The milk-ejection reflex in the sheep: an anatomical study on the afferent pathway

The present study is an attempt to reveal the spinal and supraspinal organization of the ascending branch of the milk-ejection reflex in the ewe by means of a tract-tracing technique. For this purpose, injections of horseradish peroxidase (HRP) were performed into the lateral cervical nucleus (LCN) and into the hypothalamic paraventricular nucleus (PVN). Peroxidase injections into the LCN revealed retrogradely labelled neurons in the medial part of laminae I-III of the ipsilateral L3 and L4 spinal segments, while injections of HRP into the PVN revealed retrogradely labelled cells in the contralateral LCN and the medial cuneate nucleus. Taking into account the results obtained, it is concluded that the transmission of the afferent input from the nipples to the PVN is accomplished by at least two pathways: one employing a single relay station located in the medial cuneate nucleus, and another possessing two relay stations located in the medial part of laminae I-III of the dorsal horn of L3 and L4 spinal segments and in the LCN.