Physiological and pharmacological investigations on synaptic transmission in sympathetic ganglia

Abstract

In vertebrates, outside the spinal column, lies the long sympathetic chain of nerve fibres studded with ganglia whose neurones innervate the glands, heart and smooth muscle. The preganglionic fibres arise from neurones in the spinal cord and leave it in the ventral roots. Thence by way of the white rami they reach the sympathetic trunk and sometimes run up or down several segments of the chain before ending synoptically on ganglion cells. The sympathetic ganglion cells were shown by the historical studies of Cajal (1911), Castro (1932) and others to possess a branching dendrite system which is practically enclosed in a thick capsule, quite unlike the wide and free distribution of the motoneuronal dendrites. The intracapsular dendrites from an open network uniformly distributed around the cells and end in curious shaped swellings either on the capsule itself or amongst the small cells in the capsular sheath called satellite cells by Cajal (1911) and Castro (1932). There is good experimental evidence that there are no interneurons in the ganglia between the preganglionic fibres and the ganglion cells. Thus Ranson and Billingsley (1918) found that section of the cervical sympathetic, i.e. the preganglionic fibres, produced degeneration of all the fine branching fibres in the ganglion, only the ganglion cells and their axons surviving. Secondly the uniformity of the preganglionic endings and the ganglion cells made it seem very unlikely that interneurons were present (Cajal, 1911 and Castro, 1932; also Ranson and Billingsley, 1918). Since the number of postganglionic fibres far exceeds the number of preganglionic fibres, each preganglionic fibre must innervate several ganglion cells (Langley, 1900). Billingsley and Ranson (1918a) estimated a 1:32 ration on histological counts though later work indicated a ratio (Wolf, 1941). The following additional evidence also relates to the simple synaptic structure of the ganglion. When nicotine was painted on the superior cervical ganglion (Langley, 1900), transmission through the ganglion was blocked. Similarly section of the preganglionic nerve trunk no degeneration beyond the ganglion, i.e. degeneration could affect only the preganglionic trunk and not the internal carotid nerve (Ranson and Billingsley, 1918). Electrical stimulation of the postganglionic nerve produced no activity in the preganglionic nerve – a very good indication, as Brown (1934) points out, for unidirectional conduction through ganglia. The sympathetic ganglion thus provides the simplest example in vertebrates of a synaptic system between two groups of nerve cells, the axons of the preganglionic neurones ending on the ganglion cells without the complication of interneurons.