Neurons

Every cell type has the same organelles that make it up, but those organelles are arranged in different ways to provide for the individual cell's function. You saw that with muscle fibers, the cells were huge and multinucleate, containing bundles of cytoskeleton and lots of SR and mitochondria. Neurons are not quite as complicated internally, but they do have special regions that are described for you here.

This page contains the following:

	Neurons are cells made up of three major regions.
	By moving the regions around or adding to them a bit, you can dramatically change the shape of a neuron. This leads to their classification by cell shape.
	Since neurons have to make communicate with each other or with the world, there are different functional groups of neurons (classification by what they do).

General layout of a neuron:

Every neuron can be described to have three major regions:

Cell body (may also be called the soma)

	The part that seems like a regular cell, since that's where all the main organelles like the nucleus, rER, sER, ribosomes, and Golgi apparatus are found
	Only one per neuron
	Information gets integrated here in the neuron

Axon

	Only one per neuron
	Very thin diameter cellular extension (like only 1 micrometer in diameter)
	Information output region of the neuron
	Can branch at its terminal to supply many postsynaptic cells
	May be very long (even one full meter long!)

Dendrites

	Usually many dendrites project off the cell body, but some neurons may have only one.
	Cellular extensions, but larger in diameter than the axon
	Information input region of the neuron
	Dendrites may have many branches to receive a lot of information

Here's one neuron drawn for you:

neuron.jpg (17582 bytes)

Dendrites--> Cell Body---> Axon---> Axon Terminals

The regions and the direction of information flow on the neuron in the diagram is indicated for you above in green. The dendrites stick off the cell body; in this drawing, there are 6 major dendrites off the cell body, and 3 of those dendrites are seen to branch. The cell body is where the nucleus (with its nucleoli) is found. The other organelles in the cell body are so abundant that neurons tend to look all spotty (light blue); the spotty area is called the Nissl substance under the light microscope (we'll discuss that in our next lab). The axon is the narrowest branch off the cell body. It can also be extremely long, and end in numerous terminals.

Question for thought: How would you classify the neuron drawn above in terms of its shape? Click here to check your answer!

What is not on the diagram:

	What are the dendrites getting their information from? What do the tips of dendrites look like? How many times do the dendrites branch?
	What do the axon terminals look like and what are they giving the information to?

   Let's start in the middle. Let's assume that the neuron we are considering has decided to send information out. We don't know why yet, but if you just make this assumption here, we'll get back to it.

    The information has to get sent from this neuron to another cell. This "information" is electrical. While we write information down on paper or on hard drives, cells use either chemical or electrical information. To send this information, it has to get turned into an action potential. You have already learned that neurons send their information via action potentials in the last few units on muscle... and you will learn all about the action potential in the next nervous system unit.

    If a neuron has enough electrical information coming into it for it to produce an action potential, the action potential has to begin somewhere. The place where the action potential begins is called the axon hillock, which is just a fancy term for the beginning of the axon.

    The action potential starts at the axon hillock and then runs all the way down the axon. The concept of this is same as for the electrical current for your toaster-- it starts at the plug in the wall, and runs all the way down the cord to the toaster itself. In the case of the neuron, it runs all the way down the axon to reach the axon terminals...

    So, what happens at the axon terminals? Well, the axon ends. So the action potential also ends, because you can't go past an end, right? That's why the end of each axon terminal is part of a synapse. The action potential effect on the axon terminal is just like we saw with the NMJ: the action potential causes calcium to flood inside the terminal. We'll talk about this more when we do the synapse in detail. But it is that calcium that signals the vesicles to fuse (exocytosis) with the axon terminal and release their neurotransmitter into the synaptic cleft.

    The axon terminal is then the presynaptic terminal of a synapse. When neurons communicate with muscle, muscle (the end plate) is the postsynaptic terminal. But when neurons communicate with each other, the dendrite or the cell body of another neuron is the postsynaptic terminal.

    The dendrites acquire information from synapses (via neurotransmitters) and turn it into electrical information. This electrical information runs down to the cell body. The cell body also receives information from synapses (via neurotransmitters) and turns it into electrical information.

    All the electrical information received in the cell body adds up together. If there's enough, it causes the axon hillock to start an action potential to run down the axon. This summing up of the information is the "integration of information" that the cell body does.

Summary of parts of a neuron and how they work:

Dendrites receive synaptic information and it travels toward the cell body

Cell body receives synaptic information and sums it up with that received from the dendrites

If there's enough information, an action potential is generated at the axon hillock

The action potential runs down the axon hillock to the axon terminals

The information is passed to the next cell down the line via a synapse

If the next cell is a neuron, the whole set of steps repeats

Question for thought: Take a look at the summary above and tell me at which step number did my long description above begin?   Click here to check your answer!

Neurons are classified by their shape

    There's a fun web site that describes neuronal shape. It is actually a page on neurons in the Neuroscience for Kids web pages. Check it out! At the end of that page, you can even go to a "quiz about the parts of a neuron" for a little review quiz. Don't worry... I don't grade that.

    Because neurons may have many dendrites, or only a few, or only one, or even apparently none (! I'll explain below), they are categorized by their appearance. The three categories are: (look in your book or in the link above for pictures of these cells)

	multipolar-- many dendrites off the cell body
	bipolar-- only one major dendrite off the cell body
	unipolar-- no dendrites off the cell body!

We will mainly be working with multipolar and unipolar neurons in our class.

    The only one that is difficult to understand is the unipolar neuron (called a pseudounipolar neuron in the link at the Neuroscience for Kids page). You will see that these unipolar neurons do not have their cell bodies within the CNS. Because of that, they do not have other neurons around them that will be able to synapse on them (communicate with them). So, they do not need to have dendrites near their cell bodies to receive input.

    All the unipolar neurons are sensory neurons, as you will read about below.

Neurons are classified by their function

   Neurons communicate with each other. They recognize signals in the world (for example, visual signals, like a football coming toward you) and tell other neurons about them (like in your brain, so that you know it's a football and you realize you have to catch it), and then they tell other neurons that can cause some action about them (like the motor neurons to your hands and arms so that you can tell the muscles in your hands and arms to contract to catch the football).

    This means that some neurons have to receive information, others have to process the information, and still others have to cause a response to this information. Each of these types of neurons has a name based on these functions:

Sensory neurons-- they acquire the information

Interneurons-- they process the information

Motor neurons-- they cause responses to the information

The connections that these neurons make are consistent with these functions:

Sensory neurons get information from the world and send it into the nervous system:

Interneurons can get their information from sensory neurons, process it, and pass it right on to motor neurons:
In addition, because some information needs to be processed a lot, interneurons can also send information to other interneurons...

Motor neurons receive information from interneurons (and in some reflexes, directly from sensory neurons) and pass that information on to muscles through the NMJ.
Some motor neurons do not innervate muscles, but instead, innervate other effectors, like glands to cause secretion.

� 2011 STCC Foundation Press
written by Dawn A. Tamarkin, Ph.D.