The afferent path of the milk-ejection reflex in the brain of the rabbit.

Abstract

SUMMARY The afferent path of the milk-ejection reflex has been studied in the brain of the anaesthetized lactating rabbit. Electrical stimulation was applied between a monopolar electrode in the brain and an indifferent electrode in the scalp. The brain was transected at the mid-cerebellar level to eliminate sympathetico-adrenal activation, and intramammary pressure and arterial blood pressure were monitored to detect release of neurohypophysial hormones. In the mid-brain, the afferent path of the reflex is compact, lying in the lateral tegmentum of each side and passing forwards to lie medio-ventral to the medial geniculate body. On entering the diencephalon, the pathway on each side bifurcates: a dorsal path passing forwards in association with the extreme rostral central grey and periventricular region, and a ventral path ascending through the subthalamus. The dorsal and ventral paths reunite in the posterior hypothalamus. Delineating the pathway further forward in the hypothalamus, using a simple stimulation technique, was not possible because at this level it intermingles with efferent fibres descending from the paraventricular nucleus to the pituitary stalk. The afferent path of the reflex is concerned with the preferential release of oxytocin from the neurohypophysis, is not a major pathway for the release of vasopressin and its neural substrate in the mid-brain is believed to be the spinothalamic system of fibres.