An animal’s body must balance and control the amount of oxygen and carbon dioxide within its cardiovascular system (heart and lungs) to maintain good health. In mammals, this process is controlled by a feedback system between the cardiovascular system and the central nervous system. This is done by specialized chemoreceptors (proteins that sense changes in chemical concentrations) called the carotid body (also known as the carotid glomus or glomus caroticum). These chemoreceptors and other support cells form a small cluster located within the left and right carotid arteries just before they branch (bifurcate) into the internal and external carotid arteries. These four arteries are found on both sides of the throat and bring blood to the brain. The function of the carotid body is to sense the amount of oxygen within the blood. Specifically, it detects the partial pressure of oxygen by the glomus type I (chief) cells. This triggers a nerve impulse via the glossopharyngeal nerve (also called cranial nerve IX). The glossopharyngeal nerve is a paired cranial nerve that serves sensory and motor function to the head and neck. In turn, cranial nerve IX relays the partial pressure of oxygen within the blood to the medulla oblongata of the brainstem. In addition to measuring the partial pressure of oxygen, the carotid body detects changes in the amount of carbon dioxide, subtle changes in pH (the measurement of acidity or basicity of the blood), and temperature. Because of the location of the carotid body, it is suggested that persons who are working out and who want to take their pulse do so on their wrist. If they take their pulse on their neck, they may press too hard, altering their blood chemistry, and thus causing them to become lightheaded and possibly pass out.
Anatomy and Physiology
Two types of cells make up the carotid body, the glomus type I (chief) cells and glomus type II (sustentacular) cells. The glomus type I cells are similar to neurons in the central nervous system as they have the ability to release neurotransmitters, which are chemicals that transmit nerve signals. Specifically, glomus type I cells release acetylcholine, adenosine triphosphate (ATP), and dopamine to neurons located in the respiratory center of the brainstem. Glomus type II cells are support cells and have similar functions as glia (non-neuronal cells that maintain homeostasis) in the central nervous system. Because the carotid body is essential for maintaining homeostasis (balance) within the cardiovascular system, they are continually sensing the ratio of oxygen to carbon dioxide in the blood and constantly sending this information to the brain. For example, if the oxygen partial pressure is normal (about 100 mm Hg) and the pH is normal (around 7.2), then the glomus type I cells send out a relatively low output of action potentials to the brainstem. However, if the oxygen partial pressure is low (less than 60 mm Hg), then the glomus type I cells significantly increase their output, which causes the brainstem to significantly increase the person’s breathing.
In rare cases, carotid bodies can become tumorous and alter a person’s blood pH and oxygen to carbon dioxide ratios. There are three types of carotid body tumors: familial, hyperplastic, and sporadic. Although rare, familial tumors are most common in younger patients while sporadic is the most common form overall, representing about 85 percent of all carotid body tumors. Sporadic tumors are generally seen in adults over the age of 45. People who live at a high elevation, such as in Denver, Colorado (a mile high), are more prone to hyperplastic carotid body tumors. Hyperplastic tumors have been associated with chronic hypoxia, as seen in patients with chronic obstructive pulmonary disease (COPD). Nonetheless, carotid body tumors are generally slow growing and persons may be asymptomatic for years. The tumors can be palpated in the neck near the carotid bifurcation. If the tumor is large enough, it may press on the nerves that supply the muscles of speech and swallowing, leading to other symptoms, such as pain, dysphagia, and hoarseness. The best treatment for carotid body tumors is surgical removal.
Jennifer L. Hellier
See also: Cranial Nerves; Sensory Receptors
Chaaban, Mohamad. (2014). Carotid body tumors. Medscape. Retrieved from http://emedicine.medscape.com/article/1575155-overview
Schultz, Harold D., Noah J. Marcus, & Rodrigo Del Rio. (2013). Role of the carotid body in the pathophysiology of heart failure. Current Hypertension Reports, 15(4), 356—362.