Cough reflex
Encyclopedia
The cough
reflex
has both sensory (afferent
) and motor (efferent
) components. Pulmonary irritant receptors (cough receptors) in the epithelium of the respiratory tract are sensitive to both mechanical and chemical stimuli. Stimulation of the cough receptors by dust or other foreign particles produces a cough, which is necessary to remove the foreign material from the respiratory tract before it reaches the lungs.
The cough receptors, or rapidly adapting irritant receptors are located mainly on the posterior wall of the trachea
, pharynx
, and at the main carina, the point where the trachea branches into the main bronchi. The receptors are less abundant in the distal airways, and absent beyond the respiratory bronchioles. When triggered, impulses travel via the internal laryngeal nerve
, a branch of the superior laryngeal nerve
which stems from the (vagus nerve
) (CN X), to the medulla of the brain. This is the afferent neural pathway. Unlike other areas responsible for involuntary actions like swallowing, there is no clearly identifiable area that can be labeled as the cough center
in the brain.
The efferent neural pathway then follows, with relevant signals transmitted back from the cerebral cortex and medulla via the vagus and superior laryngeal nerves to the glottis, external intercostals, diaphragm, and other major inspiratory and expiratory muscles. The mechanism of a cough is as follows:
Stimulation of the auricular branch of the vagus nerve supplying the ear may also elicit a cough. This is known as Arnold's reflex. Respiratory muscle weakness, tracheostomy, or vocal cord pathology (including paralysis or anesthesia) may prevent effective clearing of the airways.
The reflex is impaired in the person whose abdominals and respiratory muscles are weak. This problem can be caused by disease condition that lead to muscle weakness or paralysis, by prolonged inactivity, or as outcome of surgery involving these muscles. Bed rest interferes with the expansion of the chest and limits the amount of air that can be taken into the lungs in preparation for coughing, making the cough weak and ineffective. This reflex may also be impaired by damage to the internal branch of the superior laryngeal nerve which relays the afferent branch of the reflex arc. This nerve is most commonly damaged by swallowing a foreign object, such as a chicken bone, resulting in it being lodged in the piriform recess (in the laryngopharynx) or by surgical removal of said object.
Cough
A cough is a sudden and often repetitively occurring reflex which helps to clear the large breathing passages from secretions, irritants, foreign particles and microbes...
reflex
Reflex
A reflex action, also known as a reflex, is an involuntary and nearly instantaneous movement in response to a stimulus. A true reflex is a behavior which is mediated via the reflex arc; this does not apply to casual uses of the term 'reflex'.-See also:...
has both sensory (afferent
Afferent
Afferent is an anatomical term with the following meanings:*Conveying towards a center, for example the afferent arterioles conveying blood towards the Bowman's capsule in the Kidney. Opposite to Efferent.*Something that so conducts, see Afferent nerve fiber...
) and motor (efferent
Efferent
Efferent is an anatomical term with the following meanings:*Conveying away from a center, for example the efferent arterioles conveying blood away from the Bowman's capsule in the kidney. Opposite to afferent....
) components. Pulmonary irritant receptors (cough receptors) in the epithelium of the respiratory tract are sensitive to both mechanical and chemical stimuli. Stimulation of the cough receptors by dust or other foreign particles produces a cough, which is necessary to remove the foreign material from the respiratory tract before it reaches the lungs.
The cough receptors, or rapidly adapting irritant receptors are located mainly on the posterior wall of the trachea
Vertebrate trachea
In tetrapod anatomy the trachea, or windpipe, is a tube that connects the pharynx or larynx to the lungs, allowing the passage of air. It is lined with pseudostratified ciliated columnar epithelium cells with goblet cells that produce mucus...
, pharynx
Pharynx
The human pharynx is the part of the throat situated immediately posterior to the mouth and nasal cavity, and anterior to the esophagus and larynx. The human pharynx is conventionally divided into three sections: the nasopharynx , the oropharynx , and the laryngopharynx...
, and at the main carina, the point where the trachea branches into the main bronchi. The receptors are less abundant in the distal airways, and absent beyond the respiratory bronchioles. When triggered, impulses travel via the internal laryngeal nerve
Internal laryngeal nerve
The internal laryngeal nerve is the internal branch of the superior laryngeal nerve. It descends to the thyrohyoid membrane, pierces it in company with the superior laryngeal artery, and is distributed to the mucous membrane of the larynx....
, a branch of the superior laryngeal nerve
Superior laryngeal nerve
The superior laryngeal nerve is a branch of the vagus nerve. It arises from the middle of the ganglion nodosum and in its course receives a branch from the superior cervical ganglion of the sympathetic....
which stems from the (vagus nerve
Vagus nerve
The vagus nerve , also called pneumogastric nerve or cranial nerve X, is the tenth of twelve paired cranial nerves...
) (CN X), to the medulla of the brain. This is the afferent neural pathway. Unlike other areas responsible for involuntary actions like swallowing, there is no clearly identifiable area that can be labeled as the cough center
Cough center
The cough center of the brain is a region of the brain which controls coughing, located in the medulla oblongata area of the brain. Antitussives and other cough medicines focus their action on the cough center....
in the brain.
The efferent neural pathway then follows, with relevant signals transmitted back from the cerebral cortex and medulla via the vagus and superior laryngeal nerves to the glottis, external intercostals, diaphragm, and other major inspiratory and expiratory muscles. The mechanism of a cough is as follows:
- DiaphragmThoracic diaphragmIn the anatomy of mammals, the thoracic diaphragm, or simply the diaphragm , is a sheet of internal skeletal muscle that extends across the bottom of the rib cage. The diaphragm separates the thoracic cavity from the abdominal cavity and performs an important function in respiration...
(innervated by phrenic nervePhrenic nerveThe phrenic nerve originates mainly from the 4th cervical nerve, but also receives contributions from the 5th and 3rd cervical nerves in humans....
) and external intercostal muscles (innervated by segmental intercostal nerves) contract, creating a negative pressure around the lung. - Air rushes into the lungs in order to equalise the pressure.
- The glottisGlottisThe glottis is defined as the combination of the vocal folds and the space in between the folds .-Function:...
closes (muscles innervated by recurrent laryngeal nerveRecurrent laryngeal nerveThe recurrent laryngeal nerve is a branch of the vagus nerve that supplies motor function and sensation to the larynx . It travels within the endoneurium...
) and the vocal cords contract to shut the larynx. - The abdominal muscles contract to accentuate the action of the relaxing diaphragm; simultaneously, the other expiratory muscles contract. These actions increase the pressure of air within the lungs.
- The vocal cords relax and the glottisGlottisThe glottis is defined as the combination of the vocal folds and the space in between the folds .-Function:...
opens, releasing air at over 100 mph. - The bronchi and non-cartilaginous portions of the trachea collapse to form slits through which the air is forced, which clears out any irritants attached to the respitory lining.
Stimulation of the auricular branch of the vagus nerve supplying the ear may also elicit a cough. This is known as Arnold's reflex. Respiratory muscle weakness, tracheostomy, or vocal cord pathology (including paralysis or anesthesia) may prevent effective clearing of the airways.
The reflex is impaired in the person whose abdominals and respiratory muscles are weak. This problem can be caused by disease condition that lead to muscle weakness or paralysis, by prolonged inactivity, or as outcome of surgery involving these muscles. Bed rest interferes with the expansion of the chest and limits the amount of air that can be taken into the lungs in preparation for coughing, making the cough weak and ineffective. This reflex may also be impaired by damage to the internal branch of the superior laryngeal nerve which relays the afferent branch of the reflex arc. This nerve is most commonly damaged by swallowing a foreign object, such as a chicken bone, resulting in it being lodged in the piriform recess (in the laryngopharynx) or by surgical removal of said object.