An electron microscope study of the retino-receptive layers of the pigeon optic tectum.

The fine structure of the superficial layer of the optic tectum of the pigeon, Columba livia, has been examined both in normal animals and after unilateral eye removal. Optic afferent terminals distributed throughout sublaminae IIa-f, are principally concentrated at three depths in the retinoreceptive zone. At first, degeneration of optic afferent terminals occurs as swelling of synaptic vesicles and accumulation of neurofilaments and later as dense degeneration. There appear to be several classes of optic afferent terminals in the retino-receptive zone, as would be expected from most previous studies of the avian tectum. The non-optic components of the neuropil, which include glomerular and pleomorphic vesicle terminals, and the synaptic interrelationships between optic and non-optic components are described. Optic afferent terminals mainly make synaptic contact with the branches of radial dendrites but also, in sublaminae IIb and IIc, with the dendrites of non-radial cells. In sublamina IIc afferent terminals synapse onto the horizontal presynaptic dendrites of spindle cells and triplet synaptic arrays including optic afferent terminals, horizontal dendrites and radial dendrite profiles and radial dendrite profiles are found. This optic projection to non-radial elements in IIb c, is discussed in relation to light microscope and electrophysiological studies.