What Does the Central Nervous System Do?

Medically Reviewed By Nancy Hammond, M.D.
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The central nervous system is the brain and spinal cord. It controls nearly everything about the human body.  The central nervous system (CNS) is a biological computer responsible for conscious and unconscious processes. It controls automatic systems, voluntary movements, thoughts, emotions, logic, and sensory interpretation.

This article describes the CNS function and structure. It also covers some of the conditions that can affect it.

Central nervous system defined

illustration of central nervous system including the brain and spinal cord
The central nervous system includes the brain and spinal cord. Illustration by Mekhi Baldwin

The CNS is the brain and spinal cord. It sounds simple, but it is the most complex system in the body. Its main role is to allow people to interact with their surroundings. It does this through:

  • communication
  • emotions
  • memory
  • movement
  • reflexes
  • responses
  • sensation
  • thought processing
  • unconscious functions

The brain is inside the skull and connects at the base of the skull to the spinal cord. A column of vertebral bones houses the spinal cord that runs down the length of the back to the tailbone. 

Both the brain and spinal cord have meninges, a protective membrane, surrounding them. Cerebrospinal fluid (CSF) fills the space inside the meninges and bathes the brain and spinal cord. CSF provides nutrients and cushioning protection.

Brain structure and key functions

Your brain weighs about 3 pounds, and there are four main areas.

Get more brain facts here.

Brainstem

The brainstem is the top part of the spinal cord inside the skull. Cranial nerves 3–12 originate from the brainstem. The 12 pairs of cranial nerves connect directly from the brain to different parts of the body. Cranial nerves 1 (olfactory nerve) and 2 (optic nerve) originate in the cerebrum.

The brainstem coordinates messaging between the brain and the rest of the body.

The three components of the brainstem include:

  • Medulla oblongata: This is the lowest part of the brainstem and is responsible for autonomic functions such as breathing. Learn about the autonomic nervous system here.
  • Midbrain: This is the uppermost part of the brainstem and is involved with reflexes and voluntary movements, including eye movement.
  • Pons: This sits between the other two components and helps with balance, posture, and unconscious processes.

Learn more about brainstem function here.

Cerebellum

The cerebellum lies at the back and bottom of the skull. It is the second largest part of the brain. 

Its main function is the coordination of smooth and fluid movements. Long-term memory movements, or “muscle memory,” develops here. However, it also has a role in cognitive and sensory-motor functions, such as speech recognition.

Diencephalon

The diencephalon sits in the middle of the brain. It contains the thalamus and hypothalamus. 

The thalamus processes sensory impulses that come in from the body. It relays and routes the information to the appropriate area. It also has a role in sleep and consciousness.

The hypothalamus is a tiny part of the brain, but it is vital for maintaining homeostasis. It signals glands to release hormones and regulates appetite, blood pressure, heart rate, temperature, and thirst.

Learn more about the hypothalamus here.

Cerebrum

The cerebrum is what people typically picture as the brain. It is the largest and most developed part of the brain. It is responsible for higher functions, including attention, language, logic, memory, problem-solving, reasoning, thinking, and understanding.

A deep groove separates the right and left hemispheres of the cerebrum. Each hemisphere controls the opposite side of the body.

The hemispheres divide into lobes:

  • frontal lobes, which lie behind the forehead 
  • parietal lobes, which sit behind the frontal lobes
  • occipital lobes, which are behind the parietal lobes at the back of the skull
  • temporal lobes, which are on the side of the head and under the frontal and parietal lobes

Covering the cerebrum is the cerebral cortex. Most of the information processing takes place here. Although thin, the many folds of the brain give the cerebral cortex a large surface area.

Learn more about the cerebrum here.

Spinal cord structure and function

The spinal cord is a two-way pathway between the body and the brain. It carries input from the body to the brain and instructions from the brain to the body. It is also responsible for reflexes, reactions that do not require thought.

The four sections of the spinal cord include:

  • cervical, or neck area
  • thoracic, or chest level
  • lumbar, or low back region
  • sacral, or tailbone area

There are 31 pairs of spinal nerves that exit the spinal cord and vertebrae at different levels to supply the body.

CNS disorders range in severity and effects, depending on the area of involvement. 

Unlike other tissues, the delicate neurons of the CNS cannot regenerate. When damage occurs, the effects can be overwhelming. However, not all disorders cause damage.

Common CNS disorders include:

Peripheral nervous system vs. central nervous system

The CNS is just one part of the nervous system as a whole. The other part is the peripheral nervous system (PNS). It includes all nerves outside of the brain and spinal cord.

The basic unit of both systems is the neuron. Nerves are bundles of neurons. Nerves in the PNS branch off the spinal cord and run throughout the body. 

Sensory nerves carry input from the body to the spinal cord and brain. They detect:

  • light
  • odors
  • pressure
  • proprioception, or the sense of your body’s motion and position in space
  • sound
  • taste
  • temperature

Motor neurons relay responses and instructions back out to the body. They tell voluntary and involuntary muscles and glands what to do.

What is the difference between white and gray matter?

White and gray matter contain different parts of the neurons.

Neurons have a cell body, an axon, and dendrites. The cell body is where processing takes place. The axon carries electrical signals from the cell body to the dendrites. The dendrites extend from the cell body and release neurotransmitters. These substances “talk” to other neuron cell bodies or muscles and glands.

White matter consists of myelinated axons. Myelin is a fatty insulating substance that appears white. It helps electrical impulses travel quickly over distances. White matter is deeper in the brain and mainly lies under gray matter, providing connections to different areas. White matter is the outer layer of the spinal cord. (All nerves in the PNS are myelinated due to their long axons).

Gray matter contains neuron cell bodies, unmyelinated axons, dendrites, and glia (cells that support neurons with various functions). The axons in gray matter are short and do not need myelin to travel quickly. In the brain, gray matter makes up the outer layer — the cortex — and is where processing occurs. In the spinal cord, it is the core.

Gray matter is most abundant around the cerebellum and cerebrum, the largest part of the brain. White matter is just as critical, allowing for lightning-fast communication between the CNS and the rest of the body.

Summary

The CNS is the body’s command center. This complex system controls and responds to everything else in the body. Most of its functions are so fast that you are not aware of it working. However, when something goes wrong, the effects can alter your life. While scientists have learned a lot about the brain and spinal cord, there is still much they do not understand. 

 

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Medical Reviewer: Nancy Hammond, M.D.
Last Review Date: 2022 Aug 8
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THIS TOOL DOES NOT PROVIDE MEDICAL ADVICE. It is intended for informational purposes only. It is not a substitute for professional medical advice, diagnosis or treatment. Never ignore professional medical advice in seeking treatment because of something you have read on the site. If you think you may have a medical emergency, immediately call your doctor or dial 911.
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