The physiology, pharmacology, and trophic effectiveness of synapses formed by autonomic preganglionic nerves on frog skeletal muscle (original) (raw)
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Competitive interaction between foreign nerves innervating frog skeletal muscle
The Journal of physiology, 1979
1. Competition between two foreign nerves innervating frog skeletal muscle has been studied by using pairs of somatic motor nerves (s.m.n.s) or one s.m.n. and the preganglionic splanchnic nerve (s.p.n.) implanted into a denervated sartorius muscle that has been transplanted to the lymph sac of the back. 2. A single s.m.n. implanted into the muscle succeeded in innervating essentially every fibre within 2--3 months; tetanic stimulation of the nerve elicited 9--100% of the maximal direct tetanus tension. Most of the e.p.p.s were suprathreshold, since a single indirect stimulus evoked a twitch 60--100% as large as that to a direct stimulus. 3. If two s.m.n.s were implanted simultaneously, tetanic stimulation of either elicited 80--100% of the maximal tension to direct stimulation. If one nerve was implanted 2--3 months before the other, the second, although usually less effective than the first, normally innervated 50--100% of the fibres, with approximately the same time course of inne...
Physiological regulation of synaptic effectiveness at frog neuromuscular junctions
The Journal of physiology, 1980
1. Nerve terminals in two different muscles of the frog, the sartorius and cutaneous pectoris (c.p.), have been found to differ sharply in safety factor. This difference is shown to be attributable to corresponding disparities in the amount of transmitter released, without evident correlated morphological differences. 2. In Ringer containing 0.3 mM-Ca2+ and 1 mM-Mg2+, quantal content of c.p. junctions exceeded that of sartorius junctions by 3-4 times. 3. When quantal content was corrected for nerve terminal size, c.p. terminals still released 2-4 times more transmitter per unit terminal length. 4. Light and electron microscopic examination of junctional morphology in the two muscles revealed no significant difference in the spacing of presynaptic active zones, the width of synaptic contact, or the density of presynaptic vesicles and mitochondria. It seems likely, therefore, that the greater release at c.p. junctions is due to a 'physiological' difference between the two popu...
The regulation of synaptic strength within motor units of the frog cutaneous pectoris muscle
The Journal of neuroscience : the official journal of the Society for Neuroscience, 1985
The physiological properties of frog neuromuscular junctions may vary widely in a single muscle. In order to understand the factors that contribute to this variation, we have studied populations of synapses belonging to individual motor units of the frog cutaneous pectoris muscle. Motor units in this muscle differ widely in twitch strength. A motor axon's synaptic contacts could be found throughout the muscle, at both singly and polyneuronally innervated endplates. Indeed, over 36% of the endplates contacted by each isolated motor axon were polyneuronally innervated. Comparisons of synapses on muscle fibers in large twitch motor units with those in small twitch motor units reveal that endplate potential amplitude, transmitter release, and muscle fiber diameter are positively correlated with the strength of the motor unit contraction. Large and small twitch motor units differ more in their transmitter release than in their nerve terminal length, indicating that larger twitch moto...
Induction of the action potential mechanism in slow muscle fibres of the frog
The Journal of Physiology, 1971
The electrical and structural characteristics of 'slow' muscle fibres of the frog were studied in normal and denervated muscles, and in muscles undergoing re-innervation by a mixed nerve containing large and small motor axons. 2. In agreement with previous studies, slow fibres in normally innervated muscles were incapable of producing action potentials. 3. Approximately 2 weeks after the sciatic nerve was transacted or crushed, slow muscle fibres acquired the ability to generate action potentials. These fibres were positively identified as belonging to the slow type, because their passive-electrical and ultrastructural characteristics remained essentially unchanged after the operations. 4. The action potential mechanism induced in slow fibres is sodiumdependent, and is blocked by tetrodotoxin. 5. After long-term re-innervation by a mixed nerve, slow fibres lose their acquired ability to generate action potentials, presumably because small motor axons re-establish connexion with the fibres. 6. It is concluded that the action potential mechanism of slow muscle fibres is under neural control, and is normally suppressed by small motor axons.
Development of the mature distribution of synapses on fibres in the frog sartorius muscle
Journal of Neurocytology, 1985
Most of the fibres in mature frog sartorius muscle possess two or more synapses separated by up to one-third the length of the muscle. The aim of the present work was to determine how the relative distances between these synapses changes during development in the frog (Limnodynastes tasmaniansis), as the fibres increase in length from 2mm (stage 56) to 20mm (1 year postmetamorphosis). At the earliest stage investigated (fibres 2.0-4.0 mm in length; stages 56-57) about 80% of the fibres were innervated at two endplates. The percentage of fibres with two endplates then remained approximately constant with further development. The polyneuronal innervation of endplates was almost eliminated by stage 57. Muscle fibres with two endplates had each situated on average about one-third the length of the fibre from a tendinous insertion; these relative positions did not change throughout development. Thus the distance between endplates increased linearly with an increase in fibre length. The size of terminals and the complexity of their branching also increased continually throughout development, independently of the location of the terminals on the fibres The observations suggest that the distance between terminals increases during development because of the intercalation of new plasma membrane and basal lamina associated with the increase in Iength and diameter of fibres.
Journal of Neurocytology, 1990
Changes in the density of intramembrane particles (IMPs) of sensory nerve terminals in the bullfrog muscle spindle were correlated with recovery in the response of the spindle to stretch during postcrush reinnervation. A few IMPs on the protoplasmic (P) face in summer experiments (June to October) reappeared by the 3rd week after the nerve crush, then rapidly increased to 110% and 120% of control values 2 and 2.5 months after the crush. Afferent responses to stretch could be recorded after the mean IMP density on the P-face in terminal branches had recovered to more than 25% of the control value. The discharge rate showed a plateau pattern during the period of the excessive IMPs. This was supplanted by a normal pattern after a myelinated branch of the sensory axon was cut. The IMPs in winter experiments (November to April) reappeared by day 90 after nerve crush, and then slowly increased. The sustained responses to stretch reappeared after 5 months, when the mean IMP density on the P-face was restored to 25% or more of the control. Neither excessive density of the IMPs nor plateau pattern of the discharge rate were observed in winter experiments. The relation between the regenerated IMP densities and the functional recovery is discussed.
Journal of Neurobiology, 1991
Synaptic size, synaptic remodelling, polyneuronal innervation, and synaptic efficacy of neuromuscular junctions were studied as a function of growth in cutaneous pectoris muscles of postmetamorphic Rana pipiens. Recently metamorphosed frogs grew rapidly, and this growth was accompanied by hypertrophy of muscle fibers, myogenesis, and increases in the size and complexity of neuromuscular junctions. There were pronounced gradients in pre-and postsynaptic size across the width of the muscle, with neuromuscular junctions and muscle fibers near the medial edge being smaller than in more lateral regions. The incidence of polyneuronal innervation, measured physiologically and histo-logically, was also higher near the medial edge. Growthassociated declines in all measures of polyneuronal innervation indicated that synapse elimination occurs throughout life. Electrophysiology also demonstrated regional differences in synaptic efficacy and showed that doubly innervated junctions have lower synaptic efficacy than singly innervated junctions. Repeated, in vivo observations revealed extensive growth and remodelling of motor nerve terminals and confirmed that synapse elimination is a slow process. It was concluded that some processes normally associated with embryonic development persist long into adulthood in frog muscles.