Respiratory-modulated and phrenic afferent-driven neurons in the cervical spinal cord (C4-C6) of the fluorocarbon-perfused guinea pig.

The potential contributions of cervical spinal interneurons to the neural control of respiration have been investigated by extracellularly recording the patterns of activity of neurons in the C4-C6 spinal cord during fictive respiration in the fluorocarbon-perfused, adult guinea pig. Two types of neurons were recorded: respiratory-modulated neurons, whose activity was modulated with respiration, and phrenic-driven neurons, which were excited by electrical stimulation of the phrenic nerve. Respiratory-modulated neurons (n = 20) could be divided into inspiratory, expiratory, and phase-spanning neurons, based on their patterns of activity during fictive respiration. Respiratory-modulated neurons showed varying dependencies on the type of breathing; during spontaneous augmented breaths, one-half exhibited patterns of activity that were significantly different to those seen during normal, fictive respiration, whereas the other half of the respiratory-modulated neurons showed similar patterns of activity during both normal and augmented breaths. Phrenic-driven neurons (n = 22) could be divided into short-latency (7-12 ms), moderate-latency (12-25 ms), and inhibited neurons, but were only occasionally and weakly modulated with respiration. The results suggest that respiratory-modulated C4-C6 spinal neurons may contribute to the neural control of respiration, with different subpopulations specialized for different types of respiratory tasks, and that phrenic-driven neurons may be interposed in sensory or reflex pathways, such as the spinothalamic tract or phrenic-to-phrenic inhibitory reflex.