all about neurones
Neurones are nerve cells carrying information. This page is based on an article featured in Epilepsy Review magazine. It looks at what neurones do, what they look like and how they pass messages.
What do neurones do?
Neurones are a particular type of cell that carry information messages or signals to and from the brain and the rest of the body. The connections from groups of neurones form nerves, a bit like telephone wires from a telephone exchange. The messages they send are called 'nerve impulses' or 'action potentials'. As all these messages are sent around the body by nerves, this is how we get the name nervous system.
The nervous system
The nervous system is made up of two different parts. The brain and the spinal cord make up the central nervous system; which is like the body's 'control centre'. The brain sends and receives messages, which the spinal cord helps to relay. The peripheral nervous system contains all the nerves for the rest of the body.
Peripheral nervous system - the nerves in the body, other than in the brain and spine.
What do neurones look like?
The size and shape of neurones varies depending on where in the body they are, but most of them share the same general features and structure. The structure of a neurone is similar to a tree, as we will see.
Every neurone has an outside coating or wall called a cell or plasma membrane. This membrane has an important role in how neurones communicate.
The neurone has a type of 'control centre' called the cell body. This contains the nucleus of the cell (like the cell's 'brain') and helps to make sure the rest of the neurone is working. This is like the top of the tree trunk.
Spreading out from the cell body are short branches called dendrites. These are the part of the neurone that receives messages from other cells. Each dendrite may have many branches, which can look like the big and small branches of the tree.
The axon of the neurone is like the trunk of the tree. It usually stretches out from the cell body, and the point where this happens is called the axon hillock. While the size of the dendrites and cell body tend to be fairly constant, the length of the axon varies. This part of the cell carries the messages away from the cell body, to the next cell.
At the end of the axon is the synaptic terminal, like the roots of the tree. This is the part of the neurone that connects to and communicates with the next cell, often the dendrites of another neurone. Where these two cells connect is called the synapse.
Each neurone can connect to many other cells. The messages from one neurone, which can be either a chemical or an electrical message, pass through this synapse from one cell to another. The synaptic terminal contains a number of small sacs, called synaptic vesicles. These vesicles contain neurotransmitters, which are used when neurones send and receive messages.
Neurones can be grouped together depending on their function, and whether they send or receive information. There are three types, which altogether make up the circuits of the nervous system.
Receptor or sensory neurones receive information from the senses (eg the ears, eyes, nose and skin) and respond by sending this information to the brain.
Effector or motor neurones carry messages from the brain to muscles and glands in the body. These messages cause something to happen in the muscles or glands, which is how the body moves and functions.
Interneurones send and receive information from sensory, motor or other interneurones. In effect they form a link between the sensory and motor neurones.
The brain and nervous system work by sending messages around the body, from one neurone to another. These messages, called nerve impulses or action potentials, are received by one neurone and carried along its length to its end. Once the message gets to the end of one neurone, it passes to the next neurone, or to another cell in the body.
Looking more closely at how neurones communicate, this involves a lot of different things happening. To make this simpler, we can break this communication down into two stages. The first stage is how messages are carried along a neurone, and the second stage is how messages are passed from one neurone to another.
The science bit!
To understand how neurones communicate, we need to know a little bit of chemistry. There are chemicals found in the human body which have an electrical charge. These are called ions. Although there are lots of different ions in the body, the important ones when looking at how neurones work are sodium, potassium, and calcium. These ions all have an electrical charge that is positive. (Some ions, such as chloride, have an electrical charge that is negative.)
Ions and the neurone
Messages are sent around the body as action potentials carried along the neurones. The action potential usually starts at the axon hillock and moves down the axon, like a message moving along a telephone wire. This movement happens due to changes in the balance of electrical charge inside and outside the cell, at points along the axon.
The electrical charge at any point along the axon depends on the amount of ions inside and outside the cell. Ions can pass through the cell membrane, and into or out of the cell, through ion channels (like gateways). The cell membrane controls what ions can pass through at any time by opening and closing the channels.