About Neurons
© Kim Taylor, September 2007
Now that you’ve reviewed a picture of the structure of a neuron, let’s talk about
each part.
Neuron – brain cells. (It is estimated that we have about 100 billion. Although
you can easily find many credible sources that might suggest a different
number, stick with the spirit of this detail – we have a whole lot of ‘em!)
Cell body – holds our DNA. (Think of this as the palm of a hand.)
Axon – sends information from neuron to neuron. There is only one per neuron,
though it may branch off. (Think of this as the arm connected to the palm
mentioned earlier.)
Axon hillock – calculates energy potential to determine if the neuron has enough
energy to fire. (Think of this as the wrist. Also, think of this as the
teacher who calculates your points to see if you get to move to the next
grade. Or, an accountant who calculates if enough money is available to
make that big purchase. No credit is allowed in the brain. More on this will
explained shortly.)
Dendrites – these branch-like structures receive information from other neurons.
(Think of these as fingers. What information is received if you put your
fingertips on a hot stove? That’s right – the message is received loud and
clear.)
Synapse – the small gap across which neurotransmitters travel.
Neurotransmitters – chemical messengers. (More on this in another lecture).
Terminal buttons – sacs that hold and release neurotransmitters.
Myelin sheath – insulating cells that are wrapped around the axon to increase
connection speed and ensure that the message does not get lost in
transit. (Why is an electrical cord covered in plastic? Damage to the
myelin sheath plays causes symptoms of multiple sclerosis. Talk through
these statements out loud so that the ideas become clear.)
The brain is sometimes referred to as “gray matter”. That is, an exposed
brain appears gray in color. Inside the brain, however, the color is white.
What are your guesses about this? Hint: it involves the myelin sheath.
Nodes of Ranvier – small gaps in the myelin sheath that expose the axon. (As
these gaps exist, what might be the result?)
In class, I will tell you a story about how these pieces fit together. This is my
mnemonic device; it may not work for you as far as helping you to understand
and remember the information. I encourage you to create your own if you don’t
want to share mine.
Let me give you a few more terms to add to this drama:
Resting potential – state in which the neuron is not transmitting (though is in a
state of readiness).
Graded potential – voltage change in a neuron’s dendrites as a result of receiving
impulse(s). A sufficient charge must be present for the neuron to fire –
this is a decision that the axon hillock makes (the accountant or teacher
calculates to see if the next step can be taken.)
Why is a neuron “not allowed” to fire unless is has enough potential?
Action potential – state in which a neuron is transmitting. Neurons fire at only
one level of intensity and its all or nothing.
If this is true (and it is), how can we account for a loud noise, soft touch
or a sharp pain? Answer this before you read on….
Did you figure it out?
Okay. Stimulus intensity is the result of the number of neurons fired and
the frequency of their firing.
Is this information making sense to you? It’s important to get these basic
concepts. After reading the lectures and discussing in class, ask me if you’re still
not clear.