Understanding these two divisions of the peripheral nervous system is essential because one helps us act on the world deliberately, while the other keeps the body's internal functions running automatically.

The Peripheral Nervous System

The peripheral nervous system (PNS) includes the nerves outside the brain and spinal cord. Its main role is to connect the body to the central nervous system (CNS) so messages can travel in both directions. Within the PNS, psychologists distinguish between two major divisions: the somatic nervous system and the autonomic nervous system.

This diagram summarises how the peripheral nervous system splits into the somatic and autonomic divisions, highlighting that somatic pathways mainly support voluntary control of skeletal muscles while autonomic pathways regulate involuntary functions. It is useful for visualising the overall “map” before learning the details of each system. Source

This distinction matters because the body does not control all responses in the same way. Some actions involve conscious choice and directed movement, while others happen automatically to keep the body functioning.

Somatic Nervous System

The first major division is the somatic nervous system.

It is closely linked to actions a person can usually control consciously. When you decide to raise a hand, write a sentence, or turn your head, the somatic system carries the signals needed for those movements. It also brings information from the outside world toward the CNS, allowing the brain to respond to touch, pressure, pain, and changes in the environment.

The somatic system is important for:

• voluntary movement

• awareness of external stimulation

• coordination between incoming sensory information and outgoing movement

Because it is associated with skeletal muscles, the somatic system is mainly involved in body movement rather than the regulation of internal organs.

Autonomic Nervous System

The second major division is the autonomic nervous system.

This system works largely without conscious control. A person does not normally decide to change heart rate, release digestive enzymes, or alter pupil size moment by moment. Instead, the autonomic system adjusts these functions automatically so the body can respond to changing demands.

The autonomic system is involved in:

• heart rate

• digestion

• gland activity

• blood vessel activity

• pupil dilation and constriction

Unlike the somatic system, which mainly targets skeletal muscles, the autonomic system acts on smooth muscle, cardiac muscle, and glands. This makes it essential for keeping the body operating in a stable and efficient way.

The autonomic system is often described as having two branches:

• the sympathetic branch, which increases bodily activity during challenge or demand

• the parasympathetic branch, which reduces arousal and supports resting functions

For this subsubtopic, the key idea is that both branches still belong to the autonomic division because their effects are automatic rather than consciously directed.

Key Differences Between the Two Systems

The somatic and autonomic systems are both part of the PNS, but they differ in several important ways.

• Type of control

  • The somatic system is mainly voluntary.

  • The autonomic system is mainly involuntary.

• Main targets

  • The somatic system controls skeletal muscles.

  • The autonomic system controls organs, glands, smooth muscle, and cardiac muscle.

• Main function

  • The somatic system helps a person interact with the external environment through movement.

  • The autonomic system regulates the internal environment of the body.

• Level of awareness

  • Somatic activity is usually easier to notice consciously.

  • Autonomic activity usually happens without awareness.

A simple way to remember the distinction is that the somatic system helps you do things, while the autonomic system helps your body keep doing things.

Communication and Control

Both divisions send information to and from the CNS, but they do so for different purposes. The somatic system allows the brain to receive information about the outside world and to produce deliberate motor responses. This supports everyday actions such as walking, speaking, or picking up an object.

The autonomic system constantly monitors and adjusts internal processes. Even when a person is sitting still or sleeping, autonomic activity continues. Heartbeat, digestion, and glandular activity do not stop when conscious attention moves elsewhere. This shows why the autonomic system is essential for survival: it provides ongoing regulation rather than occasional, chosen actions.

Although the distinction is clear, the two systems often operate at the same time. For example, a person may deliberately walk across a room using the somatic system while the autonomic system simultaneously adjusts breathing, circulation, and energy use in the background.

Why This Distinction Matters in Psychology

Psychologists need to separate these systems because human behavior includes both voluntary actions and automatic bodily responses. If someone chooses to speak, wave, or move away, the somatic system is central. If their heart begins to race, their mouth becomes dry, or digestion slows, the autonomic system is involved.

This distinction also helps explain why not all behavior is under direct conscious control. People often feel the effects of bodily activity without deliberately causing them. Recognizing the difference between the somatic and autonomic systems helps students link physical processes to behavior in a precise way.