Cranial Nerves

 

There are 12 pairs of cranial nerves. They originate from nuclei in the inferior surface of the brain. Only the 1^st and 2^nd pairs emerge from the cerebrum. Remaining 10 pairs emerge from the brain stem. Some sensory, some motor and some mixed. Their names suggest their distribution or function. They are generally related to the head and neck.

I.                Olfactory                         - sensory nerves      - sense of smell

II.              Optic                                - sensory nerves      - sense of sight

III.            Oculomotor                    - motor nerves        - moves eyes and pupil

IV.            Trochlear                        - motor nerves        - down and inward movement of eye

V.              Trigeminal                      - mixed nerves         - face sensation

VI.            Abducens                       - motor nerves        - moves eye

VII.          Facial                               - mixed nerves         - movements of facial muscles, taste

VIII.        Vestibulocochlear         - sensory nerves      - balance and hearing

IX.            Glossopharyngeal         - mixed nerves         - taste, swallow

X.              Vagus                               - mixed nerves         - heart rate, digestion

XI.            Accessory                       - motor nerves        - moves head

XII.          Hypoglossal                    - motor nerves        - moves tongue

 

Olfactory nerves (sensory)

Function is sense of smell. They emerge from the cerebrum. Their sensory receptors and nerve fibers originate in the upper part of the mucosal membrane of the nasal cavity. The nerves then proceed backwards as the olfactory tract, to the area of the perception of smell in the temporal lobe of the cerebrum.

 

Optic nerves (sensory)

Function is sense of sight. They emerge from the cerebrum. Their fibers originate in the retinae of the eyes and they combine to form the optic nerve. Impulses pass to the visual areas in the occipital lobes of the cerebrum and to the cerebellum. Sight is perceived by cerebrum.  Cerebellum; impulses from the eyes contribute to the maintenance of balance, posture and orientation of the head in space.

 

Oculomotor nerves (motor)

Function is moves eyes and pupil. Location of its nuclei is midbrain, so these nerves are arise from nuclei near the cerebral aqueduct. They supply four of the six extrinsic muscles, move the eye ball. Intrinsic (intraocular) muscles, ciliary muscles, alter the shape of the lens, changing its refractory power, circular muscles of the iris, constrict the pupil. Levator palpebrae muscles, raise the upper eyelids.

 

Trochlear nerve (motor)

Function is down and inward movement of the eye. Location of its nuclei is midbrain, so these nerves arise form nuclei near the cerebral aqueduct. They bring motor information from brain to the superior oblique muscles of the eye.

 

Trigeminal nerves (mixed)

Function is face sensation. Location of its nuclei is pons. These nerves contain motor and sensory fibers. Are among the largest of the cranial nerves. Chief sensory nerves for the face and head (including oral and nasal cavities and teeth), transmitting sensory impulses, such as pain, temperature, touch. Motor fibers stimulate the muscles for chewing (mastication).

 

Abducens nerves (motor)

Function is moving eye. Location of the nuclei is pons, so these nerves arise from nuclei lying under the floor of the 4^th ventricle. They supply the lateral rectus muscles of the eyeballs causing abduction.

 

Facial nerves (mixed)

Functions are movement of facial muscles, taste, salivary glands. Location of the nuclei is pons. Composed of both motor and sensory nerve fibers. Arising from nuclei in the lower part of the pons. Motor fibers supply the muscles of facial expression. Sensory fibers, convey impulses from the taste buds in the anterior 2/3 of the tongue to the taste perception area in the cerebral cortex.

 

Vestibulocochlear nerves (sensory)

Function is balance and hearing. Location of the nuclei is medulla. Composed of two divisions,

1.    Vestibular nerves – arise from the semicircular canals of the inner ear and convey impulses to the cerebellum. Associated with the maintenance of posture and balance.

2.    Cochlear nerves – originate in the spiral organ (of corti) in the inner ear. Convey impulses to the hearing areas in the cerebral cortex where sound is perceived.  

 

Glossopharyngeal nerves (mixed)

Functions are taste and swallow. Location of the nuclei is medulla. Motor fibers arise from nuclei in the medulla oblangata. Stimulate the muscles of the tongue and pharynx and the secretory cells of the parotid glands. Sensory fibers, convey impulses to the cerebral cortex from the posterior third of the tongue, the tonsils and pharynx and from taste buds in the tongue and pharynx. Essential for the swallowing and gag reflexes. Some fibers conduct impulses from the carotid sinus, which plays an important role in the control of blood pressure.

 

Vagus nerves (mixed)

Functions are heart rate and digestion. Location of the nuclei is medulla. Have the most extensive distribution of the cranial nerves. Pass downwards through the neck into the thorax and the abdomen. From an important part of the parasympathetic nervous system.

 

Accessory nerves (motor)

Function is moving head. Location of the nuclei is medulla.Supply the sternocleidomastoid and trapezius muscles. Branches join the vagus nerves and supply the pharyngeal and laryngeal muscles in the neck.

 

Hypoglossal nerves (motor)

Function is move tongue. Location of the nuclei is medulla. Supply the muscles of the tongue and muscles surrounding the hyoid bone and contribute to swallowing and speech.