Cranial nerves
Cranial nerves are the twelve pairs of nerves, which originate essentially in the brain and the brainstem, as opposed to in the spinal cord. They go through holes in the skull supplying predominantly the areas of the head and neck, the vagus nerve is connected to the thorax and abdomen. Special senses like smell, eye movement, taste, hearing and balance, motor control of facial expression, eyes, mastication, swallowing, speech and autonomic control of organs vital to the body are also under the control of the cranial nerves. All the cranial nerves can be sensory, motor, or combined.
CN I – Olfactory Nerve
Olfactory nerve is a pure sensory nerve which is involved in the sense of smell. It is formed by olfactory receptor cells in the nasal mucosa and enters the cribriform plate of the ethmoid bone to get into the olfactory bulb. This is a nerve that enables an individual to sense and recognize various odors. Injury to the olfactory nerve can cause a loss of smell (anosmia), taste sensation, and appetite.
CN II -Optic Nerve
Optic nerve is a sensory nerve which transmits the visual information to the brain involving the retina of the eye. It is vital to vision, such as visual acuity, color perception, and visual fields. Another role of the optic nerve in the pupillary light reflex is significant. Damage of this nerve can lead to incomplete or total blindness, visual field defects or limitation of the pupillary reaction.
CN III- Ocular Motor Nerve
The oculomotor nerve is largely a motor nerve which regulates majority of the extraocular muscles such as superior, inferior, and medial rectus muscles, and inferior oblique muscle. It also serves the levator palpebrae superiori which lifts the upper eyelid. It also has parasympathetic fibers which regulate the constriction of the pupil and lenses accommodation. A blow to this nerve can cause ptosis, diplopia, a large pupil and ataxia of eye movement upwards or to the middle.
CN IV -Trochlear Nerve.
The trochlear nerve is a motor nerve, which serves superior oblique muscle of the eye. This muscle enables the eye to move downwards and inwards particularly when one is reading or walking down the stairs. Damage to the trochlear nerve may result in vertical diplopia and inability to look down the head, which may cause the patient to tilt the head in order to compensate.
CN V- Trigeminal Nerve
The trigeminal nerve is a mixed nerve and it is the primary sensory nerve of the face. It features three divisions namely ophthalmic, maxillary and mandibular divisions. Sensory constituent provides sensation in the face, head, cornea, mouth, and teeth. Cheshing is made possible through the motor component that innervates the mastication muscles. The trigeminal nerve can dysfunction leading to numbness of the face, absence of corneal reflex or weakness of chewing muscles.
CN VI- Abducens Nerve.
Abducens nerve is a motor nerve that supplies the lateral rectus of the eye. This muscle helps the eyeball to move sideways as it is the one that abducts the eyeball. Destruction of this nerve will lead to lack of ability to move the eye outwards resulting in medial deviation of the eye and horizontal diplopia.
CN VII- Facial Nerve.
Facial nerve is a mixed nerve that has a role in facial expression in the muscles. It also transmits the sensations of taste on the anterior 2/3 of the tongue and the parasympathetic fibers to the lacrimal, submandibular and the sublingual glands. This nerve is involved in blinking, smiling, frowning, salivation and tears. Damage to the facial nerve could lead to facial paralysis as in the Bell palsy, loss of taste, dry eyes, or drooling.
CN VIII- Vestibulocochlear Nerve
The vestibulocochlear nerve is a sensory nerve that consists of two parts. This is the cochlear part that deals with hearing and the other is the vestibular part that deals with balance and space orientation. This nerve is used in keeping posture and coordination of head and eye coordination. The damage may result in hearing impairment, tinnitus, vertigo, dizziness, and loss of balance.
CN IX -The Glossopharyngeal Nerve
One such nerve is the glossopharyngeal nerve which serves the posterior one-third of the tongue with taste and sensation. It gives sensory input also representing the pharynx and helps in the gag reflex. The motor fibers serve the stylopharyngeus muscle which aids in swallowing. It also supplies parotid salivary gland with the parasympathetic fibers. The dysfunction can lead to a problem with swallowing, loss of gag reflex, and poor taste.
CN X – Vagus Nerve
Vagus nerve is a mixed nerve, also the primary parasympathetic nerve of the thoracic and abdominal organs. It provides to the muscles of the soft palate, pharynx, and larynx, the muscles necessary in swallowing and speech. There is information on the larynx and internal organs via the sensory fibers. Vagus nerve injury can lead to hoarseness, dysphagia, voice loss, and autonomic dysregulations in the heart pace and digestion
CN XI -Accessory Nerve
The accessor nerve is a motor nerve, which serves the trapezius and sterna cleavages muscles. The muscles are those involved in the rotation of the head, flexion of the neck and elevation of shoulders. Damage to this nerve can result in deficiency in shoulder shrugging and inability to move the head against gravity.
CN XII- Hypoglossal nerve.
The hypoglossal nerve is a motor nerve, and it regulates both intrinsic and extrinsic tongue muscles. It plays a vital role in the articulation of the speech, chewing, and swallowing. Damage to this nerve causes the tongue to become weak, as well as to deviate on the side of the lesion, and to cause speech and swallowing difficulties.