The inhibitory innervation of the taenia of the guinea‐pig caecum

1. The inhibitory innervation of the taenia of the guinea‐pig caecum has been studied, after blocking the responses to stimulation of excitatory cholinergic nerves with atropine. 2. Stimulation of the perivascular nerves supplying the taenia caused relaxations. These nerves had properties which were typical of sympathetic post‐ganglionic adrenergic nerves. The relaxations caused by stimulation were maximal at frequencies of stimulation above 30 pulses/sec and they were abolished by bretylium, guanethidine and 1,1‐dimethyl‐4‐phenylpiperazinium iodide (DMPP). 3. The taenia is also innervated by intramural inhibitory nerves with their cell bodies in Auerbach's plexus. These nerves can be excited by electrical stimulation of the taenia or by the application of ganglion‐stimulating drugs. 4. The intramural inhibitory nerves have different properties from sympathetic adrenergic nerves. Relaxations in response to stimulation were maximal with frequencies of stimulation of about 5 pulses/sec and they were not blocked by bretylium, guanethidine or DMPP. 5. Preganglionic cholinergic fibres in the caecal wall make synaptic connexions with the intramural inhibitory neurones. 6. The role of the intramural inhibitory neurones in intestinal activity and their possible connexions with the central nervous system have been discussed.