VENTILATORY CO2-INDUCED OPTICAL-ACTIVITY CHANGES OF CAT VENTRAL MEDULLARY SURFACE

We examined neuronal activation of the ventral medullary surface (VMS) during hypercapnic challenges using optical recording procedures. Wit h a coherent imaging probe, we assessed reflected 700-nm light from 18 VMS sites in 11 spontaneously breathing adult cats and from the supra sylvian cortex in two cats. Video frames were acquired during a baseli ne period, hypercapnic (3, 5, and 10% CO2 in O2) exposure, and recover y. Hypercapnic exposure elicited overall reflectance changes in all VM S sites, but no changes in the suprasylvian cortex. Light reflectance changes, suggesting altered neuronal activity, were reproducible, occu rred as early as 30 s after CO2 exposure, and were dose dependent. The changes persisted approximately 20-25 min beyond the stimulus, but re spiratory responses consistently recovered within 2-3 min. Although mo re rostral VMS sites tended to be associated with decreased activity a nd caudal regions with increased excitation, no uniform topographical organization was apparent across animals. The variability in VMS optic al reflectance patterns across animals during CO2 stimulation may refl ect the heterogeneous topographical distribution of responsive neurons in the structure.