Neuroanatomy, Somatic Nervous System

The somatic nervous system is a component of the peripheral nervous system associated with the voluntary control of body movements via skeletal muscles. It is responsible for all the functions we know and can consciously influence, including moving our arms, legs, and other body parts. A substantial portion of the peripheral nervous system comprises 43 different segments of nerves (12 pairs of cranial and 31 pairs of spinal nerves), which help us perform daily functions. The somatic nervous system consists of both afferent (sensory) and efferent (motor) nerves . It is also responsible for the reflex arc, which involves using interneurons to perform reflexive actions. Besides these are thousands of other associated nerves in the body (See  Somatic Nervous System). Cranial nerves are responsible for carrying information in and out of the brain. Ten cranial nerves originate from the brain stem and mainly control the voluntary movement and structures of the head, with some exceptions. The olfactory and optic nerve nuclei are located in the forebrain and thalamus and are not considered true cranial nerves. The others originating from the brainstem include oculomotor, trochlear, trigeminal, abducens, facial, vestibulocochlear, glossopharyngeal, vagus, spinal accessory, and hypoglossal. Of note is that the accessory nerve innervates the sternocleidomastoid and trapezius muscles, neither of which control muscles are used exclusively in the head. Spinal nerves carry somatosensory information and motor instructions out of the spinal cord. They arise from the spinal cord as nerve roots, merge to form a web (plexus) of interconnected nerve roots, and once again branch to form nerve fibers. The formation of nerve plexuses rather than a direct continuation of the nerve roots to peripheral nerves serves as an essential safety measure so that injury at 1 site or body region may not affect the vital functions of our body. The spinal nerves help to control the function and movement of the rest of the body. The 31 pairs of spinal nerves include 8 cervical, 12 thoracic, 5 lumbar, 5 sacral, and 1 coccygeal. Their names match the adjacent spinal vertebra from which they exit. In the cervical region, the nerve root exits above the corresponding vertebrae (the nerve root between the skull and C1 vertebrae is the C1 spinal nerve). The spinal nerve root originates below the corresponding vertebrae in the thoracic to the coccygeal region. This difference is due to the naming and location of the spinal root between the C7 and T1 vertebrae (C8 spinal nerve root). In the lumbar region, the spinal cord ends at L1 from the conus medullaris region, but the spinal nerve roots travel within the dural sac below the L2 level; this region is known as the cauda equina. Information in electrical impulses is relayed to and from the CNS (brain and spinal cord) to the neuromuscular junction (NMJ), which converts electrical signals into chemical signals for muscle contraction. Information from the periphery is detected by sensory receptors and coveted as electrical signals back to the central nervous system.