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Get The Facts

Understanding the nervous system

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Medical

Understanding the nervous system

Let's take a look at all the different parts of the nervous system - the brain, spinal cord and nerves.

We'll briefly explain what each part does, how the different parts of the nervous system communicate with each other and the rest of the body, and discuss the effects of disease or damage to it.

The brain

The brain is the body's control center. It may only weigh about 1.5kg but it is estimated to have about 100 billion cells. It controls everything we do, from basic body functions (such as breathing, heart beat and blood pressure) to our movements, speech, senses and aspects of our personality.

 

Cerebral hemispheres

The brain is divided into two cerebral hemispheres - the left hemisphere and the right hemisphere. Each hemisphere tends to specialize in certain functions but the two hemispheres work seamlessly together, sharing information:

  • The right hemisphere tends to be more visual, thinking in pictures. It sees, recognizes and organizes information for the left side to analyze and process further. Generally speaking, the right hemisphere controls muscles on the left side of the body.
  • The left hemisphere is mostly responsible for speech, language, calculations, maths and logical abilities. It generally controls muscles on the right side of the body.

 

Lobes

The brain is divided further into "lobes" that handle specific areas of function, the frontal, parietal, temporal and occipital lobes:

  • The frontal lobes look after planning, organizing, reasoning, decision-making, judgment and the emotions.
  • The temporal lobes also have a role in language, particularly in the ability to hear and understand it. They are concerned with memory, the emotions, the ability to enjoy music and to recognize and identify things we see, such as faces or objects.
  • The parietal lobes are concerned with the perception of sensations, such as touch, pressure, temperature, pain and the understanding of words and sentences, reading and writing and sometimes the ability to use numbers. They deal with spatial awareness, such as our ability to find our way around a house, to drive a car and to reach for objects.
  • The occipital lobes are primarily concerned with vision but also with our ability to recognize what we see in terms of identifying colours, locating objects in the environment and seeing objects accurately.

 

Below the cerebral hemispheres are the cerebellum and the brain stem, which connect with the spinal cord.

 

The cerebellum is involved in "doing" rather than "thinking" activities. It carries out orders from the cerebral hemispheres above and keeps a number of vital but routine functions kicking over, such as maintaining balance and ensuring our muscles move in a smooth, coordinated way.

 

The brain stem controls many vital functions including breathing, blood pressure, blood circulation, swallowing, appetite, body temperature and digestion, as well as the need for water, staying awake and sleeping. It is also the main route for nerve fibers running between the cerebral hemispheres and the spinal cord. Any damage in the brain stem can produce widespread and profound effects.

 

The Nervous system

The brain communicates messages through a complex network of nerves that travel throughout our body. Together, the brain and nerves are known as the nervous system, while the spinal cord and the brain make up the central nervous system.

 

On their own, the nerves that run throughout our body are called the peripheral nervous system. They relay information from our brain through our spinal cord to the body, and back again. The autonomic nervous system is part of the peripheral nervous system. It conveys messages from all of the organs in our chest, abdomen and pelvis. For example, it manages our "fight and flight" responses, our "rest and digest" responses. It looks after the automatic activities of our heart and blood vessels and plays an important part in sexual response and bladder control.

 

Neurones - basic building blocks

The basic building blocks of the nervous system are nerve cells or neurones. We are born with about 100 billion neurones that must last a lifetime. Unlike all the other cells in the body, neurones do not replace themselves if they die or are damaged.

 

Gray matter is formed when neurones cluster together on the outer part of the brain and inner part of the spinal cord.

 

White matter is found on the inner part of the brain and outer part of the spinal cord. It is made up of bundles of nerve fibers called axons, which are really just the long thin extensions of neurones. These axons are covered by a white, fatty substance called myelin (hence the term "white matter"), which insulates them, like the plastic coating of an electric wire. The axons then bundle together, like the individual telegraph wires in a cable, to form a nerve.

 

How it all works

The brain is in constant contact with all parts of the body, sending instructions and receiving feedback from the senses. The axons carry these messages as tiny electrical currents or nerve impulses:

  • Outgoing messages are sent from the brain to activate the muscles of the body travel along the motor pathways. The neurones that make up these pathways are called motor neurones.
  • Incoming messages  are sent from the senses back to the spinal cord and brain come along the sensory pathways. These are called sensory neurones.

 

How change affects the nervous system

Various conditions from illness (encephalitis) and incidents (heart attack, stroke) to accidents (near drowning, a skateboarding fall) can cause brain damage, which affect the way the nervous system functions by:

  • affecting brain function itself
  • affecting the brain's ability to communicate with the rest of the body
  • affecting the ability of muscles to respond to the brain's orders (nerve impulses).

 

Acquired Brain Injury

Damage to the brain is called Acquired Brain Injury. An accident, illness or incident can cause direct injury to the brain cells, and any interruption to the blood supply to the brain may also cause damage. Without a constant blood supply, the brain is unable to maintain its extraordinary level of functionality.

 

For example, a lack of oxygen (hypoxia) during near drowning affects blood supply to the brain, as does severe bleeding in other parts of the body, or any excessive pressure within the skull, which might occur due to brain swelling or bruising.

 

Changes to the brain and nervous system can lead to the following kinds of issues:

  • Medical problems - headache and epilepsy are two of the most common.
  • Sensory difficulties such as sight, hearing, touch, smell, taste, body-temperature control and awareness of body position e.g. some people may become hypersensitive to sound, heat or cold, while others may lose awareness of body position which creates problems with buttoning shirts, using a spoon or stepping off a curb safely.
  • Physical difficulties such as paralysis and limb weakness or problems with coordination, balance and tremor. Fatigue is also very common.
  • Thinking abilities including poor concentration, memory loss and difficulties in planning, organizing, problem-solving, abstract thinking and responding effectively may arise. Slowness in thinking is very common.
  • Communication and speech difficulties such as slurred words or difficulty understanding others. Some people may have trouble swallowing. Others have difficulties using language, such as finding the right words or understanding sentences.
  • Behaviour - a person may become aggressive, lack initiative or be poorly motivated. They may have difficulty regulating their own behaviour in a way that is socially acceptable e.g. a person with a brain injury may make inappropriate jokes.
  • Personality changes can occur as a result of damage to the brain. One example is having difficulty regulating emotional responses, such as becoming irritable or laughing or crying too easily. Personality change can also be the result of a person's reaction to having ABI, which is often the case with depression.

 

Some of the effects of a brain injury are obvious and profound, while others are subtle, yet disabling. The effects will vary widely from person to person and the recovery process may continue over many years.  

References and further information

This article has been reproduced with the permission of BrainLink, from their excellent brain injury resource available for free download at www.brainlink.org.au


This publication is split into sections covering medical issues, common changes after a brain injury, practical assistance and emotional issues. The website also has a wide range of fact sheets on many other issues. BrainLink is a Victorian service dedicated to improving the quality of life of people affected by conditions of the brain, and providing support to their families.

 

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