Explain The Peripheral Nervous System in detail?

The peripheral nervous system is divided into the somatic nervous system and the autonomic nervous system. The somatic nervous system includes nerves extending from the brain and spinal cord that control voluntary actions, whereas the autonomic nervous system regulates involuntary functions such as digestion and hormone release.

The peripheral nervous system contains 12 pairs of nerves which connect the brain with various structures, mostly in the head and neck. These nerves affect many functions, from the sense of smell to the movement of the tongue. The first, second, and the eighth nerves are solely sensory, while the rest - mixed nerves, carry both motor and sensory information. The vagus nerve leading from the brain to the heart and controlling rate of heartbeats is part of the autonomic nervous system.

There are 32 pairs of spinal nerves extending from the spinal cord. Each has two parts, a ventral root, containing the axons of the motor neurons, and a dorsal root, made up of sensory neurons. Clusters of cell bodiesof the sensory neurons, called dorsal root ganglia, occur outside of the spinal cord along each dorsal root.

The somatic nervous system controls voluntary activities, such as movement. It includes both sensory and motor functions. Sensory nerves convey nerve impulses from the sense organs (discussed in the next section) to the central nervous system, while motor nerves convey nerve impulses from the central nervous system to skeletal muscle effectors.

The autonomic nervous system controls regulatory activities, such as heartbeat, contraction of walls of blood vessels, digestive, urinary, and reproductive systems. The autonomic system is divided by function into sympathetic and parasympathetic systems.

The sympathetic system functions largely in times of stress, initiating the "flight or fight reaction", an increase in blood pressure, heartbeat rate, hearing, and blood flow to the muscles and the skin, while decreasing the blood flow to digestive organs and kidneys to make more available where it is needed. This response is thought to be an adaptation that allowed primitive man to survive either as a hunter, or when hunted himself.

The parasympathetic system controls involuntary functions during normal conditions. The two systems oppose each other to achieve homeostasis, the equilibrium of functions in an organism.