Figure 1. The A5 region contains two main types of CO₂-activated neurons.

A, experimental design. Units were recorded in the ventrolateral pontine reticular formation. The region of interest was identified by its location relative to the facial motor nucleus. The presence of an axonal projection to the spinal cord was used as one of the criteria to identify putative A5 noradrenergic neurons. B, location of bulbospinal (putative noradrenergic) neurons and retrotrapezoid (RTN)-like neurons plotted as a function of their stereotaxic coordinates relative to the rostral and ventral edge of the facial motor nucleus (7) as monitored by antidromic field potentials. C, properties of bulbospinal A5 area neurons. Ca, response to step changes in end-expiratory CO₂; Cb, collision test showing that this neuron had an axon projecting to or through the thoracic spinal cord; Cc, relationship between unit discharge rate and end-expiratory CO₂ at steady state (from measurements taken where indicated by asterisks in panel Ca); Cd, collision test for a CO₂-insensitive A5 region neuron; Ce, relationship between unit discharge rate and end-expiratory CO₂ at steady state for the bulbospinal neuron shown in Cd. D, relationship between discharge rate and end-expiratory CO₂ at steady state for bulbospinal, putative A5 neurons (n= 24) and putative RTN neurons (n= 10). E, RTN-like neuron with high dynamic range of response to CO₂. Ea, response to step changes in end-expiratory CO₂; Eb, relationship between unit discharge rate and end-expiratory CO₂ at steady state (from measurements taken at asterisks in Ea).