<P> The last four cervical spinal nerves, C5 through C8, and the first thoracic spinal nerve, T1, combine to form the brachial plexus, or plexus brachialis, a tangled array of nerves, splitting, combining and recombining, to form the nerves that subserve the upper - limb and upper back . Although the brachial plexus may appear tangled, it is highly organized and predictable, with little variation between people . See brachial plexus injuries . </P> <P> The anterior divisions of the lumbar nerves, sacral nerves, and coccygeal nerve form the lumbosacral plexus, the first lumbar nerve being frequently joined by a branch from the twelfth thoracic . For descriptive purposes this plexus is usually divided into three parts: </P> <Ul> <Li> lumbar plexus </Li> <Li> sacral plexus </Li> <Li> pudendal plexus </Li> </Ul> <P> The autonomic nervous system controls involuntary responses to regulate physiological functions . The brain and spinal cord from the central nervous system are connected with organs that have smooth muscle, such as the heart, bladder, and other cardiac, exocrine, and endocrine related organs, by ganglionic neurons . The most notable physiological effects from autonomic activity are pupil constriction and dilation, and salivation of saliva . The autonomic nervous system is always activated, but is either in the sympathetic or parasympathetic state . Depending on the situation, one state can overshadow the other, resulting in a release of different kinds of neurotransmitters . There is a lesser known division of the autonomic nervous system known as the enteric nervous system . Located only around the digestive tract, this system allows for local control without input from the sympathetic or the parasympathetic branches, though it can still receive and respond to signals from the rest of the body . The enteric system is responsible for various functions related to gastrointestinal system . </P>

Define peripheral nervous system and list its components and their functions