Cranial Nerves

Nerves supplying the body can be divided in to cranial and spinal. Cranial nerves emerge from the brain or brain stem and spinal from the spinal chord. There are 12 pairs of cranial nerves. They are components of the peripheral nervous system, with the exception of the optic nerve, as their axons extend beyond the brain to supply other parts of the body. They are named numerically from region of the nose (rostral) to back of the head (caudal). Here’s a brief overview of all twelve nerves and their basic functions.

I – The Olfactory Nerve. The cells of this nerve arise from the olfactory membrane of the nasal mucosa. The dendrites of the nerve cells project in to the olfactory mucosa. The axons of these cells combine to form the olfactory nerve. They join the brain at the olfactory bulb, located at the end nearest the nose. The fibres are short and lie deep and protected from casual injury. It is often found that loss or interference of sense of smell is due to blockage of the air passage leading to the olfactory mucosa, not due to nerve damage.

II – The Optic Nerve. This nerve connects the retina to the diencephalon of the brain. It is the only cranial nerve considered to be part of the central nervous system. This means the fibres are incapable of regeneration, hence why damage to the optic nerve produces irreversible blindness. Interestingly the eye's blind spot is a result of the absence of photoreceptor cells in the area of the retina where the optic nerve leaves the eye. I find the optic nerves easy to spot when looking at the brain from below as they form the optic chiasm. This is the point at which they cross and forms a clear ‘x’.

III- The Oculomotor Nerve. This nerve controls most of the eye’s movements including the constriction of the pupil and levitation of the eyelid. Damage to the nerve can cause double vision and inability to open the eye. A symptom of damage to this nerve is tilting of the head.

IV – The Trochlear Nerve. This nerve is a small somatic motor nerve and innervates the dorsal oblique muscle of the eye, responsible for allowing the eye to look down and up as well as internal rotations. Damage to the nerve can cause one eye to drift upwards in relation to the undamaged eye, meaning patients tilt their heads down to compensate.

V – The Trigeminal Nerve. This is the largest cranial nerve and is so called as it has three major divisions. It is sensory to the skin and deeper tissue of the face and motor to certain facial muscles, playing a large role in mastication.

VI – The Abducent Nerve. This nerve controls the movement of the lateral rectus muscle of the eye. It also plays a role in eye retraction for protection. Injury produces the inability to deviate the eyeball away from the midline of the body.

VII – The Facial Nerve. This nerve innervates the muscles of facial expression. It also functions in the conveyance of taste sensations from the front two thirds of the tongue. As well as this it can increase saliva flow through certain salivary glands.

VIII – The Vestibulocochlear Nerve. This nerve is named after the vestibular and cochlear components of the inner ear. It transmits information on sound and balance. Damage can lead to deafness, impaired balance and dizziness.

IX – The Glossopharyngeal Nerve. This nerve has any roles including the innervation of certain muscles of the palate of the mouth, certain salivary glands and the sensory mucosa of the root of the tongue, palate and pharynx. Damage can lead to difficulty swallowing as well as the loss of ability to taste bitter and sour things in humans.

X – The Vagus Nerve. This is a very important nerve and one frequently discussed when considering many important systems within the body. It is the longest of all cranial nerves and extends to supply the pancreas, spleen, kidneys, adrenals, and intestine. It has parasympathetic control of the heart and digestive tract as well as certain glands and involuntary muscles.

XI – The Accessory Nerve. This plays a role in neck turning and elevation of the scapula (shoulder). Muscle atrophy of the shoulder region indicates damage to this nerve.

XII – The Hypoglossal Nerve. This nerve’s name relates to the fact that is runs under the tongue, innervating the tongue’s internal and external musculature. It has important roles in speech, food manipulation and swallowing.