Respiration is regulated via chemical and nervous system interactions. Both the central nervous system (CNS) and the peripheral nervous system (PNS) regulate and control respiration. The peripheral nervous system consists of nerves outside of the brain and spinal cord. It has two distinct parts; the autonomic nervous system (ANS) and the somatic nervous system. The somatic nervous system regulates skeletal muscle. The autonomic nervous system controls heart and lung functions, and the two divisions often have opposite effects. Within the lungs the sympathetic nervous system can act to relax smooth bronchial muscles (bronchodilation) while the parasympathetic nervous system can act to contract smooth bronchial muscles (bronchoconstriction).4
The sympathetic nervous system regulates involuntary functions, including heart rate, blood pressure, digestion and respiration in response to emotional stress and/or physical exertion. Norepinephrine, a chemical mediator released by sympathetic cells, binds to Beta2 adrenergic receptors and acts to increase bronchodilation which improves lung function. Medications used to control asthma, such as SABAs (short-acting beta2-agonists), which act like norepinephrine likewise improve airflow but do not affect the underlying cause of bronchoconstriction and other asthma changes.
The respiratory control center in the brain stem regulates breathing in response to the stimuli of various receptors. There are four types of receptors. First the peripheral chemoreceptors respond to changes in the concentration of carbon dioxide and oxygen in the blood. Receptors in the cerebrospinal fluid respond to pH changes. Stretch receptors within the airways detect mechanical changes in the lungs and prevent hyperinflation. Irritation receptors initiate a cough reflex when stimulated.4
The autonomic nervous system can also act locally to control mucus secretion. Afferent sensory nerves transmit signals to the brain and spinal cord, while the efferent nerves may bring messages from the brain to the respiratory organs. Neurotransmitters are chemicals, which are released from efferent nerve endings and then bind to receptors on bronchial smooth muscles. Agonists and antagonists are types of neurotransmitters. Agonists bind to receptors to stimulate a specific response, while antagonists bind to receptors to block a specific response.4