1.7 The Peripheral Nervous System

PNS splits into two branches

The peripheral nervous system (PNS) connects the brain and spinal cord to the rest of the body. It has two divisions: the somatic nervous system, which controls voluntary skeletal muscle movements and relays sensory information, and the autonomic nervous system, which regulates involuntary functions like heart rate, digestion, and glands.

Autonomic nervous system: two modes

The autonomic nervous system has two complementary branches. The sympathetic nervous system triggers the "fight-or-flight" response—arousing the body during stress by increasing heart rate and releasing energy. The parasympathetic nervous system promotes "rest-and-digest" activity, calming the body and conserving energy after arousal.

Working together

The somatic and autonomic systems work together to let organisms sense, react, and maintain balance. For example, seeing a threat (sensed via somatic pathways) can trigger sympathetic arousal, while parasympathetic activity restores calm afterward. This balance is called homeostasis.

Key terms

Peripheral nervous system (PNS)

The nervous system division connecting the brain and spinal cord to the rest of the body; includes somatic and autonomic systems.

Somatic nervous system

Controls voluntary skeletal muscle movements and relays sensory information to the CNS.

Autonomic nervous system

Regulates involuntary functions like heart rate, digestion, and glands; divided into sympathetic and parasympathetic branches.

Sympathetic nervous system

Triggers the fight-or-flight response, arousing the body during stress.

Parasympathetic nervous system

Promotes rest-and-digest activity, calming the body and conserving energy after stress.

Try an AP-style question

Which division is active?

1. Parasympathetic
2. Somatic
3. Sympathetic
4. Central