Principal mechanisms controlling penile retraction and protrusion in rabbits.

The effects on penile volume of nerve stimulations and drugs injected into the systemic circulation were studied plethysmographically. Dilator responses at selective perfusion of the penile artery were studied by measuring the perfusion pressure. The main results and conclusions are: The penis has an adrenergic vasoconstrictor supply coming from the sacrococcygeal parts of the sympathetic chains. A very low (0.2 Hz) vasomotor tone keeps the penis relaxed. If this tone is interrupted the penis will protrude but autoregulation will soon take over and eventually produce hyperinvolution of the penis. Two vasodilator paths, both with pelvic ganglionic relays, were found. 1) The pelvic parasympathetic nerves, probably having mainly non-cholinergic postganglionic neurons and operating quite effectively at low frequencies. 2) The sympathetic hypogastric nerves, presumably having at least partly cholinergic postganglionic neurons which, apart from muscarinic dilation of minute inflow resistance vessels to the erectile tissue, may also work by suppression of excitatory adrenergic neurotransmission. The pelvic and hypogastric vasodilator outflows work synergistically. The vasoconstrictor nerves are very strong and efficient antagonists of the vasodilator nerves.