5.10 The Nervous System

Two main divisions

The nervous system has two divisions: the central nervous system (CNS) — brain and spinal cord — and the peripheral nervous system (PNS), which connects the CNS to the rest of the body. The PNS splits into the somatic nervous system (controls voluntary muscle movement) and the autonomic nervous system (controls involuntary functions like heart rate).

Sympathetic vs. parasympathetic

The autonomic nervous system further divides into the sympathetic nervous system, which arouses the body for fight-or-flight (increases heart rate, releases adrenaline), and the parasympathetic nervous system, which calms the body and conserves energy (rest-and-digest), slowing heart rate and aiding digestion after a threat passes.

Reflexes and the spinal cord

The spinal cord relays messages between brain and body and controls simple reflexes (e.g., the knee-jerk or pulling away from a hot stove) without needing brain input first, via a reflex arc. This allows faster reactions to protect the body from harm.

Key terms

Central nervous system (CNS)

Brain and spinal cord; processes and directs information for the body.

Peripheral nervous system (PNS)

Connects the CNS to the rest of the body; includes somatic and autonomic divisions.

Somatic nervous system

Controls voluntary movements of skeletal muscles.

Sympathetic nervous system

Arouses the body for fight-or-flight responses (increases heart rate, releases adrenaline).

Parasympathetic nervous system

Calms the body after arousal, conserving energy (rest-and-digest).

Try an AP-style question

Which system is dominant?

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