Ganglionic Blocking Drugs and Nicotine

Transmission through autonomic ganglia is more complex than neurotransmission at the neuromuscular and postganglionic neuroeffector junctions and is subject to numerous pharmacological and physiological influences. In some ganglionic synapses, especially at parasympathetic ganglia, there is a simple presynaptic to postsynaptic cell relationship; in others, the presynaptic to postsynaptic cell relationship may involve neurons interposed between the presynaptic and postsynaptic elements (interneurons). In a variety of sympathetic and certain parasympathetic ganglion cells (e.g., vagal ganglia in the sinoatrial node), cells exhibiting the characteristic catecholamine fluorescence spectrum have been found. These cells are referred to as small intensely fluorescent (SIF) cells. At some autonomic ganglia, the SIF cell is a true interneuron, receiving afferent innervation from preganglionic cholinergic neurons and forming efferent synapses with postganglionic neurons. At other autonomic ganglia, its function is not completely understood, but the SIF cell is believed to play a role in the modulation of ganglionic transmission. Many SIF cells are thought to contain dopamine or norepinephrine as their neurotransmitter. Unlike the receptors at postganglionic neuroeffector junctions or at skeletal neuromuscular junctions, both types of cholinergic receptors, that is, nicotinic and muscarinic, are present on the cell bodies of the postganglionic neurons. Stimulation of the preganglionic neuron results in the release of acetylcholine (ACh) from the preganglionic nerve terminal, which in turn activates postganglionic cholinergic receptors and leads ultimately to the formation of a propagated action potential down the postganglionic axon. At the more complicated synapses, the release of ACh from preganglionic neurons results in the appearance of complex postsynaptic potential changes consisting of several temporally arranged components. There is an initial fast excitatory postsynaptic potential (EPSP) followed by a succession of much slower postsynaptic potential changes, including a slow EPSP that lasts for 2 to 5 seconds, a slow inhibitory postsynaptic potential (IPSP) lasting about 10 seconds, and a late slow EPSP lasting for 1 to 2 minutes. There is considerable diversity among nicotinic acetylcholine receptors, and at least one source of this diversity is the multiplicity of acetylcholine receptor genes. Cholinergic–nicotinic receptors in skeletal muscle are different from those in autonomic ganglia and the central nervous system.