Neuron

00:03 Well, when we look at a nerve fibre in more detail, we can see that there are other structures that we need to describe and recognize. Here is a section again of some neurons found in the brain region. The left-hand image, you can see long elongated structures and on the right-hand side, you can see a very high magnification of another neuron.

00:31 So I would describe each of these. They are motor neurons. Motor neurons are those that bring about movement of skeletal muscle in the somatic division of the nervous system or they bring about movement of smooth muscle in the autonomic division of the nervous system. On the left-hand side, the brown stained neurons you see are called pyramidal cells. They consist of a cell body. We call the cell body a soma when we refer to the cell body of a neuron.

01:09 And if we have a look at this cell body in high magnification, as we can see on the right-hand image, we see details of the cell body in more recognizable position, a more stained illustration than you see on the left-hand side and so is also a high magnification. Now this ventral horn cell that you see labelled on the right-hand side has got lots and lots of dark stained material in the cell body, in the soma. It is called nissil substance and that reflects all the granula or rough endoplasmic reticulum inside the cell body, the soma. Because neurons are not just like electrical cables and transmit nerve impulses, they manufacture lots of substances, lots of proteins and lots of other structures. And here is an example where this cell, the ventral horn cell which I will describe in more detail in a minute is making a lot of protein. Some of that is going to be the neurotransmitter substances that enable the impulse to transfer from one neuron across to another at a synapse, or from the motor endplate to the muscle.

02:32 So they are very busy cells manufacturing those sorts of components. And the axon extends a very long way into other parts of the body. But before the axon carries an impulse down through the nerve fibre, a lot of information is received by the cell body through a dendritic branch, a whole of little branches from the cell body called dendrites and then the transmission goes down the long process you see labelled as the axon. Now those axons are extremely long in some cases. These two cells communicate with each other. The pyramidal cell is located in the cortex of the cerebrum, in the motor cortex. So you are looking really here, on the left hand side, of an image of the cortex, the cerebral cortex of the brain, the motor cortex. And those pyramidal cells, those axons you see labelled project all the way down the spinal cord from the cerebral cortex. So that can be extremely long. They can go all the way down to the lumbar or sacral part of our spinal cord. And there they communicate with these ventral horn cells you see on the right-hand side. Those ventral horn cells sitting what we will term later on and describe later on as the ventral horn of the spinal cord. And these pyramidal cells then stimulate these ventral horn cells to then innervate skeletal muscles. And therefore those neuron processes are also very long. They can extend from the spinal cord all the way down to our toes. So these axons are extremely long and these axons from these ventral horn cells will form part of what we call a peripheral nerve. Now, I mentioned earlier that these cells are busy making neurotransmitter substance.

04:52 So within these long processes, these long axons there are certain components that allow the transport of these neurotransmitter substances and other substances down the length of the axon. Sometimes slow transport, sometimes very fast transport. And those structures are the microtubules you will see inside the axons if you look at them under the electron microscope.

05:17 Lots and lots of microtubules, they're like a railway line, a railway track that carries packages of neurotransmitter substances all the way down to the axon terminals. So it's a very important function of nerve cells to have that transport mechanism. And the transport of the cell body down towards the axonal processes at the end, the endplates or the synapses is called antegrade flow. Now things can also flow backwards from the end terminals of the axons all the way back to the cell body. That is called retrograde flow. And in fact, that's how the early neurobiologists traced where different cell bodies were located that control different parts of the body. They injected dyes etc into say peripheral muscles or peripheral regions of the body and those dyes were taken up by the axon terminals. And through this retrograde flow, those dyes or whatever markers these early neurobiologists used was taken back up into the brain to the surface of the nerve cell bodies. And there the neurobiologists were able to identify where these cell bodies were located and therefore, map out parts of the brain and regions of the brain that control various parts of the body. And that is also a bad story about that as well because some viruses can therefore, get into the end terminals of nerve fibres and travel into the central nervous system. Sometimes the rabies virus resulting from the bite, from a dog for instance can replicate in skeletal muscle over a period of 10 or 12 weeks and then finally find its way into the axon terminals and move all the way up into the central nervous system. And there the virus can move throughout the central nervous system and even travel down either axons by antegrade flow to other parts of the body and cause all sorts of problems.