Testing Cranial Nerves
Introduction
Cranial nerves arise from the brain directly (unlike spinal nerves which arise from the spinal cord). There are twelve pairs of cranial nerves, varying in length – from supplying nearby structures of the head to the Vagus nerve (X) which is the longest nerve in the body.
Cranial nerves may carry:
- Sensory information only, i.e. information from an organ to the brain
- Motor information only, i.e. information from the brain to an organ
- Both sensory and motor information
Cranial nerves may also be either:
- Afferent – Meaning to carry sensory information into the central nervous system
- Efferent – Meaning to carry motor information away from the central nervous system
| Cranial Nerve | Type of Nerve Fibre | Function |
| Olfactory (I) | Sensory | Carries sensory information from the olfactory bulb to the brain |
| Optic (II) | Sensory | Carries sensory information from the eye to the brain |
| Oculomotor (III) | Motor | Enables the eye to make small, intricate movements |
| Trochlear (IV) | Motor | Supplies the extrinsic muscles of the eye |
| Trigeminal (V) | Both | Receives sensory information from the face and supplies motor fibres involved in mastication |
| Abducens (VI) | Motor | Supplies the extrinsic muscles of the eye |
| Facial (VII) | Both | Supplies motor fibres for facial movements and receives sensory information from ‘anterior taste’ |
| Vestibulocochlear (VIII) | Sensory | Carries sensory information from the vestibule (balance) and cochlear (hearing) of the inner ear |
| Glossopharyngeal (IX) | Both | Carries sensory information from posterior taste (posterior tongue and pharynx) and supplies muscle fibres of the pharynx |
| Vagus (X) | Both | Carries sensory information from the pharynx and larynx. Supplies muscle fibres of the larynx as well as; visceral motor fibres to the heart and various thoracic and abdominal organs (including the gastrointestinal tract) |
| Accessory (XI) | Motor | Supplies muscle fibres of the neck and shoulders |
| Hypoglossal (XII) | Motor | Supplies muscle fibres of the tongue |
Testing Cranial Nerves
There are certain tests which can be done to ensure that a cranial nerve is working properly. The tests differ between the nerves due to their different functions. Each test usually has a reflex response which signifies that the cranial nerve is undamaged. The tests have been written primarily with animals in mind, but the majority of these are also observable in humans.
| Cranial Nerve | Test of Afferent Nerve | Test of Efferent Nerve |
| Olfactory (I) | A strong smell is used to test the aversion reflex. If the cranial nerve was undamaged the subject would respond to the smell | |
| Optic (II) | Avoiding creating air movement, a finger or hand is thrust towards the eye. If the optic nerve is undamaged, the subject will employ the menace reflex and close the eyelid in response to the finger/hand | |
| Oculomotor (III) | Testing eye muscles- Usually tested alongside nerves IV & VI, the movement of the eye and eyelid is observed in response to a stimulus. If this nerve is damaged, the pupils of the eye at rest point down & out
Pupillary reflex- Shining a light into the pupil of one eye should result in the constriction of both pupils |
|
| Trochlear (IV) | Tested alongside nerve III & VI, if this nerve is damaged a strabismus (abnormal eye alignment) in an up & in direction will be apparent | |
| Trigeminal (V) | Touching the skin around the eye will result in the palpebral reflex (closing of the eyelids in response to the touching of the skin). If the nerve is damaged, this will not occur. Also, touching the cornea itself should result in the corneal reflex (closing of the eyelids in response to the touching of the cornea). Again this is absent if the nerve is damaged | Should the efferent nerve become damaged, you will be able to observe a drooping jaw in the subject |
| Abducens (VI) | Tested alongside nerve III & IV, if this nerve is damaged a strabismus in a medial, inward direction will be apparent | |
| Facial (VII) | The corneal reflex may be tested to check for damage to the nerve. In animals with motile pinna (external ear – not motile in humans), the handclap reflex can be tested. If the nerve is not damaged the pinna will move in response to a loud clap | If the efferent nerve is damaged, drooping ears and facial paralysis may be observed. Ptosis (drooping of the eyelid) can also be observed. The menace and palpebral reflexes may be tested to check for nerve damage |
| Vestibulocochlear (VIII) | Testing the Cochlea- The handclap reflex is tested. If the pinna do not respond, this may indicate damage to the nerve.
Testing vestibular responses- In response to altering the orientation of an animal i.e. tilting the body down to face the floor slightly, the neck will self right the head so the head is facing forwards if the nerve is undamaged (tonic neck reflex). If the nerve is damaged, animals may tilt their head with the ear down on the side of lesion/damage. Further observations include nystagmus – spontaneous eye movement, moving slowly in a lateral direction and then returning with a quick eye movement. The direction of the slow movement indicates the side of the lesion/damage |
|
| Glossopharyngeal (IX) | Bilateral damage to the nerve results in the loss of the gag reflex. Observations that this nerve is damaged include dysphagia – difficulty swallowing | |
| Vagus (X) | Similarly to nerve IX, lack of the gag reflex and observing dysphagia can indicate damage. Laryngeal paralysis can be observed with damage, this can cause loss of ability to speak/bark etc. and loud noises when inhaling. Other respiratory and cardiovascular anomalies may arise if damaged. | |
| Accessory (XI) | ||
| Hypoglossal (XII) | If the nerve is damaged, minor dysphagia and a drooping tongue may be observed – often drooping to the side of damage/lesion if damage is unilateral |
VetSci 