Cranial nerves Part 1

The cranial nerves are so called because they are attached to the brain inside the skull (cranium). Some are sensory, some are motor and others mixed. They leave the skull through various openings in the skull called foramina.

From their superficial attachment to the surface of the brain their fibers can be traced into the substance of the brain where they culminate in special groups of cells called nuclei (singular=nucleus).

The motor nerves originate in their nuclei and these nuclei are called the nuclei of origin. They carry signals from the brain to the target organs.

The sensory cranial nerves arise in the peripheral organs. Their cell bodies are situated outside the brain. These fibers carry impulses from the peripheral organs to the sensory nuclei in the brain. These nuclei are called the nuclei of termination. Mixed nerves have separate sensory and motor nuclei.

Olfactory nerve: These are special sensory nerves and carry sensation of smell from the receptors in the nose. The olfactory epithelium containing the special cells responsible for detecting the sense of smell is situated in the roof of the nasal cavity, over the superior concha and the corresponding part of the nasal septum.

Inhaled aromatic substances dissolve in the secretion of the olfactory epithelium and stimulate the receptor cells.

The olfactory nerves start from these receptor cells and form a network in the mucus membrane. From this network about twenty branches arise, pass through the cribriform plate of the ethmoid bone and end in the olfactory bulb.

The olfactory bulb contains the cell bodies of the olfactory nerves. The axons of these cells form the olfactory tract and end in the area of the brain responsible for appreciating the sense of smell.

This area of brain communicates with several other areas of the brain for coordinated responses. For example if you detect the smell of your favorite dish your mouth starts watering. If you smell something very offensive you feel like vomiting.

Optic nerve: Optic nerves are essential for vision. Each nerve carries impulses from the retina of the eye. The optic nerves leave the eye ball from behind and pass through an opening in the lesser wing of the sphenoid bone.

They partly join together to form the optic chiasma. The fibers then continue as optic tracts. Most of the fibers in each optic tract end in a mass of neurons called the lateral geniculate body. Some fibers end in an area of the midbrain and help in bringing about the pupillary light reflex (the pupils become smaller when bright light is focused on the eyes).

The axons of the neurons of the lateral geniculate body constitute the optic radiation. Each optic radiation join the visual cortex (the area of the brain which interprets impulses from the nerves) situated at the back of the brain.

Occulomotor nerve: This nerve has two components somatic motor and visceral motor. The somatic component supplies all the extra ocular muscles except two. There are six extra ocular muscles in each eye which control the movements of the eye ball. The nerve also supplies the muscle responsible for elevating the upper eyelid.

The visceral motor component supplies the ciliary muscles responsible for accommodation and the muscles needed for constricting the pupils. The nuclei of these nerves are situated in the midbrain.

Trochlear nerve: The nerve supplies the superior oblique muscle of the eye ball. The nucleus of this nerve is situated in the midbrain below that of the occulomotor nerve. It is a pure motor nerve.

Trigeminal nerve: This nerve has a sensory and a motor root. It is the thickest cranial nerve. As the name indicates it has three branches-ophthalmic, maxillary and mandibular.

The sensory root supplies the head and face. The fibers of the motor root travel in the mandibular branch.

The ophthalmic branch passes through the superior orbital fissure in the sphenoid bone then through the orbit to supply the skin of forehead and top of head.

The maxillary nerve passes through another opening in the sphenoid bone called the foramen rotundum and supplies the skin of cheek, upper teeth, nasal cavity, hard palate, soft palate and pharynx.

The mandibular nerve passes through another opening in the sphenoid bone called the foramen ovale. It supplies the lower teeth, skin over the temporal area, lower lip and lower part of the face. It also supplies the mucus membrane of the anterior two thirds of the tongue. The motor component supplies the muscles needed for chewing.

Abducent nerve; This nerve arises from its nucleus in the pons and passing through the superior orbital fissure supplies the lateral rectus muscle of the eye ball. It is a pure motor nerve.

Some fibers from the nucleus join the occulomotor nerve of the opposite side to supply the medial rectus muscle of the other eye. This is essential as the lateral rectus muscle of one eye and the medial rectus muscle of the other eye have to contract simultaneously if we have to look to one side.

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