MS ExplainedThe central nervous system

MS is a condition in which damage occurs within the central nervous system.

The nervous system

The nervous system is the means by which the body communicates messages to and from muscles and organs and maintains awareness of the outside world through the senses.

diagram of the central nervous system showing the brain and spinal cord

The nervous system is divided into two areas

the central nervous system (CNS) consists of the brain and spinal cord and is enclosed within the skull and backbone
the peripheral nervous system (PNS) comprises all other nerves.

The peripheral nervous system

The peripheral nervous system is the cabling that transmits information to and from the central nervous system from the rest of the body.

There are two main types of nerve in the peripheral nervous system:

Sensory nerves - these collect information from the body's sense organs - responding to touch, temperature, pain, position, smell, sound and sight.
Motor nerves - these pass messages from the central nervous system. When information has been processed centrally, these nerves carry instructions for action to the muscles controlling movements, speech, internal organs, and to the various glands, such as the sweat glands in the skin.

The central nervous system

The central nervous system receives, processes and stores information and initiates instructions for bodily activities.

The central nervous system is divided into two major parts:

the brain analyses and stores information and directs the action of the body
the spinal cord passes information to and from the brain and is responsible for reflex reactions. Reflex reactions are automatic responses to stimuli, such as pulling the hand away if it touches something hot. By responding immediately to stimulation before a signal has travelled to the brain for processing, we can react to situations that need instant actions without wasting time waiting for the brain to analyse the problem.

The central nervous system is made up of two types of cells - nerve cells and support cells.

Nerve cells

Nerve cells are called neurons. The human brain and central nervous system contain about 100 billion neurons.

The brain develops rapidly in the unborn child and in the first years of life. By the age of two, a person has most of the neurons that they will have during their lifetime. Although other cells die and are replaced, many neurons are never replaced when they die. As a result, damage caused by MS that leads to the damage or destruction of neurons can cause permanent disabilities.

Neurons have specialised extensions called dendrites and axons. A neuron usually has a number of dendrites but only one axon.

Information enters the neuron via the dendrites, passes through the cell body and then along the axon. At the end of the axon, chemicals called neurotransmitters carry the message over a small gap called a synapse to a dendrite of another neuron.

The axon can be as long as a metre, making neurons some of the longest cells in the body. It is surrounded by a sheath of fatty protein called myelin, which acts as insulation. The myelin sheath has short gaps about one micrometre apart known as nodes of Ranvier, which assist fast conduction of messages. Nerve messages pass along the axon from node to node. The thickness of the myelin sheath and the size of the gap between nodes determine the speed of messages, which can travel as fast as 120 metres/second (268mph).

Support cells

About 40 per cent of the total volume of the brain and spinal cord is made up of cells that support neurons in various ways but which do not carry information themselves.

The collective name for these support cells is glial cells. Glia comes from the Greek word for glue and one of the roles of these cells is to hold the nerve cells in place. Other functions include transporting nutrients to neurons, cleaning up debris and digesting parts of dead neurons.

Glial cells also provide the insulation to neurons through the production of myelin. The specific glial cells that produce myelin in the central nervous system are called oligodendrocytes. Each oligodendrocyte can supply myelin for several axons and each axon can be supplied by several oligodendrocytes. The myelin produced by oligodendrocytes wraps around the axons in thin sheets like a Swiss roll.

diagram of an oligodendrocyte myelinating serveral axons

Damage to myelin, and the interruption or delay to nerve messages along the axon that this causes, leads to the symptoms of MS. If the protection of myelin is lost, the axon itself will be destroyed, which can cause permanent problems.

Cerebrospinal fluid and the blood brain barrier

The entire surface of the central nervous system is surrounded by a clear, colourless liquid called cerebrospinal fluid (CSF). This acts to cushion the brain and spinal cord within the bony casing of the skull and backbone and helps support the weight of the brain.

The cerebrospinal fluid and the central nervous system are surrounded by a layer of cells called the blood brain barrier. This prevents larger molecules circulating in the blood from reaching the central nervous system. In MS, the blood-brain barrier is breached, allowing immune cells to move across from the blood stream and into the central nervous system.

Next page - The immune system