What does the central nervous system do

This page outlines the basic physiology of the central nervous system, including the brain and spinal cord. Separate pages describe the nervous system in general, sensation, control of skeletal muscle and control of internal organs.

1. The central nervous system CNS is responsible for integrating sensory information and responding accordingly. It consists of two main components:
   1. The spinal cord serves as a conduit for signals between the brain and the rest of the body. It also controls simple musculoskeletal reflexes without input from the brain.
   2. The brain is responsible for integrating most sensory information and coordinating body function, both consciously and unconsciously. Complex functions such as thinking and feeling as well as regulation of homeostasis are attributable to different parts of the brain.
2. The brain and spinal cord share some key anatomic features:
   1. Living nervous tissue has the consistency of jelly and requires special protection from physical damage. The entire CNS is encased in bone. The brain is within the cranium, while the spinal cord runs within a canal through the vertebrae.
   2. Within its bony case, the entire CNS is bathed in a cerebrospinal fluid (CSF), a colorless fluid produced by special structures in the brain. CSF provides a special chemical environment for nervous tissue, as well as an additional buffer against physical damage.
   3. The special chemical environment of nervous tissue is maintained by the relatively impermeable membranes of capillaries in the CNS. This feature is known as the blood-brain barrier.
   4. There are two general types of tissue in the CNS:
      1. Gray matter consists of nerve cell bodies, dendrites, and axons. Neurons in gray matter organize either in layers, as in the cerebral cortex, or as clusters called nuclei.
      2. White matter consists mostly of axons, causing it to look white due to the myelin sheathing of the axons.
3. In the early embryo, the CNS forms as a relatively uniform tube. The major regions of the brain develop as enlargements at the head end of this tube:
   1. The medulla oblongata appears as a swelling at the upper end of the spinal cord. Besides being a conduit for fibers running between the spinal cord and higher regions of the brain, it contains control centers for involuntary functions such as blood pressure, breathing, swallowing and vomiting.
   2. Just above the medulla are the pons and cerebellum. The pons relays information between higher regions of the brain and the cerebellum, which processes sensory information and helps coordinate movement.
   3. The next segment, the midbrain, is primarily responsible for eye movement.
   4. Above the midbrain lies the diencephalon, which is composed of two major parts:
      1. The thalamus processes and integrates all sensory information going to the higher regions of the brain.
      2. The hypothalamus is critical for homeostasis, the maintenance of the body's internal environment. It influences nervous control of all internal organs and also serves as the master regulator of endocrine function by its control over the pituitary gland.
   5. The highest region of the brain is the cerebrum, which includes both the cerebral cortex that is visible on the outside of the brain as well as other internal structures. The cerebrum is responsible for conscious sensation and voluntary movement, as well as advanced functions such as thinking, learning and emotion.

The nervous system is made up of all the nerve cells in your body. It is through the nervous system that we communicate with the outside world and, at the same time, many mechanisms inside our body are controlled. The nervous system takes in information through our senses, processes the information and triggers reactions, such as making your muscles move or causing you to feel pain. For example, if you touch a hot plate, you reflexively pull back your hand and your nerves simultaneously send pain signals to your brain. Metabolic processes are also controlled by the nervous system.

There are many billions of nerve cells, also called neurons, in the nervous system. The brain alone has about 100 billion neurons in it. Each neuron has a cell body and various extensions. The shorter extensions (called dendrites) act like antennae: they receive signals from, for example, other neurons and pass them on to the cell body. The signals are then passed on via a long extension (the axon), which can be up to a meter long.

The nervous system has two parts, called the central nervous system and the peripheral nervous system due to their location in the body. The central nervous system (CNS) includes the nerves in the brain and spinal cord. It is safely contained within the skull and vertebral canal of the spine. All of the other nerves in the body are part of the peripheral nervous system (PNS).

Regardless of where they are in the body, a distinction can also be made between voluntary and involuntary nervous system. The voluntary nervous system (somatic nervous system) controls all the things that we are aware of and can consciously influence, such as moving our arms, legs and other parts of the body.

The involuntary nervous system (vegetative or autonomic nervous system) regulates the processes in the body that we cannot consciously influence. It is constantly active, regulating things such as breathing, heart beat and metabolic processes. It does this by receiving signals from the brain and passing them on to the body. It can also send signals in the other direction – from the body to the brain – providing your brain with information about how full your bladder is or how quickly your heart is beating, for example. The involuntary nervous system can react quickly to changes, altering processes in the body to adapt. For instance, if your body gets too hot, your involuntary nervous system increases the blood circulation to your skin and makes you sweat more to cool your body down again.

Both the central and peripheral nervous systems have voluntary and involuntary parts. However, whereas these two parts are closely linked in the central nervous system, they are usually separate in other areas of the body.

The involuntary nervous system is made up of three parts:

• The sympathetic nervous system

• The parasympathetic nervous system

• The enteric (gastrointestinal) nervous system

The sympathetic and parasympathetic nervous systems usually do opposite things in the body. The sympathetic nervous system prepares your body for physical and mental activity. It makes your heart beat faster and stronger, opens your airways so you can breathe more easily, and inhibits digestion.

The parasympathetic nervous system is responsible for bodily functions when we are at rest: it stimulates digestion, activates various metabolic processes and helps us to relax. But the sympathetic and parasympathetic nervous systems do not always work in opposite directions; they sometimes complement each other too.

The enteric nervous system is a separate nervous system for the bowel, which, to a great extent, autonomously regulates bowel motility in digestion.

Sources

• Menche N. (ed.) Biologie Anatomie Physiologie. Munich: Urban & Fischer/ Elsevier; 2012.

• Pschyrembel W. Klinisches Wörterbuch. Berlin: De Gruyter; 2014.

• Schmidt R, Lang F, Heckmann M. Physiologie des Menschen: mit Pathophysiologie. Heidelberg: Springer; 2011.

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