[1] The main function of the PNS is to connect the CNS to the limbs and organs, essentially serving as a relay between the brain and spinal cord and the rest of the body.

[2] Unlike the CNS, the PNS is not protected by the vertebral column and skull, or by the blood–brain barrier, which leaves it exposed to toxins.

[6] The autonomic nervous system exerts involuntary control over smooth muscle and glands.

The somatic nervous system is under voluntary control, and transmits signals from the brain to end organs such as muscles.

There are twelve cranial nerves, ten of which originate from the brainstem, and mainly control the functions of the anatomic structures of the head with some exceptions.

For descriptive purposes this plexus is usually divided into three parts: The autonomic nervous system (ANS) controls involuntary responses to regulate physiological functions.

[8] The most notable physiological effects from autonomic activity are pupil constriction and dilation, and salivation of saliva.

[8] The sympathetic system is activated during a "fight or flight" situation in which mental stress or physical danger is encountered.

[8] Neurotransmitters such as norepinephrine, and epinephrine are released,[8] which increases heart rate and blood flow in certain areas like muscle, while simultaneously decreasing activities of non-critical functions for survival, like digestion.

[9] Primarily using the neurotransmitter acetylcholine (ACh) as a mediator, the parasympathetic system allows the body to function in a "rest and digest" state.

[9] Consequently, when the parasympathetic system dominates the body, there are increases in salivation and activities in digestion, while heart rate and other sympathetic response decrease.

[9] 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.

Common symptoms of carpal tunnel syndrome include pain and numbness in the thumb, index and middle finger.