Study topic · about 3 minutes

The nervous system

The nervous system has two main parts. The central nervous system is the brain and spinal cord, where decisions are made. The peripheral nervous system is all the nerves that link them to the body. It has a somatic part you control, like moving your arm, and an autonomic part that runs by itself, like your heartbeat.

Key points

Central nervous system (CNS): the brain and the spinal cord.
Peripheral nervous system (PNS): 12 pairs of cranial nerves, 31 pairs of spinal nerves, and all their branches.
The somatic nervous system handles voluntary movement and the senses.
The autonomic nervous system runs automatic functions, and has two sides: sympathetic (fight or flight) and parasympathetic (rest and digest).
The hypothalamus and the brainstem are the main controllers of the autonomic system.

The map of the nervous system

Division	Made of	Job
Central nervous system	Brain and spinal cord	Takes in information, decides and sends commands
Peripheral nervous system	Every nerve outside the brain and spinal cord	Carries messages between the CNS and the body
Somatic	Sensory nerves and nerves to skeletal muscles	The senses, and movements you choose
Autonomic	Nerves to organs, blood vessels and glands	Automatic body functions
Sympathetic	Part of the autonomic system	Gears the body up for action: fight or flight
Parasympathetic	Part of the autonomic system	Calms and restores the body: rest and digest

The central nervous system

The brain takes in information, makes sense of it and decides what to do. The spinal cord, a cable about 45 centimeters long inside the backbone, carries messages between the brain and the body and handles some reflexes on its own. Both are protected by bone, by three membranes called the meninges, and by a cushion of cerebrospinal fluid.

The peripheral nervous system

Nerves are bundles of axons wrapped together, like wires in a cable. Twelve pairs of cranial nerves leave the brain directly, mostly serving the head and neck, and 31 pairs of spinal nerves leave the spinal cord for the rest of the body. Sensory (afferent) fibers carry signals toward the central nervous system, and motor (efferent) fibers carry commands away from it.

The somatic nervous system

The somatic nervous system connects the brain to the skin, muscles and joints. Its sensory side reports touch, pain, temperature and the position of your limbs; its motor side moves your skeletal muscles when you choose to, like typing or walking. Movement commands start in the motor cortex and travel down the corticospinal tract to the spinal cord.

The autonomic nervous system

	Sympathetic	Parasympathetic
Nickname	Fight or flight	Rest and digest
Heart	Beats faster and harder	Slows down
Pupils	Widen	Narrow
Airways	Open wider	Narrow
Digestion	Slows down	Speeds up
Where its nerves leave	The middle of the spinal cord (thoracic and lumbar)	The brainstem and the bottom of the spinal cord (cranial and sacral)

The two sides usually push in opposite directions and balance each other. The vagus nerve (X), cranial nerve X, carries most of the parasympathetic signals to the heart, lungs and gut. The hypothalamus and centers in the medulla oblongata steer both sides, for example raising your heart rate when you are afraid.

Some textbooks add a third division, the enteric nervous system: a web of neurons in the wall of the gut that can run digestion on its own.

Reflexes: when the spinal cord decides

A reflex is a fast, automatic response that doesn't wait for the brain. In the knee-jerk reflex, a tap on the tendon below the kneecap stretches the thigh muscle; a sensory neuron signals a motor neuron directly in the spinal cord, and the leg kicks. Other reflexes, like pulling your hand away from something hot, pass through an interneuron in between. Read more in neurons and nerve signals.

Quick quiz

5 questions. Your answers are saved and come back in your daily review.

Which two structures make up the central nervous system?
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The brain and spinal cord. The central nervous system is the brain and the spinal cord; everything else is peripheral.
Your heart races when you see a bear. Which division is at work?
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Sympathetic. The sympathetic nervous system prepares the body for fight or flight.
Which nerve carries most of the parasympathetic signals to the heart, lungs and gut?
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Vagus nerve. The vagus nerve (X) (X) wanders from the brainstem down into the chest and belly.
You decide to kick a ball. Which part of the nervous system do you use?
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Somatic. The somatic nervous system controls the skeletal muscles you move on purpose.
Which brain area is a main controller of the autonomic nervous system?
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Hypothalamus. The hypothalamus steers heart rate, temperature, hunger and the stress response.

In these study guides

MCATNeurons, the nervous system and hormones
AP Psychology1.2 Overview of the nervous system
Intro psychologyNeurons and the nervous system (sections 3.2 and 3.3)
Anatomy and physiologyNervous tissue (chapter 12)
NursingNeuro basics
A-level PsychologyThe divisions of the nervous system
GCSE BiologyThe nervous system and reflexes (4.5.2.1)

Take it further in 3D

The brainstem and cranial nervesLife support and the nerves of the head. A short guided lesson in the 3D app.
Moving and feelingThe motor and sensory strips. A short guided lesson in the 3D app.

Sources

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