The structural basis for electrotonic coupling in the avian ciliary ganglion. A study with thin sectioning and freeze-fracturing

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Abstract
Each ‘ciliary’ neurone in the ciliary ganglion of adult birds receives its innervation from a single myelinated fibre of the oculomotor nerve by means of a dual mode of synaptic action, electrical and chemical. The preganglionic fibre branches repeatedly around the postganglionic axon but the extracellular compartment is large. The preterminal fibres, most of which are unmyelinated, end with large boutons on the axon hillock, a few on short dendrites and on the portion of the perikaryon of the ciliary neurone from which the axon emerges. This synaptic apparatus is enveloped by a glial sheath, mainly consisting of satellite cell bodies and loose myelin lamellae. The nonsynaptic portion of the ciliary perikaryon is covered by a sheath consisting mainly of compact myelin. The ciliary neurone has an initial axon segment like that of C.N.S. neurones. The area of each neurone apposed to boutons measures about 16 ooo μm2. Approximately 9% is specialized for chemical transmission and 0.17% for electrical transmission. Each neurone has about 280 000 gap junctional particles. Assuming that each particle represents one channel, the electrical resistance provided by these junctions is estimated to be of the order of 100 k Ω. The electrical coupling between the preganglionic fibre and the ciliary neurone may therefore be of resistive nature.