The peripheral nervous system

The peripheral nervous system is made up of the nerves that connect the brain, spinal cord, and other body parts (PNS). This system consists of cranial and spinal nerves, which link the brain and spinal cord to peripheral structures like the skin and skeletal muscles, respectively. One division of the PNS is known as the autonomic nervous system (ANS). The motor neurons that supply the internal organs make up the ANS. The ANS joins the brain and spinal cord to a number of bodily glands as well as the smooth and cardiac muscle in the chest and belly.

Nerves

Outside of the central nervous system, a nerve is a group of nerve cell fibers (in the PNS). Tracts are collections of nerve cell fibers in the CNS. To carry impulses to and from the brain, tracts are found both inside the spinal cord and in the brain. An electric cable made up of numerous wires can be compared to a nerve or tract. Axons in nerves are typically myelinated and only rarely unmyelinated, and they are sheathed in several connective tissue layers. Only sensory fibers on a few of the cranial nerves carry signals to the brain. These are referred to as afferent or sensory nerves. A handful of the cranial nerves are referred to as motor, or efferent, nerves because they solely have motor fibers for conducting impulses away from the brain. The remaining cranial nerves and all spinal nerves, on the other hand, are referred to as mixed nerves because they have both sensory and motor fibers.

Cranial Nerves

The nerves that connect to the brain are known as the cranial nerves. The cranial nerves come in 12 pairs. According to the order of their attachment from the front to the rear of the brain, they are labeled with the Roman numerals I through XII. Additionally, they have names that are connected to their roles. The head is supplied by the first 9 pairs and the 12th pair. Only sensory impulses are carried by some cranial nerves. Because they contain fibers from both types, a number of cranial nerves are referred to as mixed nerves.

Table: Cranial nerves

Spinal Nerves

The spinal cord is the source of the spinal nerves. The numbering of the 31 pairs of spinal nerves corresponds to the level of the spinal cord from which they emerge. The dorsal root and the ventral root are each a part of a neuron that connects to the spinal cord. Efferent (motor) fibers leave the cord on the ventral (anterior) root, while afferent (sensory) fibers enter the cord on the dorsal (posterior) root. Through an intervertebral foramen, each nerve exits the vertebral canal. Several spinal nerves are:

• Cervical nerves: 8 pairs
• Thoracic nerves: 12 pairs
• Lumbar nerves: 5 pairs
• Sacral nerves:5 pairs
• Coccygeal nerves: 1 pair

Dorsal and ventral are the two principal rami (branches) of each spinal nerve. The deep muscles and skin of the dorsal surface of the trunk are supported by the dorsal rami. The muscles, skeletal components, and skin covering the ventral surfaces of the trunk are all supported by the ventral rami.

Branches of the Spinal Nerves

Only a very small distance separates each spinal nerve from the spinal cord before it splits into small posterior divisions and relatively large anterior divisions. The more substantial anterior branches join together to form plexus networks, which distribute branches to the body parts. These are the three main plexuses' descriptions:

• The cervical plexuses transmit motor impulses to the neck's muscles and receive sensory information from the head's back and neck. This plexus gives rise to the phrenic nerve, which contracts the diaphragm.
• Numerous branches from the brachial plexus travel to the shoulder, arm, forearm, wrist, and hand. The brachial plexus is where the radial nerve leaves.
• The lower extremities' nerves are supplied by the lumbosacral plexus. The sciatic nerve, which emerges from the dorsal portion of the pelvis, travels beneath the gluteus maximus muscle and descends the back of the thigh, is the greatest of these branches.