Neurotransmission at Parasympathetic Nerve Endings

The concepts of neurohumoral transmission and the receptor sites for the neurotransmitter are illustrated more thoroughly with particular reference to sympathetic neurones. The neurotransmitter at the parasympathetic nerve terminal with smooth muscle, cardiac muscle or glandular tissue is acetylcholine (Ach). Dale was struck by the coincidence between the activity of Ach and responses to parasympathetic nerve stimulation and coined the term parasympathomimetic to describe its effects. The release of Ach from parasympathetic nerve endings induces a response of the effector by interacting with cholinoceptors of the muscarinic subtype. The terminal portion of the autonomic parasympathetic neurone, in common with the sympathetic nerve, has varicosities distributed along its length which are filled with agranular electron-luscent vesicles. Muscarinic receptor agonists inhibit the release of Ach from parasympathetic nerves evoked by electrical stimulation and by depolarization induced by high K+. This effect has been observed in a wide range of tissues including the heart, gastrointestinal tract, bronchi, bladder and iris.