Abstract
1. Orally projecting, cholinergic interneurones are important in mediating ascending excitatory reflexes in the small intestine. We have shown that there is just one major class of orally projecting interneurone, which we have characterized using retrograde labelling in organ culture, combined with immunohistochemistry, intracellular recording and dye filling. 2. Orally projecting interneurones, previously shown to be immunoreactive for choline acetyltransferase, tachykinins, enkephalin, calretinin and neurofilament protein triplet, have axons up to 14 mm long and are the only class of cells with orally directed axons more than 8.5 mm long. 3. They are all small Dogiel type I neurones with short dendrites, usually lamellar in form, and a single axon which sometimes bifurcates. Their axons give rise to short varicose collaterals in myenteric ganglia more than 3 mm oral to their cell bodies. 4. Orally projecting interneurones receive prominent fast excitatory post synaptic potentials (fast EPSPs). A major source of fast EPSPs is other ascending interneurones located further aborally. They also receive fast EPSPs from circumferential pathways. 5. In the stretched preparations used in this study, orally projecting interneurones were highly excitable, firing repeatedly to depolarizing current pulses and had negligible long after-hyperpolarizations following their action potentials. They did not receive measurable non-cholinergic slow excitatory synaptic inputs. 6. Ascending interneurones had a characteristic inflection in their membrane responses to depolarizing current pulses and their first action potential was typically delayed by approximately 30 ms. Under single electrode voltage clamp, ascending interneurones had a transient outward current when depolarized above -70 mV from more hyperpolarized holding potentials. Ascending interneurones also consistently showed marked inward rectification under both current clamp and voltage clamp conditions. 7. This class of cells has consistent morphological, neurochemical and electrophysiological characteristics and are important in mediating orally directed enteric reflexes.