Carotid chemoreflex. Neural pathways and transmitters.

The chemoreceptor projection site encompasses a midline area (0-0.5 mm caudal, 0-0.5 mm lateral and 0.3-0.5 deep with respect to the calamus scriptorius) which includes the commissural subnucleus of NTS. The chemosensitive neurons located in this site fire tonically, without any rhythmic relation to the phrenic nerve bursts, and are not responsive to baroreceptor stimulation. These neurons may serve as a pre-central pattern generator for respiration. The mechanisms which mediate respiratory responses (i.e., PN responses) to carotid chemoreceptor stimulation are as follows. Activation of carotid chemoreceptors results in the release of an excitatory amino acid (probably glutamate) in the chemoreceptor projection site; both NMDA and non-NMDA receptors in this site mediate the responses to chemoreceptor stimulation. The chemosensitive neurons send projections (which have not been identified yet) to respiratory neurons. Unlike in the neonatal rat, the inspiratory drive to the phrenic motoneurons is mediated by both NMDA and non-NMDA receptors. The PN response to chemoreceptor stimulation are also mediated by NMDA and non-NMDA receptors located in the phrenic motor nucleus. Cardiovascular responses to chemoreceptor stimulation are mediated by neural pathways connecting the chemosensitive neurons to the rostral ventrolateral medullary pressor area. These projections are glutamatergic. Activation of the sympatho-excitatory neurons in the rostral ventrolateral medulla results in pressor and tachycardic responses characteristic of chemoreceptor activation.