The Central Nervous System (CNS)

The central nervous system consists of the brain and spinal cord. It is referred to as “central” because it combines information from the entire body and coordinates activity across the whole organism.

This article gives a brief overview of the central nervous system (CNS). We will look at the types of cells involved, different regions within the brain, spinal circuitry, and how the CNS can be affected by disease and injury.

Fast facts on the central nervous system

Here are some key points about the central nervous system: More details and supporting information are in the main article.

  • The brain and spinal cord make up the central nervous system.
  • The brain is the most complicated organ in the body, consuming 20% of the oxygen we inhale.
  • There are an estimated 100 billion neurons in the brain, each of which is connected to thousands of others.
  • The temporal, parietal, occipital, and frontal lobes are the four primary lobes of the brain.

What is the central nervous system?

Central nervous system

The brain and spinal cord make up the central nervous system. The skull (cranial cavity) protects the brain, and the spinal cord runs from the back of the brain down the centre of the spine to the lumbar region of the lower back. The meninges are a protective triple-layered membrane that surrounds the brain and spinal cord.

Anatomists and physiologists have researched the central nervous system extensively, yet it still retains many secrets; it regulates our thoughts, movements, emotions, and desires.

It also regulates our respiration, heart rate, hormone secretion, body temperature, and many other functions.

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 brain

The cerebral cortex (the outermost region of the brain and the largest by volume) contains an estimated 15–33 billion neurons, each of which is connected to thousands of other neurons, making it the most complex organ in the human body.

The human brain is made up of approximately 100 billion neurons and 1,000 billion glial (support) cells. Our brain consumes about 20% of our whole body energy.

The brain is the body’s central control module, coordinating activity. From physical movement to hormone secretion, memory formation, and emotional sensations, the human body is a complex system.

Some parts of the brain are specifically designed to carry out these activities. However, many higher processes, like as thinking, problem-solving, and creativity, require collaboration between various areas.

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.

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 associated with an underactive Broca’s area.

Corpus callosum: a broad band of nerve fibers that joins 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 cord

Spinal cord

The spinal cord, which runs virtually the whole length of the back, not only transmits information between the brain and the body, but also performs additional functions.

31 spinal nerves enter the spinal cord from the brainstem, where it joins the brain.

It links with nerves of the peripheral nervous system (PNS) that travel through the skin, muscles, and joints along its length.

Motor commands from the brain flow from the spine to the muscles, whereas sensory information passes from sensory tissues like the skin to the spinal cord and then to the brain.

Certain reflexive responses, such as the involuntary movement of your arm if your finger touches a flame, are controlled by circuits in the spinal cord.

More sophisticated movements, such as walking, can be generated by the circuits within the spine. The spinal nerves can coordinate all of the muscles required for walking even without brain input.

When a cat’s brain is detached from its spine, the brain has no contact with the body, and the cat is placed on a treadmill, it will begin walking voluntarily. Only the brain is necessary to stop and start the procedure, or to make alterations if an object arrives on your route, for example.

White and gray matter

The CNS can be roughly divided into the 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.

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.

Central nervous system diseases

Tumor on tissue

Below are the major causes of disorders that affect the CNS:

Trauma: Depending on the site of the injury, symptoms can vary widely, from paralysis to mood disorders.

Autoimmune disorders: an individual’s immune system can destroy healthy cells in some situations. Acute disseminated encephalomyelitis, for example, is marked by an immune response directed towards the brain and spinal cord, which attacks myelin (the nerve insulation) and thereby destroys white matter.

Infections: fungi, such as cryptococcal meningitis; protozoa, such as malaria; bacteria, such as leprosy; and viruses can all infect the CNS.

Degeneration: The spinal cord or brain can degenerate in some circumstances. Parkinson’s disease, for example, is characterized by the slow degeneration of dopamine-producing cells in the basal ganglia.

Birth deformities: such as anencephaly, in which sections of the skull, brain, and scalp are absent at birth, are the most frequent structural defects.

Tumors: can affect sections of the central nervous system, both malignant and noncancerous. Depending on where they originate, both types can cause damage and produce a variety of symptoms.

A 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.

The difference between the CNS and peripheral nervous system

Any element of the nervous system outside of the brain and spinal cord is referred to as the “peripheral nervous system.” Although the two systems are interrelated, the CNS and the peripheral nervous system are independent. There are several distinctions between the CNS and the PNS, one of which is the cell size.

The CNS has significantly shorter nerve axons, which are the slender projections of nerve cells that transport impulses.

Nerve axons in the PNS can be up to 1 meter long (for example, the nerve that operates the big toe), whereas they are rarely longer than a few millimeters in the CNS.

Another significant distinction between the CNS and the PNS is regeneration (regrowth of cells). A large portion of the PNS can regenerate; for example, if a nerve in your finger is severed, it can regrow. The CNS, on the other hand, lacks this ability.

The central nervous system’s components are further divided into a variety of segments. We’ll go through some of these aspects in greater depth below.

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