What Is the Somatic Nervous System? Functions & Examples

Every time you walk, write, speak, or pick up a cup, your nervous system is coordinating the action. Most of these voluntary movements run through the somatic nervous system. The somatic nervous system is the division of the peripheral nervous system responsible for voluntary movement and conscious sensation. It carries sensory information from the skin, muscles, and joints to the brain and spinal cord, and it carries motor commands from the central nervous system back out to skeletal muscles. It is how you consciously sense the world and act on it.

Linking the brain to muscles and skin

The peripheral nervous system has two major functional divisions. The somatic nervous system connects the central nervous system, the brain and spinal cord, with skeletal muscles and with sensory receptors throughout the body. The autonomic nervous system, by contrast, regulates involuntary functions such as heart rate and digestion. The somatic system mainly controls movements that can be started voluntarily, though, as we’ll see, it also handles some fast automatic reflexes.

Its reach is specific. The somatic nervous system controls skeletal muscles only. Smooth muscle, cardiac muscle, and glands are controlled mainly by the autonomic nervous system.

Two directions of traffic

Picture a two-lane road between the body and the brain. On the incoming lane, the sensory or afferent pathway, receptors detect changes outside and inside the body: touch, pressure, pain, temperature, vibration, and proprioception, the sense of body position and movement. Sensory neurons carry this information through peripheral nerves to the spinal cord and brain, where it is processed and interpreted.

On the outgoing lane, the motor or efferent pathway, the central nervous system sends commands to skeletal muscles through somatic motor neurons, producing movements like walking, writing, speaking, reaching, and smiling. Each somatic motor neuron connects directly to skeletal muscle fibers at a neuromuscular junction. This is a key difference from the autonomic system: the somatic motor pathway uses a single motor neuron to reach the muscle, while the autonomic pathway uses two. The road picture is a simplification, since the two lanes constantly influence each other, but it captures the basic layout.

A continuous loop

The somatic nervous system works as a continuous cycle. Receptors in the skin, muscles, tendons, and joints detect a stimulus or a change in body position. Sensory neurons carry the signal to the spinal cord and brain. The central nervous system integrates the information and decides on a response. Motor neurons then carry commands from the spinal cord to skeletal muscles, producing movement. This loop runs constantly whenever you interact with your surroundings.

Voluntary, but not only voluntary

It is a common misconception that the somatic nervous system handles only voluntary movement. It also carries somatic reflexes, such as the knee-jerk, or patellar, reflex and the withdrawal reflex. These are processed mainly in the spinal cord and happen quickly without conscious awareness, but the sensory and motor pathways involved belong to the somatic system. So reflexes don’t all belong to the autonomic nervous system. Many familiar ones are somatic reflexes involving skeletal muscle.

Voluntary also doesn’t mean every movement is consciously planned. Many learned movements, like walking or typing, become highly automatic with practice. They still run through the somatic nervous system, because they involve skeletal muscles and can be started or changed at will.

Why it matters, and what goes wrong

The somatic nervous system lets you perceive your surroundings and move your body on purpose. Without it, you couldn’t walk or run, hold objects, speak, keep your posture, feel pain or touch, or sense your body’s position as you move. It underpins everyday activities, sports, fine hand control, and coordinated movement.

Disorders affecting it can impair movement or sensation. Examples include peripheral nerve injury, radiculopathy, which is irritation or compression of a nerve root, peripheral neuropathy, amyotrophic lateral sclerosis affecting somatic motor neurons, and poliomyelitis affecting lower motor neurons. Depending on where the damage is, people may have muscle weakness, paralysis, numbness, reduced sensation, or lost reflexes.

This article is educational background, not a diagnostic guide. New weakness, numbness, or loss of sensation should be evaluated by a clinician.

Related: What Is the Peripheral Nervous System? · What Is the Autonomic Nervous System? · What Is a Motor Neuron? · What Is a Reflex Arc?

References

1. Akinrodoye MA, Lui F. Neuroanatomy, Somatic Nervous System. StatPearls. Updated November 7, 2022.
2. Neuroanatomy, Motor Neuron. StatPearls. NCBI Bookshelf.
3. Neuroanatomy, Spinal Cord Myotatic Reflex. StatPearls. NCBI Bookshelf.