Download PDF How the brain works

By Dr Ava Easton, Encephalitis Society and reviewed by Dr Ashik Mohamed Babu, Royal Liverpool University Hospital

In order to understand the effects of encephalitis on the brain, it can be helpful to understand how the brain works.

The brain

The brain is an amazing organ: it controls everything you think, feel and do. The brain is made up of billions of nerve cells (neurons). A neuron has a cell body containing a nucleus and an axon which carries the impulse away from the cell body. Each neuron makes thousands connections to other neurons. This ‘neural network’ is similar to the way roads connect to make road networks. In some instances, neurons are encased in a substance called myelin. This coating acts like the insulation around a wire, making impulses faster and more precise.

The brain has several areas with different functions and damage to these discrete regions give rise to certain symptoms. These areas of the brain are called lobes. The frontal lobe is involved in decision making, personality and complex movement. The parietal lobe is responsible for sensation and the occipital lobe for sight. The temporal lobe is responsible for memory and aspects of speech.  Each of these lobes are made up of the neurons described above and act as the computer processing unit, sending and receiving information from all over the body.

The brain and infection

The brain is protected by a blood-brain barrier which prevents any large molecules passing from the blood into the brain. The blood-brain barrier acts very effectively to protect the brain from many common infections. Thus, infections of the brain are very rare.

The outcome of any infection is dependent upon the ability of the infection to cause disease and the response of the immune system. The immune response protects organisms against injury and infection by delivering white blood cells to sites of injury to kill potential pathogens and promote tissue repair. However, the powerful inflammatory response also has the capacity to cause damage to normal tissue. Unfortunately the immune response to an infection of the brain can contribute more to the disease process than the infection itself.

The brain and encephalitis

In infectious encephalitis, viruses entering neurons utilise components of the cell in order to replicate (make copies of themselves). This uses up energy stores and oxygen, damaging the cell. In post-infectious / autoimmune encephalitis it is the immune system that causes damage to neurons or other brain cells. In both types of encephalitis, by-products of the immune system’s actions (fluid, white blood cells, the contents of dead nerve cells and disabled viruses) can significantly alter the fluid surrounding neurons and affect their functioning. Cell death causes a fluid build up in the brain. This can cause increased pressure within the skull sometimes causing reduced consciousness

The extra unwanted fluids build up rapidly, and glial cells (cells that support neurons) try to absorb the unwanted chemicals and fluids in order to protect neurons from harm, and in the process they swell up too. Glial cells act as sponges and scavengers of toxic by-products, caused by the inflammation but when they become overloaded, they die and then re-release the toxic chemicals back into the fluid, where they kill additional neurons. The extremely high levels of these substances are sufficient to kill vulnerable and weakened neurons by damaging their membranes or by exciting them to a point where they “burn out” and die.

At the site of inflammation and in nearby tissue, there is biological chaos, as the brain tries to adjust and fight the consequences of the damage. The dying cells give off chemicals that activate macrophages (white blood cells), which move from the bloodstream into the injury area, to absorb and eliminate debris. Glial cells and their helpers, which have gathered at the site to clean it up, now begin to form the scar tissue that will remain a part of the brain's new architecture. Sometimes, the glial barriers prevent healthy, remaining neurons from restoring axonal connections. In other cases, nerve terminals cannot pass the scar, and abnormal activity is then generated that can lead to epileptic seizures.

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FS064V3 How the brain works

Page created: August 2002/ Last update: August 2019/ Review date: August 2022

Disclaimer: We try to ensure that the information is easy to understand, accurate and up-to-date as possible. If you would like more information on the source material the author used to write this document please contact the Encephalitis Society. None of the authors of the above document has declared any conflict of interest which may arise from being named as an author of this document.