Search PubMed⌕ Search

Biomedical subjects

J G Colebatch

Publications and source records attributed to J G Colebatch.

71 records · Page 4Linked to original sources
PubMed1990

Cortical outflow to proximal arm muscles in man.

Corticospinal influences on two antagonistic muscles of the shoulder were studied in 6 normal volunteers. Recordings were made of the discharges of single motor units from the deltoid and pectoralis major muscles and poststimulus time histograms constructed following either anodal electrical or magnetic cortical stimulation via the scalp. Contralateral anodal stimulation caused narrow short-latency peaks of excitation in motor units from both muscles, probably indicative of the arrival of the D or direct corticospinal volley. The size of the peaks led to estimates of the underlying excitatory postsynaptic potentials (EPSP) amplitude of 5.5 mV for the deltoid and 2.2 mV for the pectoralis motor units. Magnetic stimulation over the vertex caused multiple early peaks of excitation in motor units from both muscles but these peaks began at a greater latency than those produced by anodal stimulation. Additionally, magnetic cortical stimulation (and, occasionally, contralateral anodal stimulation) caused a previously undescribed period of medium latency excitation in both muscles. These medium latency periods of excitation differed from the earlier events in that they occurred independently of the direction of current flow in the magnetic coil. This raises the possibility that they are the consequence of stimulation of bilaterally distributed cortical motor outflow pathways. The short-latency excitation strongly suggests that, in man, corticomotoneuronal projections exist to proximal as well as distal muscles of the contralateral arm. The strength of the projection to deltoid assessed by this method is similar to that to an intrinsic muscle of the hand and significantly larger than that to its antagonist, pectoralis. The possible bilateral organization of the additional medium-latency projection to these proximal arm muscles may contribute to two characteristic clinical features after a unilateral cerebral lesion, such as stroke: the relative sparing of proximal arm strength compared with distal and, secondly, ipsilateral weakness.

Adult↗
PubMed1989

The distribution of muscular weakness in upper motor neuron lesions affecting the arm.

The strength of 12 muscle groups of the arm was measured to determine the distribution of upper motor neuron weakness in man. Three groups of subjects were studied: 14 intact volunteers (both sides recorded), 10 patients with unilateral arm paresis (both sides recorded) and 6 patients with severe paralysis of the arm (the 'intact' arm only measured). On the side contralateral to the causative lesion the pattern of weakness was not the same in all patients. Shoulder muscles were relatively spared while the wrist and finger flexors were relatively severely affected, the difference being statistically significant. In hemiparetic and hemiplegic patients the strength of muscles ipsilateral to the lesion was reduced compared with normal controls. These observations refine previous clinical descriptions of upper motor neuron weakness and have implications for its pathophysiology.

Arm↗
PubMed1989

Maintenance of visual fixation using a reaction-time task.

The VEP to pattern reversal was recorded in eight healthy subjects who were instructed to fixate on a central LED in the stimulating screen. The LED could be extinguished at irregular intervals approximately once every 10 s and the subjects were then required to re-light it as rapidly as possible in a reaction-time task. In the first series of recordings subjects were reminded verbally to maintain visual fixation throughout the averaging sequences. When the reaction-time paradigm was used to maintain visual fixation, the VEP was slightly less variable in latency but otherwise identical to that recorded when the subjects maintained visual fixation on a constantly illuminated LED. In a separate series of 8 consecutive averaging runs, subjects were given the reaction-time task but no verbal reminders. VEP parameters differed from those in the first series when verbal reminders had been given. There was no correlation between VEP parameters and the reaction time or its variability in individual subjects or in the group. The reaction-time task is a satisfactory means for ensuring accurate visual fixation but does not obviate the need to provide subjects with verbal encouragement to maintain full attention.

Evoked Potentials, Visual↗
PubMed1987

'Long-latency' responses occurring with startle in the conscious monkey.

A monkey was noted to have an intermittent startle response to tendon tapping and vibration of proximal arm muscles. The latency of the first phase of this response, averaging 34 ms in triceps (and of average duration 49 ms), was similar to that described for the long-latency reflex response to muscle stretch in the same muscles. Unrecognised startle responses could explain some of the reported differences in long-latency reflexes in proximal and distal muscles.

Animals↗
PubMed1987

Maintenance of constant arm position or force: reflex and volitional components in man.

1. Normal subjects, with closed eyes, attempted to keep constant either the force exerted at the wrist or the position of the wrist against an elastic load. The load was attached to the wrist 275 mm from the axis of rotation of the elbow joint. During recording, the far end of the elastic load was displaced slowly enough that it was not immediately perceived but far enough for perception to occur before its completion. 2. The over-all relation between wrist force and position for the two conditions was approximately linear and could be described in terms of effective stiffness. The effective stiffness for the constant-position task averaged 2.8 N/mm (210 N m/rad), while for the constant-force task the mean effective stiffness was -0.028 N/mm (-2.1 N m/rad), indicative of slight over-compensation. 3. Averaging the performance at the onset of the imposed disturbance indicated that the subjects' behaviour consisted of two parts: an initial, small-range response followed by a second phase over the remainder of the displacement. The transition corresponded to the subjects' threshold for detection of the disturbance. 4. The stiffness measured for the response prior to perception was taken as a measure which included the tonic stretch reflex. The stiffness was altered appropriately for the two tasks, being lower when the subjects tried to maintain the force exerted constant (average 1.1 N/mm, 83 N m/rad) than when they attempted to keep the position constant (average 2.3 N/mm, 170 N m/rad). A small degree of co-contraction occurred but could be dissociated from the stiffness changes. 5. Scaling the results allowed comparison of the initial stiffness with values for the decerebrate cat. When analysed in this way, the values recorded in man during the constant-position task were similar to those reported for short-range stiffness in the decerebrate cat. 6. The thresholds for detection of the disturbance were much lower than those reported for subjects with relaxed muscles. 7. The stretch reflex in man has a direct role in compensating for small disturbances during motor tasks. It may also function to improve detection of applied disturbances by magnifying the corresponding force change. Once the stimulus is perceived and voluntary intervention is possible, a greater contrast is seen between the subjects' performance of the two tasks.

Arm↗
PubMed1986

Voluntary muscle strength in hemiparesis: distribution of weakness at the elbow.

Maximal voluntary strength (torque) of the flexors and extensors of the elbow was measured in 56 normal subjects and 18 hemiparetic subjects. In normal subjects the ratio of extension to flexion strength averaged 55% and did not differ significantly between sides or sexes. The ratio of maximal extensor to flexor strength on the clinically unaffected side of hemiparetic subjects was the same as that for the normal subjects but it was significantly increased on the affected side. This increase indicates that the elbow flexors were relatively more weakened than the extensors on the hemiparetic side, a conclusion contrary to conventional clinical teaching. The increase in the ratio was not the result of co-contraction of either muscle group. A possible physiological basis for the observed distribution of weakness is suggested.

Adult↗
PubMed1986

Measurement of capillary pressure in humans using a venous occlusion method.

The venous occlusion technique was used to measure capillary pressure in the forearm and foot of man over a wide range of venous pressures. In six recumbent subjects venous pressure (Pv) in the forearm (mean +/- SE) was 9.3 +/- 1.4 mmHg and the venous occlusion estimate of capillary pressure (Pc) was 17.0 +/- 1.6 mmHg, whereas in another six subjects Pv in the foot was 17.1 +/- 1.2 mmHg and Pc was 23.4 +/- 2.5 mmHg. Venous pressure in the limbs was increased either by changes in posture or by venous congestion with a sphygmomanometer cuff. On standing Pv in the foot increased to 95.2 +/- 1.5 mmHg and Pc rose to 112.8 +/- 3.1 mmHg. The relationship established between venous pressure and capillary pressure in the forearm is Pc = 1.16 Pv + 8.1, whereas in the foot the relationship is Pc = 1.2 Pv + 1.6. The magnitude and duration of the changes in capillary pressure were also recorded during reactive hyperemia. The venous occlusion method of measuring capillary pressure is simple and easily applied to studies in humans.

Blood Pressure↗
PubMed1985

Responses of precentral cells during cooling of post-central cortex in conscious monkeys.

A cooling plate was implanted over the forelimb representation in area 2 of the post-central region of cerebral cortex in two monkeys. Recordings were made of the discharges of thirty-seven movement-related neurones (thirty-four precentral and three post-central) in the forelimb motor representation of the cerebral cortex during active and passively imposed limb movements before, during and after cooling area 2 and local surrounding regions. Perfusion of the cooling plate with ice-cooled water for 3-5 min caused marked clumsiness of the conscious animal's forelimb movement and anaesthesia of the contralateral hand. Cooling of area 2 did not reduce the responses of area 4 cells to passive joint movements, nor did it alter the over-all pattern of activity of these cells during self-initiated lever pulling while that could still be performed. Cooling of area 2 did cause a significant increase in background cellular discharge in area 4 while the animal was at rest. Afferent impulses which are generated by passive joint movement and which have been shown to influence cells in area 4 of the conscious monkey at short latencies are probably not transmitted through cortico-cortical connexions from area 2.

Animals↗
PubMed1984

Effects of externally imposed elastic loads on the ability to estimate position and force.

Normal adult subjects attempted, with vision excluded, to match the position of the left index finger by pointing to it with the right one, using only flexion-extension movements at the right elbow. An elastic load applied at the right wrist was altered from trial to trial. When instructed to align the fingers, subjects were found to select the position of the right forearm by taking into account both a position signal and some measure of the force exerted by the elbow flexors. When instructed to match a target force, instead of position, the subjects were able to give greater weighting to signals of force than in the position matching task.

Adult↗
PubMed1983

Benign intracranial hypertension.

Benign intracranial hypertension is caused by defective cerebrospinal fluid (CSF) absorption which may sometimes be secondary to partial superior sagittal sinus obstruction. It may appear in obese young women for obscure hormonal reasons or may be a reaction to head injury, infections or certain medications. It commonly presents with headache and papilloedema, sometimes with associated ocular palsies. Conservative management comprises repeated lumbar puncture, salt and fluid restriction and the use of diuretics or steroids. Surgical treatment such as CSF shunting procedures, optic nerve decompression or subtemporal decompression may become necessary. The results of treatment have not been subjected to adequate evaluation, but the patient usually recovers well, although some 10% suffer severe visual impairment and 10% are subject to recurrence.

Adolescent↗
PubMed1979

Subject instruction and long latency reflex responses to muscle stretch.

1. Surface electromyographic recordings were made in eight normal subjects from the isometrically contracting elbow flexors before and during forcible extension of the elbow through 7 degrees in 50-150 msec. 2. Weh the subjects were instructed prior (2-5 sec) to a forthcoming stretch to 'resist' or to 'let go', they could reliably enhance or suppress e.m.g. activity occurring between 40 and 70 msec from commencement of the stretch. Such e.m.g. activity represents a 'long-latency' (or 'M2') reflex response: it occurs with a latency longer than the spinal segmental monosynaptic reflex, but shorter than a voluntary reaction time. When the subjects were given their instructions (by means of a light) at the moment the stretch commenced, however, none of them could adjust the long-latency reflex appropriately. 3. It is concluded that central, evaluative processes commencing at the time of a perturbation cannot influence long-latency reflex responses to that perturbation.

Elbow↗
PubMed1979

Sural nerve effects on medial gastrocnemius motoneurones in the cat.

1. Excitability cycles for medial gastrocnemius motoneurones were recorded following sural nerve stimuli with amplitudes of 1.5, 3, 5, 10 and 40 times the nerve threshold. The monosynaptic reflex was time-integrated to quantify the motoneuronal output and some implications of this technique are discussed. In particular it is calculated that this method specifically examines the 15-25% most excitable motoneurones of the medial gastrocnemius pool.2. In most cases high amplitude stimulation of the sural nerve caused a triphasic (excitatory-inhibitory-excitatory) change in excitability. Arguments are given to support the conclusion that this represents a corresponding post-synaptic alteration in medial gastrocnemius motoneurone potential. It is concluded that this pattern illustrates the nature of the sural nerve projections to this portion of the medial gastrocnemius pool.3. The first period of excitation began after a latency, corrected to be compatible with intracellular recordings, of 2.8 ms and had a mean duration of 4.0 ms. The minimum stimulus level necessary for this effect lay between 1.5 and 3 times the nerve threshold. The maximum amplitude of this facilitation occurred with stimuli of 3-5 times threshold and was 60-90% of the amplitude of the monosynaptic facilitation which followed stimulation of the lateral gastrocnemius-soleus nerve.4. A period of inhibition followed immediately after the period of excitation and had a mean duration of 16 ms. The minimum stimulus level necessary for this effect lay between 1.5 and 3 times the nerve threshold and the degree of inhibition increased with stimuli up to 40 times threshold.5. The second period of excitation lasted a mean period of 50 ms and was due to activity in high threshold fibres. On average its amplitude was 50% that of the initial excitation.6. Exceptions to this triphasic variation of excitability were found. This result is interpreted as indicating the presence of projections with opposing actions which were simultaneously activated by sural stimuli.

Animals↗
PubMed1977

Reduction in inspiratory activity in response to sternal vibration.

It has been shown previously that there is a reduction in tidal volume in response to longitudinal sternal vibration at 100 Hz. In the present study it was shown that the effect of such vibration is to reduce tidal volume (VT) and prolong inspiratory time (ti) in such a way that points from vibrated and non-vibrated breaths fall on the same VT:ti curve. This indicates that the normal mechanisms which terminate inspiration are unaffected by vibration. The effect of vibration is simply to reduce the rate at which inspiration proceeds. This was illustrated here when vibration reduced the rate of fall of intrapleural pressure during inspiration, and also reduced the instantaneous ventilation at any level of chemical drive. Electrophysiological recordings made here from phrenic motoneurones support these findings. It is concluded that sensory nerves in the chest wall, which can be excited by vibration, can inhibit inspiration.

Animals↗