Peripheral nervous system

The peripheral nervous system, or PNS, is a vital component of the nervous system in bilateral animals. It works alongside the central nervous system (CNS), which consists of the brain and spinal cord. While the CNS acts as the primary command center, the PNS consists of nerves and ganglia located outside the brain and spinal cord. Its fundamental role is to serve as a relay between the CNS and the rest of the body, including limbs and organs. Unlike the CNS, the PNS lacks the protection of the skull or vertebral column. It also does not have the protection of the blood-brain barrier, which means it is more exposed to toxins.

To understand how the PNS functions, we can divide it into two main branches: the somatic and the autonomic divisions. Each of these divisions contains both sensory and motor sectors. The somatic nervous system is under voluntary control. It transmits signals from the brain to end organs, such as muscles, to facilitate movement. It also includes the sensory nervous system, which carries information from senses like touch and taste back to the brain. The autonomic nervous system, however, is a self-regulating system. It manages involuntary functions, such as heart rate and digestion, by influencing smooth muscle and glands.

The autonomic nervous system is always active, but it shifts between different states. The sympathetic nervous system is triggered during "fight or flight" situations involving stress or physical danger. In this state, the body releases neurotransmitters like epinephrine and norepinephrine. These chemicals increase heart rate and blood flow to muscles while slowing down non-critical functions like digestion. Conversely, the parasympathetic nervous system manages a "rest and digest" state. It primarily uses the neurotransmitter acetylcholine to increase salivation and digestive activity while lowering the heart rate. There is also a specialized enteric nervous system located around the digestive tract. This system allows for local control of the gastrointestinal system without needing direct input from other branches.

In the head and neck, the PNS utilizes cranial nerves to carry somatosensory data. There are twelve cranial nerves in total, and ten of them originate from the brainstem. Most of these control the anatomical structures of the head. However, some are unique. The vagus nerve is notable because it receives sensory information from organs in the abdomen and thorax. The accessory nerve is also unique, as it innervates the trapezius and sternocleidomastoid muscles, which are not located exclusively in the head. It is important to note that the optic nerve and the retina are considered part of the CNS due to their developmental origin, rather than being true peripheral nerves.

For the rest of the body, spinal nerves are responsible for transmitting somatosensory information. These nerves arise from the spinal cord, often forming an interconnected web of roots known as a plexus. In humans, there are 31 pairs of spinal nerves organized by region. These include 8 cervical, 12 thoracic, 5 lumbar, 5 sacral, and 1 coccygeal pair. The naming of these nerves depends on the vertebrae they are adjacent to. In the cervical region, roots emerge above the vertebrae. In the thoracic to coccygeal regions, they emerge below. This creates a unique naming situation where the nerve between C7 and T1 is called the C8 spinal nerve.

Specific nerve groups perform highly specialized tasks. The first four cervical nerves (C1–C4) split and recombine to serve the neck and the back of the head. For example, the phrenic nerve arises from the C3, C4, and C5 nerve roots. This nerve is essential for survival because it supplies the thoracic diaphragm to enable breathing. If the spinal cord is transected above the C3 level, spontaneous breathing becomes impossible. Further down, the C5 through T1 nerves and the first thoracic nerve combine to form the brachial plexus. This is a highly organized and predictable array of nerves that supports the upper back and upper limbs.

In the lower body, the lumbosacral plexus is formed by the anterior divisions of the lumbar, sacral, and coccygeal nerves. This plexus is often divided into three parts: the lumbar plexus, the sacral plexus, and the pudendal plexus.

Because the PNS is exposed, it is susceptible to various diseases. Damage to a single nerve or root is called a mononeuropathy, which can result from trauma, tumors, or compression. For instance, carpal tunnel syndrome occurs when a nerve is trapped in a fixed space, causing pain and numbness in the middle, index, and thumb fingers. More widespread damage is known as peripheral neuropathy. This can be caused by toxins, heavy metals, alcohol, or medical conditions like diabetes. Peripheral neuropathy often presents as a sensory loss in a "glove and stocking" distribution, meaning it starts at the extremities and progresses upward. If the autonomic system is affected, it is referred to as autonomic neuropathy.