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What is a Cranial Nerve?

“On Old Olympus’ Topmost Top A Fat-Eared German Viewed A Hop”. Or perhaps “Oh, Oh, Oh, Tell Teacher About Fred And Give Vic Special Honours”. Hopefully, not incoherent ramblings but two mnemonics for medical students to remember the names of the cranial nerves. More colourful mnemonics are also used! So why remember them? What are they? Why do they matter?

We have twelve pairs of cranial nerves which arise from the underside of the brain (not from

the spinal cord as other nerves do). They pass through individual holes (foramina) in the base of the skull and they supply the organs of special sense, sensation and motor (muscle activity) of the head and sensory and motor functions of various other parts of the body. We take them for granted as they work ceaselessly day in and day out and we only become aware of one or more of them if disease strikes and disrupts their function.

Each pair of nerves is individually named and, generally, the name gives a clue to what they do. They are each assigned a

number, using Roman numerals rather than Arabic numbers.

I The Olfactory Nerve These nerves mediate the sense of smell. Not only does it enable us to smell food and other pleasant, or indeed unpleasant, smells but it has a warning function (e.g. gas) and a sexual function (pheromones). Humans have about 10 sq. cm. of specialised mucous membrane lining the nose. Molecules of an odour dissolve in the mucus in the nose and are picked up by the olfactory receptors within the mucosa. The stimuli created pass along nerves and through the cribriform plate (a flat piece of bone pierced by multiple holes through which the nerves pass) in the base of the skull. From there they enter the front part of the brain in the olfactory cortex.

The loss of the sense of smell is called anosmia. It may occur as a result of temporary blockage of the nose as a result of infection such as a cold, or it may be the result of other infections or head injury.

Medicine for Managers Dr Paul Lambden BSc MB BS BDS FDSRCSEng MRCS LRCP DRCOG MHSM FRSM

Medicine for Managers articles are not intended to be a source of medical advice. Their purpose is to familiarise the non-medical reader about current key medical disorders. Any medical or medicinal products mentioned by name are examples only and should not be regarded as an endorsement of their use.

II The Optic Nerve The optic nerves are the nerves that supply vision. Damage to an optic nerve may cause blindness in one eye or may cause partial visual losses. A visual defect is called anopsia.

The actual optic tracts are complicated. Retinal (light sensitive) fibres supplying the right (outer) side of the right eye and the right (inner) side of the left eye (fibres from both eyes shown red in the illustration) come together to form the right optic tract (remember on this diagram that the brain is upside down so that the eye on the left is in fact the right eye - sorry!) Similarly, the left (outer) fibres of the left eye and the left (inner) fibres of the right eye (shown green in the illustration) come together to form the left optic tract.

Therefore, the fibres from the outer side of the right eye and the outer side of the left eye remain on the same sides. Fibres from the inner (nasal) side of the right eye and the inner (nasal) side of the left eye cross over to the other side of the brain. This point is called the optic chiasma. The fibres end in the visual cortex on each side.

Sorry this sounds so complicated.

The key significance of the optic chiasma is that the pituitary gland

lies in close proximity to the optic chiasma. If the pituitary gland becomes enlarged, as when a tumour of the pituitary develops (the pituitary adenoma), the gland applies pressure to the chiasma which affects the function of the nerves. The nerves disturbed are those which cross over from the nasal half of each retina, causing a visual defect which affects the peripheral vision in both eyes and is called bi-temporal hemianopia (pronounced: bi-tem-pour-al hem-e-an-o-pee-ah). The black areas in the diagram represent the lost are of visual field in each eye.

III Oculomotor Nerve IV Trochlear Nerve VI Abducens Nerve These three pairs of nerves are described together and control and co-ordinate eye movements. If these nerves are damaged the result is compromise of the eye movements on the affected side. In order to test them, a doctor will ask the patient to move the eyes in all directions and the speed of movement and the co-ordination is monitored

Oculomotor means ‘eye-mover’. It supplies most of the muscles that move the eye. Damage to the oculomotor (III) nerve may cause diplopia (double vision), strabismus (inability to co-ordinate eye movements), mydriasis (dilation of the pupil) and ptosis [pronounced toe-sis] (drooping of the eyelid).

The word ‘trochlear’ means ‘pulley’. It is so named because the muscle which it operates (superior oblique) hooks through a pulley-shaped ligament. Damage to the trochlear

(IV) nerve will also cause double vision but, specifically, when the eye is moved upwards and inwards because the nerve supplies the muscle which is responsible for that movement.

Damage to the abducens (VI) nerve may also cause double vision when the eye is moved outwards (hence the name because it ‘abducts’ the eye) because the function of the lateral rectus muscle, innervated by the abducens nerve is compromised.

V Trigeminal Nerve These are large and have three principal divisions.

The upper division (shown yellow in the illustration) is called the ophthalmic division (Va) and collects sensory information from the skin of the scalp, forehead, upper eyelid, nose and cornea.

The middle division (shown purple on the illustration) is called the maxillary division (Vb) and collects sensory information from the skin of the cheek and upper lip, the lower eyelid and also from the inside of the nose, the palate and the upper teeth.

The lower division (shown blue in the illustration) is called the mandibular division (Vc) and collects sensory

information from the side of the head (temporal region), the chin, part of the tongue and the lower teeth. In addition, the nerves control the function of the muscles associated with chewing (mastication) and they include the big temporal muscles on the side of the head and the powerful masseter muscles located on the side of the face. It is Vb and Vc that are of importance to dentists because it is parts of those nerves which are anaesthetised with local anaesthetic in order to allow them to carry out dental work.

Disturbances of nerve function may need to numbness or pain and it may be subject to various conditions. Shingles (herpes zoster) may affect any of the divisions but, when it involves the ophthalmic division, the characteristic blisters may affect the cornea of the eye and, in severe untreated cases, can lead to blindness. Other conditions affecting the trigeminal nerve are cluster headaches and trigeminal neuralgia. This is a very unpleasant condition, normally affecting the maxillary or mandibular divisions of the nerve. It tends to occur in older people and traditionally causes shooting pains, often described as like an electric shock, lasting only a very short time but causing considerable distress. There may be a ‘trigger area’ which can generate the pain and sufferers become anxious that a severe sharp pain will suddenly occur. The condition may be treated with carbamazepine (Tegretol) will good effect in many patients.

VII The Facial Nerve The facial nerves are mixed nerves which control the facial muscles (their motor function) but also collect sensory (taste) information from the tongue. The facial nerves have a course through the petrous temporal bone (the word petrous coming from the

Latin ‘petra’, meaning a stone, because it is so dense) and enters the face through the stylomastoid foramen and the parotid gland.

The motor part of the nerve divides into five branches where it enters the face at a point

fancifully called by bygone anatomists the pes anserinus (the goose’s foot). The five branches, called the temporal (A), the zygomatic (B), the buccal (C), the mandibular (D) and the cervical (E), supply the muscles of the head and face including the muscles of facial expression. Any disorder or damage to the facial nerve will manifest as a facial palsy. Many will know Bell’s palsy which is a partial or complete paralysis of facial muscles, which is usually only temporary but which prevents the movements of facial expressive muscles on the side of the lesion, including the forehead, the eyebrow and eyelid and the muscles around the mouth. A similar appearance may be caused by a stroke as well as by such things as tumours of the parotid salivary gland or the brain.

The sensory function of the facial nerve is confined to the collection of information from taste buds on the anterior (front) two-thirds of the tongue carried to the brain in a nerve called the chorda tympani.

VIII Vestibulocochlear Nerve (Pronounced vest-ib-you-low-cock-lee-are) The eighth cranial nerve is comprised of vestibular and cochlear fibres.

The vestibular nerve supplies the vestibular system in the inner ear, which detects balance. The vestibular apparatus senses changes in the head position. The information collected contributes, with the information provided by the eyes, to the co-ordination of balance.

If the vestibular nerve becomes inflamed (vestibular neuritis), a condition which is thought to be associated with reactivation of the herpes simplex virus, the result is vertigo, a sensation of spinning, loss of balance, nystagmus, an involuntary to-and-fro movement of the eyes, and nausea and vomiting. The condition is usually self-limiting and is treated symptomatically with drugs that damp down the vertigo and relieve the nausea and vomiting.

The cochlear nerve innovates the cochlea in the inner ear, which serves the sense of hearing. The mechanism is complex but essentially the sound generates vibration in a membrane which is converted to electrical activity. The louder the sound, the greater the vibration.

Labyrinthitis is an inflammation of the labyrinth which damages both the vestibular and the cochlear branches of the nerve. The result may be symptoms similar to those described above but with hearing loss and tinnitus, a ringing or buzzing in the ears.

IX The Glossopharyngeal Nerve (Pronounced gloss-o-far-in-jeel) The ninth cranial nerves are called ‘mixed’ nerves because they carry sensory fibres (i.e. they collect

information) from the pharynx (throat), back of the tongue and the tonsils. They also carry motor fibres which instruct the parotid salivary glands to work and innervate small muscles.

The name of the nerve is derived from the word ‘glosso’ meaning tongue and ‘pharyngeal’ being the beginning of the alimentary tract. It was first described by Galen in about 150 AD, although he thought that the IX, X and XI nerves were all one.

Damage to the nerve through injury or inflammation impairs swallowing and taste at the back of the tongue.

X The Vagus Nerve The pair of vagus nerves are the longest in the body. Because of the range of areas supplied, the vagus nerve name was derived from the Latin word vagus meaning literally ‘wandering’. They are the only cranial nerves to extend beyond the head and neck and they ‘wander’ into the thorax and abdomen.

Its function is complex and difficult to describe. The nerves supply motor parasympathetic fibres to virtually all the organs between the neck and the lower part of the colon. This

results in the vagus nerve influencing or controlling the muscles of the mouth, speech, heart rate, and peristalsis (the rhythmic contractions of the bowel which move the contents

along). It is also involved in such functions as vomiting and the feeling of fullness after eating. It also has a sympathetic function controlling and increasing blood pressure.

If damaged it may lead to a variety of symptoms ranging from hoarseness and difficulty swallowing to impaired bowel activity.

XI The Accessory Nerve

This pair of nerves travel from the skull and down the neck to supply motor functions to the large sternomastoid muscle (a powerful prominent muscle in the neck on each side) and the

trapezius muscles (named according to their shape from the word trapezoid) which are attached to the skull, the spine and shoulder blade on each side. The trapezius moves the shoulder blade and supports the arm.

XII The Hypoglossal Nerve

The hypoglossal nerves provide innervation for virtually all the muscles of the tongue. Its sole function is to make the muscles work.

It is important to achieve speech and swallowing and it controls other tongue movements including sticking the tongue out. Its function is disturbed

The name ‘hypoglossal’ comes from the Latin meaning ‘below the tongue’ which is descriptive of the course that it runs from the brain.

So the Cranial Nerves, generally named according to their functions, maintain the host of functions of the head, face and neck, controlling the organs of special sense and managing the finer tasks that make the species successful; vision, speech, smell and hearing. Damage to the nerves causes incapacity and increases the vulnerability of affected individuals. They are truly amazing.

paullambden@compuserve.com