Somatic Nervous System

The somatic nervous system is the branch of the peripheral nervous system that controls voluntary movements of skeletal muscles, carrying sensory information to the brain and motor commands back out to the body.

Verified for the 2027 AP Psychology examLast updated June 2026

What is the Somatic Nervous System?

The somatic nervous system is one of the two main branches of the peripheral nervous system (the other being the autonomic nervous system). Its job is voluntary movement. When you decide to raise your hand, type a text, or kick a soccer ball, that's the somatic system doing its thing by activating your skeletal muscles.

It works in two directions. Sensory neurons carry information from your senses and body inward toward the central nervous system, and motor neurons carry commands back outward to your muscles. So the somatic system is the two-way road between your brain and the muscles you control on purpose. Think of it as the part of your nervous system you can actually boss around.

Why the Somatic Nervous System matters in AP Psychology

This term lives in Unit 2: Cognition, specifically topic 2.3, Overview of the Nervous System and the Neuron. It's part of the foundational map of how the nervous system is organized, which you need before anything about behavior, sensation, or memory makes sense. Understanding the somatic system means understanding how a thought becomes a physical action. You can't explain reflexes, motor control, or the difference between voluntary and automatic responses without first knowing which branch handles what.

How the Somatic Nervous System connects across the course

Autonomic Nervous System (Unit 2)

These are the two halves of the peripheral nervous system, and they split by control. The somatic system handles what you do on purpose (moving muscles), while the autonomic system runs the stuff you don't think about, like heart rate and digestion.

Motor Neurons and Sensory Neurons (Unit 2)

The somatic system is basically these two neuron types working as a loop. Sensory neurons bring info in, motor neurons send commands out to skeletal muscles, and that round trip is the somatic system in action.

Reflex Arc (Unit 2)

A reflex shows the somatic pathway moving fast. Sensory neurons detect a hot stove, the signal jumps through the spinal cord, and motor neurons yank your hand back before your brain even fully registers it.

Is the Somatic Nervous System on the AP Psychology exam?

On multiple choice, you'll most often see this as a straight identification question, like "Which system is responsible for controlling voluntary movements of our skeletal muscles?" The answer is the somatic nervous system. The trap is sorting it from the autonomic system and its fight-or-flight responses, so MCQ stems love to test all of these together. Your move is to slot each system into the right spot on the nervous system tree: peripheral splits into somatic (voluntary) and autonomic (involuntary), and autonomic splits again into sympathetic and parasympathetic.

The Somatic Nervous System vs Autonomic Nervous System

Both are branches of the peripheral nervous system, but the difference is control. The somatic system runs voluntary skeletal muscle movement (you choosing to walk), while the autonomic system runs involuntary functions like heart rate, breathing, and the fight-or-flight response. Quick test: if you can decide to do it, it's somatic.

Key things to remember about the Somatic Nervous System

  • The somatic nervous system is the part of the peripheral nervous system that controls voluntary movement of your skeletal muscles.

  • It carries information both ways: sensory neurons bring signals in, and motor neurons send commands out to muscles.

  • The peripheral nervous system splits into the somatic (voluntary) and autonomic (involuntary) systems, and that split is the single most tested distinction.

  • Reflexes run through the somatic system, which is why they can fire through the spinal cord before the brain consciously processes them.

  • If you can choose to do an action, the somatic system controls it; if your body does it automatically, that's the autonomic system.

Frequently asked questions about the Somatic Nervous System

What is the somatic nervous system in AP Psych?

It's the branch of the peripheral nervous system that controls voluntary movements of your skeletal muscles, using sensory neurons to send information in and motor neurons to send commands out.

Does the somatic nervous system control fight-or-flight?

No. Fight-or-flight is run by the sympathetic branch of the autonomic nervous system, not the somatic system. The somatic system handles voluntary muscle movement, like deciding to run, while the autonomic system controls the involuntary stuff like your racing heart.

How is the somatic nervous system different from the autonomic nervous system?

Both are branches of the peripheral nervous system, but the somatic system controls voluntary skeletal muscle movement and the autonomic system controls involuntary functions like heart rate and digestion. If you can choose to do it, it's somatic.

Is the somatic nervous system part of the central or peripheral nervous system?

It's part of the peripheral nervous system, which is everything outside the brain and spinal cord. The peripheral nervous system divides into the somatic and autonomic systems.

What neurons make up the somatic nervous system?

Sensory neurons and motor neurons. Sensory neurons carry information from the body toward the central nervous system, and motor neurons carry movement commands out to skeletal muscles.