Cranial Nerves I-VI

1. Introduction to Cranial Nerves
2. Cranial Nerve I - Olfactory
3. Cranial Nerve II - Optic
4. Movement of the Eyeball - Cranial Nerves III, IV and VI
5. Cranial Nerve III - Oculomotor
6. Cranial Nerve IV - Trochlear
7. Cranial Nerve VI - Abducens
8. The H Test
9. Cranial Nerve V - Trigeminal
10. Quiz

• There are 12 pairs of cranial nerves. I and II are associated with the forebrain (the front part of the brain, which includes the cerebrum); III and IV emerge from the midbrain (the top of the brainstem). V emerges from the pons (the middle of the brainstem), whereas VI, VII and VIII emerge at the pontomedullary junction (where the pons meets the medulla). IX, X and XII emerge from the medulla (the lowest part of the brainstem, which continues into the spinal cord). The spinal component of XI arises from the upper cervical spinal cord; its traditional cranial component joins the vagus nerve.
• The cranial nerves have unique motor, general sensory (touch, pain and temperature) and special sensory (vision, taste, smell, hearing and balance) functions. Cranial nerves may have a function which is purely motor, purely sensory, special sensory or a combination.
• The olfactory nerve has a special sensory function in facilitating the sense of smell (olfaction).
• The optic nerve has a special sensory function in facilitating the sense of vision.
• The oculomotor, trochlear and abducens nerves supply motor innervation (the nerve signals that make muscles move) to muscles within the orbit (the eye socket) to control the movements of the eye.
• The trigeminal nerve supplies sensation to the face, and motor innervation to the muscles of mastication (chewing).

Introduction to Cranial Nerves

The cranial nerves are 12 pairs of nerves with various motor, sensory, special sensory and autonomic functions. Most are connected with the brain or brainstem, but the spinal component of the accessory nerve arises in the upper cervical spinal cord.

For clarity, a special sensory function is the carrying of one of the special senses: vision, taste, smell, hearing and balance. General sensory functions carry other sensations, such as touch, pain and temperature.

The 12 cranial nerves and their functions are as follows. Helpful tips for remembering these are shown in the blue columns:

Table - Cranial nerve functions and apparent emergence or attachment. V emerges from the pons; VI, VII and VIII emerge at the pontomedullary junction. Spinal XI arises in the upper cervical spinal cord. The functional categories are broad summaries; III, VII, IX and X also carry parasympathetic fibres.

SimpleMed Original by Dr. Sachin Sudhakaran

The image below shows the anatomical origins of the cranial nerves.

The first 2 nerves, the olfactory and optic nerves, arise from the forebrain (the front part of the brain), not the brainstem.

The next 2 nerves emerge from the midbrain - oculomotor and trochlear.

The trigeminal nerve (V) emerges from the pons itself. The abducens (VI), facial (VII) and vestibulocochlear (VIII) nerves emerge at the pontomedullary junction, where the pons meets the medulla.

The glossopharyngeal (IX), vagus (X) and hypoglossal (XII) nerves emerge from the medulla. The spinal accessory nerve (XI) arises from the upper cervical spinal cord and ascends through the foramen magnum (the large opening at the base of the skull). It then leaves the skull through the jugular foramen (another opening in the skull base). The traditionally described cranial component arises from the medulla and joins the vagus nerve; this does not make spinal XI a medullary nerve.

Diagram - Inferior view showing cranial nerve attachments. VI, VII and VIII attach at the pontomedullary junction; the ascending spinal roots of XI arise in the cervical spinal cord. The traditional cranial component of XI joins X (the vagus nerve).

Creative commons source by OpenStax [CC BY 4.0]

This article will cover the path and function of the first six cranial nerves.

Cranial Nerve I - Olfactory

Function

These nerves are used in providing the sense of smell (olfaction).

Pathway

Found at the roof of the nasal cavity, fibres from the olfactory nerve pass through the cribriform foramina (literally ‘sieve holes’) of the cribriform plate. This plate is a feature of the ethmoid bone of the skull, a thin bone between the nose and the brain.

Once inside the skull, these nerve fibres enter the olfactory bulb, an oval structure where they synapse (connect) with the next nerve cells in the pathway. From the olfactory bulb, a bundle of fibres called the olfactory tract runs backwards to the primary olfactory cortex (the area that first receives smell signals), which lies in the temporal lobe of the brain.

Diagram - Illustration showcasing the path of the olfactory nerve from the roof of the nasal cavity. Remember that the olfactory tract extends posteriorly to the temporal lobe

Creative commons source by Patrick J. Lynch [CC BY-SA 4.0 (https://creativecommons.org/licenses/by-sa/4.0)]

Pathology

Pathology of this nerve has several causes. Anything that blocks odours from reaching the nerve can cause loss of smell, for example inflammation, infection or nasal polyps (soft swellings in the lining of the nose). When the nose and sinuses are not involved, infection and head injury are the most common causes. Sudden trauma to the nerve, such as a fracture of the ethmoid bone in a road traffic accident, is one example. External compression, for example by a tumour near the olfactory bulb, can also damage the nerve. Affected patients will report a loss of smell (anosmia).

Testing

To test this nerve, ask the patient about any change in their sense of smell and, where appropriate, test each nostril separately using a familiar, non-irritant odour.

Cranial Nerve II - Optic

Function

These nerves are used in providing the sense of vision.

Pathway

Very simply put, light enters the eye and hits the photoreceptor cells (the light-detecting cells) of the retina (the light-sensitive layer at the back of the eye). This generates graded electrical signals, which ultimately lead to action potentials in the retinal ganglion cells (the nerve cells whose axons form the optic nerve). The action potentials travel along the optic nerve and then along the rest of the visual pathway (described below) to the visual cortex of the brain. There they are interpreted as vision. The optic nerve leaves the orbit (the eye socket) through the optic canal, a passage in the sphenoid bone (the butterfly-shaped bone at the base of the skull).

To properly understand how the optic nerves of a patient are functioning it is important to understand the concept of visual fields.

Visual Fields

When describing the eye, the terms temporal (closer to the temporal bone, on the outer side) and nasal (closer to the nose, on the inner side) are used in place of lateral and medial. The visual fields are also divided into 4 quadrants. A visual field is the area of the world that a person can see. Each part of the visual field is detected by a matching part of the retina.

When light enters the eye, it will strike the contralateral side of the retina, meaning the side opposite to where the light came from. As shown in the diagram below, light from an object to the side of the patient (temporal vision) will hit the nasal surface of the retina, and vice versa.

To further complicate things the same occurs with light coming from above or below the patient. Light coming from above the patient will hit the inferior surface of the retina, and vice versa.

In summary, light from an object in the inferior temporal visual field (below and towards the temple) will hit the superior nasal part of the retina.

We do not perceive the world upside down or back to front, because the brain corrects for this effect. Nevertheless, the way light lands on the retina is an important fact to keep in mind when assessing for lesions (areas of damage) along the visual pathways.

Diagram - How light enters the eye and hits the contralateral side of the retina, and how this leads to the visual fields corresponding to the retinal areas (with the same areas shown in the same colours), e.g. the orange object is above and lateral to the patient, but the image is seen on the inferior nasal quadrant

SimpleMed original by Dr. Maddie Swannack

The optic nerve fibres on the surface of the retina converge to form the optic nerve. The two optic nerves then travel posteriorly to the optic chiasm (the point where they meet). At this chiasm, fibres from the nasal retina decussate (swap sides) and fibres from the temporal retina remain on the same side. Exiting this chiasm are the optic tracts. Each tract carries information from the ipsilateral temporal retina (on the same side as the tract) and the contralateral nasal retina (in the opposite eye). For example, the left optic tract carries the information from the left half of both retinas. Together these detect the right visual field (the right-hand half of what both eyes see). This is shown in the image below.

Diagram - The pathway of the visual system

Creative commons source by Wiley [CC BY-SA 4.0 (https://creativecommons.org/licenses/by-sa/4.0)]

The optic tracts extend to the lateral geniculate nuclei (the relay stations for vision, one on each side) in the thalamus. From each lateral geniculate nucleus, two bundles of fibres called optic radiations run to the primary visual cortex in the occipital lobe (at the back of the brain).

There are two radiations - the superior optic radiations which travel through the parietal lobes and the inferior optic radiations which travel through the temporal lobes. The superior radiations carry fibres from the superior retina, and so from the inferior half of the visual field.

Likewise the inferior radiations carry fibres from the inferior retina, and so from the superior half of the visual field.

Pathology

Lesions (areas of damage) along the visual pathway give different patterns of visual loss depending on the site. For example, direct compression of the optic chiasm by a pituitary tumour could cause bitemporal hemianopia, which is loss of the outer half of the visual field in both eyes. This will be covered more in the Neuroanatomy Unit.

Testing

The optic nerve is tested in several ways: visual acuity, colour vision, pupillary responses to light, and the visual fields. Visual acuity means how sharp the vision is, for example with Snellen charts (which are letter charts).

It is equally important to look at the retina through fundoscopy (examining the back of the eye), using an ophthalmoscope (a handheld instrument with a light and lenses).

Movement of the Eyeball - Cranial Nerves III, IV and VI

Primary Functions

There are six muscles that control movement of the eyeball. The four easiest to understand are called superior rectus, inferior rectus, medial rectus and lateral rectus. When they contract, they pull the eyeball in the direction of their name e.g. the superior rectus moves the eye superiorly (looking upwards).

The two remaining muscles are called the superior oblique and inferior oblique. These muscles attach to the eye at an oblique angle. As a result, when they contract, they rotate the eyeball. This means that superior oblique is responsible for the intorsion (internal rotation) of the eyeball, and inferior oblique is responsible for the extorsion (external rotation) of the eyeball. This movement is key in keeping the visual image stable when the head is tilted to the side.

All the muscles work antagonistically to each other to keep the eye in position. If a muscle loses its tone, the other muscles will pull away from it and move the eye out of position. For example, if the superior rectus is weak, the eye will be pulled inferiorly.

These are all shown in the diagram below:

Diagram - The extra-ocular (eye-moving) muscles of the right eye. Note the oblique paths of the superior and inferior oblique muscles

Creative commons source by OpenStax College [CC BY-SA 4.0 (https://creativecommons.org/licenses/by-sa/4.0)]

Secondary Functions

Some of the muscles also have secondary directions of pull, shown in the table below:

Table - The secondary functions of the extraocular muscles

This table can be quite complicated - the important thing to remember is the primary function.

Also remember that if the function of a muscle is lost, the eye will move in the opposite direction because the pull of the other muscles is unopposed.

Innervations

The six muscles which move the eye are innervated by one of three cranial nerves.

• Superior Rectus - Oculomotor (III)
• Superior Oblique - Trochlear (IV)
• Medial Rectus - Oculomotor (III)
• Inferior Rectus - Oculomotor (III)
• Inferior Oblique - Oculomotor (III)
• Lateral Rectus - Abducens (VI), so called as the lateral rectus abducts the eye

Which nerve supplies which muscle is difficult to remember. The mnemonic ‘LR6 SO4’ may act as an aid. This will help with knowing that the lateral rectus is innervated by cranial nerve VI (abducens) and that superior oblique is innervated by cranial nerve IV (trochlear). The rest are innervated by cranial nerve III (oculomotor).

Cranial Nerve III - Oculomotor

Function

The oculomotor nerve innervates four of the extra-ocular muscles (listed above and shown in image below). In addition, it innervates levator palpebrae superioris (responsible for elevating the eyelid) and sphincter pupillae (constricts the pupil). It also carries autonomic parasympathetic fibres.

Diagram - The extraocular muscles of the eye. The highlighted muscles are innervated by the oculomotor nerve

Creative commons source by OpenStax College, edited by Dr. Sachin Sudhakaran [CC BY-SA 4.0 (https://creativecommons.org/licenses/by-sa/4.0)]

Pathway

Originating from the midbrain, fibres from the oculomotor nerve travel through the cavernous sinus (a blood-filled channel beside the pituitary gland). They then pass through the superior orbital fissure (a gap at the back of the orbit) to reach the muscles that the nerve supplies.

Pathology

Pathology of this nerve has several causes. In a population-based study from the USA, the most common cause was presumed microvascular disease, at 42% of cases. This means damage to the small blood vessels that supply the nerve, for example in diabetes or high blood pressure. Next came trauma (12%), external compression by a tumour (11%), brain surgery (10%) and compression by an aneurysm (a bulge in an artery wall, 6%). Raised intracranial pressure (pressure inside the skull) and cavernous sinus thrombosis (a blood clot in the cavernous sinus) can also affect the nerve.

Patients with pathology of this nerve usually have diplopia (double vision) and severe ptosis (drooping of the upper eyelid), due to loss of innervation to the levator palpebrae superioris muscle. They may also have a dilated (blown) pupil, due to loss of parasympathetic impulses to the sphincter pupillae muscle. A dilated pupil is more likely when the nerve is compressed (for example by an aneurysm) than in microvascular disease, but a normal pupil does not rule out compression. A new third nerve palsy needs urgent medical assessment, especially if the pupil is dilated.

The patient will present with the affected eye in a ‘down and out’ position (lateral and inferior) due to the paralysis of four of the extra-ocular muscles. This leaves only lateral rectus and superior oblique functioning, pulling the eye laterally and inferiorly.

Image - Patient presenting with a ‘down and out’ position of the left eye. Notice that the eyelids have to be retracted (pulled open); this is most likely because of ptosis of the left eye secondary to an oculomotor nerve lesion (damage to the nerve)

Creative commons source by Wang Y, Wang XH, Tian MM, Xie CJ, Liu Y, Pan QQ, Lu YN [CC BY-SA 4.0 (https://creativecommons.org/licenses/by-sa/4.0)]

Testing

To test this nerve:

• Inspect the eyelids and pupils (check for asymmetry)
• Test the eye movements (using the H test - see below)
• Test the pupillary reflexes

Preview the Cranial Nerves I-VI quiz