The central nervous system (CNS) consists of the brain and spinal cord. It is referred to as "central" because it integrates information from the entire body and coordinates activity across the whole organism.
This article will give a brief overview of the systems at work. We will look at the types of cells involved, different areas within the brain, spinal circuitry and how the CNS can be affected by disease and injury.
Contents of this article:
Fast facts on the central nervous system
Here are some key points about the central nervous system. More detail and supporting information is in the main article.
- The CNS consists of the brain and spinal cord
- The brain is the most complex organ in the body and uses 20% of the total oxygen we breathe in
- The retina, optic nerve, olfactory nerves and olfactory epithelium are sometimes considered to be part of the central nervous system
- The CNS is often divided into white and gray matter
- The brain consists of an estimated 100 billion neurons, with each connected to thousands more
- CNS support cells, called glial cells, outnumber neurons 10 to 1
- The brain can be divided into four lobes: temporal, parietal, occipital and frontal
- Circuits within the spinal cord are capable of controlling motor coordination without input from the brain
- The brain makes up around 2% of a human's entire weight.
What is the central nervous system?
The CNS consists of the brain and the spinal cord. The brain is housed within and protected by the skull (the cranial cavity) and the spinal cord flows from the back of the brain, down the center of the spine in the spinal canal, stopping in the lumbar region.
The brain is the most complex organ of the body.
The brain and spinal cord are both housed within a protective triple-layered membrane called the meninges.
The central nervous system has been thoroughly studied by anatomists and physiologists, but it still holds many secrets. Our thoughts, our movements, our emotions and our desires are all generated within.
The retina, optic nerve, olfactory nerves and olfactory epithelium are sometimes considered to be part of the CNS alongside the brain and spinal cord. This is because they connect directly with brain tissue without intermediate nerve fibers.
The CNS is considered a separate entity from the peripheral nervous system (PNS), although the two systems are highly intertwined. The term PNS refers to any part of the nervous system that lies outside of the brain and spinal cord.
There are a number of differences between the CNS and PNS; one difference is the size of the cells. The nerve axons of the CNS - the slender projections of nerve cells that carry impulses - are significantly shorter. PNS nerve axons can be up to 1 m long (for instance, the nerve that innervates the big toe) whereas within the CNS they are rarely longer than a few millimeters.
Another major difference between the CNS and PNS involves regeneration. Much of the PNS has the ability to regenerate; if a nerve in your finger is severed, it can regrow. The CNS, however, does not have this capability.
The components of the central nervous system are further split into a myriad of parts. Below, we will describe some of these sections in a little more detail.
White and gray matter
The CNS can be roughly divided into white and gray matter. As a very general rule, the majority of the brain consists of an outer cortex of gray matter and an inner area consisting of tracts of white matter.
Both types of tissue contain glial cells which protect and support neurons. White matter mostly consists of axons (nerve projections) and oligodendrocytes - a type of glial cell - whereas gray matter consists predominantly of neurons.
Central glial cells
Also called neuroglia, glial cells are generally referred to as support cells for neurons. They carry out a wide range of tasks and outnumber nerve cells in the brain by a factor of 10; this gives a good indication of their importance.
Without glial cells, developing nerves are often unable to navigate to their appropriate destinations, and, if they do find their way, they are unable to form functioning synapses.
Glial cells are present in both the CNS and the PNS but each system has its own specific subtypes. The following are brief descriptions of the CNS glial cell types:
- Astrocytes: these cells have numerous projections and anchor neurons to their blood supply. They also regulate the local environment by removing excess ions and recycling neurotransmitters. Astrocytes are further split into two distinct groups - protoplasmic and fibrous
- Oligodendrocytes: responsible for creating the myelin sheath - this thin layer coats nerve cells, allowing them to send signals quickly and efficiently
- Ependymal cells: lining the spinal cord and the brain's ventricles (fluid-filled spaces), these create and secrete cerebrospinal fluid and keep it circulating using their whip-like cilia
- Radial glia: these act as scaffolding for new neuronal cells during the creation of the embryonic nervous system.
The circuitry of the spinal cord can coordinate the muscles of locomotion without neural input.
The spinal cord, running almost the full length of the back, carries information between the brain and body, but also carries out other tasks. From the brainstem, where the spinal cord meets the brain, 31 spinal nerves enter the cord.
Along its length, it connects with the nerves of the PNS that run in from the skin, muscles and joints.
Motor commands from the brain travel from the spine to the musculature and sensory information travels from the sensory tissues - such as the skin - toward the spinal cord and finally up to the brain.
The spinal cord contains circuitry that mediates reflexive responses such as the involuntary movement your arm might make if your finger was to come into contact with a flame.
The circuits within the spine can also generate more complex movements such as walking. Even without input from the brain, the spinal nerves can coordinate all of the muscles necessary to walk. For instance, if the brain of a cat is separated from its spine so that its brain has no contact with its body, it will start spontaneously walking when placed on a treadmill. The brain is only required to stop and start the process, or make changes if, for instance, an object appears in your path.1
The cranial nerves are 12 pairs of nerves that arise directly from the brain and pass through holes in the skull rather than traveling along the spinal cord. These nerves collect and send information between the brain and parts of the body, predominantly the neck and head.
Of these 12 pairs, the olfactory, optic and terminal nerves all arise from the forebrain and are considered to be part of the central nervous system:
- Olfactory nerves (cranial nerve I): transmit information regarding odor from the upper section of the nasal cavity to the olfactory bulbs on the base of the brain
- Optic nerves (cranial nerve II): carry visual information from the retina to the primary visual nuclei of the brain. Each optic nerve consists of around 1.7 million nerve fibers
- Terminal nerves (cranial nerve 0): the smallest of the cranial nerves, their role is not yet clear. Some believe they may be vestigial (an evolutionary byproduct with no remaining function) or involved in sensing pheromones (secreted hormones that cause responses in social animals).2
On the next page, we look at specific areas of the brain and diseases of the central nervous system.