There is twelve pair of nerves that spring from the tissues of the brain. These originate directly from the brain stem. These are called cranial nerves. All these nerves have to take entry or exit via cranium. The name cranial nerve is derived from cranium. It acts as a link between the muscles and sensory organs of head and chest to the brain via the skull openings. Twelve pairs of cranial nerves are present in all mammals, birds and reptiles. Amphibians have ten pairs each. As opposed to this, the spinal nerves originate from parts of the spinal cord.
Any nerve that transmits information from the brain to the periphery is called motor and if the information is transmitted from the periphery to the brain, they are called sensory fibers. Sensory fibers are further classified as somatic, visceral and special sensory.
Somatic sensory: These originate from the muscle or the skin.
Visceral Sensory: These originate from the soft internal organs of the body like stomach, intestines, thoraisic.
Special Sensory: These originate from the sensory organs like eyes and ears.
Motor fibers are further classified in to somatic motor, general visceral motor and special visceral motor.
Somatic Motor: These transmit message to the somatic muscles.
General Visceral motor: These transmit messages to glands, smooth muscle and the heart muscle.
Special Visceral Motor: These transmit messages to visceral striated muscles.
A cranial nerve could comprise of one or many fibers. A cranial nerve with many fibers is called mixed nerve.
The cranial nerves are as follows:
Olfactory Nerve: The olfactory nerve is a special sensory organ. Its function is to know and distinguish smell.
Optic Nerves: This too comes under the category of special sensory organ. Its function is to impart vision.
Oculomotor Nerve: It performs and controls most of the eye movements like bating of eyelid, blinking, dilation of pupil etc.
Trochlear Nerve: Its function is to control the movement of the eye ball Like moving the eye ball up and down.
Trigeminal Nerve: It receives sensation from the face and transmits it to the brain.
Abducens Nerve: It is related to the retina
Facial Nerve: Its function is to control the expressions of the face. It also performs the function of taste and the salivary glands.
Vestibucochlear Nerve: It controls the balance and the movement of the body.It also senses sounds.
Glossopharyngeal Nerve: It receives signals of taste from the one third of the tongue located at the end. It also senses hot or cold, and pain. Palantine tonsils will also transmit certain sensation to the brain.
Vagus nerves: It transmits special taste sense. It controls the voice muscles.
Accessory Nerve: It controls the neck muscles and also performs certain functions of the vagus nerves. For example, it helps in shrugging the neck.
Hypoglossal nerve: Its function includes swallowing and formation of speech.
These are the total 12 pairs of cranial nerves. In addition to these twelve pair of nerves, all the mammals have an extra pair of nerve called the terminal nerve.