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Biomedical subjects

M H Evans

Publications and source records attributed to M H Evans.

At least 37 records · Page 2Linked to original sources
PubMed1976

Stimulation of the rabbit hypothalamus: caudal projections to respiratory and cardiovascular centres.

1. Electrical stimulation of a small area of the hypothalamus, lying about 2 mm dorsal to the lateral margin of the mammillary bodies, evoked a characteristic complex of effects in the anaesthetized rabbit. 2. The observed effects included apneusis followed by tachypnoea, increased arterial blood pressure, and vagally mediated bradycardia. Exophthalmost and pupillodilatation was commonly seen. The ears were erected and the vibrissae drawn forward. Somatic effects were weaker, but often included increased extensor tone, especially in the forelimbs. 3. The intensity of these effects was dependent upon the frequency of stimuli within the trains. Effects were weak or absent below 20 shocks/sec, and were near-maximal above 60 shocks/sec, when employing constant-current rectangular wave stimuli of 100-300 muA, 1 msec pulse duration and 5-9 sec train duration. 4. The effects were not dependent upon connexions with the cerebral cortex, but were mediated by pathways that projected ipsilaterally from each side of the hypothalamus through the mesencephalon. 5. The evidence suggests that this region of the rabbit hypothalamus is partly homologous with that hypothalamic area associated with the defence reaction in cat. However, the over-all responses to stimulation in the anaesthetized rabbit differed considerably from those described in the anaesthetized carnivore, suggesting that their integrated physiological responses to emotional threat are dissimilar.

Animals↗
PubMed1973

Some effects of dimethyl sulphoxide (DMSO) on the frog neuromuscular junction.

1. Dimethyl sulphoxide (DMSO) partially reversed neuromuscular blockade brought about by the action of (+)-tubocurarine or Mg(2+) on the frog sartorius nerve-muscle preparation.2. The amplitude and duration of the endplate potential (e.p.p.) were increased by DMSO at concentrations of 70 mM or greater.3. Miniature endplate potentials were raised in frequency, prolonged in duration and increased in amplitude by DMSO at concentrations of 141 mM or greater, but the increase in amplitude was generally less than in the case of the e.p.p.4. The resting muscle membrane potential was significantly depolarized by DMSO at 70 mM or greater concentrations, both at the endplate and remote from an endplate.5. The reversal of neurmuscular blockade by DMSO can be explained in terms of its previously reported ability to inhibit cholinesterase activity, together with the depolarizing action on muscle.

Animals↗
PubMed1973

Sensory receptors in the sheep's foot.

1. Median and ulnar nerves of anaesthetized Soay sheep have been investigated in vivo.2. The compound action potentials of these nerves indicate that the most rapidly conducting fibres had mean velocities of 87 m/sec in the median nerve and 57 m/sec in the ulnar nerve. The mean conduction velocities of the axons responsible for the Group A gammadelta wave in the median nerve were 23-30 m/sec and those responsible for the Group A delta wave in the ulnar nerve conducted at 23-26 m/sec.3. Single myelinated afferents were isolated by microdissection of median and ulnar nerves in the metacarpal region, and action potentials were recorded from hair follicle units, cutaneous mechanoreceptors, deeply situated proprioceptors, and from units of unknown function which were spontaneously active.4. The hair follicle units fell into two categories: ;diffuse' units which innervated many hairs in an area of about 1-3 cm(2) anywhere on the hairy skin above the hoof, and ;localized' units which innervated a few hairs within small areas of the skin adjacent to the hoof. The ;diffuse' units generally resembled the ordinary hair follicle units of other animals except that in the sheep the afferents had higher conduction velocities. The ;localized' units also had large myelinated axons, but their physiological properties differed in several aspects from the guard hair, tylotrich and carpal hair units which have been studied in other animals.

Action Potentials↗
PubMed1971

Comparison of the actions of saxitoxin and tetradotoxin on the motor end-plate of frog muscle.

End-plate potentials (e.p.p.) and responses to ionophoretic application of acetylcholine (ACh potentials) have been recorded in frog muscle. Saxitoxin (STX) and tetrodotoxin (TTX) abolished the e.p.p. at concentrations of 10(-8) g/ml. In almost all cases there was no effect on the ACh potentials even when the concentrations were raised, STX to 1.2 x 10(-6) g/ml (3.2 x 1O(-6)M), or TTX to 5 x 10(-6) g/ml (1.5 x 10(-5)M). STX, 10(-6) g/ml (2.7 x 10(-6)M) often caused a slow rise in the frequency of miniature e.p.p., without significant effect on amplitude.

Acetylcholine↗
PubMed1970

Two toxins from a poisonous sample of mussels Mytilus edulis.

1. A crude preparation of toxin was extracted from a sample of mussels Mytilus edulis, part of a batch responsible for many cases of paralytic shellfish poisoning.2. The crude extract was partially purified by absorption on sodium Amberlite ion-exchange resin. Two toxins were recovered by elution from the Amberlite, and purified further by gel filtration.3. One toxin closely resembled saxitoxin in its behaviour on Amberlite and in its biological effects.4. The other toxin behaved quite differently on the Amberlite. Its molecule was small, comparable in size with saxitoxin. It was not tetrodotoxin. Its biological effects were similar, but not identical, to those of saxitoxin: it paralysed muscular contraction and inhibited conduction along nerves; it caused death of experimental animals by producing a peripheral paralysis of respiration; it did not depolarize the membrane of frog skeletal muscle fibres, but acted by preventing a stimulus from initiating a conducted action potential.5. The biological effects of the second toxin suggest that, like saxitoxin and tetrodotoxin, it is an inhibitor of inward sodium ion movement through electrically excitable membranes.

Animals↗
PubMed1969

The effects of saxitoxin and tetrodotoxin on nerve conduction in the presence of lithium ions and of magnesium ions.

1. It has been shown that nerve fibres from rat cauda equina will conduct action potentials after immersion in saline in which lithium chloride is substituted for sodium chloride.2. Both saxitoxin and tetrodotoxin inhibit lithium-generated action potentials. The concentration of toxin needed to inhibit the lithium-generated action potentials is similar to that needed to inhibit sodium-generated action potentials.3. If magnesium chloride is added to the saline to give a concentration of 10-15 mM there is usually a slight fall in amplitude of the compound action potential. Saxitoxin and tetrodotoxin now inhibit the action potential to a greater degree than in the absence of magnesium ions.

Action Potentials↗