Localization of the phrenic nucleus in the spinal cord of the rabbit.

The motor fibres of the phrenic nerve arise in the cervical portion of the spinal cord from a group of lower motor neuron cell bodies called the 'phrenic nucleus'. The phrenic nucleus of the rabbit has been located by cutting the right phrenic nerve (phrenicotomy) in the neck and observing retrograde degeneration (chromatolysis) in the cells of the ventral grey horn of the spinal cord 2-4 weeks later. The phrenic nucleus in the rabbit was found to extend from the fourth to the sixth cervical segment. It lay in the caudal half to three fourths of the fourth, the whole of the fifth and the cranial half to two thirds of the sixth segment. The phrenic nucleus in the rabbit is a longitudinal column of cells lying between the ventromedial and the ventrolateral columns of the ventral grey horn of the spinal cord. In the fourth and fifth cervical segments, the phrenic nucleus of the rabbit is dorso-medial to the ventrolateral column and dorsolateral to the ventromedial column. In the sixth segment it is dorsomedial to the ventrolateral column but lateral to the ventromedial column. The total number of chromatolysed cells found in the right phrenic nucleus (after right phrenicotomy in the neck) varied between 1496 and 1754. The chromatolysed cells had a peculiar and characteristic orientation in that in a frontal section they appeared longer than broad with their long axes almost parallel ti the longitudinal axis of the spinal cord.