The preceding two chapters have examined the mechanisms of fast synaptic transmission, which involves the opening of ion channels by the direct action of the neurotransmitter. In this chapter we consider the slower responses produced by indirect mechanisms, where the neurotransmitter receptor does not contain its own intrinsic channel. Activation of the receptor, we shall see, sets in train a series of changes in one or more other proteins, leading eventually to the opening or closing of a particular set of ion channels. Let us look first of all at the electrical phenomena that need to be explained.

Slow synaptic potentials

In the sympathetic nervous system of vertebrates there is a chain of ganglia lying near to the spinal cord. These contain the cell bodies of the postganglionic fibres which terminate on smooth muscle or gland cells. The preganglionic fibres arise in the spinal cord and form synapses with the cell bodies of the postganglionic fibres in the ganglia.

Bullfrog sympathetic ganglia contain B cells and C cells, the B cells being the larger. Each is innervated by preganglionic fibres which form numerous synaptic boutons on the neuronal soma. They show a variety of different types of synaptic activity (Kuffler, 1980; Adams et al.9 1986), as is shown in fig. 9.1.

A single stimulus applied to the preganglionic fibres produces a fast EPSP in both B and C cells, and this may be large enough to produce an action potential in the postganglionic fibres (fig. 9.1a).