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:
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
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 are connected to thousands of other neurons.
In total, around 100 billion neurons and 1,000 billion glial cells make up the human brain.3
The brain is the central control module of the body and coordinates a multitude of tasks. From physical motion to the secretion of hormones, the creation of memories and the sensation of emotion.
To carry out these roles, some sections of the brain have dedicated roles. However, many higher functions - reasoning, problem solving, creativity - involve different areas working together in networks and recruit different modules at different times.
The brain is roughly split into four lobes, a simplified description of each is given below:
The brain can be roughly split into four lobes, shown as green, purple, yellow and pink in this picture.
- Temporal lobe (green): the temporal lobe is important for processing sensory input and assigning it emotional meaning. It is also involved in laying down long-term memories in conjunction with the hippocampus. Some aspects of language perception are also housed here
- Occipital lobe (purple): the occipital lobe is the visual processing region of the mammalian brain, housing the visual cortex. Damage to the primary visual cortex can cause blindness
- 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.
The following are some specific brain regions of note with a summary of their theorized functions. The list is by no means exhaustive:
- Basal ganglia: involved in the control of voluntary motor movements, procedural learning and decisions about which motor activities to carry out.4 Diseases that affect the area include Parkinson's disease and Huntington's disease
- Cerebellum: (shown in blue in the diagram above) predominantly involved in fine, precise motor control, but also involved 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 associated with an underactive Broca's area5
- 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 and left-handed people, ambidextrous people and musicians typically have larger ones6
- Medulla oblongata: extending below the skull, it is essential for a number of 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 a variety of responses including body temperature control, thirst and hunger
- Thalamus: sitting centrally in 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.
Central nervous system diseases
A system as complex and sizable as the CNS can malfunction for a number of reasons. Below are the major causes of disorders that effect the CNS:
The CNS is susceptible to many diseases and injuries, ranging from infection to cancer.
- Trauma: any significant injury to the brain or spinal cord can cause negative health consequences. Depending on the site of the injury, symptoms can vary widely, from paralysis to mood disorders
- Infections: various micro-organisms and viruses can invade the CNS. These include fungus such as cryptococcal meningitis; protozoa, including malaria; bacteria, as is the case with leprosy, or viruses
- Degeneration: the spinal cord or brain can degenerate, causing different issues dependent on which areas degenerate. One example is Parkinson's disease which involves the gradual degeneration of dopamine-producing cells in the substantia nigra, part of the basal ganglia
- Structural defects: the most common examples within this category are birth defects; these include anencephaly, where major parts of the skull, brain and scalp are missing at birth
- Tumors: both cancerous and noncancerous tumors can impact parts of the central nervous system. Both types can cause damage and yield an array of symptoms depending on where they develop
- Autoimmune disorders: in some cases, an individual's immune system can mount an attack on healthy cells. For instance, acute disseminated encephalomyelitis is characterized by an immune response against the brain and spinal cord, attacking myelin (the nerves' insulation) and, therefore, destroying white matter
- Stroke: a stroke is an interruption of blood supply to the brain; the resulting lack of oxygen causes tissue to die in the affected area.7
Neurosurgeons have used a laser probe to open the brain's protective cover long enough to deliver chemotherapy drugs for treating glioblastoma, the most common and aggressive form of brain cancer. The results are published in PLOS One.
New research, published this week in Neurology, confirms that remaining physically and mentally stimulated helps to stave off cognitive decline. However, the underlying brain alterations involved in Alzheimer's disease do not seem be affected in most individuals.