Postsynaptic specializations at excitatory and inhibitory cholinergic synapses.

In serial sections of neurons in the paravertebral ganglia of the frog (Limmodynastes dumerili), the postsynaptic structures termed 'postsynaptic bar' (PSB) and 'junctional subsurface organ' (JSO) were never observed in the same ganglion cell. Further, PSBs were found mostly in small ganglion cells (less than 22 micrometer), while JSOs were found mostly in large ganglion cells (up to 45 micrometer). Between 10 and 22 PSBs were located at both 'spine' and 'non-spinous' somatic synapses of the smaller ganglion cells; while 8 to 16 JSOs were located largely in the axon hillock region of the larger ganglion cells. Based on these observations, it is suggested that the two ganglion cell populations represent the B and C cell types defined according to electrophysiological data. Further, since the nerve terminals adjacent to both these postsynaptic structures appear to be cholinergic according to their vesicular content, this provides some basis for suggesting that JSOs are associated with slow excitatory synapses, while PSBs are present at slow inhibitory synapses.