Analysis of quantal content and quantal conductance in two populations of neurons in the avian ciliary ganglion.

The avian ciliary ganglion contains two populations of parasympathetic cells, termed the ciliary and choroid neurons. We have estimated the quantal contents of nicotinic excitatory postsynaptic potentials in both populations of neurons by several methods. The singly innervated ciliary neurons have quantal contents of 15-30. In contrast, the multiply innervated choroid cells have quantal contents of 4-7. Quantal conductance was also determined, using a parallel conductance model which takes into account the capacitance of the cell membrane. This analysis indicates that in both populations of neurons one quantum activates approximately 100 postsynaptic receptors. It is concluded that in autonomic ganglia singly innervated cells demonstrate a larger quantal content, consistent with a higher safety factor for neurotransmission, while quantal content in multiply innervated cells is generally much lower, allowing for considerable summation of presynaptic inputs. Further, in autonomic neurons many fewer postsynaptic receptors are activated by a single quantum than is the case at the neuromuscular junction.