The Lobster Optic Lamina

The following interpretations are based on the assumption that the vesicles are presynaptic. Synapses between retinula cells are symmetrical contacts, with cisternae attached to both thickened membranes and the cleft is 8-10 mµ wide. Synapses from retinula terminal bags to the numerous invaginating spines of the ganglion cell axon have presynaptic ribbons and filaments but few vesicles; the cleft is 7.5-13 mµ wide. Synapses from retinula cell bags to secretory horizontal fibres have postsynaptic spines, typical vesicles one side and thickened presynaptic membrane (cleft Io-17 µ wide). Synapses from retinula fibres to empty (long) transverse fibres are similar. Synapses from secretory or empty transverse fibres to ganglion cell axons are axon-to-axon contacts; there are vesicles one side but both membranes are thickened; the cleft is 11-13 mµ wide. Between empty transverse fibres the synapses are similar but symmetrical; from empty to secretory transverse they have vesicles one side. Synapsesfrom secretory fibres to each other (symmetrical) or to empty transverse fibres (vesicles on one side and with only the postsynaptic membrane thickened) reveal a sharp distinction between synaptic vesicles and secretory vesicles. Serial synapses occur (a) from empty transverse fibre to secretory fibre to another empty transverse fibre, and (b) from retinula cell to secretory fibre to ganglion cell fibre. On account of its curious structure the optic cartridge probably has complex synaptic properties. Retinula terminals are probably inhibitory. Their light mitochondria, contrasting with the dense ones of the ganglion cells, are interpreted as aged.