Cranial Nerves

Cranial nerves are 12 pairs of nerves seen on the ventral surface of the brain. It is a known fact that brains have control over the body and its functions in humans and animals. Based on the instructions sent by the brain to different body parts through nerves, body parts act accordingly. The signals through nerves are very fast to make the body parts react immediately. The nerves that carry the signals are dedicated to their work only. If a nerve is to carry the signals for hands, it cannot carry signals for legs. 12 pairs of nerves are dedicated to different parts of the body. These nerves, known as Cranial Nerves, are the part of the central nervous system that is located above the first vertebrae of the vertebral column.

Cranial nerves are found in pairs, one carries the signal to the body part and the other one takes the signals back to the central nervous system. Because of this arrangement, sending the to and fro signals become very fast and the reaction time is very less.

Types and Functions of Cranial Nerves

There are 12 pairs of cranial nerves and they are described with roman numerals from I to XII. Some scientists consider terminal nerves as the 13^th pair of cranial nerves and term it as zero or terminal nerve. Though there are no proofs available researchers hypothesize that this terminal nerve may trigger hormonal responses and regulate the production and secretion of the sex steroid hormones, estrogen (in females) and testosterone (in males). The 12 cranial nerves (CN) and their functions are:

1. CN I – Olfactory nerve: Sense of smell
2. CN II – Optic nerve: Sense of vision
3. CN III – Oculomotor nerve: Movement of muscles like upper eyelids, moving of eyeballs. These muscles automatically adjust the pupil depending intensity of the source of light. If the light is bright, pupillae muscles constrict the pupil to allow adequate light to enter the eye and in dark these muscles relax the pupil to allow more light to see.
4. CN IV- Trochlear nerve: This nerve also involves the eye’s movement in an inward or downward direction.
5. CN V – Trigeminal nerve: This nerve has both motor and sensory functions. The motor function helps to chew and clench the teeth whereas its sensory function gives three-way sensations. It’s ophthalmic (V1) part gives sensation to the eyes (cornea), eyelids, forehead, mucosa in the nose and skin on the nose. Also, its maxillary (V2) part gives sensation to upper teeth, side of the nose, the lower eyelid and middle third of the face. Its mandibular (V3) part gives sensation to lower teeth, the lower third of the face, the tongue, mucosa of the mouth and side of the nose.
6. CN VI – Abducens nerve: This nerve helps in the movement of the eye. It controls the lateral rectus muscle that turns the gaze outward.
7. CN VII – Facial nerve: This is a facial nerve and has both motor and sensory functions. Its motor function controls and produces facial expressions and its sensory function gives the sensation of taste and external ear.
8. CN VIII – Vestibulocochlear nerve: this nerve controls hearing and balance. This nerve has two components, vestibular and cochlear nerves. The vestibular nerve senses the change in the position of the body against gravity and due to this, the body maintains balance. The cochlear nerve helps to hear and with help of specialized inner hair cells and vibration in the basilar membrane, it determines the frequency and magnitude of the sound.
9. CN IX – Glossopharyngeal nerve: This nerve has both motor and sensory functions. The motor function helps stylopharyngeus muscle to widen or shorten the throat. Its sensory function receives sensation from the throat, tonsils, middle ear and back of the tongue. This also senses the taste from the back of the tongue.
10. CN X – Vagus nerve: This nerve plays multifunctional roles. It has sensory, motor and parasympathetic functions. The sensory function receives sensation from the outer part of the ear, the throat, the heart and abdominal organs. It also helps to sense the taste. The motor function supports the movement to the throat and soft palate. The parasympathetic function helps to regulate heartbeats, muscles of the airway, lungs and gastrointestinal tract.
11. CN XI – Accessory nerve: This nerve has a motor function that controls the sternocleidomastoid and trapezius muscles that regulate the shoulder elevation head-turning functions.
12. CN XII – Hypoglossal nerve: This nerve helps in the movement of the tongue.

Disorders and Malfunctioning in Cranial Nerves

All the 12 cranial nerves provide sensory and motor functions to the head and neck. Though some nerves help at the common part of the body their functionalities are different from each other. Any neurological or psychiatric disorders can be identified by testing their functionalities. Many causes prohibit these nerves to function properly. As all the nerves are in the brain, the major cause of dysfunction of nerves includes tumour, intracranial or skull base cancer, intracranial pressure, the mass effect of an intracerebral haemorrhage.

The problem in smelling disorder occurs due to the damage in the olfactory nerve (CN I). Damages in CN II, CN III, CN IV and CN VI nerves create the problem of double vision. Damages in the CN III nerve also cause an inability to coordinate and movements in both eyes. Also, damages in the CN IV nerve cause a problem in eye movement in the downward direction. The damage in CN V (Trigeminal) nerve leads to loss of sensation in the affected area. Patients having damage in the facial nerve (CN VII) are not able to move muscles of either or both sides of the face. Due to this, they have a drooping mouth on the affected side and face problems in chewing from the affected face jaw.

Patients with abnormality in the vestibulocochlear nerve (CN VIII) can feel a sensation of spinning and dizziness and damage in the cochlear nerve can cause partial or complete deafness in the affected ear. Difficulties in swallowing (damage in CN X nerve) and failure of the gag reflex (damage in CN IX nerve) can create problems at the time of eating or swallowing something.

Frequently Asked Questions on Cranial Nerves

Q.1: What are the functions of cranial nerves?

Answer: Cranial nerves have both motor and sensory functions. Some of these nerves help the person to see, taste, hear and smell. While other nerves help to control the muscle movements in the head and neck. There are 12 numbers of cranial nerves and each has its specific name based on its functionality. These are numbered based on roman numerals. They are Olfactory (I), Optic (II), Oculomotor (III), Trochlear (IV), Trigeminal (V), Abducens (VI), Facial (VII), Vestibulocochlear (VIII), Glossopharyngeal (IX), Vagus (X), Accessory (XI) and Hypoglossal (XII) nerves.

Q.2: What are problems a patient may face when there is damage in the accessory nerve (CN XI)?

Answer: Patient suffering from damage in accessory nerve may not be able to shrug the shoulder and feel the weakness on the same side. In this condition, the shoulder blade may protrude in a winged position.

Q.3:  What will happen if the Hypoglossal nerve is damaged?

Answer: The damage in the Hypoglossal nerve leads to weakness on the affected area and the tongue muscle doubles itself on to the other. This affects the speaking and paralysis of the tongue. This occurs, most often, on one side of the tongue.