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12.5: Cranial Nerves

  • Page ID
    35915
  • By the end of this section, you will be able to:

    • Describe the structures found in the PNS
    • Distinguish between somatic and autonomic structures, including the special peripheral structures of the enteric nervous system
    • Name the twelve cranial nerves and explain the functions associated with each

    The PNS is not as contained as the CNS because it is defined as everything that is not the CNS. Some peripheral structures are incorporated into the other organs of the body. In describing the anatomy of the PNS, it is necessary to describe the common structures, the nerves and the ganglia, as they are found in various parts of the body. Many of the neural structures that are incorporated into other organs are features of the digestive system; these structures are known as the enteric nervous system and are a special subset of the PNS.

    Nerves

    Bundles of axons in the PNS are referred to as nerves. These structures in the periphery are different than the central counterpart, called a tract. Nerves are composed of more than just nervous tissue. They have connective tissues invested in their structure, as well as blood vessels supplying the tissues with nourishment. The outer surface of a nerve is a surrounding layer of fibrous connective tissue called the epineurium. Within the nerve, axons are further bundled into fascicles, which are each surrounded by their own layer of fibrous connective tissue called perineurium. Finally, individual axons are surrounded by loose connective tissue called the endoneurium (Figure \(\PageIndex{1}\) and Figure \(\PageIndex{2}\)). These three layers are similar to the connective tissue sheaths for muscles. Nerves are associated with the region of the CNS to which they are connected, either as cranial nerves connected to the brain or spinal nerves connected to the spinal cord.

    Spinal nerve as a circle with superficial layer of epineurium. Circles inside are fascicles.
    Figure \(\PageIndex{1}\): Nerve Structure. In (a), diagram of the organization of a nerve. The structure of a nerve is organized by the layers of connective tissue on the outside, around each fascicle, and surrounding the individual nerve fibers. In (b), micrograph of a nerve showing fascicles covered by perineurium, and the nerve covered by epineurium (tissue source: simian). LM x 40. (Image credit: "Nerve Structure" by OpenStax is licensed under CC BY 3.0/Micrograph provided by the Regents of University of Michigan Medical School 2012)

    Perineurium encircles bundle of axons, which are small circles with darker edges
    Figure \(\PageIndex{2}\): Close-Up of Nerve. Magnified micrograph of a nerve trunk where individual axons are visible as circular structures covered by endoneurium (tissue source: simian). Axons are bundled into a fascicle covered by perineurium. LM x 1600. (Image credit: "Nerve Mag" by OpenStax is licensed under CC BY 4.0/Micrograph provided by the Regents of University of Michigan Medical School 2012)

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    Peripheral Nerve

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    View a virtual slide of a nerve in longitudinal section at the University of Michigan WebScope to explore the tissue sample in greater detail. With what structures in a skeletal muscle are the endoneurium, perineurium, and epineurium comparable?

    Answer

    The endoneurium surrounding individual nerve fibers is comparable to the endomysium surrounding myofibrils, the perineurium bundling axons into fascicles is comparable to the perimysium bundling muscle fibers into fascicles, and the epineurium surrounding the whole nerve is comparable to the epimysium surrounding the muscle.

    Cranial Nerves

    The nerves attached to the brain are the cranial nerves, which are primarily responsible for the sensory and motor functions of the head and neck (with the exception of one that targets organs in the thoracic and abdominal cavities as part of the parasympathetic nervous system). There are twelve cranial nerves, which are designated CNI through CNXII for “Cranial Nerve,” using Roman numerals for 1 through 12, based on the anatomical location on the inferior view of the brain, from anterior to posterior (Figure \(\PageIndex{3}\)). They can be classified as sensory nerves, motor nerves, or a combination of both, meaning that the axons in these nerves originate out of sensory ganglia external to the cranium or motor nuclei within the brainstem. Sensory axons enter the brain to synapse in a nucleus. Motor axons connect to skeletal muscles of the head or neck. Three of the nerves are solely composed of sensory fibers; five are strictly motor; and the remaining four are mixed nerves.

    Inferior view of brain with cranial nerves emerging from it. 1 to 12 from anterior to posterior.
    Figure \(\PageIndex{3}\): Cranial Nerves. The anatomical arrangement of the roots of the cranial nerves observed from an inferior view of the brain. Cranial nerves are numbered with Roman numbers from 1 to 12 based on their exit from the brain, from anterior to posterior. (Image credit: "The Cranial Nerves" by OpenStax is licensed under CC BY 4.0)

    Learning the cranial nerves is a tradition in anatomy courses, and students have always used mnemonic devices to remember the nerve names. A traditional mnemonic is the rhyming couplet, “Oh Oh Oh To Touch And Feel Very Good Velvet Ah Heaven,” in which the initial letter of each word corresponds to the initial letter in the name of each nerve. The names of the nerves have changed over the years to reflect current usage and more accurate naming. An exercise to help learn this sort of information is to generate a mnemonic using words that have personal significance. The names of the cranial nerves are listed in Table \(\PageIndex{1}\) along with a brief description of their function, their source (sensory ganglion or motor nucleus), and their target (sensory nucleus or skeletal muscle). The olfactory nerve and optic nerve are responsible for the sense of smell and vision, respectively. The oculomotor nerve is responsible for eye movements by controlling four of the extraocular muscles. It is also responsible for lifting the upper eyelid when the eyes point up, and for pupillary constriction. The trochlear nerve and the abducens nerve are both responsible for eye movement, but do so by controlling different extraocular muscles. The trigeminal nerve is responsible for cutaneous sensations of the face and controlling the muscles of mastication. The facial nerve is responsible for the muscles involved in facial expressions, as well as part of the sense of taste and the production of saliva. The vestibulocochlear nerve is responsible for the senses of hearing and balance. The glossopharyngeal nerve is responsible for controlling muscles in the oral cavity and upper throat, as well as part of the sense of taste and the production of saliva. The vagus nerve is responsible for contributing to homeostatic control of the organs of the thoracic and upper abdominal cavities. The accessory nerve is responsible for controlling the muscles of the neck, along with cervical spinal nerves. The hypoglossal nerve is responsible for controlling the muscles of the lower throat and tongue.

    Four of these cranial nerves make up the cranial component of the autonomic nervous system responsible for pupillary constriction (oculomotor nerve), salivation and lacrimation (facial and glossopharyngeal nerves), and the regulation of the organs of the thoracic and upper abdominal cavities (vagus nerve).

    Three of the cranial nerves also contain autonomic fibers, and a fourth is almost purely a component of the autonomic system. The oculomotor, facial, and glossopharyngeal nerves contain fibers that contact autonomic ganglia. The oculomotor fibers initiate pupillary constriction, whereas the facial and glossopharyngeal fibers both initiate salivation. The vagus nerve primarily targets autonomic ganglia in the thoracic and upper abdominal cavities.

    Another important aspect of the cranial nerves that lends itself to a mnemonic is the functional role each nerve plays. The nerves fall into one of three basic groups. They are sensory, motor, or both (see Table \(\PageIndex{1}\)). The sentence, “Some Say Marry Money But My Brother Says Brains Beauty Matter More,” corresponds to the basic function of each nerve. The first, second, and eighth nerves are purely sensory: the olfactory (CNI), optic (CNII), and vestibulocochlear (CNVIII) nerves. The three eye-movement nerves are all motor: the oculomotor (CNIII), trochlear (CNIV), and abducens (CNVI). The accessory (CNXI) and hypoglossal (CNXII) nerves are also strictly motor. The remainder of the nerves contain both sensory and motor fibers. They are the trigeminal (CNV), facial (CNVII), glossopharyngeal (CNIX), and vagus (CNX) nerves. The nerves that convey both are often related to each other. The trigeminal and facial nerves both concern the face; one concerns the sensations and the other concerns the muscle movements. The facial and glossopharyngeal nerves are both responsible for conveying gustatory, or taste, sensations as well as controlling salivary glands. The vagus nerve is involved in visceral responses to taste, namely the gag reflex. This is not an exhaustive list of what these combination nerves do, but there is a thread of relation between them.

    Table \(\PageIndex{1}\): Cranial Nerves
    Mnemonic # Name Function (S/M/B) Central connection (nuclei) Peripheral connection (ganglion or muscle)
    Oh I Olfactory Smell (S) Olfactory bulb Olfactory epithelium
    Oh II Optic Vision (S) Hypothalamus/thalamus/midbrain Retina (retinal ganglion cells)
    Oh III Oculomotor Eye movements (M) Oculomotor nucleus Extraocular muscles (other 4), levator palpebrae superioris, ciliary ganglion (autonomic)
    To IV Trochlear Eye movements (M) Trochlear nucleus Superior oblique muscle
    Touch V Trigeminal Sensory/motor – face (B) Trigeminal nuclei in the midbrain, pons, and medulla Trigeminal
    And VI Abducens Eye movements (M) Abducens nucleus Lateral rectus muscle
    Feel VII Facial Motor – face, Taste (B) Facial nucleus, solitary nucleus, superior salivatory nucleus Facial muscles, Geniculate ganglion, Pterygopalatine ganglion (autonomic)
    Very VIII Vestibulocochlear Hearing/balance (S) Cochlear nucleus, Vestibular nucleus/cerebellum Spiral ganglion (hearing), Vestibular ganglion (balance)
    Good IX Glossopharyngeal Motor – throat Taste (B) Solitary nucleus, inferior salivatory nucleus, nucleus ambiguus Pharyngeal muscles, Geniculate ganglion, Otic ganglion (autonomic)
    Velvet X Vagus Motor/sensory – viscera (autonomic) (B) Medulla Terminal ganglia serving thoracic and upper abdominal organs (heart and small intestines)
    Ah XI Accessory Motor – head and neck (M) Spinal accessory nucleus Neck muscles
    Heaven XII Hypoglossal Motor – lower throat (M) Hypoglossal nucleus Muscles of the larynx and lower pharynx

    Cranial Nerve Ganglia

    A ganglion (ganglia for plural) is a group of neuron cell bodies in the peripheral nervous system. Ganglia can be categorized, for the most part, as either sensory ganglia or autonomic ganglia, referring to their primary functions. A type of sensory ganglion is a cranial nerve ganglion. The roots of cranial nerves are within the cranium, whereas the ganglia are outside the skull. For example, the trigeminal ganglion is superficial to the temporal bone whereas its associated nerve is attached to the mid-pons region of the brainstem. The neurons of cranial nerve ganglia are also unipolar in shape with associated satellite cells.

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    Vision Loss

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    Read this article about a man who wakes with a headache and a loss of vision. His regular doctor sent him to an ophthalmologist to address the vision loss. The ophthalmologist recognizes a greater problem and immediately sends him to the emergency room. Once there, the patient undergoes a large battery of tests, but a definite cause cannot be found. A specialist recognizes the problem as meningitis, but the question is what caused it originally. How can that be cured? The loss of vision comes from swelling around the optic nerve, which probably presented as a bulge on the inside of the eye. Why is swelling related to meningitis going to push on the optic nerve?

    Answer

    The optic nerve enters the CNS in its projection from the eyes in the periphery, which means that it crosses through the meninges. Meningitis will include swelling of those protective layers of the CNS, resulting in pressure on the optic nerve, which can compromise vision.

    AGING AND THE...

    Nervous System: Anosmia

    Anosmia is the loss of the sense of smell. It is often the result of the olfactory nerve being severed, usually because of blunt force trauma to the head. The sensory neurons of the olfactory epithelium have a limited lifespan of approximately one to four months, and new ones are made on a regular basis. The new neurons extend their axons into the CNS by growing along the existing fibers of the olfactory nerve. The ability of these neurons to be replaced is lost with age. Age-related anosmia is not the result of impact trauma to the head, but rather a slow loss of the sensory neurons with no new neurons born to replace them.

    Smell is an important sense, especially for the enjoyment of food. There are only five tastes sensed by the tongue, and two of them are generally thought of as unpleasant tastes (sour and bitter). The rich sensory experience of food is the result of odor molecules associated with the food, both as food is moved into the mouth, and therefore passes under the nose, and when it is chewed and molecules are released to move up the pharynx into the posterior nasal cavity. Anosmia results in a loss of the enjoyment of food.

    As the replacement of olfactory neurons declines with age, anosmia can set in. Without the sense of smell, many sufferers complain of food tasting bland. Often, the only way to enjoy food is to add seasoning that can be sensed on the tongue, which usually means adding table salt. The problem with this solution, however, is that this increases sodium intake, which can lead to cardiovascular problems through water retention and the associated increase in blood pressure.

    Concept Review

    The PNS is composed of the groups of neurons (ganglia) and bundles of axons (nerves) that are outside of the brain and spinal cord. Nerves are organized into structures by layers of connective tissue that cover them. The epineurium covers the nerve, the perineurium covers the fascicles and the endoneurium covers the individual axon.

    Nerves are classified as cranial nerves or spinal nerves on the basis of their connection to the brain or spinal cord, respectively. The twelve cranial nerves can be strictly sensory in function, strictly motor in function, or a combination of the two functions. The olfactory nerve (CN I) and optic nerve (CN II) are responsible for the sense of smell and vision, respectively. The oculomotor nerve (CN III) is responsible for eye movements, lifting the upper eyelid and size of the pupil. The trochlear nerve (CN IV) and the abducens nerve (CN VI) are both responsible for eye movement, but do so by controlling different extraocular muscles. The trigeminal nerve (CN V) is responsible for cutaneous sensations of the face and controlling the muscles of mastication. The facial nerve (VII) is responsible for the muscles involved in facial expressions, as well as part of the sense of taste and the production of saliva. The vestibulocochlear nerve (VIII) is responsible for the senses of hearing and balance. The glossopharyngeal nerve (IX) is responsible for controlling muscles in the oral cavity and upper throat, as well as part of the sense of taste and the production of saliva. The vagus nerve (CN X) is responsible for contributing to homeostatic control of the organs of the thoracic and upper abdominal cavities. The accessory nerve (CN XI) is responsible for controlling the muscles of the neck, along with cervical spinal nerves. The hypoglossal nerve (CN XII) is responsible for controlling the muscles of the lower throat and tongue.

    Ganglia are of two types, sensory or autonomic. Sensory ganglia contain unipolar sensory neurons and are associated with many of the cranial nerves.

    Review Questions

    Q. What type of ganglion contains neurons that control homeostatic mechanisms of the body?

    A. sensory ganglion

    B. dorsal root ganglion

    C. autonomic ganglion

    D. ventral root ganglion

    Answer

    C

    Q. Which ganglion is responsible for cutaneous sensations of the face?

    A. otic ganglion

    B. vestibular ganglion

    C. geniculate ganglion

    D. trigeminal ganglion

    Answer

    D

    Q. What is the name for a bundle of axons within a nerve?

    A. fascicle

    B. tract

    C. nerve root

    D. epineurium

    Answer

    A

    Q. Which cranial nerve does not control organs in the head and neck?

    A. olfactory

    B. trochlear

    C. glossopharyngeal

    D. vagus

    Answer

    D

    Critical Thinking Questions

    Q. Why are ganglia and nerves not surrounded by protective structures like the meninges of the CNS?

    A. The peripheral nervous tissues are out in the body, sometimes part of other organ systems. There is not a privileged blood supply like there is to the brain and spinal cord, so peripheral nervous tissues do not need the same sort of protections.

    Q. Testing for neurological function involves a series of tests of functions associated with the cranial nerves. What functions, and therefore which nerves, are being tested by asking a patient to follow the tip of a pen with their eyes?

    A. The contraction of extraocular muscles is being tested, which is the function of the oculomotor, trochlear, and abducens nerves.

    Glossary

    abducens nerve
    sixth cranial nerve; responsible for contraction of one of the extraocular muscles
    accessory nerve
    eleventh cranial nerve; responsible for contraction of neck muscles
    cranial nerve
    one of twelve nerves connected to the brain that are responsible for sensory or motor functions of the head and neck
    cranial nerve ganglion
    sensory ganglion of cranial nerves
    endoneurium
    innermost layer of connective tissue that surrounds individual axons within a nerve
    epineurium
    outermost layer of connective tissue that surrounds an entire nerve
    facial nerve
    seventh cranial nerve; responsible for contraction of the facial muscles and for part of the sense of taste, as well as causing saliva production
    fascicle
    small bundles of nerve or muscle fibers enclosed by connective tissue
    glossopharyngeal nerve
    ninth cranial nerve; responsible for contraction of muscles in the tongue and throat and for part of the sense of taste, as well as causing saliva production
    hypoglossal nerve
    twelfth cranial nerve; responsible for contraction of muscles of the tongue
    oculomotor nerve
    third cranial nerve; responsible for contraction of four of the extraocular muscles, the muscle in the upper eyelid, and pupillary constriction
    olfactory nerve
    first cranial nerve; responsible for the sense of smell
    optic nerve
    second cranial nerve; responsible for visual sensation
    perineurium
    layer of connective tissue surrounding fascicles within a nerve
    trigeminal ganglion
    sensory ganglion that contributes sensory fibers to the trigeminal nerve
    trigeminal nerve
    fifth cranial nerve; responsible for cutaneous sensation of the face and contraction of the muscles of mastication
    trochlear nerve
    fourth cranial nerve; responsible for contraction of one of the extraocular muscles
    vagus nerve
    tenth cranial nerve; responsible for the autonomic control of organs in the thoracic and upper abdominal cavities
    vestibulocochlear nerve
    eighth cranial nerve; responsible for the sensations of hearing and balance

    Contributors and Attributions

    OpenStax Anatomy & Physiology (CC BY 4.0). Access for free at https://openstax.org/books/anatomy-and-physiology