Cranial Nerves – applied anatomy

Dr Ameer Khalid BHMS,MD(Hom)

Olfactory Nerve CN – I
Purely sensory and are concerned with smell.
The olfactory cells reside in the mucosa of the superior nasal concha & the upper part of the nasal septum.
Nerve fibre arising in this mucosa collect bundles that together constitute an olfactory nerve. The axons of these cells pass through the sieve like cribriform plate of the ethmoid bone, pierces the duramater & arachnoid of the brain to reach the overlying olfactory bulb to the medial surface of the cerebral hemisphere & the temporal lobe. The most important of these is entorhinal area made up of the uncus and the anterior part of the paraphippocampal. gyrus.

Lesions of the olfactorty Nerve results in –
Lloss of sense of smell – Anosmia (Olfactory anaesthesia). Smell is also responsible for the finer appreciation of taste.

Causes of Anosmia
1)Head Injuries : A sudden shift of the cerebral hemispheres results in avulsion of Olfactory bulb from the brain or from the cribriform plate.
The olfactory Nerve may also be damaged by a fracture of the ciribriform plate. The subarachnoid space on the cranial side of the cribriform plate is located in close approximation to the olfactory mucosa. A fracture through the cribriform plate that tears the mucosa can result in leak of CSF through the nose – CSF Rhinorrhoea.
This condition can result in meningitis caused by spread of micro organisms from the nose to CSF
2) CVA-Effusion of blood into base of frontal lobe.
3) Tumours of the Frontal lobe, or those arising near the pituitary gland
4) Infections –c/c tuberculous meningitis & other infections like common cold, Viral Hepatitis, syphilis, osteomyelitis of frontal or ethmoidal regions.
Unilateral Anosmia may be of diagnostic significance in localizing brain lessons.

Optic Nerve – CN II

• Special sensory nerve conveying information related to special senses.
• (Olfactory Nerve, optic Nerve & Cochlear Nerve)
• The nerve mediate vision.

Optic Nerves-

• Not strictly a nerve, but a tract of nerve fibres of CNS. Leave the retina at the optic disc (Blind Spot) courses through the optic canal to Optic Chiasm
• About 5cm in length; is slightly longer than the distance it travels; that it permits free movement of eyeballs.
• These tracts are complete with dura, arachnoid, subarachnoid space & pia.
• There is a myelin sheath for the fibres of this nerve, but there is no neurolemmal sheath. So there is no regeneration of Optic Nerve Fibres after injury or destructive diseases.
• Lesions of optic nerve distal to chiasm cause Ipsilateral blindness
• Some disease processes involving optic nerve can affect some fibres but spare other & instead of total blindness there usually are areas of lost function in the central or peripheral parts of fields of vision of each eye. An area of depressed function in the visual field is called a scotoma

At the optic chiasm
Here pathways from both retinas are sorted so that the same visual fields project the opposite side of the brain.

• (a) Pathways from temporal ½ of each retina remain ipsilateral. Pathways from nasal ½ cross in optic chiasm.
• (b) ) Hemianopsias HEMI =1/2 .AN – No, OPSIS –Vision

Optic chiasm sits above pituitary fossa, so that a pituitary tumour may press on optic chiasm
(i) Bitemporal Heteronymous Hemianopsia (Tunnel Vision)-Results from medial compression of optic chiasm,interrupting input from the nasal half of retina of both eyes with loss of both temporal visual fields.
(ii) Ipsilateral Nasal Hemianopsia -: Results from lateral compression of uncrossed nerves, interrupting input from temporal retina of one eye with loss of one nasal visual field. A Localised dilation or Aneurysm of Internal carotid Arterty, superior to cavernous sinus, could exert this king of pressure on optic chiasm which cause nasal hemianopsia

Optic tracts
Pass posteriorly from optic chiasm conveying common visual field to the lateral geniculate body of thalamus (LGB’s)

Optic Radiations :
From each LGB, project to the primary visual area of each occipital lobe in the region of calcarine fissure.
Lesions of optic tracts, optic radiations or occipital cortex result in Cortical Blindness with Contralateral homonymous hemianopsia.(loss of contralateral ½ of the visual fields of both eyes.)
Optic Nerve is peculiarly liable to Neuritis (Inflammation)-
Papillitis – Inflammation of optic Nerve head
Retrobulbar Neuritis – Neuritis behind the eyeball optic neuritis results in atrophy.
Retrobulbar neuritis involving the optic nerve/tract is commonly caused by Multiple sclerosis.
Optic Atrophy – Any damage of optic Nerve is followed by its atrophy. Results in diminished visual aquity or blindness.
Primary or simple optic atrophy is caused by processes that involve optic nerve such as adjacent tumours.

Tumours
Rare : Glioma & Meningioma
Papilloedema– Non inflammatory oedema of optic disc.

Vestibulocochlear Nerve-CN XIII

• A sensory nerve consisting of 2 components :
• Fractures of the middle cranial fossa which involve the internal acoustic meatus may cause permanent deafness.
• Lesions of the Cerebellopontine angle :-
• An acoustic neuroma is a slowly growing tumour that arises from Schwann cells in the sheath of nerve VIII close to the attachment of the nerve to the brainstem. Tumour exerts pressure on the lateral region of the caudal part of the pons near the cerebellopontine angle.
• At first, symptoms are those of VIII Nerve damage
• Progressive deafness, spontaneous horizontal nystagmus and if tested, absence of normal labyrinthine (Vestibular) response (By placing cold or warm water in the ear& observing the patient for nystagmus)
• Later, Cerebellar ataxia appears on the side of the lesion owing to compression of cerebellar peduncles.
• If tumour becomes extremely large, damage to spinal tract & nucleus of Nerve V can occur, abolishing corneal reflex & causing decreased pain & temperature sensibility over the face on the side of injury.
• A peripheral type of facial paralysis, also on the side of the lesion, can result from damage to fibres of Nerve VII.

Trigeminal Nerve – CN V

• Largest of the 12 cranial Nerve
• Called so because it consist of 3 main divisions
• The principal general sensory nerve to head, particularly the face and it is the motor nerve to the muscles of mastigation.

1. Ophthalmic Nerve:
Superior division of trigeminal nerve
Smallest of the 3 branches
Wholly sensory; supplies the area of skin derived from embryonic frontonasal prominence
Divides into 3 branches
a) Nasociliary
b) Frontal
c) Lacrimal
Takes part in the sensory supply to the skin of the forehead, upper lid & nose.

II . Maxillary Nerve:
Intermediate division of trigeminal
Has 3 cutaneous branches that supply the area of skin derived from the embryonic maxillary prominence

III. Mandibular Nerve :
Inferior division of Trigeminal.It has 3 sensory branches that supply the area of skin derived from embryonic manidbular prominence of first branchial arch.
Also supplies Motor Fibres to muscles of mastication.

Trigeminal Neuraligia (TIC Douloureaux) – Condition characterized by sudden attacks of excruciating pain, brought on by a mere touch in area of distribution of one of the divisions of trig nerve usually CNV2. It mimics toothache
Sensory & Motor nuclei of CNV may be involved in degenerative or other lesions in brain stem or intracranial positions of nerve may be affected by trauma or a tumour.
If motor fibres are affected, muscles of mastication will be weakened or paralysed causing deviation of mandible to affected side.
When mouth is opened, In paralysis of pterygoid muscle of one side, chin is pushed to paralysed side muscle of side.

Lesions of peripheral branches of CNV are not common, but may result from
(a) Traumatic injury to face
(b) Tumours
(c) Fractures of bones of skull
Infra orbital nerve is commonly injured in fracture of maxilla , Mandibular, Nerve may be damaged by Fracture of ramus of mandible.

Herpes Zoster – A virus infection also affects the trigeminal ganglia. Inflammation of ganglia may result in necrosis of some ganglion cells, which usually produces typical herpetic eruptions in one or more of the 3 divisions of the nerve Herpes zoster of the face usually involves the region supplied by ophthalmic nerve.
Aneurysm of Internal Carotid Artery
May involve CNV particularly lesion interior to anterior clinoid process. When aneurysm is located near the foramen lacerum, it often affects all 3 divisions of CNV

Pain arising in structure supplied by nerve may be felt in an area of skin supplied by another branch- Referred pain
(a) Caries tooth in lower jaw (supplied by inferior alveolar nerve) causes pain in ear (Auriculotemporal nerve)
(b) If ulcer of Cancer on tongue (Lingual Nerve affected) Causes pain over ear & temple)
(c) Frontal sinusitis – Sinus supplied by a branch from supra orbital nerve – pain referred to forehead (Skin supplied by supra orbital nerve)

Facial Nerve – VII
Has a large MOTOR a small sensory root
The fibres forming the motor root arise from the motor nucleus which lies in the lower part of the pons.
Facial nerve runs superficially within parotid gland before giving rise to 5 terminal branches
-Temporal
-Zygomatic
-Buccal
-Mandibular
-Cervical
It supplies the superficial muscles of the neck ie the platysma, muscles of facial expression, auricular muscles & scalp muscles (occipitofrontalis muscle.The fibres of Sensory root (also called the Nerves are the axons of cells situated in the Facial (Geniculate) Ganglion which lies within a canal in the temporal bone.

Clinical Application-
1. Peripheral facial paralysis may be caused by chilling of the face, middle ear infections, tumours, fracture & other disorders. About 75% of all Facial Nerve lesions are of the type. Types of signs & symptoms depend upon location of the lesion.
2. As the Facial nerve runs superficially within the parotid gland, it may be infiltrated by malignant cells (eg: from a carcinoma of the parotid gland). This commonly result in incomplete paralysis (paresis of the muscles of facial expression). Care must be taken to preserve the facial nerve & its branches during excision of these tumours.
3. On rare occasions, parotid inflammation (eg : Mumps) may temporarily affect the Facial Nerve.
4. There is fan like distribution of the branches of Facial Nerve. So avoid vertical incisions in the parotid gland (eg : for drainage of pus. Short incisions parallel to the courses of the branches are less likely to produce a noticeable effect on the functioning of facial muscles. Weakness rather than complete paralysis of muscle usually results from injury to the branches of facial nerve because of the overlapping distribution of its branches
5. As the Mastoid process of the temporal bone is not present at birth,facial nerve may be easily injured by forceps during delivery of a baby.
6. Following a stroke or CVA, one side of the body is paralysed however there is usually some preservation of movement of muscles of forehead on the paralysed side because superficial part of the motor nucleus of the facial nerve is bilaterally controlled from the cerebral cortex.
7. Sudden facial paralysis often follows exposure to cold- BELL’S PALSY- An infra nuclear type of paralysis . Motor supply to the muscles of one half is severely affected. Paralysis of muscles of facial expression causes an expressionless look. On that side of his face, inability to whistle, puff his cheek or close that eye.

Vagus Nerve – CN X
The vagus, as its name implies, is a wandering or vagrant nerve, in the sense that it travels far afield its branches go to many parts of the body. passes through the thorax into the abdomen, in its course give branches to lungs, heart, stomach, intestines etc.
Contains –
• Motor
• Sensory
• Secretory &
• Vasodilator fibres

Vagal Trunk branches takes part in the formation of cardiac plexus.Function of vagus nerve is to slow the heart and reduce the force of its beat.
Recurrent laryngeal nerve which is a branch of vagus is motor to all muscles of larynx except cricothyroid Damage to recurrent laryngeal nerve – will result in the cord being pulled by the cricothyoid muscle into the paramedian position. This decreases the airway.
If both recurrent laryngeal nerves are injured, vocal cord is fixed in the intermediate or cadaveric position. Airway is good but patient is unable to approximate the vocal cords , resulting in a poor voice.

Vagus& recurrent laryngeal nerve-
Damage to these nerves may produce an alternation in the voice.
Temporary paralysis of these nerves may occur as a result of post-operative oedema.
As the vagus nerve largely control the secretion of acid by parietal cells of stomach ,an excess acid secretion is associated with peptic ulcers. Section of vagus nerve (Vagotomy) as they enter the abdomen is sometimes performed to reduce acid production. Often only the gastric branches of vagus nerve are cut (selective vagotomy)
Thereby avoiding adverse effects on other organs (eg : dilatation of gall bladder)

Glossopharyngeal Nerve-IX CN
Arises from the lateral aspect of medulla just caudal to pons
Predominantly a sensory nerve, but also contains voluntary motor fibres to the stylopharyngus muscle& Preganglionic parasympathetic fibre
Lesions : Rare in isolation
Interruption of all fibres result in
(A) Loss of sensation including that of taste in posterior 1/3rd of tongue
(B) Unilateral loss of gag reflex (produced by stimulating the posterior pharyngeal wall)
(Glossopharyngeal Nerve : Afferent to that region
Vagus Nerve : Efferent to that region)
(C) difficulty with swallowing
Glossopharyngeal Neuralgia : Very rare. Paraxysms of pain felt in sensory distribution of nerve.
Glossopharyngeal Nerve to the tongue accompanies the tonsillar artery on the lateral wall of pharnyx. As the wall is thin, CN IX is vulnerable to injury during tonsillectomy. Also odema around this nerve following tonsillectomy may result in temporary loss of taste.

Accessory Nerve – CN XI
Has 2 distinct parts
1) Spinal Root
2) Cranial Root
Lesion of the nerve is rare

May be damaged by
1) Traumatic Injury
2) Tumours at the base of the skull
3) Fractures involving the Jugular foramen
4) Neck laceration
Injury to spinal accessory nerve results in paralysis of sternocleido- mastoid causing weakness on turning the head against resistance away from the paralysed muscle.
Paralysis of Trapezius muscle causes downward & outward rotation of the upper part of the scapula, sagging of the shoulder & weakness on attempting to shrug the shoulder.The normal nuchal ridge formed by Trapezius muscle is also depressed & dropping of the shoulder is an obvious sign of injury to spinal root of nerve.
Isolated Lower Motor Neuron lesion of Accessory Nerve may occur after Lymph node dissection of the neck. The function of CN XI may also be interfered with by inflamed lymph nodes in the neck. This can cause a/c torticollis (WRY NECK) or drawing of the head to one side.

Hypoglossal Nerve – CN XII

• A purely motor nerve
• CN XII supplies all the intrinsic & extrinsic muscles of the tongue except palatoglossus
• To test the nerve : – Ask the patient to protrude the tongue. In a normal person, protruded tongue lies in the midline. If the CN XII is paralysed, tongue deviates to the paralysed side.
• Trauma such as fractured jaw may injure the CN XII causing paralysis of tongue musculature.
• Neck Lacerations – often with major vessel damage, and basal fracture of skull also cause CN XII injuries. Moderate dysarthria may result.

Occulomotor, Trochlear & Abducent Nerves – III, IV, VI CN
All the 3 cranial nerves supply the muscles of the eyeball

Occulomotor Nerve
Occulo Motor = Related to movements of eye
This nerve is the motor nerve to the levator palpebrae superioris muscle of the eyelid ,the superior, medial & inferior rectus & inferior oblique muscles of the eyeball.

Lesions of the Occulomotor nerve results in-
1. Ptosis : Levator palpebrae muscle is involved & upper eyelid is in complete ptosis ie, the eye is partially closed & upper lid cannot be raised voluntarily (unlike ptosis due to lesions of cervical sympathetic trunk where the voluntary motor supply is intact).
2. Outward deviation (abduction) of the eye (External strabismus) because of the unopposed action of the lateral rectus muscle & inability to turn the eye vertically or inward.
3. Dilatation of the pupil (Mydriasis) because of the unopposed action of the radial muscle fibres of the iris, which are supplied by the sympathetic system. A light shone in either eye produces no constriction of the dilated pupil.
4. Patient is unable to look up, down or medially with the affected eye.
5 .The patient with a lesion of the occulomotor nerve complains of a drooping lid & double vision. Incomplete lesions produce partial effects. There may be some weakness of all functions, or one symptom may appear without the others (eg : dilatation of the pupil without paralysis of eye movements)
Patient with diabetes mellitus are prone to develop vascular lesions of the occulomotor nerve with loss of all functions except for papillary responses.

Abducens Nerve – VI CN
This is the motor nerve to the lateral rectus muscle. It has the longest intracranial course of cranial nerve & can be damaged in the brain stem or more often in its intracranial course.

Lesions of Abducens nerve –
Abducens nerve paralysis result in the patient being unable to turn the eyeball laterally on the affected side. The imposed pulls of the medial rectus muscles cause the eye to turn inward (adduct) thereby producing on internal strabimus. Strabrimus / Squint, is an abnormality of eye movement in which the axis of the eyes are not parallel. When strabimus occurs from a nerve VI lesion visual images do not fall on corresponding points of the left & right retinas, as a result, the images cannot be fused properly. The result is Diplopia (Double Vision), which worsens when the patient attempts to gaze to the side of the lesion. The 2 images are seen side by side, thus the disorder is termed Horizontal Diplopia.

The patient usually attempt to minimize the diplopia by rotating the head so that the chin turns towards the side of lesion Tentorial herniation may result in abducens nerve paralysis . It is seen more commonly with posterior fossa lesion, such as a slowly accumulating posterior fossa haematoma which results in reversed tentorial coning. The patient may remain conscious while the nerve palsy occurs which makes diplopia an important symptom.
A carotiocavernous fistula may also result in abducens nerve paralysis.

Trochlear Nerve – IV
The trochlea is a motor nerve which supplies the superior oblique muscle. Paralysis of superior oblique muscle –
The ocular muscles do not act individually, but contract in groups and the action of a muscle at any moment will depend on the position of the eye when muscle contraction occurs.
When the trochlear nerve is injured, there is limitation of movement when the patient is asked to look downward with the eye adducted; in this position the action of the inferior rectus is minimal while that of the superior oblique muscle should be maximal. The patient is unable to look at the tip of the nose. Complaints of vertical diplopia & tilts the head in order to align the eyes & thereby eliminate the diplopia .

References-
1.Clinically oriented anatomy by Keith.L.Moore.
2.Gray’s Anatomy
3.Grant’s method of Anatomy
4. Bailey&Love’s short practice of surgery
5.Last’s Anatomy