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3.2. Cranial Nerves
What is a Cranial Nerve?
Welcome to the pinnacle of anatomy – the cranial nerves! The cranial nerves are peripheral nerves that largely supply the head and neck with various sensory and motor functions. These nerves have different characteristics; they can be sensory or motor in function, and general or special in nature. Many nerve fibres in the head and neck are autonomic – these regulate the vast number of glands, mucus membranes and smooth muscle fibres that are present there. The nerve fibres emerge directly from the brainstem and generally travel a very short distance to their targets. There are 12 cranial nerves. Their nerve fibre types and functions are listed below and you can see the 12 cranial nerves labelled on the brainstem in Figure 3.2.1.
The 12 Cranial Nerves
1. Olfactory Nerve
Contains special sensory neurons. This nerve detects smell molecules, called odorants, using nerve endings in the roof of the nasal cavity.
2. Optic Nerve
Contains special sensory neurons. This nerve detects light using nerve endings at the front of the retina in the eye. As the nerves on each side travel to the cerebrum, they partially cross (also called 'decussate') at a structure called the optic chiasm. This means that the left side of the cerebrum is responsible for processing the right visual fields, and vice versa.
3. Oculomotor Nerve
Contains general motor neurons. This nerve provides motor innervation to four of the six 'extraocular muscles' which move the eyeball in the eye socket, namely inferior rectus, superior rectus, medial rectus and inferior oblique. However, this nerve is also responsible for dilating (sympathetic) and constricting (parasympathetic) the pupil.
4. Trochlear Nerve
Contains general motor neurons. This nerve provides motor innervation to one of the six 'extraocular muscles', called the superior oblique muscle. This muscle is responsible for turning the eyeball down and out (also called depression and abduction).
5. Trigeminal Nerve
Contains general sensory neurons. This nerve carries sensory information from the anterior and lateral sides of the face. The fibres detect pain, temperature, touch and pressure. This nerve also contains special motor neurons – these fibres provide motor control to the muscle of mastication (i.e. the four main muscles involved in chewing, namely, temporalis, masseter, lateral pterygoid and medial pterygoid). These muscles have a special 'visceral' embryological origin.
6. Abducent nerve (also called 'abducens')
Contains general motor neurons. This nerve provides motor innervation to one of the six 'extraocular muscles', called the lateral rectus muscle. This muscle is responsible for turning the eye inwards (also called adduction).
7. Facial nerve
Contains special motor neurons. This nerve provides motor control to the muscles of facial expression (examples include the muscles surrounding the mouth and eye, namely orbicularis oris and orbicularis oculi). These muscles also have a unique 'visceral' embryological origin. This nerve also contains general motor neurons – these provide motor control to the lacrimal (eye) and certain salivary (sublingual and submandibular) glands in the mouth. This nerve also contains special sensory neurons, that receive taste information from the anterior two thirds of the tongue (i.e. the main, flat surface of the tongue that comes into contact with food during chewing).
8. Vestibulocochlear nerve
Contains special sensory neurons. This nerve receives sound information via the cochlea. It also receives balance/position information from the semicircular canals.
9. Glossopharyngeal nerve
Contains special motor neurons. This nerve provides motor innervation to one of the pharyngeal muscles, called stylopharyngeus. It also contains general visceral motor neurons that provide motor control to one of the salivary glands, called the parotid gland. This nerve also contains special sensory neurons – these receive taste information from the posterior third of the tongue (i.e. the part that lies vertically).
10. Vagus nerve
Contains general sensory neurons. This nerve receives sensory information from the thoracic and abdominal organs. It also contains general motor neurons – provide motor innervation to the thoracic and abdominal organs. Taste from the epiglottis is carried by this nerve too.
11. Spinal accessory nerve
Contains special motor neurons. This nerve provides motor control to muscles of the neck (specifically trapezius and sternocleidomastoid).
12. Hypoglossal nerve
Contains general motor neurons. This nerve provides motor control to most of the muscles of tongue (i.e. all the intrinsic and most of the extrinsic muscles. The most important extrinsic muscles are the paired genioglossi muscles).
Contains special sensory neurons. This nerve detects smell molecules, called odorants, using nerve endings in the roof of the nasal cavity.
2. Optic Nerve
Contains special sensory neurons. This nerve detects light using nerve endings at the front of the retina in the eye. As the nerves on each side travel to the cerebrum, they partially cross (also called 'decussate') at a structure called the optic chiasm. This means that the left side of the cerebrum is responsible for processing the right visual fields, and vice versa.
3. Oculomotor Nerve
Contains general motor neurons. This nerve provides motor innervation to four of the six 'extraocular muscles' which move the eyeball in the eye socket, namely inferior rectus, superior rectus, medial rectus and inferior oblique. However, this nerve is also responsible for dilating (sympathetic) and constricting (parasympathetic) the pupil.
4. Trochlear Nerve
Contains general motor neurons. This nerve provides motor innervation to one of the six 'extraocular muscles', called the superior oblique muscle. This muscle is responsible for turning the eyeball down and out (also called depression and abduction).
5. Trigeminal Nerve
Contains general sensory neurons. This nerve carries sensory information from the anterior and lateral sides of the face. The fibres detect pain, temperature, touch and pressure. This nerve also contains special motor neurons – these fibres provide motor control to the muscle of mastication (i.e. the four main muscles involved in chewing, namely, temporalis, masseter, lateral pterygoid and medial pterygoid). These muscles have a special 'visceral' embryological origin.
6. Abducent nerve (also called 'abducens')
Contains general motor neurons. This nerve provides motor innervation to one of the six 'extraocular muscles', called the lateral rectus muscle. This muscle is responsible for turning the eye inwards (also called adduction).
7. Facial nerve
Contains special motor neurons. This nerve provides motor control to the muscles of facial expression (examples include the muscles surrounding the mouth and eye, namely orbicularis oris and orbicularis oculi). These muscles also have a unique 'visceral' embryological origin. This nerve also contains general motor neurons – these provide motor control to the lacrimal (eye) and certain salivary (sublingual and submandibular) glands in the mouth. This nerve also contains special sensory neurons, that receive taste information from the anterior two thirds of the tongue (i.e. the main, flat surface of the tongue that comes into contact with food during chewing).
8. Vestibulocochlear nerve
Contains special sensory neurons. This nerve receives sound information via the cochlea. It also receives balance/position information from the semicircular canals.
9. Glossopharyngeal nerve
Contains special motor neurons. This nerve provides motor innervation to one of the pharyngeal muscles, called stylopharyngeus. It also contains general visceral motor neurons that provide motor control to one of the salivary glands, called the parotid gland. This nerve also contains special sensory neurons – these receive taste information from the posterior third of the tongue (i.e. the part that lies vertically).
10. Vagus nerve
Contains general sensory neurons. This nerve receives sensory information from the thoracic and abdominal organs. It also contains general motor neurons – provide motor innervation to the thoracic and abdominal organs. Taste from the epiglottis is carried by this nerve too.
11. Spinal accessory nerve
Contains special motor neurons. This nerve provides motor control to muscles of the neck (specifically trapezius and sternocleidomastoid).
12. Hypoglossal nerve
Contains general motor neurons. This nerve provides motor control to most of the muscles of tongue (i.e. all the intrinsic and most of the extrinsic muscles. The most important extrinsic muscles are the paired genioglossi muscles).
Summary
Cranial nerves 3, 4, 6 and 12 are purely motor nerves.
Cranial nerves 1, 2 and 8 are purely sensory nerves.
Cranial nerves 5, 7, 9, 10 and 11 are mixed (i.e. motor and sensory).
Cranial nerves 1, 2 and 8 are purely sensory nerves.
Cranial nerves 5, 7, 9, 10 and 11 are mixed (i.e. motor and sensory).
Clinical Top Tip:
Cranial Nerve Exam
A cranial nerve examination is a physical examination performed on a patient by a clinician to determine if there are any problems with the functioning of the patient's cranial nerves. This test involves asking the patient to open their jaw against resistance (CN5, responsible for the muscles of mastication, make various facial expressions (CN7, responsible for innervating the muscles of facial expression) and stick their tongue out (CN12, responsible for innervating the intrinsic and extrinsic muscles of the tongue, among other physical tests. If the patient cannot complete these tests, there may be damage (or what neuroscientists call a 'lesion') to the cranial nerves or their cell bodies in the brainstem. For example, if the patient is asked to stick out their tongue and it flops weakly to the left, there is likely to be a lesion of the left hypoglossal nerve or nucleus in the brainstem. If the patient sticks out their tongue and it is continually contracting (this is called 'spastic paralysis') and deviated to the right, there may be a lesion in the neurons running between the left cerebral cortex and left brainstem.
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