Cough Reflex
The cough reflex is a coordinated neural and muscular response to the irritation of the respiratory system. As a reflex action, coughing does not require conscious direction or control. Coughing is a nociceptive reflex, designed to protect the body from injury.
Coughing is initiated by the sensory neurons lining the respiratory passages. Afferent neural impulses produced in respiratory structures travel via the vagal nerve to the medulla of the brain and result in the appropriate muscular excitations to produce an acute expulsion of air designed to clear the respiratory passages. Irritation of the respiratory passages may result from contact with a foreign body, mechanical obstruction of the airway, or by an excessive buildup of fluid that obstructs a portion of an airway.
The coordinate cough reflex involves the inspiration of air and the subsequent closure of the epiglottis and the vocal cords that traps the air in the lungs, as the abdominal muscles, diaphragm, and intercostal muscles (muscles between the ribs) involved in normal expiration violently contract. The forceful contraction of these muscles causes the pressure in the lungs to rise until the pressure suddenly and explosively forces open the epiglottis and vocal cords. Air is then expelled at speed ranging up to 100 miles per hour through the bronchi and trachea.
The transmission of neural reflexes in reflex arcs is based upon the same mechanisms of electrical and chemical mechanisms of transmission as all other forms of neural impulses. Within the neural cell body (axon) the impulse travels electrically as an action potential and at the synapse (the intercommunicating gap or space between neurons) the neural signal is transmitted through the release, diffusion, and binding of neurotransmitters. Reflex impulses are limited to the same conduction speeds as any other form of neural impulse and are the result of simplified neural connections as opposed to a special form of neural transmission. Accordingly, cough reflexes and the muscles involved in the cough reflex are subject to reflex rebound and reflex fatigue.
The cough reflex is vital to the maintenance of respiratory integrity, the free-flow of air necessary for life. Although irritation of any of the respiratory structures may produce a cough, certain structures are more sensitive then others. The smaller terminal bronchial structures and lung alveoli are, for example, easily subject to irritation by corrosive gases and chemicals, and low levels of such substances will usually induce a cough reflex to expel the irritating agent.
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