Chapter 15 - Cranial Nerves

 

     Twelve pairs of peripheral nerves connect to the brain rather than the spinal cord. These nerves are designated by Roman numerals preceded by N. In addition, each pair has a name.
     The cranial nerves can be classified as (primarily) sensory, special sensory, (primarily) motor, or mixed sensory and motor.

 

Olfactory Nerve (N I)
     The olfactory nerve is a special sensory cranial nerve that is responsible for the sense of smell. The receptors of these neurons are found in the olfactory epithelium and the axons pass through the cribriform plate of the ethmoid bone. These neurons synapse within the olfactory bulb which lies directly above the cribriform plate.
     Information concerning olfaction goes directly to the cerebrum in the olfactory tract. This is the only cranial nerve whose sensory information goes directly to the cerebrum rather than being relayed by the thalamus.

 

Optic Nerve (N II)
     The optic nerve is a special sensory cranial nerve that carries visual information. The ganglion cells of the retina have axons that pass through the optic nerves. The optic nerve from each eye meet at the optic chiasm where some fibers cross to the optic tract on the contralateral side while the remaining fibers remain in the optic tract on the same side. Most of the neurons travel to the lateral geniculate body of the diencephalon.

 

Oculomotor Nerve (N III)
     The oculomotor nerve is a motor neuron that controls all but two of the extraocular eye muscles, the levator palpebrae superioris and the intrinsic eye muscles.
     The nerves originate in the mesencephalon and pass through the superior orbital fissure on its way to the eye.

 

Trochlear Nerve (N IV)
     The trochlear nerve is a motor neuron that innervates the superior oblique muscle.
     The nerve originates in the mesencephalon and pass through the superior orbital fissure.

 

Trigeminal Nerve (N V)
     The trigeminal nerve is a mixed sensory and motor nerve. The trigeminal is the largest cranial nerve and has three major branches:
  1. Ophthalamic branch (sensory) This passes through the superior orbital fissure. It provides sensory information from the orbit and the forehead.
  2. Maxillary branch (sensory) This branch passes through the foramen rotundum. It provides sensory information from the upper lip, cheeks and nose and the gums and teeth of the upper jaw.
  3. Mandibular branch (mixed sensory and motor) This branch passes through the foramen ovale. The sensory component provides sensory information from lips, gums and teeth of the lower jaw and the anterior portion of the tongue. The motor component innervates the muscles of mastication.

 

Abducens Nerve (N VI)
     The abducens nerve is a motor nerve that innervates the lateral rectus muscle.
     The nerve originates in the pons but emerges at the border between the pons and medulla. It passes through the superior orbital fissure.

 

Facial Nerve (N VII)
     The facial nerve is a mixed sensory and motor nerve. The motor neurons originate from the pons.
     The somatic motor neurons innervate the muscles of facial expression and visceral motor neurons innervate the lacrimal and nasal mucous glands and the submandibular and sublingual salivary glands.
     The sensory neurons receive taste information from taste receptors in the anterior two-thirds of the tongue.
     The facial nerve passes through the internal acoustic meatus and the stylomastoid foramen.

 

Vestibulocochlear Nerve (N VIII)
     The vestibulocochlear nerve is a special sensory nerve. The nerve has two branches:
  1. Vestibular branch This branch carries information from receptors in the inner ear responsible for equilibrium and balance.
  2. Cochlear branch This branch carries information from receptors in the cochlea responsible for hearing.
     Both branches leave the inner ear and reach the brain through the internal acoustic meatus. The ultimate destination of the afferent neurons are vestibular and cochlear nuclei in the pons and medulla.

 

Glossopharyngeal Nerve (N IX)
     The glossopharyngeal nerve is a mixed sensory and motor nerve.
     The sensory nerves carry general sensory information from the pharynx and the soft palate. Sensory neurons also convey information from taste and general sense receptors in the posterior one-third of the tongue, and from receptors in the carotid sinus that convey information about pressure and blood gases.
     The somatic motor neurons innervate pharyngeal muscles involved in swallowing and visceral motor neurons that innervate the parotid salivary glands.
     Both sensory and motor neurons are associated with nuclei in the medulla and the nerve passes through the jugular foramen.

 

Vagus Nerve (N X)
     The vagus nerve is a mixed motor and sensory nerve.
     The sensory neurons convey information from the inferior pharynx, larynx and thoracic and abdominal organs.
     Somatic motor neurons innervate muscles in the soft palate, pharynx and intrinsic laryngeal muscles.
     Visceral motor neurons innervate thoracic and abdominal viscera.
     The nuclei associated with the vagus nerve resides in the medulla and the nerve passes through the jugular foramen.

 

Accessory Nerve (N XI)
     The accessory nerve is primarily a motor nerve. It innervates the sternocleidomastoid and trapezius muscles, originate from nuclei in the cervical spinal cord,  and passes through the jugular foramen.

 

Hypoglossal Nerve (N XII)
     The hypoglossal nerve is primarily motor. It innervates intrinsic and extrinsic muscles of the tongue. Its nuclei reside in the medulla and it leaves the cranium through the hypoglossal canal.