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Biomedical subjects

P Ashby

Publications and source records attributed to P Ashby.

At least 19 recordsLinked to original sources
PubMed1992

Ipsilateral fast corticospinal pathways do not account for recovery in stroke.

We tested the hypothesis that in adult humans, recovery from stroke results from the emergence of ipsilateral, fast-conducting corticospinal pathways. In 10 patients recovering from stroke, the unaffected hemisphere was stimulated with an electromagnet and changes in the firing probability of single biceps motor units were used to derive postsynaptic potentials in single biceps motoneurons. Stimuli sufficient to excite the neurons of the fast-conducting corticospinal pathway (as shown by short-latency facilitation of contralateral biceps motoneurons) did not produce short-latency depolarization of ipsilateral biceps motoneurons. The hypothesis is therefore not supported.

Action Potentials↗
PubMed1992

Single fiber EMG in the frontalis muscle in ocular myasthenia: specificity and sensitivity.

Patients (n = 41) with isolated weakness of the eyelids or extraocular muscles, who had been referred for single fiber electromyography (SFEMG), were followed up after 4 to 24 months, At follow-up the patients were classified as "definite ocular myasthenia gravis" (MG), "definite other diagnosis," or "no definite diagnosis" on the basis of the completed investigations and subsequent course. The original SFEMG findings in the frontalis muscle were then reviewed. The specificity and sensitivity of SFEMG for "definite ocular MG" could be maximized by using as criteria for abnormality greater than 8/20 pairs with jitter greater than 45 microseconds, or a mean jitter of 20 pairs of greater than 50 microseconds. Patients with abnormal SFEMG according to these criteria have MG, and are likely to require treatment in the immediate future. Patients who have normal SFEMG according to these criteria (and no other demonstrated disorder) may have MG, but it is so mild that they are unlikely to require treatment. Two patients whose final diagnosis was progressive external ophthalmoplegia had normal SFEMG according to these criteria.

Adult↗
PubMed1992

Corticospinal projections to lower limb motoneurons in man.

The projections of cortical neurons activated by transcranial magnetic stimulation to single lower limb spinal motoneurons were examined in 34 normal subjects. Peristimulus time histograms of the discharge times of single, voluntarily activated motor units were used to derive information about postsynaptic potentials in single spinal motoneurons produced by magnetic stimuli applied over the contralateral scalp. All tibialis anterior motor units and the majority of motoneurons innervating the small muscles of the foot showed strong short latency facilitation. About half of the motoneurons of proximal lower limb muscles showed this facilitation. Short latency facilitation of the motoneurons of soleus and medial gastrocnemius was only rarely observed and when present was weak. The short latency facilitation is attributed to the projections of the fast corticospinal pathway with monosynaptic projections to motoneurons. The relative strength of the facilitation in different motoneuron pools is considered to reflect the density of corticospinal projections to that motoneuron pool. The observed pattern of projections in man shows some differences from the pattern of projections in subhuman primates that might reflect the different use of the limb.

Cerebral Cortex↗
PubMed1992

The transcortical nature of the late reflex responses in human small hand muscle to digital nerve stimulation.

The hypothesis that long-latency reflex activity in human small hand muscles in response to stimulation of digital nerves involves a transcortical pathway was tested by combining digital nerve stimulation and magnetic stimulation over the motor cortex in 12 studies on nine normal subjects. Postsynaptic events in human single first dorsal interosseous (FDI) motoneurones were derived from changes in the firing probability of voluntarily activated single motor units. Electromagnetic stimulation over the contralateral motor cortex resulted in a short-latency, brief facilitation of FDI motor units considered to be due to the activation of "fast" corticospinal neurones making monosynaptic projections to motoneurones. Stimulation of the digital nerves of the index finger produced a period of reduced firing probability (I1), a period of increased firing probability (E2) and a further period of reduced firing probability (I2) in FDI motor units. When the two stimuli were given separately and then together, timed so that the magnetic stimulus occurred at the predicted transit time of the E2 through the cortex, the facilitation of FDI motoneurones by the combined stimulation was often less than the algebraic sum of the facilitations from each stimulus alone. Thus, in contrast to the results of similar studies on the late response to muscle stretch, there is no confirmation that the E2 from digital nerve stimulation is due to a transcortical reflex.

Adult↗
PubMed1992

Stimulus-sensitive myoclonus in akinetic-rigid syndromes.

The cutaneous reflexes of upper limb muscles were studied in five patients with Parkinson's disease and 10 patients with stimulus-sensitive myoclonus associated with akinetic-rigid syndromes. The middle finger was stimulated with ring electrodes and rectified electromyographs were averaged from seven upper limb muscles and orbicularis oculi. Responses from subjects with Parkinson's disease without stimulus-sensitive myoclonus were similar to those of normal subjects. The responses from patients with stimulus-sensitive myoclonus associated with Parkinson's disease or multiple system atrophy had the normal pattern except that a long latency facilitation, which is present in normal subjects and known as E2, was greatly exaggerated. Patients with stimulus-sensitive myoclonus associated with cortical-basal ganglionic degeneration had a completely different pattern of responses. There was synchronous activation of all recorded upper limb muscles with latencies substantially shorter than those of the long latency facilitation (E2) in normal subjects. Cutaneous reflex testing may therefore be useful in the differentiation of akinetic-rigid syndromes.

Aged↗
PubMed1992

Corticospinal projections to upper limb motoneurones in humans.

1. Magnetic stimulation was applied over the motor cortex in forty-five normal human subjects and peristimulus time histograms (PSTHs) of the discharges of single motor units were used to record changes in the firing probability of individual spinal motoneurones of contralateral upper limb muscles. Recordings were obtained from 153 motor units from fourteen upper limb muscles. 2. For the majority of motor units the initial effect was a short latency facilitation. The estimated central conduction velocities and the rise times of the underlying excitatory postsynaptic potentials (EPSPs) were compatible with monosynaptic facilitation by a fast corticospinal pathway. In some motor units the initial effect was a short latency inhibition. Other units showed no statistically significant changes in firing probability. The proportion of the tested motor units in each of these categories depended on the muscle. All of the sampled units of first dorsal interosseous (1DI) showed short latency facilitation, as did the majority of units in the forearm and the biceps brachii. More than half of the sampled motor units of triceps brachii and deltoid showed either no effect or were inhibited. 3. To compare the net short latency actions of the neurones activated by magnetic stimulation on various motoneurone pools, the magnitude of the short latency facilitation or inhibition in a given motor unit was normalized to the magnitude of the short latency facilitation in the 1DI motor unit of the same subject at the same stimulus intensity, and these data were pooled for a number of subjects. 4. 1DI motoneurones received strong net facilitation (estimated mean EPSP amplitude 2.9 +/- 0.2 mV), the motoneurones of forearm muscles and biceps brachii received weaker net facilitation and triceps brachii and deltoid received no net effect. 5. It is concluded that the short latency corticospinal projections to upper limb motoneurones in humans have a distinct pattern which is similar to that in other primates.

Adult↗
PubMed1992

Evidence that a long latency stretch reflex in humans is transcortical.

1. The hypothesis that the long latency reflex response to muscle stretch in humans uses a transcortical pathway was tested by looking for convergence onto cortical neurones in eleven normal subjects. 2. Postsynaptic events in single flexor pollicis longus (FPL) motoneurones were derived from changes in the firing probability of individual FPL motor units. 3. Extension of the terminal phalynx of the thumb resulted in both short latency and long latency facilitations of individual FPL motoneurones. These were not reproduced by electrical stimulation of afferents in the terminal phalynx. Magnetic stimulation over the contralateral motor cortex produced strong, short latency facilitation of FPL motoneurones. 4. When the facilitation produced by stimulation over the cortex was superimposed on the long latency facilitation produced by extension of the thumb, the facilitation produced by both stimuli was greater than the sum of the individual facilitations produced by either stimulus given alone. This was not the case when the superimposition occurred on the short latency response to stretch. 5. We conclude that afferent systems excited by the stretch of FPL converge onto cortical neurones which are known to facilitate motoneurones. Thus the cortex is likely to contribute to the long latency stretch reflex in humans.

Adult↗
PubMed1992

Changes in corticospinal facilitation of lower limb spinal motor neurons after spinal cord lesions.

The projections from the cortex to the motor neurons of lower limb muscles were examined in 33 normal subjects and 16 patients with incomplete spinal cord lesions. Corticospinal neurons were excited by transcranial magnetic stimulation and the effects on single spinal motor neurons determined from peristimulus time histograms (PSTHs) of single tibialis anterior (TA) and soleus (SOL) motor units. In normal subjects magnetic stimulation produced a short latency facilitation of TA motor units but had little or no effect on SOL motor units. In the patients with spinal cord lesions magnetic stimulation also produced facilitation of TA but not SOL motor units; however, the mean latency of the TA facilitation was significantly longer (by about 14 ms) in the patient group. The F wave latencies were normal in all patients tested, suggesting that central rather than peripheral conduction was slowed. The duration of the period of increased firing probability (in TA motor units) was also significantly longer in the patients with spinal cord lesions. These changes may reflect the slowing of conduction and dispersal of conduction velocities in the corticospinal pathways as a consequence of the spinal cord lesion. No significant correlations were found between the delay of the TA facilitation and the clinical deficits in this group of patients.

Adolescent↗
PubMed1992

Intermittent claudication caused by compression of tibial vessels as a result of calf muscle hypertrophy: case report.

We present a case with findings suggestive of popliteal artery entrapment in a patient with intermittent claudication and localized muscle hypertrophy in the calf muscles after removal of a herniated disk. Angiography failed to demonstrate popliteal artery entrapment but instead revealed compression of the tibial vessels caused by calf muscle hypertrophy. The concept of muscle hypertrophy caused by denervation is also discussed.

Humans↗
PubMed1990

Corticospinal projections to upper and lower limb spinal motoneurons in man.

The pattern of muscle activation produced by magnetic stimulation over the human motor cortex has been examined in normal subjects. Magnetic stimulation elicited short latency compound muscle action potentials (CMAPs) from upper limb muscles contralateral to the stimulus and from lower limb muscles bilaterally. In the upper limb, the hand and forearm muscles had the lowest thresholds for activation and the largest CMAPs. There was little or no activation of the more proximal muscles, biceps and tricepts. In the lower limb the extensor digitorum brevis and tibialis anterior had low thresholds and large CMAPs. There was little or no short latency activation of soleus. This pattern of muscle activation remained the same even when the stimulating coil was moved to various locations within a radius of 3 cm from the vertex. Recordings from individual tibialis anterior and soleus motor units indicated that the differential activation of these muscles reflected the relative amplitudes of the short latency postsynaptic potentials produced by magnetic stimulation in their motoneurons. The 'jitter' in the activation of single muscle fibers by magnetic stimulation suggests that these projections are monosynaptic. It is argued that the pattern of muscle activation reflects the projections from the cortex to motoneurons rather than the projections of afferents onto cortical neurons. We conclude that magnetic stimulation can be used to identify the projections of the corticospinal pathways in man.

Action Potentials↗
PubMed1979

Midbrain asterixis.

Asterixis is usually a manifestation of metabolic encephalopathy. It was the only skeletal motor sign in a patient with ophthalmoplegia caused by midbrain infarction; no metabolic abnormality was present. The asterixis was accompanied by signs of damage to the mesencephalic reticular formation. We propose that episodic lapses of postural control by the reticular formation are responsible for midbrain asterixis and suggest that this asterixis is a segmental form of drop attack.

Aged↗
PubMed1978

Post-translational regulation of lipoprotein lipase activity in adipose tissue.

Changes in adipose-tissue lipoprotein lipase activity that are independent of protein synthesis were investigated in an incubation system in vitro. Under appropriate conditions at 25 degrees C a progressive increase in the enzyme activity occurs that is energy-dependent. Part of the enzyme is rapidly inactivated when the tissue is incubated with adrenaline or adrenaline plus theophylline. The mechanism of this inactivation appears to be distinct from, and to follow, the activation of the enzyme. A hypothesis is presented to account for the results in terms of an activation of the enzyme during obligatory post-translational processing and a catecholamine-regulated inactivation of the enzyme as an alternative to secretion from the adipocyte.

Adipose Tissue↗
PubMed1978

The heterogeneity of the lipoprotein lipase of rat epididymal adipose tissue.

Lipoprotein lipase is heterogeneous, and it was suggested that the enzyme in adipose tissue is transformed from a species of mol. wt. approx. 120000 to forms of much higher molecular weight as it is secreted from the fat-cell. This paper demonstrates that the forms of higher molecular weight are probably artifacts. Enzyme preparations were characterized by gel filtration, by density-gradient centrifugation and by affinity chromatography. The results indicate that the enzyme forms of mol. wt. greater than 120000 result from an association of the enzyme with particulate material. It is therefore necessary to reconsider schemes that have recently been proposed for the synthesis and export of lipoprotein lipase.

Adipose Tissue↗
PubMed1978

Disulfiram neuropathy.

A 35-year-old man developed a distal sensorimotor polyneuropathy after taking disulfiram, 500 mg daily for five months. His symptoms improved after the drug therapy was discontinued. Clinical, electrophysiological, and pathological observations during the acute stage and during recovery suggest that disulfiram produces a distal axonopathy.

Adult↗
PubMed1978

Synaptic connections to individual tibialis anterior motoneurones in man.

The characteristics of post-synaptic potentials in a single human motoneurone can be derived from the profiles of post-stimulus time histograms (PSTH) of that neurone when it is firing rhythmically. We have used this method to explore the synaptic connections to individual tibialis anterior motoneurones in man. Agonist group 1 volleys produced short latency excitation of 85% of tibialis anterior motoneurones probably representing the 1a EPSP. Excitation, at a latency of 70 ms, is attributed to a long loop reflex. Antagonist group 1 volleys produced short latency inhibition of 65% of tibialis anterior motoneurones probably representing the 1a IPSP. Excitation, at a latency of 97 ms, may represent spino-bulbo-spinal reflex. None of these effects are due to the excitation of cutaneous afferent nerve fibres.

Action Potentials↗
PubMed1978

Hemiplegic spasticity: neurophysiologic studies.

The excitability of segmental reflex pathways in normal subjects and in patients with hemiplegia has been examined by conditioning the monosynaptic H reflex with a 200 msec burst of vibration applied to the tendo Achilles. In 6 normal subjects, the burst of vibration produced a short-latency facilitation of soleus motoneurons (attributed to monosynaptic excitation) which was followed by a longer-latency inhibition. A similar response was observed in 8 patients with hemiplegia, but the late inhibition was significantly less. The loss of an inhibitory mechanism may contribute to the exaggerated reflexes observed in patients with hemiplegia.

Adult↗