Figure 1.

Roles for propriospinal neurons in control of respiratory muscles. Respiratory rhythm is generated by three oscillators located in discrete regions of the brainstem—the pre-Bötzinger complex (PBC, green) for inspiration, the lateral parafacial region (pFL, turquoise) for active expiration, and the post-inspiratory complex (PiCo, blue) for the post-inspiratory phase of breathing. Excitatory and inhibitory interactions (with inhibition typically dominating) between the three oscillators ensure coordination of each phase (dark gray arrows = known connections, light gray arrows = hypothesized connections). The inspiratory drive is transmitted from the brainstem to the spinal cord via bulbospinal neurons of the rostral ventral respiratory group (dark green arrows), whereas expiratory drive is transmitted via bulbospinal neurons of the caudal ventral respiratory group (turquoise arrows). Bulbospinal pathways can provide drive directly to phrenic motor neurons (blue), other inspiratory motor neurons (yellow, e.g., external intercostal, scalene, etc.), or expiratory motor neurons (orange, e.g., abdominal muscles). In addition, bulbospinal pathways can activate propriospinal neurons (excitatory = green, inhibitory = red) that modulate respiratory motor activity. Sensory afferents (purple) from respiratory muscles or limb muscles can project to propriospinal neurons as well as the brainstem. For clarity, not all neurons/connections are shown. The roles of propriospinal neurons in controlling breathing include: (1) coordinating different respiratory motor pools; (2) patterning the activity of respiratory motor neurons within the same motor pool; (3) processing and transmitting afferent input; and (4) coordinating locomotor and respiratory activity.