Hi this is Peter from AnatomyZone, and this is another neuroanatomy basics tutorials and in this short tutorial, we will take a look at the different types of neuron.

So you've got three main different classification of neurons, you've got unipolar neurons, you've got bipolar neurons, and multipolar neurons.

 

Unipolar Neuron

We will begin by taking a look at the unipolar neuron. So this neuron as the name suggests, only has one process, so “uni” referring to one. And this type of neuron is not truly found in humans, it's only found in invertebrates.

 

Pseudounipolar Neuron

What humans have is something called a pseudo-unipolar neuron. “Pseudo” means false - it's not actually a true unipolar neuron, but this is what we have in humans. Looking at the diagram in the far left, you can see that there is only one process extending from the cell body. Now if this was a true unipolar neuron, this process would extend and you'd have the axonal endings emerging from this one single process. But what happens in a pseudo-unipolar neuron is that the process leaves the cell body and it splits into these two axonal branches, so you've got one branch extending to one side, and one branch extending to the other.

 

Now in the case of humans, this type of neuron is exclusive to sensory neurons. What you mustn't get confused about is axons and dendrites, what these branches are, are actually axons. There aren't any dendrites in a pseudo-unipolar neuron. You've got this one process leaving from the cell body and you've got these two axon branches. One extends from the periphery, and one extends into the centre, so into the spinal cord.

 

It's easiest to illustrate this with an example really. What we're looking at here, is on the right hand side of the screen, we've got this dorsal root ganglion, containing the cell body which you can see in the left hand diagram of the pseudo-unipolar neuron. And we've got a section of the spinal cord, and we've got a brain slice at the top. The little round blue blob over here is the cell body of the pseudo-unipolar neuron, and just like in the diagram on the left, you've got the axons extending peripherally to the hand, and centrally to the spinal cord.

 

So you've got one axon picking up the peripheral sensation with various different sensory receptors, in this case you would have some thermoreceptors picking up the sensation, transmitting it down the axon and right into the spinal cord. You've got this peripheral branch, and you've got this central branch.

 

Bipolar Neuron

So just coming back to the other types of neuron, the next one is the bipolar neuron, and that name gives you a clue as to its structure. Bi refers to two, you've got words like bicycle, binoculars, that all refers to two. You've got these two processes extending from the cell body. In this case you've got one axon, and one dendrite. An example of this type of cell, is the olfactory epithelium, and you've also got these cells forming part of the retina. These cells aren't very common in the body, they're exclusive to the structures I've mentioned, so the unipolar neuron is found in sensory neurons, and you've got the bipolar neuron found in various areas, such as the olfactory epithelium and the retina.

 

Multipolar Neuron

The final neuron is the multipolar neuron. This is neither unipolar, nor bipolar, and this accounts for all the rest of the neurons in the body, and it's by far the most common type of neuron. So the multipolar neuron has one axon as you can see, extending from the cell body here, and its got several dendrites, which I've highlighted in red.

 

Anaxonic Neuron

One final type of neuron which you might hear mentioned, is the anaxonic neuron. The name again gives a clue, so "an" means without, so it's without axons. There are no axons in this type of neuron, it merely has a few projections from the cell body. This type of neuron can't produce action potentials, but what happens is that it undergoes local potentials, or grades potentials. So it doesn't have any axons to send out action potentials.

 

That's the basic classification of neurons. You’ve got those three main types: unipolar, bipolar, and multipolar.

 

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