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Biomedical subjects

R F Mark

Publications and source records attributed to R F Mark.

At least 55 records · Page 3Linked to original sources
PubMed1985

Innervation pattern of muscles of one-legged Xenopus laevis supplied by motoneurons from both sides of the spinal cord.

In Xenopus tadpoles one limb bud was removed before innervation and motoneurons from both sides of the spinal cord were induced to innervate the remaining limb. When examined after metamorphosis the motor innervation of the limb had the following characteristics. In agreement with previous findings a large proportion of contralateral motoneurons survived (51-82% of the ipsilateral numbers) and sent axons to the limb. By acetylcholinesterase staining and intracellular recording from muscle fibers of the response to electrical stimulation of the two limb innervations, the neuromuscular junctions from contralateral motoneurons were indistinguishable from those from the ipsilateral side in their morphology, spacing along the fiber, and physiological properties. Many single muscle fibers shared innervation from both sides of the cord by symmetrically placed spinal nerves. By the same techniques junctions in one-legged frogs were morphologically indistinguishable from those in normal frogs, but the quantal content of transmitter release was increased by up to 63%. Recording twitch and tetanic tensions from individual motor units from the gastrocnemius muscle showed that the one-legged animals had many more and smaller motor units than do normal frogs. We confirm that the hind-limb musculature has the ability, normally unexpressed, to sustain, through the period of normal developmental cell death, up to twice the usual number of motoneurons. In maturity, motoneurons accommodate themselves to the limb muscles by making fewer than the normal number of synapses. The above suggests that developmental motoneuron death is not primarily a mechanism for adjusting the number of motoneurons to the size of the peripheral musculature and is likely to be related to mechanisms for securing specific neuromuscular connections.

Animals↗
PubMed1983

The effect of striatal lesions in the chick on haloperidol-potentiated tonic immobility.

The potentiating effects of haloperidol on tonic immobility in chickens were reduced in birds given bilateral injections of kainic acid into the paleostriatum. Aspiration of tissue above the lamina medullaris dorsalis, which includes the neostriatum and hyperstriatum, had no significant effect on haloperidol-potentiated tonic immobility. The results support the suggestion that the paleostriatum of birds is homologous to the basal ganglia of mammals.

Animals↗
PubMed1982

Long-term effects on motor activity induced by apomorphine or scopolamine after intracerebral injections of cycloheximide or glutamate shortly after hatching in chickens.

Compared to controls, bilateral forebrain injections of cycloheximide or glutamate in 2-day-old chicks resulted in significantly less activity in response to intraperitoneal injections of the dopaminergic agonist, apomorphine, or the cholinergic antagonist, scopolamine, when tested on stabilimeters four weeks later. These preliminary results are consistent with the idea that amongst other effects neonatal injections of cycloheximide or glutamate may alter dopaminergic and cholinergic neurotransmitter mechanisms involved in motor behavior in birds.

Animals↗
PubMed1982

Operant, open-field and tonic immobility behaviours in chickens with forebrain injections of cycloheximide or glutamate.

Chickens that had received bilateral injections of cycloheximide or glutamate into the forebrain on day 2 of life and tested 4 weeks later showed no deficit in acquisition, performance or extinction of continuously reinforced appetitive key-pecking as compared to control birds injected with saline. However, chickens that had received injections of cycloheximide and were subsequently tested in an open-field apparatus took longer to leave the first square, defaecated more, and pecked, preened and moved about less than controls. They also showed longer durations of tonic immobility. Those injected with glutamate inhibited similar behaviour but were not significantly different from controls in the open field latency to leave the first square, defaecation, or tonic immobility tests. The above treatments have previously been described as producing permanent slowed learning in chickens on a pebble-floor task. Our results suggest that learning mechanisms may not be disrupted as shown by normal performance in a simple operant task but that enhanced emotionality or fear of novelty as revealed in the open field tests may interfere with the expression of learning behaviour in some situations.

Animals↗
PubMed1982

Brief monocular deprivation leaves subthreshold synaptic input on neurones of the cat's visual cortex.

1. We have examined neurones in area 17 of monocularly deprived kittens for subthreshold input in response to visual stimulation through the deprived eye during reversible abolition of activity from the non-deprived eye and during increased excitability of cortical neurones induced by ionophoresis of DL-homocysteic acid (DL-H).2. After two or three days of monocular deprivation, beginning five weeks post-natally, most cortical neurones were dominated by the non-deprived eye. From a sample of forty-three neurones, from three kittens, driven exclusively by the non-deprived eye, only 16% developed responses to stimulation through the deprived eye as a result of reversible pressure blinding of the non-deprived eye. The responses through the deprived eye during pressure blinding usually developed over a period of several minutes and were always transitory: in no cases did the response persist for more than a few minutes after the return of the normal response through the non-deprived eye. Occasionally cells became responsive through the deprived eye during a short period of heightened general excitability after the release of pressure.3. Ionophoretic application of DL-H usually increased the spontaneous activity of cortical neurones. From two kittens monocularly deprived for three days during the fourth or fifth week of life, out of a sample of sixteen cortical neurones initially responsive only through the non-deprived eye, 63% showed responses to visual stimulation through the deprived eye during DL-H application. From a kitten monocularly deprived for 12 d from post-natal day 38, 28% of eighteen initially monocular neurones developed responses through the deprived eye during DL-H application.4. Preliminary results from intracellular recording showed apparently monocular neurones with excitatory input from the deprived eye producing subthreshold synaptic activity in response to moving bars of the same orientation that gave a suprathreshold response through the non-deprived eye.

Action Potentials↗
PubMed1981

Body weight and dietary factors in Huntington's disease patients compared with matched controls.

A longitudinal study using medical records has confirmed that, even when eating high energy diets, loss of body weight is characteristic of Huntington's disease patients in comparison with a group of matched control subjects. Further analysis showed that although some Huntington's disease patients initially may gain weight rapidly after a period in hospital, they eventually lose weight to below their admission levels. This pattern was not seen in the control group. The findings are discussed in terms of striatal pathology.

Adult↗
PubMed1981

Behavioral and electrophysiological study of cutaneous trigeminal nerves in axolotls. I. Normal innervation and reinnervation following nerve section.

Behavioral responses to light tactile stimulation of the skin of the head of axolotls were variable in occurrence but were of a constant pattern. Retraction of the eyes and gills followed stimulation of dorsal but not ventral skin. Ventral skin yielded lowering of the head. Touching near the lips caused turning of the head and a snap towards the stimulator. Following section and regeneration of divisions of the trigeminal Vth nerve, normal behavioral responses returned in 7-17 weeks. Afferent activity during cutaneous stimulation was recorded from sensory fibres in division of the cranial trigeminal nerve. The shapes of corresponding innervation fields on the left and right side were similar in normal animals. Following section and regeneration of one or more of the peripheral divisions of the trigeminal nerve, the pattern of fields of innervation of the skin of the head returned to normal, as judged by electrophysiological recordings from nerves central to the point of section. Receptive fields of individual fibres were roughly oval in shape and many overlapped with adjacent fields. Unit activity evoked by tactile stimulation of the face skin was also recorded from the region of the principal trigeminal nucleus in the medulla. Topographical organization was not apparent in the cutaneous receptive fields of medullary units, but a large proportion of the fields tended to be located on dorsal skin towards the front of the head and around the upper lip, innervated by the ophthalmic division.

Ambystoma↗
PubMed1977

Fiber counts of regenerating peripheral nerves in axolotls and the effect of metamorphosis.

Counts have been made of myelinated and unmyelinated fibers in the sixteenth nerve root of the axolotl, before, during and after regeneration following nerve section. There is no loss of myelinated fibers. Many collateral sprouts are formed and eventually withdrawn. Regeneration during metamorphosis is similar but there is more sprouting. There is no fiber loss from cut nerves in metamorphosis. This suggests that rising thyroid hormone levels are not in themselves detrimental to nerves without peripheral connections. Other factors that may govern cell loss in limb development are discussed.

Ambystoma↗
PubMed1977

Electrophysiological experiments on the mechanism and accuracy of neuromuscular specificity in the axolotl.

The supracoracoideus muscle of the axolotl shoulder girdle is innervated by two nerves, the supracoracoideus nerve (SC) supplying most of the muscle and the posterior supracoracoideus (PSC) supplying the posterior corner. All the muscle fibres are multiply innervated and at the border between the two innervations many muscle fibres, when penetrated by a microelectrode, show junction potentials from both nerves. In such cases one junction potential is often very small, below the threshold for exciting muscle contraction, the other large and effective at exciting the muscle. If the SC nerve is cut, the territory of the PSC nerve expands over several weeks. Upon regrowth of the cut nerve it reinnervates its old muscle fibres and removes the previous foreign innervation, the borderline between the two nerve territories being established exactly as before. This depends upon two processes, sprouting of nerves and a competitive repression of transmission from nerves ending on foreign muscle fibres.

Action Potentials↗