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Biomedical subjects

M R Bennett

Publications and source records attributed to M R Bennett.

At least 289 records · Page 16Linked to original sources
PubMed1974

The formation of synapses in reinnervated and cross-reinnervated striated muscle during development.

1. A study has been made of the formation of synapses in reinnervated and cross-reinnervated developing striated muscles which normally receive either a focal or distributed innervation, using histological, ultrastructural and electrophysiological techniques.2. The focally innervated mammalian tibialis anterior muscle, denervated soon after birth, was reinnervated at both the original end-plates as well as on the new muscle added during the period of denervation; but not on the muscle present at the time of denervation. Nearly all the synapses which had formed, other than at the original end-plates, disappeared by 6 weeks post-natal.3. The avian anterior latissimus dorsi muscle (ALD), which receives a distributed innervation, was denervated during the first week post-hatched, and became reinnervated both at the original synaptic sites as well as on the new muscle added during the period of denervation; all these synapses were spaced approximately 200 mum apart along the length of individual muscle cells.4. The myofibres of the ALD muscle cross-reinnervated at hatching with the superior brachialis nerve, which contains fast motor axons that normally form a focal innervation, were each focally innervated by a single ;en plaque' terminal; these synapses had the same electrical properties as normal synapses formed by fast motor axons.5. Many of the myofibres of the avian posterior latissimus dorsi (PLD), which normally receive a focal innervation, received a distributed innervation from ;en grappe' terminals when cross-reinnervated with the ALD nerve at hatching.6. It is suggested that during development the nerve type determines the pattern of synapses over an effector; this is achieved by the nerve, after forming the initial synaptic contact, making the rest of the muscle cell membrane refractory to further synapse formation for some distance, this distance being determined by the nerve type.

Action Potentials↗
PubMed1973

An electrophysiological analysis of the storage and release of noradrenaline at sympathetic nerve terminals.

1. An electrophysiological analysis has been made of the storage and release of noradrenaline (NAd) in the sympathetic nerve terminals of the isolated vas deferens of the mouse. The amplitude of the excitatory junction potentials (e.j.p.s) recorded intracellularly in smooth muscle cells was taken as a measure of the NAd output per impulse from the terminals of sympathetic axons.2. During short trains of impulses (< 100), the amplitude of the e.j.p. increased with successive impulses at the beginning of a train, and then either continued to increase until a steady-state amplitude was reached (frequencies < 1 Hz), or decreased until a depressed steady amplitude was reached (frequencies > 1 Hz).3. During trains of impulses lasting for several minutes, the amplitude of the e.j.p. continually declined (frequencies > 1 Hz) until a steady-state amplitude was reached after 8 min of stimulation. This steady-state amplitude is smaller, the higher the frequency of stimulation.4. During short trains of impulses in the presence of high magnesium solutions, the amplitude of successive e.j.p.s increased until a steady state was reached, no matter what the frequency of stimulation. This growth of the e.j.p. amplitude during a train could be quantitatively predicted in terms of the linear summation of the individual facilitatory effects introduced by each impulse in the train.5. During trains of impulses lasting for several minutes, in the presence of a NAd synthesis blocker, the amplitude of the e.j.p. continually declined along a curve which could be described as the sum of two exponential components: one with a time constant of 1 min and the other of 10 min.6. These results suggest that NAd is released from a small pool of transmitter in sympathetic nerve terminals, which is replenished from two stores, which are in turn replenished by the synthesis of new NAd.

Animals↗
PubMed1973

An electrophysiological analysis of the uptake of noradrenaline at sympathetic nerve terminals.

1. An electrophysiological analysis has been made of the uptake of NAd in the sympathetic nerve terminals of the isolated vas deferens of the mouse. The amplitude of the excitatory junction potentials (e.j.p.s) recorded intracellularly in smooth muscle cells was taken as a measure of the NAd output per impulse from the terminals of sympathetic axons.2. Neuronal uptake blockers (desipramine and cocaine) greatly depressed the amplitude of all e.j.p.s after the first in a short train (< 100) at high frequencies (>/= 1 Hz).3. Blocking neuronal uptake did not affect the time course of decline in amplitude of the e.j.p. during long trains of stimulation over several minutes, apart from the immediate depression in the e.j.p. following the first few impulses.4. The time course of decline of the e.j.p. amplitude during continual stimulation, when both neuronal uptake and synthesis was blocked, was similar to that when only synthesis was blocked, apart from the immediate depression following the first few impulses.5. Phenoxybenzamine reversed the normal depression in e.j.p. amplitude observed at high frequencies (>/= 1 Hz) to facilitation. This facilitation lasted for several minutes of high frequency stimulation.6. A model has been proposed of the sympathetic nerve terminal, in which NAd is released by each nerve impulse in a train from a small pool in the nerve terminal, which is principally replenished by uptake of the NAd released by the immediately preceding impulses in the train. The pool is replenished to a less extent by transmitter located in two stores in the terminal which are in turn replenished by transmitter synthesis.

Animals↗
PubMed1973

The formation of synapses in reinnervated and cross-reinnervated adult avian muscle.

1. A study has been made of the formation of synapses in spontaneously reinnervated and cross-reinnervated anterior latissimus dorsi (ALD) and posterior latissimus dorsi (PLD) muscles of adult fowls.2. Denervated ALD and PLD muscle fibres have a uniform and high sensitivity to iontophoretically applied acetylcholine (ACh). During early reinnervation the sensitivity distribution to ACh of the ALD muscle fibres begins to return to normal before synaptic potentials can be evoked. The normal ACh sensitivity distribution of PLD muscle fibres is also restored after reinnervation. After cross-reinnervation of the ALD and PLD muscles the ACh sensitivity distribution of many of the muscle fibres is again restored to normal.3. Reinnervating and cross-reinnervating ALD nerve terminals showed a greater than normal degree of facilitation of transmitter release when a test impulse was applied at various intervals after a conditioning impulse. Cross-reinnervating PLD nerve terminals showed facilitation of transmitter release rather than the normal depression in a conditioning-test impulse sequence.4. The distribution of nerve terminals over the surface of spontaneously reinnervated and cross-reinnervated ALD and PLD muscle fibres has been determined from an examination of the sensitivity distribution to applied ACh, the graded versus all-or-none nature of the evoked potential and the distribution of cholinesterase stained synapses.5. The results suggest that the innervation pattern of individual ALD and PLD muscle fibres is restored both after spontaneous reinnervation and cross-reinnervation.

Acetylcholine↗
PubMed1973

The formation of synapses in reinnervated mammalian striated muscle.

1. The hemidiaphragm of the adult rabbit has a single band of end-plates running around the middle of the muscle. A study has been made of the formation of synapses during spontaneous reinnervation of this muscle, using histological, ultrastructural and electrophysiological techniques.2. Following spontaneous reinnervation, silver-stained nerve terminals were found in association with cholinesterase-stained end-plates only in the region of the muscle corresponding to the original innervation band.3. The regenerated nerve terminals were observed with the electronmicroscope in positions overlying or adjacent to the old synaptic folds.4. Spontaneous miniature end-plate potentials and evoked synaptic potentials were recorded only in the middle of the muscle fibres after reinnervation.5. The growth of the regenerating axons was not oriented towards the end-plate zone but followed muscle fibres and blood vessels in random directions.6. It is concluded that, in adult mammalian striated muscle, the old end-plate region is preferentially reinnervated as a consequence of some special property of the muscle fibre at this site.

Animals↗
PubMed1973

The formation of synapses in mammalian striated muscle reinnervated with autonomic preganglionic nerves.

1. A study has been made of the formation of synapses during reinnervation of the hemidiaphragm of adult rabbits with preganglionic fibres of the thoracic vagus, using histological, ultrastructural and electrophysiological techniques.2. Following reinnervation with preganglionic axons, silver-stained nerve terminals were found in association with cholinesterase-stained end-plates only in the region of the muscle corresponding to the original innervation band.3. The fine preganglionic axons retained their normal structure in striated muscle, but were found to synapse over discrete areas of dimensions not larger than those of the original end-plates.4. The regenerated varicose preganglionic nerve terminals were observed with the electronmicroscope in positions either overlying or in the vicinity of the old synaptic folds.5. Spontaneous potentials and evoked synaptic potentials were recorded only in the middle of the muscle fibres after vagus reinnervation.6. In a few cases, multiple synaptic potentials with similar time courses were recorded, suggesting that several axons had formed synapses at a single site on a muscle fibre.7. It has been shown that, during reinnervation of adult mammalian striated muscle fibres with nerves other than those of the somatic system, synapses are formed preferentially in the region of the old end-plates.

Animals↗
PubMed1972

An electrophysiological analysis of the storage of acetylcholine in preganglionic nerve terminals.

1. An electrophysiological analysis has been made of the storage of acetylcholine (ACh) in the preganglionic nerve terminals of the isolated superior cervical ganglion of the guinea-pig. The mean amplitude of excitatory post-synaptic potentials recorded intracellularly was taken as a measure of the ACh output per impulse from the terminals of a preganglionic axon.2. Prolonged repetitive stimulation of the cervical sympathetic trunk at 10 and 20 Hz in the presence of hemicholinium No. 3 led to an exponential decline of ACh output as the transmitter formed before the beginning of stimulation was depleted.3. The rate of decline of ACh output during stimulation at 20 Hz (tau = 0.83-1.95 min) was about twice as fast as that at 10 Hz (tau = 1.00-6.83 min).4. The results suggest that, during prolonged stimulation, ACh is released from a single store in the preganglionic nerve terminal and that each impulse releases a constant fraction of the store of the transmitter.

Acetylcholine↗
PubMed1972

An electrophysiological analysis of the synthesis of acetylcholine in preganglionic nerve terminals.

1. An electrophysiological analysis has been made of the synthesis of acetylcholine (ACh) in the preganglionic nerve terminals of the isolated superior cervical ganglion of the guinea-pig. The mean amplitude of excitatory post-synaptic potentials recorded intracellularly was taken as a measure of the ACh output per impulse from the terminals of a preganglionic axon.2. Prolonged repetitive stimulation of the cervical sympathetic trunk at 10 and 20 Hz led to a decline in ACh output over the first 5-15 min and then a maintained output for periods of up to an hour. The mean level of maintained output was 0.4 of the peak initial output.3. The maintained level of output was shown to be equal to the rate of synthesis of new transmitter and was not dependent on the addition of choline (3 x 10(-5)M) to the fluid in the organ bath.4. The ACh output per minute was shown to be directly proportional to the frequency of stimulation.5. A model has been proposed of the storage and synthesis of ACh in preganglionic nerve terminals during prolonged stimulation, in which choline from the hydrolysis of released ACh is the main source of substrate for synthesis of new transmitter, and the rate at which synthesis proceeds is controlled by the rate at which transmitter is released.

Acetylcholine↗
PubMed1967

An analysis of the surface fixed-charge theory of the squid giant axon membrane.

The observed shift in threshold potential, after perfusion of the squid giant axon with solutions of low ionic strength, can be predicted by assuming a fixed negative charge on the inside of the membrane. The constant field equation, together with the double-layer potential due to this charge, has been used to determine the change in resting potential during perfusion with solutions of low ionic strength. Neither the modified constant field equation nor Planck's diffusion equation can successfully predict the observed shift in resting potential. It is suggested that a positive charge distribution exists about the sodium channel on the outside of the membrane. The double-layer potential due to this positive charge, together with the independence principle, has been used to predict the relationship between sodium current and membrane potential when the ionic strength and sodium activity of the external solution are decreased. These predictions have been compared with the available experimental observations.

Animals↗
PubMed1967

A study of the innervation of the taenia coli.

An electrophysiological and anatomical study of the guinea pig taenia coli is reported. Changing the membrane potential of single cells cannot modulate the rate of firing action potentials but does reveal electrical coupling between the cells during propagation. The amplitude of the junction potentials which occur during transmission from inhibitory nerves is unaffected in many cells during alteration of the membrane potential, indicating electrical coupling during transmission. The taenia coli is shown to consist of smooth muscle bundles which anastomose. There are tight junctions between the cells in the bundles, and these probably provide the pathway for the electrical coupling. The smooth muscle cells towards the serosal surface of the taenia coli are shown electrophysiologically to have an extensive intramural inhibitory innervation, but a sparse sympathetic inhibitory and cholinergic excitatory innervation. These results are in accordance with the distribution of these nerves as determined histochemically. As single axons are only rarely observed in the taenia coli, it is suggested that the only muscle cells which undergo permeability changes during transmission are those adjacent to varicosities in the nerve bundles. The remaining muscle cells then undergo potential changes during transmission because of electrical coupling through the tight junctions.

Action Potentials↗
PubMed1967

The effect of intracellular current pulses in smooth muscle cells of the guinea pig vas deferens at rest and during transmission.

The effect of intracellular current pulses on the membrane of smooth muscle cells of the guinea pig vas deferens at rest and during transmission was studied. Two main response types were identified: active response cells, in which a spike was initiated in response to depolarizing currents of sufficient strength and duration; passive response cells, in which depolarizing currents gave only electrotonic potential changes. These cells were three times more numerous than the active response cells. During the crest of the active response the input resistance fell by about 25% of the resting value. Comparison of the active response with the action potential due to stimulating the hypogastric nerve showed that the former was smaller in amplitude and had a slower rate of rise and higher threshold. Electrical coupling occurred between the smooth muscle cells during the propagation of the action potential. Depolarizing current pulses had no effect on the amplitude of the excitatory junction potential (E.J.P.) in passive response cells, but in general did decrease its amplitude in active response cells. These results are discussed with respect to the mechanism of autonomic neuroeffector transmission.

Action Potentials↗
PubMed1967

The effect of cations on the electrical properties of the smooth muscle cells of the guinea-pig vas deferens.

1. A study has been made of the effects of Na(+) and Ca(2+) on the responses generated by intracellular current pulses in smooth muscle cells of the guinea-pig vas deferens and upon the propagated action potential.2. Reduction of the extracellular Na(+) activity (a(Na)) to less than 30 mM did not alter the characteristics of the spike-like response to intracellular current pulses (the active response) or of the propagated action potential.3. Reduction of the extracellular Ca(2+) activity (a(Ca)) to 0.1 mM decreased the resting input resistance of the cells but increased the input resistance during the crest of the active response. Reduction of Ca(2+) decreased the overshoot of the action potential by 22 mV per tenfold change in Ca(2+) and the resting potential by 25 mV per tenfold change.4. Reduction of the extracellular Ca(2+), keeping the product a(Ca)/a(2) (Na) constant, did not change the resting potential, but decreased the action potential overshoot by 20 mV per tenfold change.5. It is suggested that part of the current responsible for the rising phase of the action potential is carried by Ca(2+).

Action Potentials↗
PubMed1966

Transmission from perivascular inhibitory nerves to the smooth muscle of the guinea-pig taenia coli.

1. Membrane potential changes of the smooth muscle cells of the taenia coli were recorded during stimulation of the perivascular inhibitory nerves.2. Some spontaneous action potentials were preceded by a slow pacemaker-like potential. Others began at or near the maximum level of the membrane potential and were not preceded by pacemaker-like potentials.3. There were no changes in the membrane potential of smooth muscle cells when the inhibitory nerves were stimulated with a single pulse. Stimulation at frequencies greater than 5 pulses/sec caused a hyperpolarization of the smooth muscle membrane. This resulted in a decrease in spike frequency and relaxation.4. When the frequency of stimulation of the inhibitory nerves was increased there was an increase in the amplitude and rate of rise of the hyperpolarization and a decrease of the latency. The latency varied from 150 to 300 msec, and the largest hyperpolarization recorded was 16 mV.5. The effect of the hyperpolarization due to nerve stimulation in cells showing pacemaker-like activity was to increase the level of the membrane potential at which the action potentials began and to increase the membrane potential to which the action potentials repolarized. Action potentials which occurred during hyperpolarizations of the membrane had greater rates of rise and fall and larger amplitudes than did the action potentials which occurred before hyperpolarization.6. The effect of the hyperpolarization due to nerve stimulation in cells which did not show pacemaker-like activity depended on the amplitude of the hyperpolarization. Small hyperpolarizations exposed small depolarizations of the membrane which occurred when an action potential would normally have been expected. Large hyperpolarizations blocked the action potentials entirely.7. Action potentials did not begin firing again at the normal rate immediately after stimulation ceased. The time taken for the rate of firing of action potentials to return to normal increased with increasing frequency of stimulation.8. The hyperpolarization in response to perivascular inhibitory nerve stimulation was blocked by guanethidine and bretylium.

Animals↗
PubMed1966

Transmission from intramural inhibitory nerves to the smooth muscle of the guinea-pig taenia coli.

1. Membrane potential changes of smooth muscle cells were recorded during stimulation of the intramural inhibitory nerves to the taenia coli.2. Stimulation across the taenia coli with single pulses of 200 musec duration excites the intramural nerves and not the muscle directly.3. The membrane potential changes due to stimulation of the intramural inhibitory nerves were different from those produced by perivascular inhibitory nerve stimulation in the following ways: hyperpolarizations (i.j.p.'s) of up to 25 mV were produced in response to single pulses; the latency, i.e. the time taken for the membrane to hyperpolarize after a stimulus of maximal strength, was as short as 80 msec; when the nerves were stimulated repetitively the membrane was hyperpolarized by up to 35 mV and all spontaneous activity was abolished; the mean hyperpolarization due to repetitive stimulation increased with the frequency of stimulation up to 10 pulses/sec and then remained constant; the hyperpolarization due to stimulation at frequencies greater than 5 pulses/sec was not maintained but decreased after 3-5 sec of stimulation; and finally when stimulation had ceased action potentials commenced firing at frequencies greater than normal.4. The amplitude and rate of hyperpolarization of the i.j.p. increased with increasing strength of stimulation until a maximum amplitude and rate of hyperpolarization was reached. The recovery or depolarizing phase of the i.j.p. was exponential with a time constant which varied from about 250 msec to 500 msec and could not therefore be due to the discharge of the membrane capacitance. In some cases there was an inflexion on this depolarizing phase and in these cases recovery led directly into an action potential.5. Spontaneous hyperpolarizations of the membrane were seen in some cells, and these hyperpolarizations were similar to those recorded on submaximal stimulation of the intramural nerves.6. There were no changes in the characteristics of the i.j.p. in the presence of guanethidine or bretylium.

Animals↗