There are 12 cranial nerves, which originate in the brain, in contrast to the 31 spinal nerves which originate in the spinal cord. The cranial nerves along with the spinal nerves are parts of the peripheral nervous system. Each cranial nerve has a distinct numbering and a distinct function.
The first nerve is olfactory nerve or cranial nerve I. It is a purely sensory nerve which mediates the sensation of smelling. It originates in the ethmoid bone of the nasal cavity, and it ends in the brain, in the olfactory bulb. It synapses there with axons that extend to the olfactory part of the temporal cortex. The olfactory bulbs are situated at the bottom of the frontal lobe.
Cranial nerve II or the optic nerve is also a purely sensory nerve that originate in the rods and cons of the retina and ends in the lateral geniculate nucleus of the thalamus (see the fuctions of thalamus). There it synapses with axons that terminate in the visual cortex of the occipital bone. There are two optic nerves which merge to form the optic chiasm. Each optic nerve transmits visual signals to the opposite side of the visual cortex. The optic nerve passes through the optic foramen of the skull.
In addition to the optic nerve which mediates visual sensation from the eyes, there are also three other nerves that supply motor innervation to the voluntary muscles of the eye. These nerves are: the oculomoter nerve or cranial nerve III, the trochlear nerve or cranial nerve IV, and the abducens nerve or cranial nerve VI. Each one of these nerves supplies different type of eye muscles.
The oculomoter nerve has both sensory and motor functions. Its main function is motor innervation. It passes through the superior orbital fissure. It supplies several muscles of the eye. Besides, it supplies parasympathetic innervation to the ciliary muscle of the eyeball.
The trochlear nerve or cranial nerve IV is the smallest of the cranial nerves. It has both sensory and motor function. Its main function is however motor innervation. It supplies motor innervation to the eye muscle that is called the superior oblique muscle. This nerve passes also through the superior orbital fissure.
The abducens nerve or cranial nerve VI has long fibers inside the cranial cavity. Therfore this nerve can be affected by many clinical conditions. It supplies the lateral rectus muscle. Pathology to this nerve makes the lateral movement of the eye not possible.
Another nerve that one of its branches is related to structures in the eye is the trigeminal nerve or cranial nerve V. This nerve is the largest of all the cranial nerves. Its branches are the ophthalamic nerve and the maxillary nerve and the mandibular nerve. The trigeminal nerve is mainly sensory nerve but has motor function as well.
The motor function of the trigeminal nerve is related to function of the muscles of mastication and chewing. The maxillary and mandibular nerves are related clinically to dentistry, where their branches are usually anesthesized during dental surgery.
The facial nerve or cranial nerve VII is both motor and sensory nerve, which supplies the muscles of the face with innervation. In addition, it supplies the anterior two thirds of the tongue with innervation.
The vestibulocochlear nerve or cranial nerve VIII is both sensory and motor nerve. It has two branches which are called: the vestibular nerve and the cochlear nerve. The cochlear nerve is related to the sensation of hearing and the vestibular nerve is related to the sensation of body equilibrium and vertigo.
The hypoglossal nerve or cranial nerve XII is both sensory and motor nerve that supplies innervation to the muscles of the tongue.
The accesory nerve or cranial nerve XI is both sensory and motor nerve that supplies two muscles of the neck with innervation. These two muscles are the sternocleidomastoid muscle and the trapezius muscle. These muscles are responsible for rotating the neck and raising the shoulder. Any damage to this nerve in manifested clinically as dysfunction of these two muscles.
The vagus nerve or cranial nerve X is both sensory and motor nerve. It is the longest of all the cranial nerves. It has autonomic function related to gastrointestinal organs in addition to sensory function.
The glossopharyngial nerve or cranial IX is both motor and sensory nerve. It has functions related to the mechanism of swallowing, in addition to motor innervation to the stylopharyngeous muscle which raises the pharynx and larynx.