Cranial Nerves: Anatomy and Function PDF

  **[[Facial nerve]](https://www.britannica.com/science/facial-nerve) (CN VII or 7)** The [[facial nerve]](https://www.britannica.com/science/facial-nerve) is composed of a large root that innervates facial muscles and a small root (known as the intermediate nerve) that contains sensory and autono...

  **[[Facial nerve]](https://www.britannica.com/science/facial-nerve) (CN VII or 7)** The [[facial nerve]](https://www.britannica.com/science/facial-nerve) is composed of a large root that innervates facial muscles and a small root (known as the intermediate nerve) that contains sensory and autonomic fibers.   Injury to the facial nerve at the brainstem produces a paralysis of facial muscles known as [[Bell palsy]](https://www.britannica.com/science/Bell-palsy) as well as a loss of taste sensation from the anterior two-thirds of the tongue. If damage occurs at the stylomastoid foramen, facial muscles will be paralyzed but taste will be [[intact]](https://www.britannica.com/dictionary/intact).   Auditory receptors of the [[cochlear division]](https://www.britannica.com/science/cochlear-nerve) are located in the [[organ of Corti]](https://www.britannica.com/science/organ-of-Corti) and follow the spiral shape (about 2.5 turns) of the [[cochlea]](https://www.britannica.com/science/cochlea). Air movement against the eardrum initiates action of the ossicles of the ear, which, in turn, causes movement of fluid in the spiral cochlea.    Lesions of the vestibular root result in eye movement disorders ([[nystagmus]](https://www.britannica.com/science/nystagmus)), unsteady gait with a tendency to fall toward the side of the lesion, nausea, and vertigo. Damage to the cochlea or cochlear nerve results in complete deafness, ringing in the ear ([[tinnitus]](https://www.britannica.com/science/tinnitus)), or both.     **[[Glossopharyngeal nerve]](https://www.britannica.com/science/glossopharyngeal-nerve) (CN IX or 9)** The ninth cranial nerve, which exits the skull through the jugular foramen, has both motor and sensory components. Cell bodies of motor neurons, located in the nucleus ambiguus in the [[medulla oblongata]](https://www.britannica.com/science/medulla-oblongata), project as special [[visceral]](https://www.merriam-webster.com/dictionary/visceral) efferent fibers to the stylopharyngeal muscle.     Sensory fibers in the carotid branch detect increased [[blood pressure]](https://www.britannica.com/science/blood-pressure) in the carotid sinus and send impulses into the medulla that ultimately reduce [[heart rate]](https://www.britannica.com/science/heart-rate) and arterial pressure; this is known as the [[carotid sinus reflex]](https://www.britannica.com/science/carotid-sinus-syncope).     **[[Vagus nerve]](https://www.britannica.com/science/vagus-nerve) (CN X or 10)** The [[vagus nerve]](https://www.britannica.com/science/vagus-nerve) has the most extensive distribution in the body of all the cranial nerves, innervating structures as [[diverse]](https://www.merriam-webster.com/dictionary/diverse) as the external surface of the eardrum and internal abdominal organs. The root of the nerve exits the cranial cavity via the jugular foramen.      **[[Accessory nerve]](https://www.britannica.com/science/spinal-accessory-nerve) (CN XI or 11)** The accessory nerve is formed by fibers from the [[medulla oblongata]](https://www.britannica.com/science/medulla-oblongata) (known as the cranial root) and by fibers from cervical levels C~1~--C~4~ (known as the [[spinal root]](https://www.britannica.com/science/spinal-root)).      **[[Hypoglossal nerve]](https://www.britannica.com/science/hypoglossal-nerve) (CN XII or 12)** The hypoglossal nerve innervates certain muscles that control movement of the [[tongue]](https://www.britannica.com/science/tongue). From the hypoglossal nucleus in the medulla oblongata, general somatic efferent fibers exit the cranial cavity through the hypoglossal canal and enter the neck in close proximity to the accessory and vagus nerves and the internal [[carotid artery]](https://www.britannica.com/science/carotid-artery).     The sympathetic nervous system normally functions to produce localized adjustments (such as [[sweating]](https://www.britannica.com/science/sweat) as a response to an increase in [[temperature]](https://www.britannica.com/science/body-heat)) and reflex adjustments of the cardiovascular system. Under conditions of stress, however, the entire sympathetic nervous system is activated, producing an immediate, widespread response called the [[fight-or-flight response]](https://www.britannica.com/science/fight-or-flight-response).  [**[Parasympathetic nervous system]**](https://www.britannica.com/science/parasympathetic-nervous-system) The parasympathetic nervous system primarily modulates [[visceral]](https://www.merriam-webster.com/dictionary/visceral) organs such as glands. Responses are never activated en masse as in the fight-or-flight sympathetic response. While providing important control of many tissues, the parasympathetic system, unlike the sympathetic system, is not crucial for the maintenance of life. [**[Receptors]**](https://www.britannica.com/science/receptor-nerve-ending) Receptors are biological transducers that convert energy from both external and internal [[environments]](https://www.merriam-webster.com/dictionary/environments) into electrical impulses. They may be massed together to form a sense organ, such as the eye or ear, or they may be scattered, as are those of the skin and viscera. Receptors are connected to the [[central nervous system]](https://www.britannica.com/science/central-nervous-system) by afferent nerve fibers. The region or area in the [[periphery]](https://www.merriam-webster.com/dictionary/periphery) from which a neuron within the central nervous system receives input is called its [[receptive field]](https://www.britannica.com/science/receptive-field). Receptive fields are changing and not fixed entities.       Receptors are also classified according to the kinds of stimulus to which they are sensitive. Chemical receptors, or [[chemoreceptors]](https://www.britannica.com/science/chemoreceptor), are sensitive to substances taken into the mouth (taste or gustatory receptors), inhaled through the nose (smell or olfactory receptors), or found in the body itself (detectors of [[glucose]](https://www.britannica.com/science/glucose) or of acid-base balance in the blood). Receptors of the skin are classified as thermoreceptors, mechanoreceptors, and [[nociceptors]](https://www.britannica.com/science/nociceptor)---the last being sensitive to stimulation that is noxious, or likely to damage the tissues of the body. [[Thermoreceptors]](https://www.britannica.com/science/thermoreceptor) are of two types, warmth and cold. Warmth fibers are excited by rising temperature and [[inhibited]](https://www.merriam-webster.com/dictionary/inhibited) by falling temperature, and cold fibers respond in the opposite manner. [[Mechanoreceptors]](https://www.britannica.com/science/mechanoreceptor) are also of several different types. Sensory nerve terminals around the base of hairs are activated by very slight movement of the hair, but they rapidly adapt to continued stimulation and stop firing. In hairless skin both rapidly and slowly adapting receptors provide information about the force of mechanical stimulation.      In seeking information about the environment, the nervous system presents the most-sensitive receptors to a stimulating object. At its simplest, this action is reflex. In the retina a small region about the size of a pinhead, called the fovea, is particularly sensitive to color. When a part of the periphery of the visual field is excited, a reflex movement of the head and eyes focuses the light rays upon that part of the fovea. A similar reflex turns the head and eyes in the direction of a noise. As the English physiologist [[Charles Sherrington]](https://www.britannica.com/biography/Charles-Scott-Sherrington) said in 1900, "In the limbs and mobile parts, when a spot of less discriminative sensitivity is touched, [[instinct]](https://www.britannica.com/dictionary/instinct) moves the member, so that it brings to the object the part where its own sensitivity is delicate."