It also mediates reflex actions, where an afferent nerve fiber is nearly directly connected to a motor nerve fiber, to quickly generate a response to a stimulus. It controls the voluntary muscular movement of skeletal muscles in the limbs, back, shoulders, neck, and face. The second functional division of the PNS is the somatic nervous system. Thus the sensory nervous system consisting of the receptor and neural pathway deliver information about the intensity, location, type, and duration of a stimulus to the CNS. This signal, when it reaches a certain threshold, is transmitted as an action potential along an afferent neuron, and relayed to the CNS, where the signal is perceived and interpreted. Sensory receptors can transduce a physical stimulus such as pressure, sound waves, electromagnetic radiation, or chemical composition, into an electrochemical signal. Nerve fibers that carry this information are part of the afferent division. The first is the sensory nervous system, carrying signals from the viscera, sense organs, muscles, bones and joints towards the CNS. The functional classification of the PNS divides it into three categories. At the same time, it mediates the response of the body to noxious stimuli, quickly removing the body from the injurious stimulus, whether it is extremes in temperature, pH, or pressure. The PNS innervates the muscles surrounding sense organs, so it is involved in chewing, swallowing, biting and speaking. The peripheral nervous system also controls the release of secretions from most exocrine glands. It is necessary for all voluntary action, balance, and maintenance of posture. It can increase or decrease the strength of muscle contractility across the body, whether it is sphincters in the digestive and excretory systems, cardiac muscles in the heart, or skeletal muscles for movement. Thus, the PNS regulates internal homeostasis through the autonomic nervous system, modulating respiration, heart rate, blood pressure, digestion reproduction, and immune responses. When the CNS integrates these varied signals and formulates a response, motor nerves of the PNS innervate effector organs and mediate the contraction or relaxation of skeletal, smooth or cardiac muscle. This is accomplished through nerves that carry information from sensory receptors in the eyes, ears, skin, nose, and tongue, as well as stretch receptors and nociceptors in muscles, glands and other internal organs. The primary function of the peripheral nervous system is to connect the brain and spinal cord to the rest of the body and the external environment. While it receives signals from the autonomic nervous system, it can function independently and contains nearly five times as many neurons as the spinal cord. The enteric nervous system, which surrounds the gastrointestinal tract, is another important part of the peripheral nervous system. Both cranial and spinal nerves can have sensory, motor, or mixed functions. Anatomically, the PNS can be divided into spinal and cranial nerves, depending on whether they emerge from the spinal cord or the brain and brainstem.
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