Neurons are classified in two different ways. They're classified either based on structure or based on function. So let's take a look at the structural classification first.
There are three different types of neurons based on structure. Unipolar, bipolar, and multipolar. And the way that these differ is just in the number of processes that are attached to the cell body.
Unipolar has a single process, bipolar two processes, and multipolar, more than two, many. In a unipolar neuron, there is a single process that projects off of the cell body, which you can see here. The type of neuron that they're showing us is unipolar in our book.
Sometimes other books will call this a pseudo-unipolar neuron, and that's because that single process is branching. Another possibility is there are two processes that come up. off of the cell body. This is known as bipolar.
And in a multipolar neuron, you can see that there are many processes that come off of the cell body. So anything more than two, we would call it multipolar. Here we can see some variance between multipolar neurons, bipolar neurons, and unipolar neurons.
But regardless of what they look like, All three of the structural types of neurons work in the same way. So you can see in the multipolar neuron that there is a region that receives the incoming information, a region that will generate and conduct the action potential, and then a region that will convey that action potential to the next cell across the synapse in color coding, using color coding. Same thing with bipolar. There's going to be a region that receives the information, generates and conducts the action potential. conveys the action potential to the next cell and so on.
So bipolar, unipolar, and multipolar neurons, even though they look different, they have the same parts and they work the same way. A second way that we classify neurons is based on function, and this has primarily to do with the direction of information flow. If a neuron is carrying information into the central nervous system, if it's contained within the central nervous system, or if it's carrying information out of the central...
nervous system. So let me show you what I mean. When we look at the functional classification of neurons, all we care about is the direction of information flow.
So sensory neurons carry information into the central nervous system. Interneurons are completely contained within the central nervous system. They don't project in or project out.
And then motor neurons carry information from the central nervous system out into the body. So taking a closer look at these three types of neurons, this is a sensory neuron. It's bringing sensory information from the skin into the spinal cord.
Here are some interneurons. They have structures different from what we've looked at. Interneurons are just contained within the brain and spinal cord, and so you can think of them as being like the internal wiring of the CNS. And here we can see a motor neuron carrying information from the spinal cord. out to the body.
So trying to put this information together, unipolar neurons are typically sensory neurons bringing information into the CNS. Motor neurons are typically multipolar neurons bringing information from the CNS out to the body. It would be great if interneurons were bipolar. Unfortunately, that's not the case. Bipolar neurons are the most rare type of neuron in the body.
They're found in special senses. like you can see here, a row of bipolar neurons in the retina. And interneurons don't look like these three structural categories. And the interneurons are the most numerous, and they make the internal connections in the brain and spinal cord.