Intramembrane organization of synapses in the lobster stretch receptor organ

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Abstract
The intramembrane organization of axodendritic and neuromuscular synapses in the lobster stretch receptor organ was investigated by freeze-fracturing. Based on ultrastructural criteria which are known to be correlated with physiological properties, we identified three types of synapse: the inhibitory axodendritic, the inhibitory neuromuscular, and the excitatory neuromuscular synapse. Although these synapses have some features in common, each has a characteristic arrangement of intramembrane particles in both the presynaptic and postsynaptic membranes. All three have, in their presynaptic membranes, aggregates of P-face particles and associated depressions representing sites of synaptic vesicle exocytosis, features which together define active zones. However, in the inhibitory axodendritic synapse the P-face contains short ridges in this region. These ridges may occur singly or in pairs oriented in V-shaped configurations. The ridges are decorated with particles along their entire length. In the inhibitory neuromuscular synapse, no ridges are present. Clusters of particles are present, but they are scattered randomly over a large expanse of presynaptic membrane. In the excitatory neuromuscular synapse, isolated clusters of particles are associated with the P-face and are occasionally located on circular elevations of the membrane. The postsynaptic membrane also shows structural diversity in the three types of synapse. In the inhibitory axodendritic synapse, there is no apparent specialization. However, in the inhibitory neuromuscular synapse, P-face particles are arranged in double rows which are separated by particle-free strips of membrane. In the excitatory neuromuscular synapse, particles are confined to a narrow band that borders the synaptic cleft. This band is demarcated by a single intermittent strand of particles arranged in the direction of the long axis of the muscle fibre. Therefore, intramembrane specializations of both the presynaptic and postsynaptic membranes are sufficiently distinctive that three different types of synapse can be recognized.