Electrical and Synaptic Properties of Myenteric Plexus Neurones in the Terminal Large Intestine of the Guinea-pig

1. Intracellular recording methods were used to investigate the cellular neurophysiology of ganglion cells in the myenteric plexus of the guinea-pig rectum. The rectum is a region of the gastrointestinal tract with specialized functions that include reflex relaxation of the internal and sphincter during defaecation. Electrical and synaptic properties of myenteric neurones in the rectum had not previously been studied. Therefore, the overall aim of the work was to describe electrical and synaptic behaviour and identify neurophysiological properties of rectal neurones that might be related to specialization of function in this region of the gut. 2. Thirty-four (58%) of fifty-nine impaled cells had electrophysiological properties of AH/type 2 enteric neurones. Eighteen (30%) behaved like type 3 neurones and only two (3%) behaved like S/type 1 enteric neurones. Three per cent were presumably glial cells and the remainder were unclassifiable. 3. Nicotinic cholinergic fast EPSPs were recorded in six of thirty-four (18%) of AH/type 2 neurones and in all eighteen of the type 3 cells. Slow EPSPs occurred in five of thirty-four (15%) of AH/type 2 neurones and were absent from type 3 neurones. 4. Type 3 neurones received fast synaptic inputs from axons in each of the interganglionic fibre tracts that were connected with the ganglion containing the neurone. 5. AH/type 2 neurones were apparently multipolar neurones with long processes projecting into each of the interganglionic fibre tracts connected with the ganglion. Type 3 neurones did not have processes leaving the ganglion in the fibre tracts. 6. Run-down in amplitude of the fast EPSPs in type 3 neurones occurred when they were evoked at frequencies of 5 Hz or greater. 7. Presynaptic muscarinic cholinergic receptors behaved like autoreceptors in suppressing the release of acetylcholine at nicotinic synapses on type 3 neurones. 8. Application of 5-hydroxytryptamine, acetylcholine, substance P and forskolin mimicked the characteristics of slow EPSPs in the AH/type 2 neurones. 9. The principal difference between neuronal behaviour in the rectum and small intestine, colon and stomach was in the relative proportions of AH/type 2, S/type 1 and type 3 neurones. This is discussed in terms of the functional specialization of the rectum.