THE ROLE OF CHEMORECEPTORS IN THE VENTILATORY RESPONSE AND IN REHABILITATION

Abstract

Oxygen homeostasis is essential to the survival of all O₂ consuming species. The carotid body (CB) is the site in the body that triggers awareness of changes in blood oxygen pressure and consist of oxygen sensitive cells that generate electric signals in response to a decrease in arterial oxygen pressure. The electric signals are used by the central nervous system to regulate the functions of respiratory muscles and cardiovascular system. At altitude, CB plays a primary role undergoing structural changes that are fundamental to surviving at extreme situations. With the lowering of arterial PO₂ at high altitude, there is an immediate increase in firing of afferents from chemoreceptors in the carotid body. After peaking over the next few minutes, the firing rate of afferents begins to rise again within hours until a steady state is reached. This secondary increase occurs along with increase in neurotransmitter synthesis and release and altered gene expression followed by hypertrophy of carotid body glomus cells. Further exposure to hypoxia eventually leads to blunting of the ventilatory response. During acclimatization, resting ventilatory sensitivity to hypercapnia is gradually increased and is maintained thereafter, it has also been reported that resting hypoxic sensitivity is maintained after exposure to hypoxia for many months and years. The response to chronic hypoxia suggests that a number of CB factors involved with growth and neurotransmission are released with time. It is also important to consider that oxygen-sensing mechanisms occur in a tissue-specific fashion, for example, the carotid body responds to hypoxia promptly by eliciting a ventilatory response, whereas oxygen sensitive mechanism in kidney, with the erythropoietin production in response to hypoxia requires more time, involving new expression of genes. The chemoreceptors are the gates that carry information concerning the status of systemic arterial oxygen, therefore, CB is important not only for oxygen sensitive cells, but also for systemic reflexes involved in general homeostasis, respiratory centers, autonomic nervous system, phrenic nerve, pharyngeal muscles, upper airways etc.