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Biomedical subjects

P Ashby

Publications and source records attributed to P Ashby.

At least 19 recordsLinked to original sources
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↗
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↗
PubMed1977

Ethanol and spinal presynaptic inhibition in man.

Ethanol, 0.65 gm per kilogram of body weight, was administered orally to 6 normal subjects. The mean blood ethanol levels ranged from 0.74 to 0.99 gm per liter over a subsequent 40-minute testing period. The proportion of the soleus motoneuron pool activated by the Achilles tendon reflex was reduced. Vibratory inhibition of the monosynaptic reflex was used as an estimate of spinal presynaptic inhibition. It was unaffected by ethanol.

Adult↗
PubMed1977

Effects of extensor and flexor group I afferent volleys on the excitability of individual soleus motoneurones in man.

The contour of the postsynaptic potential (PSP) produced in a neurone by an afferent volley can be derived from the contour of the post-stimulus time histogram (PSTH) of that neurone when it is discharging rhythmically. In the present study the PSTH of the firing of individual soleus motor units after stimulation of the popliteal or peroneal nerve was used to explore the effects of extensor and flexor group I afferent volleys on the excitability of single soleus motoneurones in man. Extensor group I volleys resulted in an early peak of increased impulse density in the PSTH of 75% of soleus motoneurones. The latency suggests an analogy with the Ia EPSP. The mean duration of the peak of increased impulse density, equivalent to the rise time of the EPSP, was 3.6 ms. Flexor group I volleys result in a period of reduced impulse density in the PSTH of five out of nine soleus motoneurones. The latency suggests an analogy with the Ia IPSP. We conclude that this method could be used to explore the afferent connections to single motoneurones in man and to derive some of the characteristics of the postsynaptic potentials from a variety of afferent nerve fibres in single human motoneurones.

Action Potentials↗
PubMed1977

Diazepam effect on reflex activity in patients with complete spinal lesions and in those with other causes of spasticity.

The effects of diazepam on reflex pathways in patients having complete spinal lesions and in patients having incomplete lesions of the spinal cord or multiple sclerosis are compared to determine whether diazepam has an action at spinal level. The drug produced no significant alteration in the excitability of the monosynaptic arc in the patients with complete spinal lesions. In contrast, in the group with incomplete spinal lesions or multiple sclerosis, diazepam reduced the excitability of the monosynaptic arc. This action did not appear to result from a reduction in fusimotor drive. Diazepam also reduced the tonic vibration reflex in this group. It is postulated that these effects may be due to a supraspinal action of the drug.

Achilles Tendon↗
PubMed1976

Spinal reflexes and the concentrations of 5-HIAA, MHPG, and HVA in lumbar cereborspinal fluid after spinal lesions in man.

Descending bulbospinal pathways that employ specific neurotransmitter substances are known to be capable of modulating segmental reflex activity in the experimental animal. To determine whether this might also occur in man correlations have been sought between the activity in spinal reflex pathways and the lumbar cerebrospinal fluid (CSF) concentrations of 5-hydroxyindolacetic acid (5-HIAA), 3 methoxy-4-hydroxyphenylglycol (MHPG), and homovanillic acid (HVA) in 12 patients with complete or virtually complete spinal lesions. The concentrations of 5-HIAA and MHPG in lumbar CSF ARE REDUCED AFTER COMPLETE OR VIRTUALLY COMPLETE SPINAL LESIONS IN MAN. This may occur within 18 days of the lesion. MHPG concentrations appear to be inversely related to the level of the lesion. The HVA concentration in lumbar CSF is reduced when there is obstruction of the CSF pathways. No relationship could be demonstrated between the concentrations of 5-HIAA or MHPG in lumbar CSF and the activity in the spinal monosynaptic pathway (estimated from the proportion of the motoneurone pool activated by the Achilles tendon reflex or H reflex) or the activity of a spinal inhibitory mechanism (estimated by the degree of vibratory inhibition of the monosynaptic reflex). Patients with a tonic vibration reflex (TVR) tended to have higher MHPG levels. There appeared to be an association between low CSF HVA and enhanced vibratory inhibition of the monosynaptic reflex in the nine patients whose spinal lesions were complete.

Adolescent↗
PubMed1976

Conduction velocity in the proximal segments of a motor nerve in the Guillain-Barré syndrome.

Conduction velocity from spinal cord to axilla (estimated using the F wave) has been compared with conduction velocity from axilla to wrist (measured in the conventional manner) in the motor fibres of the ulnar nerve in 17 controls subjects and in 11 patients with the Guillain-Barré syndrome (GBS). In the patients with GBS the conduction velocity was, in general reduced to a similar extent in both the proximal and the distal portions of the motor fibres, suggesting that the disease process is usually diffuse. In two patients, however, the conduction velocity in the proximal segment was disproportionally reduced and in one of these the conduction velocity in the distal segment was normal. It is concluded that the estimation of conduction velocity in the proximal segments of motor nerves may be of value in the assessment of patients with GBS.

Adolescent↗
PubMed1976

Neurophysiologic changes in hemiplegia. Possible explanation for the initial disparity between muscle tone and tendon reflexes.

In an attempt to clarify the neurophysiologic changes that may follow a cerebral lesion in man, we have studied patients with recent and with long-standing hemiplegia from cerebral infarction. In patients with recent cerebral lesions, inhibition of the monosynaptic reflex by vibration is enhanced. In patients with long-standing cerebral lesions, this inhibitory mechanism is less effective and a comparison of the electrically and mechanically induced monosynaptic reflexes suggests that fusimotor drive may be increased. Related clinical findings are reduced muscle "tone" immediately after the lesion and increased muscle "tone" and exaggerated tendon jerks in patients with long-standing hemiplegia.

Adult↗
PubMed1976

Effect of diazepam on muscle contraction in spasticity.

The direct effect of diazepam on skeletal muscle has been examined in 15 patients with neurological lesions resulting in spasticity. Diazepam 15-30 mg. IV reduced the amplitude of the compound action potential of the direct muscle response (M response) and the isometric twitch tension. It is postulated that diazepam may affect the contractile properties of muscle and, possibly, the electrical properties of the muscle membrane. These peripheral effects may contribute to the reported clinical benefits of the drug in patients with spasticity including those patients with complete spinal lesions.

Action Potentials↗