Peripheral nervous system

Your body has many tiny wires.

These wires connect your brain to your parts. They help you move your arms and legs. They also help you feel things like touch. Some wires work on their own. They help your heart beat and your tummy work. These wires help you breathe too. They are not inside your skull. This means they can get hurt easily. They help your whole body work together.

Your body has a special set of wires. We call this the peripheral nervous system. These nerves sit outside your brain and spinal cord. Their main job is to act like a relay. They carry messages between your brain and your limbs or organs.

This system has two main parts. One part is the somatic nervous system. This part helps you move your muscles on purpose. It also carries feelings like touch to your brain. The other part is the autonomic nervous system. This part works on its own without you thinking. It controls things like your heart rate and digestion.

The autonomic system has two different states. The sympathetic state helps during danger. It speeds up your heart to help you fight or run. The parasympathetic state helps you rest and digest food. There is also a small part called the enteric nervous system. It stays near your gut to help with digestion.

Because these nerves are not inside your skull, they can get hurt. Toxins or injuries can cause damage. This can lead to pain or numbness in your hands and feet.

A view of the nerve web in the lower body.

Your body has a complex network of connections called the peripheral nervous system. This system is one of two main parts of your nervous system. The other part is the central nervous system, which includes your brain and spinal cord. The peripheral nervous system consists of nerves and ganglia. These sit outside your brain and your spinal cord. Its main job is to act as a relay. It connects your brain to your limbs and your organs.

The way this system works depends on the type of message being sent. It has two main divisions called the somatic and autonomic systems. The somatic nervous system is under your voluntary control. It sends signals from your brain to your muscles so you can move. It also has a sensory part that sends feelings like touch to your brain. The autonomic nervous system works on its own without you thinking. It manages things like your heart rate and your digestion. One part, the sympathetic system, helps during danger. The other part, the parasympathetic system, helps you rest and digest.

Scientists study how these nerves are organized in the body. In the head and neck, twelve cranial nerves carry information. Ten of these nerves start from the brainstem. One special nerve is the vagus nerve. It carries sensory information from organs in your chest and abdomen. Another is the accessory nerve. It helps move specific muscles in your neck and shoulders. For the rest of your body, spinal nerves do the work. These nerves grow out from your spinal cord. They often form a web of connected roots called a plexus.

There are many specific numbers to know about these nerves. In humans, there are 31 pairs of spinal nerves. These include 8 cervical nerves and 12 thoracic nerves. There are also 5 lumbar nerves and 5 sacral nerves. Finally, there is 1 coccygeal nerve. The cervical nerves C3, C4, and C5 help form the phrenic nerve. This nerve is essential for survival because it helps you breathe. If the spinal cord is cut above C3, a person cannot breathe on their own.

You can think of these nerves like the wiring in a house. The central nervous system is like the main control panel. The peripheral nervous system is the wiring that reaches every room. Because these nerves are not protected by a skull, they can be hurt. Damage to a single nerve is called a mononeuropathy. This might happen from an injury or a tumor. Sometimes nerves get trapped in tight spaces. This can cause carpal tunnel syndrome in the hand. This often leads to pain or numbness in your fingers.

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.