Electrical and mechanical properties and neuro-effector transmission in the smooth muscle layer of the guinea-pig ileocecal junction.

Electrical and mechanical properties and neuroeffector transmission were studied in circular strips of smooth muscle taken from the ileocecal junction of guinea-pigs in relation to sphincter action, using the microelectrode, and tension recording methods. The membrane potential of the smooth muscle was low (-43mV) compared with the membrane potential of circular muscle cells of the ileum or caecum (-58 mV or 62 mV). Only small populations of the muscle cells (about 5%) generated spontaneous action potentials. Field stimulation of the tissue produced an initial slight relaxation followed by a contraction, and the mechanical responses were accompanied by membrane hyperpolarization (i.j.p.) followed by repolarization with rebound spikes. Treatment with atropine increased the amplitude of i.j.p.s and decreased the amplitude of the rebound repolarization. Propranolol or phentolamine did not affect the amplitude of i.j.p., however, phentolamine slightly reduced the amplitude of the rebound repolarization. These results indicate that the ileocecal junction is predominantly controlled by non-adrenergic, non-cholinergic inhibitory nerve fibres and that the distribution of adrenergic and cholinergic excitatory nerve fibres is sparse.