Correlation between presynaptic dense bodies and transmitter output at lobster neuromuscular terminals by serial section electron microscopy

Abstract

Lobster neuromuscular terminals releasing comparatively small (low-output type) and large (high-output type) amounts of transmitter but arising from the single excitatory motor axon to the proximal accessory flexor muscle were serially sectioned for electron microscopy. The three-dimensional reconstruction showed the two types of terminals to have a complex branching pattern in which thin branches of the motor axon often enlarged into synapse bearing terminal regions. Quantitative comparison showed that the mean surface area of a synapse is similar in the two types of terminals. However, the low-output terminal has a higher synaptic density and devotes a greater part of its surface area to synapses compared to its high-output counterpart suggesting that transmitter output is not directly related to synaptic area. The mean surface area of a presynaptic dense body is not significantly different between low- and high-output synapses, but there is a significantly greater density of these active zones in the high-output terminal. This results in the ratio of mean dense body area to mean synaptic area being approximately 3 X greater in the high-output synapses than the low-output ones. This significant difference in the surface area of presynaptic dense bodies between low- and high-output synapses correlates with the difference in transmitter output at these two synapses, and implicates the dense bodies in the mechanism of transmitter release at lobster neuromuscular synapses.