Human Physiology/The Nervous System

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Other thoracic spinal nerves (T3-T12)

The remainder of the thoracic spinal nerves, T3 through T12, do little recombining. They form the 

intercostal

nerves

, so named because they run between the ribs. For points of reference, the 7th intercostal nerve terminates at

the lower end of the sternum, also known as the xyphoid process. The 10th intercostal nerve terminates at the

umbilicus, or the belly button.

The 

somatic nervous system 

is that part of the peripheral nervous system associated with the voluntary control of

body movements through the action of skeletal muscles, and also reception of external stimuli. The somatic nervous

system consists of afferent fibers that receive information from external sources, and efferent fibers that are

responsible for muscle contraction. The somatic system includes the pathways from the skin and skeletal muscles to

the Central Nervous System. It is also described as involved with activities that involve consciousness.

The basic route of the efferent somatic nervous system includes a two neuron sequence. The first is the upper motor

neuron, whose cell body is located in the precentral gyrus (Brodman Area 4) of the brain. It receives stimuli from

this area to control skeletal (voluntary) muscle. The upper motor neuron carries this stimulus down the corticospinal

tract and synapses in the ventral horn of the spinal cord with the alpha motor neuron, a lower motor neuron. The

upper motor neuron releases acetylcholine from its axon terminal knobs and these are received by nicotinic receptors

on the alpha motor neuron. The alpha motor neurons cell body sends the stimulus down its axon via the ventral root

of the spinal cord and proceeds to its neuromuscular junction of its skeletal muscle. There, it releases acetylcholine

from its axon terminal knobs to the muscles nicotinic receptors, resulting in stimulus to contract the muscle.

The somatic system includes all the neurons connected with the muscles, sense organs and skin. It deals with sensory

information and controls the movement of the body.

The Autonomic System

The 

Autonomic system 

deals with the visceral organs, like the heart, stomach, gland, and the intestines. It regulates

systems that are unconsciously carried out to keep our body alive and well, such as breathing, digestion (peristalsis),

and regulation of the heartbeat. The Autonomic system consists of the 

sympathetic 

and the 

parasympathetic

divisions. Both divisions work without conscious effort, and they have similar nerve pathways, but the sympathetic

and parasympathetic systems generally have opposite effects on target tissues (they are antagonistic). By controlling

the relative input from each division, the autonomic system regulates many aspects of homeostasis. One of the main

nerves for the parasympathetic autonomic system is Cranial Nerve X, the Vagus nerve.