescent↗
PubMed1983

Autogenetic inhibition of extensor gamma-motoneurones revealed by electrical stimulation of group I fibres in the cat.

Forty functionally single gamma-efferents (20-42 m/s) to the triceps surae were isolated in ventral root filaments of the decerebrated and paralysed cat in order to study the effects of group I muscle afferents on their own fusimotor neurones. All the efferents studied were spontaneously active. During splitting the continuity of the efferent fibre was preserved so that the destination of the target muscle of the efferent could be determined by antidromic stimulation of the muscle nerve using the collision block technique. Thereafter the filament was cut so that the reflex response, uncontaminated by antidromic impulse invasion, could be recorded from the central end. Sixteen of forty gamma-efferents to the triceps were inhibited by repetitive stimulation (range -3 to -40 impulses/s) of the homonymous nerve within the group I range. Raising the stimulus strength above the group II threshold produced no further increase in inhibition. Twelve of these sixteen cells were also tested by stretching the triceps; ten showed marked inhibition, and two were not influenced. In ten of the sixteen inhibited cells, the autogenetic inhibition at maximum group I stimulus strength was larger than the maximum antidromic inhibition elicited by stimulation of the remainder of the cut ventral roots L7 and S1. Since both effects were additive, it is concluded that Renshaw inhibition is at least not solely responsible for the autogenetic inhibition. Consistent with this assumption is the observation that some cells receiving electrically evoked autogenetic inhibition were not susceptible to inhibition induced by small-amplitude vibration of the triceps. Since small-amplitude vibration is known to excite most of the Ia afferents of the vibrated triceps, Ib afferents must be involved in the autogenetic fusimotor inhibition. A contribution of Ia afferents to the autogenetic inhibition (via alpha-motoneurones and Renshaw cells or via Ib inhibitory interneurones) seems likely since inhibition, induced by small-amplitude vibration, was detectable in many of the cells receiving autogenetic group I inhibition. All of these cells were susceptible to antidromic inhibition. Ten of the forty cells tested responded with tonic facilitation to the homonymous nerve stimulation. Some arguments favour the view that static gamma-motoneurones are involved in the low-threshold autogenetic inhibition. The results strongly support earlier work suggest a regulatory function of low-threshold muscle receptors on their own gamma-motoneurones.

Animals↗
PubMed1981

Interaction between pre-activity and stretch reflex in human triceps brachii during landing from forward falls.

1. Electromyographic (e.m.g.) profiles of proximal arm muscles were studied in human subjects falling forward onto a platform. 2. The stretching of the triceps lasted 200-300 msec for deep falls, and immediately after impact angular velocities of the elbow joint up to 1000 degrees sec-1 were reached. 3. For angles of fall between 50 and 90 degrees, more than half of the subjects exhibited marked short-latency e.m.g. responses of the triceps brachii. Such responses began 20-30 msec after touchdown, arising from a more or less plateau-like activity which started about 130 msec before impact. In some cases distinct later responses were found, the second peak having a latency of 60-80 msec after touchdown. 4. The early e.m.g. response even appeared when the subject was blindfolded and when the depth of the fall was randomly varied. 5. It is concluded that both the pre-existing activity and the spinal stretch reflex contribute significantly to the over-all activity of the triceps during stretch after impact.

Adult↗
PubMed1980

Cyclist's palsy: Neurological and EMG study in 4 cases with distal ulnar lesions .

Four cases of cyclist's palsy -- a distal compression syndrome of the ulnar nerve which can develop after long cycling tours -- are described. In two patients extensive neurological and electromyographic (EMG) studies were performed in order to document the degree and time course of recovery of the motor and sensory deficiencies. In contrast to either EMG-studies it could be demonstrated that not only the deep motor branch of the ulnar nerve is involved, but also that a severe lesion of the superficial branch supplying the skin of the 4th and 5th finger can occur, as indicated by a complete loss of the orthodromic sensory action potential recorded above the ulnar nerve in both cases. The results furthermore show that a functionally significant paresis of the hand muscles can persist for several months. The pathogenesis and some practical implications will be discussed.

Action Potentials↗
PubMed1980

Autogenetic excitation of extensor gamma-motoneurones by group II muscle afferents in the cat.

Triceps surae gamma-efferents have been isolated in intact ventral root filaments in order to study the autogenetic Group II effects evoked by stimulation of the nerve to the triceps surae in decerebrate cats. Half of the 22 cells tested were facilitated by repetitive stimulation of this nerve in the Group II range. Six cells showed a mixture response pattern with an initial facilitation followed by declining activity during the ongoing stimulation. Eight cells were not influenced within the Group II range. The results suggest that autogenetic Group II facilitation of gamma-motoneurones plays a functional role in the gain control of muscle spindle endings.

Animals↗
PubMed1979

Static and dynamic fusimotor interaction and the possibility of multiple pace-makers operating in the cat muscle spindle.

The interaction of static and dynamic fusimotor activation on the firing of primary muscle spindle afferents has been studied in the cat soleus muscle at constant length and during sinusoidal stretching. Cycle histogram analysis revealed summation of static and dynamic action during the peak of the afferent response to sinusoidal stretching, while static action completely occluded the dynamic effect during the trough of the response. Occlusion was complete as long as, for single fusimotor activation, the static-induced trough response exceeded the dynamic-induced one by about 25%. The investigation of inter-spike interval distributions obtained at constant muscle length revealed occlusion of dynamic by static action in 8 out of 13 cases. A model of multiple spike generation in primary spindle afferents is considered which is based on two or more pacemakers arranged in parallel, with a common pacemaker in series.

Afferent Pathways↗
PubMed1979

Early detection of retinal involvement in diabetes by vitreous fluorophotometry.

Vitreous fluorophotometry is a new quantitative method for evaluation of the blood-retinal barrier (BRB). The application of this method to a series of diabetic patients with apparently normal fundi revealed the presence of a significant breakdown of the BRB in the early stages of retinal involvement in diabetes. This alteration of the BRB appears to be the earliest clinically detectable change to occur in the retina in diabetes. Higher vitreous fluorophotometry values, indicating a more marked breakdown of the BRB, occurred in patients who were under worse metabolic control. Higher vitreous fluorophotometry values were also associated with the development of visible retinal lesions.

Diabetes Mellitus↗
PubMed1979

Neuronal mechanisms of human locomotion.

1. The surface electromyogram (EMG) of human leg muscles was recorded during running at different speeds; The EMG was compared with the simultaneously recorded vertical force exerted by the foot and with the angle of the ankle joint. 2. During running, the electrical activity of the gastrocnemius muscle increased sharply 35--45 ms after ground contact and reached its maximum at the end of muscle stretch; This activity was superimposed on a slowly increasing level of activation, which began 120--180 ms before ground contact. At the end of the stance phase, gastrocnemius became inactive and, simultaneously, there was a sudden increase in tibialis anterior activity. The assumption that the steep increase in the gastrocnemius EMG reflects the spinal stretch reflex of alpha-motoneurons is supported by the following findings. 3. The peak level of gastrocnemius EMG in the stance phase of fast running was 2--3 times higher than the activity during maximum voluntary contraction. 4. With stimulation of the tibial nerve at different rates, the maximum isometric force was about 30--40% higher than the maximum voluntary isometric contraction. 5. The increase in EMG at 35--45 ms after ground contact was markedly diminished during running, after partial blockage of Ia afferents by ischemia, at a time when the strength of voluntary contraction was shown to be uninfluenced by the ischemia. 6. The gastrocnemius activity during running was simulated by electrical stimulation of the tibial nerve. The rate of stimulation was varied so as to approximate to the EMG profile during running. This indicated that a spinal stretch reflex could become mechanically effective within the shortest stance phase measured in a fast sprint (ca. 120 ms).

Adult↗
PubMed1977

Static and dynamic fusimotor action on the response of Ia fibres to low frequency sinusoidal stretching of widely ranging amplitude.

1. Single fusimotor fibres were stimulated repetitively to test their action on the responsiveness of muscle spindle primary endings in the cat soleus to sinusoidal stretching of both large and small amplitude. Frequencies of 0.06-4 Hz were used at amplitudes from 10 mum to 3 mm.2. The response was assessed by fitting a sinusoid to the cycle histogram of the afferent firing throughout the course of the cycle; this linear approximation measures the fundamental of the response and ignores any harmonics. The sine was allowed to project to negative values and any empty bins in the histogram were ignored when fitting.3. With small amplitudes of stretching the histograms were reasonably sinusoidal, but with large amplitudes they showed appreciable distortion of the wave form for the passive ending and during dynamic fusimotor stimulation. Non-linearity of response manifested itself also, with increasing amplitude of stretching, by an increase in the phase advance of the response, by increasing r.m.s. deviation of the histogram points from the fitted sine and (for dynamic stimulation) by an increase in the mean value of the fitted sine.4. With increasing amplitude the response modulation ceased to increase proportionately with the stimulus, so that the sensitivity of the ending to a large stretch (defined as afferent modulation/stretch amplitude) was appreciably less than for a small stretch. This effect was most pronounced for the passive ending.5. Whatever the amplitude of movement the modulation during static stimulation was less than that for the passive or during dynamic stimulation. For small amplitudes the response during dynamic stimulation was less than that of the passive, but for large amplitudes the response during dynamic stimulation was always the greater. At some intermediate cross-over amplitude the two responses were the same size, though still differing slightly in other respects. The value of the cross-over amplitude was usually about 200 mum at 1 Hz, and increased on lowering the frequency. Thus dynamic fusimotor action does not uniformly produce either an increase or a decrease in the sensitivity of the ending in relation to the passive.6. Bode plots, for each amplitude, of sensitivity and phase against frequency suggested that(a) under all conditions the ending is relatively insensitive to frequency in the range studied, for the slope of the log-log sensitivity lines was only 0.15-0.2 (3.5-6 db/decade);(b) the mechanism which makes for non-linearity is not particularly frequency sensitive;(c) static fusimotor stimulation does not change the frequency sensitivity of the ending;(d) dynamic fusimotor stimulation very slightly increases the frequency sensitivity of the ending for large amplitudes.In reaching these conclusions more attention was paid to the slope of the sensitivity lines than to the values of phase.7. It appears that the major effect of fusimotor action, whether static or dynamic, is to regulate the sensitivity of the primary ending to stretching for all amplitudes of movement (i.e. gain) rather than to control the relative values of its sensitivity to length and to velocity (i.e. crudely, the damping in a feed-back loop).

Action Potentials↗
PubMed1977

Effects of combining static and dynamic fusimotor stimulation on the response of the muscle spindle primary ending to sinusoidal stretching.

1. A pair of fusimotor fibres, one static and the other dynamic, were stimulated simultaneously to test their combined action on the response of muscle spindle primary endings in the cat soleus to sinusoidal stretching. A frequency of 1 Hz was chiefly used, with a wide range of amplitudes (10 micronm-2 mm). The response of the ending was assessed from the parameters of the sine fitted to its firing averaged throughout the course of the cycle; this was felt useful even though the responses to the larger stretches showed certain non-linear features. 2. With small stretches (up to about 50 micronm amplitude) static action dominated, and the modulation of firing during conbined stimulation was little or no larger than that found during the static stimulation on its own, and much smaller than that found during the static stimulation on its own, and much smaller than that found during the dynamic stimulation. The phase of the response was, however, much the same for all three conditions. 3. With larger stretches the modulation with combined stimulation was intermediate between the values found on stimulating either fusimotor fibre on its own; the dynamic contribution increased progressively with the amplitude of stretching. 4. With larger stretches the phase of the response during combined stimulation was appreciably closer to that for static action than to that for dynamic action. But the differences between the various conditions were small (below 20 degrees) and seem attributable to various distortions of the response wave from away from a true sinusoid, rather than betokening a difference in the ratio of velocity to length sensitivity under the various conditions. This view was supported by the effects on phase of grading the rate of stimulation of one fusimotor fibre while holding that of the other constant. 5. Detailed comparison of the cycle histograms obtained under different conditions showed an interestingly asymmetrical pattern of summation and occlusion of the effects of the two kinds of fusimotor fibre. At the peak of the response to a large stretch static action summed with dynamic action, which was here the stronger, so that at this phase of the cycle the firing was greater with the combined stimulation than with either fibre on its own. But, in the trough of the response to the same stretch static action occluded any dynamic action, which was now the weaker, so that at this phase of the cycle the firing with combined stimulation was virtually the same as that with static stimulation on its own. With a small stretch, static action normally occluded dynamic action throughout the cycle; this is in line with the firing during static action now usually being greater than that during dynamic action for all phases of the cycle.

Action Potentials↗