The central nervous system (CNS) controls most functions of the body and mind and consists of the brain and spinal cord. The brain contains approximately 100 billion neurons and glial cells that support and protect neurons. Glial cells in the CNS include astrocytes, oligodendrocytes, ependymal cells, and radial glia. The spinal cord carries signals between the brain and body and contains circuits that control reflexes. The CNS is made up of gray matter containing neurons and white matter containing axons and glial cells.

1. Cells of the central nervous system
Neurons connect with one another to send and receive messages in the brain and spinal
cord. Many neurons working together are responsible for every decision made, every emotion
or sensation felt, and every action taken.
The complexity of the central nervous system is amazing: there are approximately 100 billion
neurons in the brain and spinal cord combined. As many as 10,000 different subtypes of
neurons have been identified, each specialized to send and receive certain types of
information. Each neuron is made up of a cell body, which houses the nucleus. Axons and
dendrites form extensions from the cell body.
Astrocytes, a kind of glial cell, are the primary support cells of the brain and spinal cord. They
make and secrete proteins called neurotrophic factors. They also break down and remove
proteins or chemicals that might be harmful to neurons (for example, glutamate, a
neurotransmitter that in excess causes cells to become overexcited and die by a process
called excitotoxicity).
Astrocytes aren't always beneficial: after injury, they divide to make new cells that surround
the injury site, forming a glial scar that is a barrier to regenerating axons.
Microglia are immune cells for the brain. After injury, they migrate to the site of injury to help
clear away dead and dying cells. They can also produce small molecules called cytokines that
trigger cells of the immune system to respond to the injury site. This clean-up process is likely
to play an important role in recovery of function following a spinal injury.
Oligodendrocytes are glial cells that produce a fatty substance called myelin which wraps
around axons in layers. Axon fibers insulated by myelin can carry electrical messages (also
called action potentials) at a speed of 100 meters per second, while fibers without myelin can
only carry messages at a speed of one meter per second.
The central nervous system (CNS) controls most functions of the body and mind. It consists
of two parts: the brain and the spinal cord.
The brain is the center of our thoughts, the interpreter of our external environment, and the
origin of control over body movement. Like a central computer, it interprets information from
our eyes (sight), ears (sound), nose (smell), tongue (taste), and skin (touch), as well as from
internal organs such as the stomach.
The spinal cord is the highway for communication between the body and the brain. When the
spinal cord is injured, the exchange of information between the brain and other parts of the
body is disrupted.

2. The brain
The brain is the most complex organ in the human body; the cerebral cortex (the outermost
part of the brain and the largest part by volume) contains an estimated 15–33 billion neurons,
each of which is connected to thousands of other neurons.
In total, around 100 billion neurons and 1,000 billion glial (support) cells make up the human
brain. Our brain uses around 20 percent of our body’s total energy.
The brain is the central control module of the body and coordinates activity. From physical
motion to the secretion of hormones, the creation of memories, and the sensation of emotion.
To carry out these functions, some sections of the brain have dedicated roles. However,
many higher functions — reasoning, problem-solving, creativity — involve different areas
working together in networks.
The brain is roughly split into four lobes:
Temporal lobe (green): important for processing sensory input and assigning it emotional
meaning.
It is also involved in laying down long-term memories. Some aspects of language perception
are also housed here.
Occipital lobe (purple): visual processing region of the brain, housing the visual cortex.
Parietal lobe (yellow): the parietal lobe integrates sensory information including touch,
spatial awareness, and navigation.
Touch stimulation from the skin is ultimately sent to the parietal lobe. It also plays a part in
language processing.
Frontal lobe (pink): positioned at the front of the brain, the frontal lobe contains the majority
of dopamine-sensitive neurons and is involved in attention, reward, short-term memory,
motivation, and planning.

3. Brain regions
Next, we will look at some specific brain regions in a little more detail:
Basal ganglia: involved in the control of voluntary motor movements, procedural learning,
and decisions about which motor activities to carry out. Diseases that affect this area
include Parkinson’s disease and Huntington’s disease.
Cerebellum: mostly involved in precise motor control, but also in language and attention. If
the cerebellum is damaged, the primary symptom is disrupted motor control, known as ataxia.
Broca’s area: this small area on the left side of the brain (sometimes on the right in left-
handed individuals) is important in language processing. When damaged, an individual finds it
difficult to speak but can still understand speech. Stuttering is sometimes associatedTrusted
Source with an underactive Broca’s area.
Corpus callosum: a broad band of nerve fibers that join the left and right hemispheres. It is
the largest white matter structure in the brain and allows the two hemispheres to
communicate. Dyslexic children have smaller corpus callosums; left-handed people,
ambidextrous people, and musicians typically have larger ones.
Medulla oblongata: extending below the skull, it is involved in involuntary functions, such as
vomiting, breathing, sneezing, and maintaining the correct blood pressure.
Hypothalamus: sitting just above the brain stem and roughly the size of an almond, the
hypothalamus secretes a number of neurohormones and influences body temperature
control, thirst, and hunger.
Thalamus: positioned in the center of the brain, the thalamus receives sensory and motor
input and relays it to the rest of the cerebral cortex. It is involved in the regulation of
consciousness, sleep, awareness, and alertness.
Amygdala: two almond-shaped nuclei deep within the temporal lobe. They are involved in
decision-making, memory, and emotional responses; particularly negative emotions.
SPINAL CHORD
The spinal cord carries information from the brain to the rest of the body.

4. 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 peripheral nervous system (PNS) that run
in from the skin, muscles, and joints.
Motor commands from the brain travel from the spine to the muscles 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 circuits that control certain reflexive responses, such as the
involuntary movement your arm might make if your finger was to touch 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 requiredTrusted Source to stop and start the process, or make
changes if, for instance, an object appears in your path.
White and gray matter
The CNS can be roughly divided into white and gray matter. As a very general rule, the brain
consists of an outer cortex of gray matter and an inner area housing 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 often called support cells for neurons. In the brain, they
outnumber nerve cells 10 to 1.

5. Without glial cells, developing nerves often lose their way and struggle to form functioning
synapses.
Glial cells are found in both the CNS and PNS but each system has different types. 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.
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 (CSF) and keep it circulating using their whip-like cilia.
Radial glia: act as scaffolding for new nerve cells during the creation of the embryo’s nervous
system.
Cranial nerves
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 – mostly the neck and head.
Of these 12 pairs, the olfactory and optic nerves arise from the forebrain and are considered
part of the central nervous system:
Olfactory nerves (cranial nerve I): transmit information about odors 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.

6. Difference between the CNS and peripheral nervous system
The term peripheral nervous system (PNS) refers to any part of the nervous system that
lies outside of the brain and spinal cord. The CNS is separate from the peripheral nervous
system, although the two systems are interconnected.
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 much shorter. PNS nerve axons can be up to 1 meter long (for
instance, the nerve that activates 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 (regrowth of
cells). 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 ability.
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.