Nerves

Nerves are that part of our nervous system which connect the brain and spinal cord (a.k.a. central nervous system) with the rest of the body. They are like cables, which comprise of bundles of neurons, that transmit electrical impulses between our central nervous system and various body parts, including the skin, muscles and internal body organs. The neurons are covered over by specialized cells called the Schwann cells which form a myelin sheath around them to facilitate faster conduction of neural impulses.

From a functional perspective, the nerves could be either sensory or motor, while some even being mixed; transmitting both sensory and motor neural impulses. The sensory nerves communicate sensory information of pain, temperature, touch, pressure, vibration and proprioception–from skin, muscle and various body organs–to the brain and spinal cord. They also send sensory signals to the brain to help us see, taste, hear and smell. The motor nerves, on the other hand, carry signals to various effector organs, such as, the muscles and the glands.

The nerves could also be categorized into cranial and spinal nerves. Together they constitute the peripheral nervous system.

There are 12 pairs of cranial nerves which emerge from the base of the brain. These include the ‘olfactory (1st cranial) nerve’ linked with sense of smell and the ‘optic (2nd cranial) nerve’ for processing visual information. The 3rd, 4th and 6th cranial nerves help contract the extraocular muscles for moving the eyeball. These are called the ‘oculomotor, trochlear and abducent nerves’, respectively. The ‘oculomotor nerve’ also serves to constrict the pupil and accommodates for near vision. Next comes the ‘trigeminal (5th cranial) nerve and its three subdivisions (i.e. ophthalmic, maxillary and mandibular nerves). They play an important role in transmitting pain, temperature, touch and vibration sensations from the face, to the brain. In addition, they also help in chewing and swallowing the food by moving the muscles of mastication. Then comes the facial (7th cranial) nerve that performs a multitude of functions, including contracting our muscles of facial expression, conducting taste impulses from the tongue and activating the lacrimal and submandibular and sublingual glands for producing tears in the eyes and secreting saliva in the oral cavity. The ‘vestibulocochlear nerve’ is the 8th cranial nerve, which transmits sensory information pertaining to hearing and balance regulation. The ‘glossopharyngeal and vagus nerves’ are the 9th and 10th cranial nerves, which innervate the muscles of larynx and pharynx. A damage to these nerves could potentially result in difficulty in swallowing and problems with the voice production. Lastly, the 11th and 12th cranial nerves include the ‘accessory and hypoglossal nerves’, respectively. The former innervates the trapezius and sternocleidomastoid muscles, while the latter innervates the muscles of the tongue.

Unlike the cranial nerves, there are 31 pairs of spinal nerves – eight cervical, twelve thoracic, five lumbar, five sacral and a single coccygeal pair of nerves. Each pair of spinal nerve originates from its respective segment of the spinal cord.

Each spinal nerve is formed by the union of anterior (ventral) and posterior (dorsal) roots, which originate from the anterior and posterior gray horns of spinal cord. The ventral root is composed of efferent motor neurons which transmit somatic and visceral motor information to skeletal muscles and glands and viscera. Their cell bodies reside in the anterior and lateral gray horns of spinal cord. The dorsal root comprises of afferent sensory neurons which transmit visceral and somatic sensory information from peripheral receptors back to the spinal cord.

The spinal nerves exit the vertebral column through the intervertebral foramina, as a mixed bundle of sensory and motor neurons. After exiting the intervertebral foramina, spinal nerves split into anterior (ventral) and posterior (dorsal) rami. The posterior rami of spinal nerves innervate posterolateral body wall, while anterior rami contribute to all major neural plexuses–cervical, brachial, lumbosacral–and are responsible for majority of body’s sensorimotor innervation.