Nerves arising directly from the brain are called cranial nerves, while those arising from the spinal cord are called peripheral nerves. These nerves pass from the brain through openings in the skull called foramina, to supply various parts of the head and neck, although some have extensions to the body.

The cranial nerves each have a name, but they are also known by their corresponding Roman numerals, which name them from the topmost to the bottommost location of origin in the brain. These nerves have various sensory, motor, and other functions, which are important to your well-being and proper functioning.

How Many Cranial Nerves Are There?

In answeringthe question, it is helpful to begin with some knowledge about the three groups of nerves coming from the brain.

There are 12 cranial nerves which are divided into nerves for the special senses, the motor nerves for the head muscles, and the nerves innervating the structures originating from thebranchial arches. The branchial arches are the primitive structures during development that give rise to your more specialized organs or structures.

• ŸThe nerves that serve your special senses are the olfactory (cranial nerve I), the ocular (cranial nerve II) and the vestibulocochlear nerve (cranial nerve VIII).
• ŸCranial nerves that innervate the muscles in the head include the oculomotor (cranial nerve III), the trochlear (cranial nerve IV), abducens (cranial nerve VI) and the hypoglossal (cranial nerve XII) nerves.
• ŸThe nerves that innervate the structures originating from the branchial arches are the trigeminal (cranial nerve V), facial (cranial nerve VII), glossopharyngeal (cranial nerve IX), vagus (cranial nerve X) and the spinal accessory (cranial nerve XI) nerves.

Functions of the 12 Cranial Nerves

So, to the question, "How many cranial nerves are there?" we enumerate and describe the functions of the 12 cranial nerves:

1. Olfactory Nerve (I)

This sensory nerve enables you to have the sense of smell (olfaction). It provides the cells in the olfactory epithelium with information, which is carried to the olfactory area in brain. It is important because when chemical substances from the environment, including food, reach the sensory cells in the nose, impulses are sent to the brain, which may help you distinguish good and bad aroma. Injury to this nerve can cause a loss of sense of smell or anosmia.

2. Optic Nerve (II)

This sensory nerve transforms information from the environment into visual images to the brain. Ganglion cells in the inner lining of the eye called retina receive the captured images and send them to the brain through the optic nerve. Serious injury to the retina or the optic nerve can lead to blindness or anopsia.

3. Oculomotor Nerve (III)

This is a motor nerve that controls the movement of the eyes. The oculomotor nerve serves to lift the eyelid, rotate the eyeball superiorly, and constrict the opening of the eye (pupil) on exposure to light. Together with the trochlear and abducens nerves, this nerve innervates the external muscles of your eyeball and controls the size of the pupil, thus protecting it from over exposure to too much light. Damage to the oculomotor nerve may result in abnormal eye movements (strabismus) or absence of pupillary light reflexes.

4. Trochlear Nerve (IV)

This is another motor nerve that functions to control the eye muscles, enabling you to turn the eyes. It is the smallest cranial nerve and it supplies the dorsal oblique eye muscle. A problem with this nerve may also result in strabismus.

5. Trigeminal Nerve (V)

This is the largest cranial nerve and it performs many sensory and motor functions. It divides into three branches called the ophthalmic nerve, maxillary nerve and mandibular nerve. The trigeminal nerve innervates structures originating from the branchial arches.

• ŸThe ophthalmic nerve or V1 has a sensory function and it further subdivides into the lacrimal, the frontal, the nasociliary and the infratrochlear branches. These nerves supply sensory fibers to the orbit of the eye and parts of the nose and tear ducts.
• ŸThe maxillary nerve or V2 is also a sensory nerve that branches further into an infraorbital, the zygomatic, andthe pterygopalatine nerves. These nerves supply sensation to parts of the teeth and palate.
• ŸThe mandibular nerve or V3 has mixed sensory and motor functions. This nerve supplies the muscles, which help you to chew, and the taste buds in the tongue, which enable you to taste. It also has branches to the salivary glands, which releases saliva and keeps your mouth moist and helps in digestion.

6. Abducens Nerve (VI)

This motor nerve supplies other muscles to the eyes and enables you to turn your eyes laterally (to the outer side). If injured, it may lose its function and result in inward deviation of the eye.

7. Facial Nerve (VII)

This is a motor nerve that is responsible for your different types of facial expression. It also performs some sensory functions, which includes the sense of touch on your face as well as in the tongue.

The facial nerve originates from branchial arches. Aside from the face and the tongue, it also supplies parts of the ear canal, the salivary glands, the lacrimal (tear) glands, the nasal cavity and the palate.

Depending on the extent and point of injury to the facial nerve, you may experience facial paralysis, loss of eye blinking, and drooling.

8. Vestibulocochlear Nerve (VIII)

This is a motor nerve that gives you the sense of hearing as well as balance. Its two components consist of the vestibular and the cochlear nerves. The vestibular nerve helps you maintain a sense of balance while the cochlear nerve enables you to hear sounds. Injury to the vestibular nerve will cause dizziness and loss of balance while injury to the cochlear nerve may result in deafness.

9. Glossopharyngeal Nerve (IX)

This sensory nerve carries information from throat (pharynx) and some areas of the palate and tongue. It also innervates the salivary glands and performs motor function that helps you swallow food.

The glossopharyngeal nerve is derived from the third branchial arch. It has several branches such as the pharyngeal, the lingual and the tympanic nerve branches. Problems with this nerve include choking and difficulty swallowing (dysphagia).

10. Vagus Nerve (X)

This is the longest nerve and is a mixed type of nerve because it carries both sensory and motor functions. It supplies areas of the pharynx, the larynx, the esophagus, the trachea, the bronchi, some portions of the heart.

The vagus nerve is derived from the fourth branchial arch. Aside from sensory and motor functions, the vagus nerve has parasympathetic functions affecting the thoracic and abdominal organs. It affects the beating of the heart, the secretory function of the glands, and more. Problems with the vagus nerve can cause abnormal heart beats, gagging, as well as changes in blood pressure, voice and breathing.

11. Spinal Accessory Nerve (XI)

As its name implies, this motor nerve contributes branches to the spinal cord and influences the function of the shoulder and neck muscles. It is also derived from the fourth branchial. Its cranial root contributes to the tenth (vagus) nerve and supplies the muscles of the throat, palate and esophagus. It also has a spinal root that branches into the dorsal and the ventral branches, which supply the back and front muscles of the neck. Problems with this nerve may result in difficulty in turning your neck.

12. Hypoglossal Nerve (XII)

This is a motor nerve that supplies the muscles of the tongue. Together with other motor nerves supplying the tongue, the hypoglossal nerve helps in the movement of this strong muscle during eating, talking and swallowing. Problems with this nerve may result in inability to move the tongue to one side.