Central nervous system

The central nervous system (CNS) is the part of the nervous system consisting primarily of the brain, spinal cord and retina. It plays a vital role in controlling and coordinating all the activities of the body. The brain integrates information from the senses and decides how the body should respond, while the spinal cord helps send these messages quickly to and from the brain.

The CNS is named for its central role in the body, acting like a command center. It is found in bilaterally symmetric and triploblastic animals—which includes most multicellular animals except simple creatures like sponges and diploblasts. This system is made of special nervous tissue arranged along the body from the rostral (nose end) to the caudal (tail end) axis.

Only certain animals such as arthropods, cephalopods and vertebrates have a true brain, although simpler creatures like onychophorans, gastropods and lancelets have smaller structures that act in a similar way. This article will focus on the central nervous system in vertebrates, which is very different from that of other animals.

Overview

In vertebrates, the brain and spinal cord are protected by layers called the meninges. These layers act as a shield against harmful substances in the blood, keeping the brain safe. The brain and spinal cord are also surrounded by a special fluid called cerebral spinal fluid, which helps protect them.

The central nervous system is located inside the body, with the brain sitting inside the skull and the spinal cord running through the vertebrae. It also includes parts like the retina and optic nerves, which connect directly to the brain, helping us see.

Structure

Main article: Neuroanatomy

The central nervous system (CNS) has two main parts: the brain and the spinal cord. The brain sits inside the skull, protected by bone. The spinal cord connects to the brain and runs down through the spine, protected by the vertebrae. It starts at the base of the skull and ends around the first or second lumbar vertebra.

Microscopically, the CNS contains white and gray matter. White matter is made of axons and oligodendrocytes, while gray matter contains neurons and other fibers. Both areas include glial cells, which support the nervous system.

The brain, including the cerebrum, midbrain, and hindbrain, has a outer layer called the cortex made of gray matter, with white matter inside forming tracts and commissures. There is also subcortical gray matter in various areas.

The spinal cord sends out nerves that connect to the skin, muscles, and joints. These nerves carry both motor commands from the CNS and sensory information to the CNS. The spinal cord also helps relay information to the brain.

The brain is the major processing unit of the CNS. While the spinal cord can handle simple reflexes, the brain processes more complex information and controls most body functions.

The brainstem, made of the medulla, pons, and midbrain, controls important functions like blood pressure, breathing, and balance. The cerebellum helps coordinate movements and balance. The thalamus and hypothalamus also play key roles in processing information and regulating basic needs like hunger and thirst.

The largest part of the brain, the cerebrum, controls many functions including thought, memory, and movement. It has several important structures like the hippocampus for memory and the amygdala for emotions.

Development

Main article: Neural development

When a baby animal is just starting to grow, a special fold forms along its back. This fold closes up to make a tube called the neural tube. Inside this tube are special cells that will become the brain and spinal cord, which make up the central nervous system (CNS).

The front part of the neural tube becomes the brain, and the back part becomes the spinal cord. As the animal grows, these parts change and form different sections of the brain and spinal cord that help control the body.

Evolution

See also: Cephalization and Archicortex

Planaria

Planarians, a type of flatworm, have a very simple nervous system. They have a small brain made of two fused parts at the front and nerve cords that run down their bodies. This setup is like the central nervous system in more complex animals.

Arthropoda

In insects and other arthropods, the central nervous system includes a nerve cord along the belly, plus special clusters of cells in the head and just below it.

Chordata

The central nervous system of animals like fish, amphibians, reptiles, birds, and mammals is placed on the back of the body, above the gut and spine. The basic design of this system stays very similar across these animals. One big change over time is that the front part of the brain, called the telencephalon, gets bigger and more important.

Mammals

Mammals have a special part of the brain called the neocortex, which helps with thinking and processing senses. This part is very simple in animals like the duck-billed platypus and spiny anteaters, as well as in marsupials like kangaroos and koalas. In animals like monkeys and humans, this area grows much larger and more folded, allowing for more complex thoughts and feelings. Even in small mammals like rats, this area exists but without the folds seen in larger animals.

Clinical significance

Diseases

The central nervous system can be affected by many different diseases and conditions. These include infections like encephalitis and poliomyelitis, disorders such as ADHD and autism, seizure disorders like epilepsy, and headache disorders such as migraine. There are also diseases that appear later in life, such as Alzheimer's disease and Parkinson's disease, as well as conditions like multiple sclerosis. Some genetic disorders, like Krabbe's disease and Huntington's disease, also affect the central nervous system. Cancers of the central nervous system can be very serious, depending on their size, location, and how quickly they grow.

Doctors usually only use special imaging tests of the brain when there is a specific medical reason to do so, rather than as a routine check-up.