The Cranial Nerves

There are 12 pairs of cranial nerves. The nerves arise directly from the brain. Some of the nerves are sensory, some are motor, those containing both sensory and motor fibres are described as mixed

Name and No.	Central Origin	Distribution	Function
1.Olfactory sensory)	Smell area in temporal lobe of cerebrum through olfactory bulb	Mucous membrane in roof of nose	Sense of smell
2. Optic (sensory)	Sight area in occipital lobe of cerebrum	Retina of the eye	Sense of sight
3. Oculomotor (motor)	Cerebellum Nerve cells near the floor of the the aqueduct of the midbrain	Superior, inferior and medial rectus muscles, inferior oblique and ciliary muscles of the eye, and circular muscle fibres of the iris	Moving the eyeball, regulating size of the pupils and focusing
4.Trochlear (motor)	Nerve cells near floor of aqueduct of midbrain	Superior oblique muscles of the eyes	Movement of the eyeball
5. Trigeminal (mixed)	Motor fibres from the pons varolii Sensory fibres from the trigeminal ganglion	Muscles of mastication Sensory to gums, cheeks, lower jaw, iris, cornea	Chewing Sensation from the face
6. Abducent (motor)	Floor of fourth ventricle	Lateral rectus muscles of the eye	Movement of the eye
7. Facial (mixed)	Pons varolii	Sensory fibres to the tongue Motor fibres to the muscles of the face	Sensation of taste Movements of facial expression
8. Auditory (sensory)	Hearing area of cerebrum	Organ of Corti in the cochlea	Sense of hearing
9. Glossopharyngeal (mixed)	Medulla oblongata	Back of tongue and pharynx Posterior third of tongue Parotid glands	Sensation of taste Secretion of saliva Movements of pharynx
10. Vagus (mixed)	Medulla oblongata	Pharynx, larynx, lungs, heart, gall bladder, stomach Small and large intestine	Movement and secretion
11. Accessory (motor)	Medulla oblongata	Sternomastoid, trapezius, laryngeal and pharyngeal muscles	Movement of the head and shoulders and pharynx and larynx
12. Hypoglossal (motor)	Medulla oblongata	Tongue	Movement of tongue

1. The olfactory nerves (sensory) The olfactory nerves are the nerves of the sense of smell. Their sensory nerve endings and fibres arise in the upper part of the mucous membrane of the nose and pass upwards through the cribriform plate of the ethmoid bone. These nerves pass to the olfactory bulb, a group of nerve cells where synapses occur and the impulse is passed to a second neurone. The nerves then proceed backwards to the area for the perception of smell in the temporal lobe of the cerebrum.

2. The optic nerves (sensory) The optic nerves are the nerves of the sense of sight. The fibres originate in the retinas of the eyes and they combine to form the optic nerves which are about 4 cm in length. They are directed backwards and medially through the posterior part of the orbital cavity. They then pass through the optic foramina of the sphenoid bone into the cranial cavity and join at the optic chiasma just above the pituitary gland. The nerves proceed backwards as the optic tracts to the lateral geniculate body. Impulses pass from these to the centre for sight in the occipital lobe of the cerebrum and to the cerebellum. In the occipital lobe sight is perceived, and in the cerebellum the impulses from the eyes contribute to the maintenance of balance.

3. The oculomotor nerves (motor) The oculomotor nerves arise from nerve cells near the aqueduct of the midbrain. They supply four of the muscles which move the eyes, namely the superior, inferior and medial recti and the inferior oblique muscles. They also supply the ciliary muscles which control the focusing power of the eyes by altering the shape of the lens, the circular muscles of the iris producing constriction of the pupils and the muscles which control the movements of the upper eyelids.

4. The trochlear nerves (motor) The trochlear nerves arise from a nucleus just behind the oculomotor nucleus. These nerves supply the superior oblique muscles of the eyes.

5. The trigeminal nerves (mixed) The trigeminal nerves contain motor and sensory fibres and are among the largest of the cranial nerves. They are the chief sensory nerves for the face and head, receiving impulses of pain, temperature and pressure. The motor fibres stimulate the muscles of mastication.

The motor fibres arise from nerve cells which are situated just above the pons varolii and pass to the muscles of mastication.

There are three main branches of the trigeminal nerves each of which has motor and sensory fibres.

1. The Ophthalmic Nerves. These supply the lacrimal glands, the conjunctiva of the eyes, the forehead, eyelids, anterior aspect of the scalp and mucous membrane of the nose.

2. The Maxillary Nerves. These supply the cheeks, the upper gums, upper teeth and the lower eyelids.

3. The Mandibular Nerves. These are the largest of the three divisions of the trigeminal nerves. They supply the teeth and gums of the lower .jaw, the pinna of the ears and the lower lip. Motor nerves from these branches supply the muscles of mastication.

6. The abducent nerves (motor) The abducent nerves arise from a group of nerve cells lying beneath the floor of the fourth ventricle. they supply the lateral rectus muscles of the eyeballs.

7. The facial nerves (mixed) The facial nerves are composed of both motor and sensory nerve fibres. The fibres arise from nerve cells in the lower part of the pons varolii. The motor fibres supply the muscles of facial expression. The sensory fibres convey impulses from the taste buds of the anterior two-thirds of the tongue to the taste perception area in the cerebral cortex.

8. The auditory nerves (sensory) The auditory nerves are composed of two distinct sets of fibres:

cochlear fibres

vestibular fibres.

The cochlear fibres originate in the cochlea or inner ear and convey impulses to the hearing area in the cerebral cortex in both hemispheres where they are perceived as sound.

The vestibular fibres arise from the semi-circular canals of the inner ear and convey impulses to the cerebellum which are associated with the maintenance of equilibrium and balance.

9. The glossopharyngeal nerves (mixed) The glossopharyngeal nerves are composed of motor and sensory fibres. The fibres arise from nuclei in the medulla oblongata. The motor fibres stimulate the muscles of the pharynx and the secretory cells of the parotid glands. The sensory fibres convey impulses from the posterior third of the tongue and from the tonsils and pharynx to the cerebral cortex

10. The vagus nerves (mixed) The vagus nerves are composed of both motor and sensory fibres. They have a more extensive distribution than any of the other cranial nerves. They arise from nerve cells in the medulla oblongata and other nuclei, and pass through a foramen in the base of the skull downwards through the neck between the internal jugular vein and the internal carotid artery into the thorax and abdomen. The motor fibres supply the muscles and secretory glands of the pharynx, larynx, trachea, heart, oesophagus, stomach, small intestine, pancreas, gall bladder, bile ducts, spleen, colon, kidneys and blood vessels in the thoracic and abdominal cavities. The sensory fibres convey impulses from the mucous membranes of the larynx, trachea, lungs, oesophagus, stomach, small intestine and gall bladder to the brain.

11. The accessory nerves (motor) The accessory nerves arise from nerve cells in the medulla oblongata and in the spinal cord. The fibres supply the sternomastoid and trapezius muscles. Branches join the vagus nerves and proceed to the pharyngeal and laryngeal muscles.

12. The hypoglossal nerves (motor) The hypoglossal nerves arise from nerve cells in the medulla oblongata. They supply the muscles of the tongue and muscles surrounding the hyoid bone.

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