Observations on the organization of the dendritic processes and receptor terminations in the abdominal muscle receptor organ of Homarus.

Abstract

AbstractThe dendritic processes and terminals of the abdominal muscle receptor organ of the lobster have been studied with the electron microscope. In both the “fast” and “slow” adapting components of the muscle receptor organ, dendritic processes ramify within the body of the stretch muscle in a zone of “relative muscle exclusion,” where the continuity of the muscle bundle is interrupted by a zone of connective tissue. The cell bodies and larger dendritic processes are ensheathed in alternate layers of connective tissue and Schwann cells, but the smallest dendritic twigs are entirely divested of their Schwann cell envelope. The membrane of the dendritic terminations is intimately applied to either connective tissue of the zone of relative muscle exclusion or to the sarcolemma of the stretch muscle. This apposition is interpreted as morphological specialization for transduction or mechanoelectric conversion. It is suggested that these two types of dendritic terminals may represent two distinct sites for transduction and may also mediate different modalities of input.The distribution of dendritic processes and terminals differs in the fast and slow systems. In the fast system the dendrites quickly ramify into a glomerular burst of processes near the junction of stretch muscle and connective tissue. In the slow system, the dendrites ramify more gradually and lie more nearly parallel to the long axis of the stretch muscle.In both the fast and slow systems, but especially in the dendritic terminals of the fast neuron, collections of vesicles 400–2000 Å in diameter have been observed. These vesicles are clearly different in fine structure from those found in the neuromuscular junction or axo‐dendritic synapses.An effort is made to relate neurophysiological characteristics of this system with the fine structural observations, with particular attention to the mechanism of transduction, or mechanoelectric conversion.