By Dr. Sachin Sudhakaran and Dr. Maddie Swannack
Next Lesson - Cranial Nerves VII-XII
Core
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 any sense other than touch specifically vision, taste, smell, hearing and balance.
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 emerge from the cerebrum - olfactory and optic.
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, ascends through the foramen magnum and leaves the skull through the jugular foramen. The traditionally described cranial component arises from the medulla and joins the vagus nerve; it 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
Creative commons source by OpenStax [CC BY 4.0]
This article will cover the path and function of the first six cranial nerves.
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.
Once inside the skull, these nerve fibres coalesce to form the olfactory bulb. From the olfactory bulb the fibres travel as a single nerve called the olfactory tract which extends to 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 is usually the result of external compression or sudden trauma to the nerve, such as a fracture of the ethmoid bone caused by a road traffic accident. 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.
Function
These nerves are used in providing the sense of vision.
Pathway
Very simply put, when light enters the eye and hits the photoreceptor cells of the retina, graded electrical signals are generated which ultimately lead to action potentials in the retinal ganglion cells. The action potential is carried along the optic nerve to the visual cortex of the brain where it is interpreted as vision. The nerve is communicated via the optic canal in the sphenoid bone.
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, lateral) and nasal (closer to the nose, medial) are used in place of lateral and medial. The visual fields are also divided into 4 quadrants. Visual fields describe the orientation of objects in the patient’s vision and retina where the light from those objects is detected.
When light enters the eye, it will strike the contralateral side of the retina. 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 will hit the superior nasal visual field.
This does not mean that we perceive the world upside down or back to front as the brain corrects for this effect. Nevertheless, it is an important fact to keep in mind when assessing for lesions 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
Once the optic nerve fibres on the surface of the retina converge to form the optic nerve they travel posteriorly and converge to form the optic chiasm. At this chiasm, the axons converge, with fibres from the nasal retina decussating (swapping sides) and fibres from the temporal retina remaining on the same side. Exiting this chiasm are the optic tracts. The tracts will carry images from the ipsilateral temporal retinal image and the contralateral nasal image (from the other side), i.e. the left optic tract will carry all the information from the left side of both retinas which is from the right temporal field. 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 in the thalamus. From here, two optic radiations are projected from each optic tract, which extend to the primary visual cortex in the occipital lobe.
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 tracts carry all the information from the superior retina and thus all the images from the inferior half of vision.
Likewise the inferior tracts carry all the information from the inferior retina and thus all the images from the superior half of vision.
Pathology
Lesions of the visual tract give different patterns of visual loss depending on the site (e.g. direct compression of the optic chiasm by a pituitary tumour could cause bitemporal hemianopia). This will be covered more in the Neuroanatomy Unit.
Testing
Visual acuity tests are used to test the optic nerve, e.g. Snellen charts, checking pupillary responses to light, and checking visual fields.
It’s equally important to visualise the retina through fundoscopy by using an ophthalmoscope.
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 extortion 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 e.g. if the superior rectus is weak, the eye will be pulled inferiorly.
These are all shown in the diagram below:
Diagram - The extra-ocular 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 muscle innervates 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 to 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, through the superior orbital fissure to innervate its respective muscles.
Pathology
Pathology of this nerve is usually the result of external compression. This can be due to a number of causes including raised intracranial pressure, aneurysms and cavernous sinus thrombosis.
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, and may have a dilated (blown) pupil due to loss of parasympathetic impulses to the sphincter pupillae muscle.
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 how the eyelids have to be retracted, this is most likely due to ptosis of the left eye secondary to an oculomotor nerve lesion
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
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