The channeling of natural stimuli that evoke the ejection of milk in the rat. Effect of transections in the midbrain and hypothalamus.

The afferent and the efferent pathways of the milk ejection reflex were studied in conscious lactating rats subjected to suckling after experiencing stereotaxically-controlled transections in the midbrain or hypothalamus. Extensive transections in midbrain or hypothalamus. Extensive transections in the midbrain or caudal hypothalamus blocked reflex milk ejection while less extensive cuts, sparing either the dorsal or the ventral fibers, did not. The frontal plane immediately caudal to the neurosecretory nuclei intersects both afferent and efferent fibers. All cuts at this plane, regardless of size, caused blockage whenever the neurosecretory fibers located at the lateral ventral hypothalamus were severed. A multiple cut in the near vicinity of both supraoptic nuclei caused blockage of milk ejection, probably by severing the combined neurosecretory tract originating in both the supraoptic and paraventricular nuclei. If the multiple cut was performed unilaterally, a blockage of milk ejection was not observed, indicating that reflex action can be sustained by preserving one side of the neurosecretory pathway. Simple cuts in the same region revealed that the efferent pathway takes on a caudo-medial direction towards the neurohypophysis. In the near vicinity of the infundibulum, simple cuts showed that the efferent fibers enter the infundibulum from the sides, and also established that the milk ejection reflex persisted even when a small number of fibers in the pituitary stalk remained intact. The results showed that the afferent pathway for the reflex is diffuse, while the efferent pathway is compact. Only complete transection of one of the two pathways caused blockage.