Cells can be considered as the fundamental unit of life because they are, to some extent, self-sufficient. That is to say that cells, whether they be in your liver or in your brain, contain all of the necessary chemicals to support the life of that cell. However, even though they may be microscopic, these little chemical factories need to be highly organised so that the chemical processes going on inside them are appropriate and effective.
Neurones have an additional communication problem to solve because part of their function is to carry information from one part of the brain to another, or even to a distant part of the body. So, for example, when you decide to reach out and pick up a cup of tea, that decision thought originates in your brain but the message then has to be sent to the appropriate part of your brain that send axons out to control the muscles of your arm and hand which will enable that to happen.
The bottom line then is that neurones may each be small, but in order to convey information over big distances they have to be stretched out to be very long. Recall the diagram of the classical neurone that we constructed in Week 1 – here we identified the axon as a process emanating from the cell body and ending in terminals.
It is the axon that can be stretched out to be very long indeed. In the diagram in the video the axon was about 10 times longer than the diameter of the cell body. In a real neurone, the diameter of the cell body, on average, is actually about 10 micrometres across.
The longest neurone in a mammal that controls muscles will have its cell body at the base of the spinal cord and its terminals in a muscle of the foot. So, in an average sized human where the distance between the bottom of the spinal cord and the foot is about 1 metre or so, then the length of the axon would be 1000000 micrometres. That’s incredibly long compared to the 10 micrometre diameter of the cell body.
Clearly, the bigger you are the longer the axon will be – for a giraffe this is in the order of 5 metres, yet the cell body will still be only 10 micrometres in diameter!
This all immediately poses the question:
- How does one end of the neurone faithfully pass a message to the other end of the neurone?
The simple answer is that the neurones use a form of electrical signalling which faithfully transmits the information over long-distances. These electrical signals are known as action potentials
We now have a rather detailed understanding of these electrical signals and their properties but this is beyond the scope of this course. However, suffice to say that the process involves ions, particularly sodium and potassium, and their ability to enter and leave cells under certain circumstances. Indeed, what sets cells that can produce action potentials apart from ones that can’t is that they have the molecular equivalent of regulatable gates in their cell membranes which can be opened or closed in response to certain signals to allow the ions to move in or out of the cells.
This process of transmitting electrical signals is optimised by the presence of glia which wrap around the axon – they were the sausage-shaped structures associated with the classical view of a neurone that we met in Week 1. They act a bit like insulation around an electrical cable to speed up the rate that the electrical signals can travel along axons. In Week 3 we will see what the functional problems can be if glial function is disrupted.
If you want to find out more about the process of generating and propagating action potentials along axons, you should be easily able to find descriptions at differing levels of complexity on the web.
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