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N Robbins

Publications and source records attributed to N Robbins.

70 records · Page 4Linked to original sources
PubMed1971

Developing neuromuscular junctions: first signs of chemical transmission during formation in tissue culture.

Electrophysiological study of rat neuromuscular junctions in the early stage of formation in tissue culture showed that chemical transmission begins with discrete, localized release of transmitter about the time when nerve-muscle contacts are first visible with light microscopy. Noncontractile myotubes with as few as three nuclei showed evidence of junctional transmission.

Acetylcholine↗
PubMed1971

Physiological studies during formation and development of rat neuromuscular junctions in tissue culture.

Developing neuromuscular junctions in tissue cultures of rat embryo spinal cord and muscle were studied with intracellular recording. Excitatory junctional potentials (ejps) were found during nerve or spinal cord stimulation, and also arising spontaneously. The time-course of the potentials tended to be shorter in cultures older than 9 days in vitro than in more recently innervated cultures. Evidence of multiple innervation was found in many cells. In order to test the hypothesis that transmitter was released in integral multiples of a quantal amount and according to a Poisson distribution, mean quantum content was calculated from the coefficient of variation of ejp amplitudes, the percentage of zero responses, and the ratio of mean amplitude to mean of the smallest class of amplitudes. The three independent measures were in fair or good mutual agreement, implying that the mechanism of transmitter release in newly developed junctions is the same as in the adult. In newly formed junctions, ejps were subthreshold for action potential generation, but afterwards mean quantum content increased and action potentials were generated by single ejps. In fibers developing both with and without innervation, the entire muscle cell surface was as sensitive to acetylcholine as the adult end plate region. Innervation was related to cross-striation: every cross-striated fiber tested was found to be innervated, and denervation in vitro led to loss of distinct cross-striations.

Acetylcholine↗
PubMed1969

Changes in contractile properties of disused soleus muscles.

1. The hypothesis that the pattern of activity of muscle can determine its contractile properties was tested in the case of the rat soleus subjected to chronic disuse.2. In order to characterize the pattern of motor activity over long periods of time, a method of chronic EMG recording with indwelling electrodes was developed. Some characteristics of the EMG recorded from unrestrained animals are described.3. Immobilization of the knee and ankle joints reduced aggregate EMG activity in the soleus to 5-15% of control and resulted in a shift from tonic to a more phasic pattern of firing.4. After 4 weeks of immobilization, speeding of the soleus mechanical properties was indicated by: shortened contraction time; decreased tetanus/twitch ratio; increased maximum rate of development of tetanic tension; and decreased fusion during a 5/sec tetanus.5. Because the contractile properties of the soleus can be altered by a period of disuse with no change in innervation, neuronal control of the contractile mechanism depends in part on impulse activity.

Animals↗
PubMed1967

Peripheral modification of sensory nerve responses after cross-regeneration.

1. Chemical responses of skin and tongue, recorded in vivo from intact and self-regenerated cutaneous or lingual nerves, were compared to responses from cutaneous nerves cross-united with distal stumps of lingual nerves. Cross-innervated tongue grafts were also studied.2. In contrast to normal tongue-nerve preparations, skin preparations usually showed responses to chemical stimulation with longer latency, higher threshold, and less variation of temporal pattern with diverse stimuli. These characteristics were similar when the nerve, having been cut, had regenerated.3. On chemical stimulation of the tongue, cutaneous nerves cross-innervating the tongue often yielded records which were ;gustatory' by all criteria. Some responses with low threshold and short latency were also obtained from dorsal cutaneous nerves innervating tongue grafts.4. Thus, cutaneous nerves can serve the same role as gustatory nerves. The functional characteristics of these sensory neurones are not predetermined and must depend on the environment of the nerve endings.

Acetates↗
PubMedDate not supplied

Fixation effects on synaptic vesicle density in neuromuscular junctions of young and old mice.

It was previously reported that in soleus neuromuscular junctions of old mice, synaptic vesicle density was decreased while transmitter release was increased (compared to results in young mice). In the present study, two hypotheses that might resolve this disparity were tested. The first was that the density of readily releasable vesicles close to the preterminal membrane, rather than those in the whole terminal, would correlate with the physiological results. This hypothesis was excluded because both vesicle density in the 200 nm region just within the presynaptic terminal membrane, and total vesicle density were similarly reduced in old soleus junctions. The second hypothesis was that more transmitter was released during fixation at old than at young neuromuscular junctions, leading to an age-related depletion of vesicles. This was tested by counting vesicles in muscles fixed after transmission block was attained in Krebs solution lacking calcium, and by direct recording of quantal release during conventional fixation. This second hypothesis was excluded: in neuromuscular junctions exposed to zero-calcium Krebs solution before fixation, the age-related reduction in vesicle density was still present, and intracellular recording revealed only a slight increase in quantal transmitter release during fixation. Therefore, as discussed, other mechanisms must be considered.

Aging↗
PubMedDate not supplied

General and strain-specific age changes at mouse limb neuromuscular junctions.

Age changes at limb neuromuscular junctions (NMJs) of CB57 and BALB/C mice were investigated and compared with previous results in a hybrid strain, in order to identify common characteristics of neuromuscular transmission and to assess the prevalence of signs of denervation and disuse. Resting membrane potential decreased with age in soleus muscles with no change in passive membrane properties. Miniature end-plate potential, amplitude and quantal content were greatly increased, with no change in muscle fiber diameter or nerve terminal morphometry. The various observed patterns of age changes were not those of disuse or denervation. Although there were diverse age changes in spontaneous transmitter release in different mouse strains, increased transmitter release per unit length of nerve terminal in limb muscle was a notably consistent finding across strains. It is apparently less subject to epigenetic variation during aging than most other reported neuromuscular physiological parameters.

Acetylcholinesterase↗
PubMedDate not supplied

Acetylcholine receptors at mature and aged mouse neuromuscular junctions.

The density and distribution of junctional and perijunctional ACh receptors (AChR) were studied in young (8-12 months) and old (24-25 months) C57 mice to determine: (1) if increased amplitude of spontaneous postsynaptic potentials previously reported in old C57 muscle was due to increased junctional AChR; (2) if increased extrajunctional AChR would be found in association with previously reported nerve terminal complexity; and (3) if extrajunctional AChR was present as in disused or denervated muscle. Microdissection of individual muscle fibers combined with I125-alpha-bungarotoxin labeling, gamma counting, measurement of surface area, cholinesterase stains, and autoradiography were used to obtain the results. In both young and old mice there was a sharp gradient in AChR between the end-plate and the perijunctional region. End-plate AChR densities and total AChR per end-plate were the same at old and young end-plates, as were perijunctional values. Thus, neither end-plate nor extrajunctional AChR density changes with age. An increased mepp amplitude reported previously in old CB57 animals must be due to other factors. The perijunctional AChR in old mice show no changes characteristic of disuse or denervation, or those which might give rise to the observed nerve terminal complexity.

Aging↗