Afferent synaptic connections between hair cells and the somata of intramacular neurons in the gravity receptor system of the statocyst of Octopus vulgaris.

In the sensory epithelium (macula) of the gravity receptor system of the statocyst of Octopus vulgaris, there are two types of afferent neurons, distinguished according to their position in the epithelium. The somata of one type lie accumulated in a ring peripheral to the hair cell layer of the epithelium; these are designated as perimacular neurons. The somata of the other type lie among the hair cells, below the level of their nuclei; these are designated as intramacular neurons. Axons of afferent neurons which touch the hair cells are postsynaptic to some of the hair cells touched. As the somata of the intramacular neurons also touch the hair cells, they were investigated by serial electron microscopic reconstruction to determine if afferent synaptic contacts between hair cells and these somata occur. An average of about 600 intramacular neurons was counted in two maculae. Afferent synapses were seen to occur between hair cells and the somata of 76% of the intramacular neurons investigated. The postsynaptic processes of the intramacular neurons' somata were of two morphological types; one with a finger-like and one with a flat postsynaptic process (average of one synapse of each type per soma). The soma of an intramacular neurons can be postsynaptic to more than one hair cell simultaneously (average of 1--2 hair cells per soma). In addition to being presynaptic to only one neuron's soma, a hair cell could be simultaneously presynaptic to the axons of one or more afferent neurons. The morphological findings are discussed as to their possible physiological consequences.