Respiratory-associated thalamic activity is related to level of respiratory drive

We recorded phrenic nerve activity and thalamic single unit firing in unanesthetized, suprathalamically decerebrated, paralyzed and ventilated cats, in which vagi and carotid sinus nerves (CSN) had been ablated. Seventy-six (14%) of 545 neurons in regions of the thalamus related to the ascending reticular system, which had been tonically firing at low respiratory drives, developed rhythmic increases of firing associated with each respiration when drive had been increased by CSN stimulation or hypercapnia. The increases of neuronal firing occurred in late inspiration/post-inspiration but sometimes lasted into expiration; the magnitude of change was graded according to the magnitude of respiratory activity. Thalamic neurons also fired with a rhythm related to ventilator-induced chest expansion, some units showing both the respiratory-associated and the ventilator-related rhythms. Simultaneously recorded mesencephalic and thalamic neurons developed similar rhythms when drive was increased. We suggest that these neuronal activities reflect the conveyance of information about respiration to the cortex, where it may lead to the sensation of dyspnea and perhaps to arousal.