Ventral respiratory column tonic expiratory neurons: Nodes for convergent afferent modulation of inspiratory drive

Abstract

The medullary ventral respiratory column (VRC) has an essential role in generating the motor pattern for breathing and inspiratory (I) drive. Contemporary models include a rostral‐to‐caudal excitatory I neuron chain extending to premotor populations that also acts upon inhibitory I neurons with recurrent and feed‐forward connections upon other VRC excitatory and inhibitory I neurons and expiratory (E) neurons. Prior work suggests (i) baroreceptor stimulation inhibits inspiratory drive in part via evoked tonic E (t‐E) neuron activity and (ii) central chemoreceptor stimulation enhances I drive via disinhibition through reduced t‐E neuron activity. A recent computational neuromechanical model (O'Connor et al. 2012) predicted that inhibition of t‐E neurons amplifies I drive during coughing ‐ a result supported by in vivo data (Segers et al. 2012). We now report recording sites of pericolumnar t‐E neurons (n=140) and show that selective stimulation of carotid chemoreceptors via close injection of CO2‐saturated saline reduces the I‐phase activity of t‐E cells coincident with increased I drive in vagotomized decerebrate cats. Spike train cross correlation and gravity clustering methods revealed short‐time scale impulse synchrony in 24 t‐E neuron pairs. We conclude that multiple afferent systems tune inspiratory drive via actions of coordinated groups of VRC t‐E neurons. Support: NIH grant NS19814.