The Nervous System
Central and peripheral divisions, the lobes of the brain and what each one costs you, the twelve cranial nerves worth knowing, the autonomic fight or flight response, and how the brain behaves when pressure inside the skull rises.
12 min read · Updated September 10, 2026
The Two Divisions
The nervous system is the body's fast control system. It senses, decides and acts in milliseconds, in contrast to the endocrine system, which does similar work over minutes to days.
Central nervous system (CNS). The brain and the spinal cord. Everything that processes and decides.
Peripheral nervous system (PNS). Every nerve outside the brain and cord. Everything that carries information in and instructions out.
The peripheral system splits again.
- Somatic division. Voluntary. Carries sensation from the skin and moves skeletal muscle on command.
- Autonomic division. Involuntary. Runs the heart, vessels, airways, gut and glands without asking you.
The autonomic division splits once more, into sympathetic and parasympathetic, which is where most of the clinically useful material lives.
Quick check
Which division of the nervous system carries sensation from the skin and moves skeletal muscle on command?
Frequently asked questions
What is the difference between the central and peripheral nervous systems?
The central nervous system is the brain and the spinal cord. The peripheral nervous system is every nerve outside those two, and it splits into the somatic division that moves skeletal muscle voluntarily and the autonomic division that runs the organs without your involvement.
What does the sympathetic nervous system do?
It is the fight or flight side. It raises heart rate and contractility, constricts blood vessels in the skin and gut, dilates the bronchioles and the pupils, releases glucose from the liver, and shuts down digestion. Nearly every early sign of shock is the sympathetic response, not the injury itself.
What is Cushing’s triad?
Rising blood pressure with a widening pulse pressure, a falling heart rate, and irregular breathing. It is a late sign of rising pressure inside the skull, and it is the opposite of the fast and weak picture you see in blood loss, which is exactly why it stands out.
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