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Ultrastructure of synapses with different transmitter-releasing characteristics on motor axon terminals of a crab, Hyas areneas


Synaptic terminals on branches of an excitatory motor axon in a spider crab (Hyas areneas) were examined by electron microscopy to determine whether differences in size, structure, and number of synapses could be correlated with differences in transmitter release. Terminals releasing relatively large amounts of transmitter during low frequencies of nerve impulses (“high-output” terminals) had larger synapses, more prominent presynaptic dense bodies (active zones), and fewer synapses per unit length than terminals releasing relatively small amounts of transmitter (“low-output” terminals). Neither the difference in synaptic area, nor the quantitative differences in the active zones, were sufficient in themselves to explain the difference in synaptic efficacy, and it is postulated that a non-linear relationship may exist between structural features of the synapse and release of transmitter by a nerve impulse, and that differences other than those apparent from the ultrastructure could be involved. Greater facilitation at low-output terminals with high frequencies of nerve impulses may be due to greater reserves of “immediately available” transmitter, and to recruitment or activation of more individual synaptic contacts.

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Author information

Correspondence to Professor H. L. Atwood.

Additional information

Supported by a grant from the Natural Sciences and Engineering Research Council of Canada. Mrs. Irene Kwan provided technical assistance in the initial stages of the work, and Ms. Tamara Sosath assisted in preparing data and illustrations.

Dr John K. Stevens provided equipment and advice for three-dimensional reconstruction of nerve terminals; his facility is supported by grants from the Medical Research Council of Canada and from the Natural Sciences and Engineering Research Council of Canada.

Dr C.K. Govind read the manuscript and offered critical comments.

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Atwood, H.L., Marin, L. Ultrastructure of synapses with different transmitter-releasing characteristics on motor axon terminals of a crab, Hyas areneas . Cell Tissue Res. 231, 103–115 (1983).

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Key words

  • Synapses
  • Ultrastructure
  • Crustacean
  • Neuromuscular synapse
  • Synaptic differentiation