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Teacher/Instructor Patrick Roisen
Patrick Roisen

M.Ed., Stanford University
Winner of multiple teaching awards

Patrick has been teaching AP Biology for 14 years and is the winner of multiple teaching awards.

Neurons are cells of the nervous system that send signals. Neurons are classified by function (sensory, motor or interneurons), by structure (multipolar, bipolar or unipolar) and by neuron communication (active potential or neurotransmitter).

The nervous system uses two different kinds of cells one is called glial cells and those are the ones that help support and maintain the other cells and those other cells are the neurons the ones that are considered they're quite essential nervous system cell and what makes them different from glial cells is they' are the ones that are specialized for sending signals. A few of the glial cells have demonstrated the ability to do some messaging back and forth but we focus in on the neurons as the ones that are involved in communication. Now there's a couple ways that you can classify them. One is by function, what do they do? Sensory neurons, they're the ones that are picking up information about the sen- from the senses and sending signals to the central nervous system i.e. sending it up to the brain or spinal cord. Motor neurons are the ones that are carrying out the orders sending out the information or signals from the brain and spinal cord to the rest of the body so a motor neuron is the one that sends the signal to the muscle saying "contract" or it's motor neuron that goes down to a gland and says "release hormone."

Interneurons are the ones that collect those signals that are coming in from the sensory neurons and then deciding whether or not to send out the signals by way of the motor neurons so they're located in the central nervous system you won't find interneurons outside in the peripheral nervous system.

Now structurally you can identify neurons based on what do they look like? Some of them are called multipolar because they have lots of collecting fibers with one sending out fiber, these collecting fibers are called dendrites, the sending out fiber is called an axon I'll got into that little bit more in depth momentarily. Bipolar have one fiber acting as the dendrite one fiber acting as an axon. Now these fibers here may branch out and there but there's one dendrite coming in ultimately one axon going out with the bipolar. Unipolar you have the main cell body sitting aside as you have the dendrite collecting in information and the cell body doesn't make any decisions about whether or not to send the signal just comes in and zips out via the a the axon.

Now let's take a look at this diagram here and focus in on the anatomy of a neuron so if you think back here's this multipolar, bipolar or unipolar well hey! there is the axon here are the dendrites there's a lot of these dendrites this is multipolar so you have the main cell body which is where you'd find the nucleus and this is where you have the basic cell machine or you'd have the majority of the endoplasmic reticulum and all those other things located there and the dendrites are here the dendrites are the parts of the neuron that collect in the signals they're like the ears of the neuron alright? They take in data the cell body is where you get a lot the processing of that data you collect in a bunch of signals from all the dendrites and then the cell body says hmm yes we shall send the signal just like your ears collecting data your brain decides I shall send out the information and to do that you use your mouth. The neuron uses this long axon here. Now you see there's these things called schwann cells wrapped around this axon. The schwann cells are filled with this kind of fat called myelin and each schwann cell named after doctor Schwann I assume, wraps itself around that and it forms the sheath of myelin, myelin is a kind of fat and what this does it greatly speeds up the transmission of electrical signals called Action Potential as they go along the neuron. Now in between each schwann cell you have these nodes called the nodes of ranvier now that's French it's not nodes or "ranvier" it's "rahn-ve-a" so an action potential or nerve signal will come along the dendrite and then the cell body will say yes send long ago and then along. This action tends to travel along membranes but they can jump from node to node to node when they reach the end of the axon called the axon terminal then it has to use another means of communication to send its signal to the next cell.

Now, actually I want to briefly highlight that, that neuron communication it can be in the form of an action potential or in the form of a neurotransmitter. An action potential is used along the surface of one neuron because it travels along it's membrane and it's a sequential in and out of sodium and potassium ions in each regional area of the neuron membrane and because one neuron doesn't share the membrane with it's neighbor, that's why it cannot use action potentials to communicate with the next cell instead it has to release the chemical at this connection between one neuron and the next neuron or a neuron and its affecter cell like a muscle or a gland and these neurotransmitters are these chemical signals. Let's take a look and see how they work so here we have one neuron a multiple polar neuron here's another multipolar neuron this must be somewhere in the brain or spinal cord since we have all these multipolar neurons communicating with each other and the action potential zips along this axon here till it reaches here this is a synapse a connection between a neuron and it's target cell. What happens is that you have the sacks of chemicals called neurotransmitters some examples of these would be dopamine or acetylcholine and when the action potential reches the end here it causes these sacks to merge with the outer membrane dumping the chemical out on the receiving neuron it has receptor proteins that have the right shape to fit that particular neurotransmitter. As a side note, a lot of chemicals actually can act like these neurotransmitters caffeine for example doesn't act like these neurotransmitters but it blocks the receptor for a neurotransmitter called amp a way that you add adenine it's how you detect whether or not you're tired by blocking that signal your brain says "I must now be tired" that's why caffeine helps wake you up. Other chemicals like cocaine blocks the ability to get rid of the neurotransmitter and that's why cocaine can cause these weirdly high levels of signals in your brain that cause the chemical high of cocaine so this is one of the reason why you got to be kind of careful with some of the chemicals that you may be using because you're interfering with the normal communication between the neurons of your brain which can interfere with all sorts of other things going on in your body.