The lateral respiratory neurones of the medulla: Their associations with nucleus ambiguus, nucleus retroambigualis, the spinal accessory nucleus and the spinal cord

Abstract

(1) The lateral respiratory neurones are concentrated in a longitudinal column which extends from C1 to beyond the obex, perhaps as far rostrally as the entry of the vagal rootlets. This column has been identified as nucleus retroambigualis (NRA) of Olszewski and Baxter37. A small minority of NRA cells are present as isolated units; these are spread dorsomedially from the lateral column towards the nucleus of the solitary tract (NST).(2) NRA consists of a caudal group of expiratory neurones and a rostral group of inspiratory neurones. The inspiratory neurones merge laterally with nucleus ambiguus (NA).(3) Virtually all NRA cells have descending spinal axons. Most of these project contralaterally.(4) NRA units do not have axons in the IX, X or XI cranial nerves.(5) NRA does not merge caudally with the spinal accessory nucleus. The spinal accessory nucleus lies medial to NRA at C1.(6) Although very few NA units have respiratory activity with deep barbiturate anaesthesia, many are active with light anaesthesia, and substantial numbers can be recruited with procedures such as CO2 elevation in the inspired air. The number of active NRA cells is not markedly affected by anaesthetic level.(7) Although some NA units have distinctive activity patterns, others are difficult to distinguish from NRA units. Because of the difficulty in stimulating all branches of the IX and X cranial nerves, it may be difficult to establish for each recorded unit whether it sends its axon to a cranial nerve (i.e. a NA unit) or down the spinal cord (i.e. a NRA unit).