In its resting state, the neuron is more negative inside compared to outside. The resting membrane potential is -70 millivolts (mV). The reason for this is because of the distribution of charged ions:
This table shows the relative concentration of positive+ and negative - ions inside and outside the neuron during the resting state.
Inside | Outside | |
organic- | high | absent |
Cl- | low | high |
Na+ | low | high |
K+ | high | low |
Neurotransmitters affect the membrane's permeability to ions. In the VRML model neurotransmitters are represented by the red and blue cones (located in the bottom left hand corner of the model).
Clicking a cone initiates two events in the model:
Three types of neurotransmitter effect are illustrated in the model:
Excitatory neurotransmitters (red
cone in VRML model ) open channels in the membrane that allow Na+
to rush into the neuron. This causes the membrane potential to change.
The increase in +ve ions inside the neuron results in the membrane
becoming depolarized. This is illustrated by a decrease
in membrane potential to a value less than -70mV.
The axon of a neuron has a threshold of -60Mv. If the
membrane is depolarized beyond this threshold, an action potential
is generated. At the peak of the action potential the membrane
potential has a value of +30mV. Then the membrane potential recovers
and there is a short period of time called the refractory period
during which the membrane potential increases beyond its
resting value of -70 mV. During the refractory period the membrane is
temporarily hyperpolarized.
Inhibitory neurotransmitters (blue cone in VRML model ) open channels in the membrane that allow Cl- to rush into the neuron. This causes the membrane potential to change. The increase in -ve ions inside the neuron results in the membrane becoming hyperpolarized. This is illustrated by a increase in membrane potential to a value greater than -70mV. Of course this means that the nerve will not generate an action potential.
Mixed neurotransmitter effects (red and blue cone in VRML model ) occur when the nerve is simultaneously influenced by excitatory and inhibitory neurotransmitter molecules. In this situation both types of membrane ion channels are opened and Na+ as well as Cl- rush into the neuron. The change in membrane potential is a reflection of both effects. In the model I have arranged things so that the hyperpolarizing effects of Cl- are only partially canceled out by the depolarizing effect of Na+. The membrane does not reach the critical threshold value (-60mV), and consequently an action potential is not generated
Graded potential changes. The number of ion channels that are opened by excitatory and inhibitory neurotransmitters is a function of the amount of neurotransmitter that reaches post-synaptic receptor sites. The greater the amount, the more channels open. Consequently whether or not the axon reaches the critical threshold is a function of how much of each type of neurotransmitter interacts with receptor sites.
In the model
How does the VRML model relate to diagrams in my textbook?
You
will find this diagram - or one very similar to it - in most textbooks.
The curvy line in this diagram shows changes in membrane potential as a
function of time after the neuron has been exposed to neurotransmitter
molecules. In the VRML model the moving white square corresponds
to this line. The line is not solid in the VRML model because the model
shows what happens from moment to moment. One thing is very clear in
the diagram, but may have escaped your attention in the VRML model.
Notice how the neuron enters a brief refractory state just after the
action potential reaches its peak value. During the refractory state
the membrane is hyperpolarized (i.e. membrane potential
momentarily drops below -70 mV before returning to the resting
potential at -70 mV.
I have added a sound track to the VRML model to emphasize the explosive
nature of the action potential, and hyperpolarization of the membrane
during the refractory period.
There are two ways to increase the size of the VRML model:
If you haven't done so already, click here to load a new VRML model that explains the
next part of the story.
If you prefer click here to display the model full screen in a new browser window
Take a quick look at the new VRML model and notice that the red cones representing excitatory neurotransmitters have been replaced by red boxes. I have done this to facilitate visualization of the concept of receptor sites. When you click on the red boxes or blue cones notice that they bind in the following way with receptor sites on the neuron:
Transmitter | Binding site |
red box | green box |
blue cone | green cone |
The binding between neurotransmitters and receptor sites is often expressed as a lock-and-key relationship because neurotransmitters fit particular receptor sites in the same way that a key fits a specific lock.
It is very important to appreciate that this chemical event (transmitter binding to receptor sites) initiates a series of electrical events. When they bind to receptors, neurotransmitters open channels in the nerve membrane that allow ions to flow in and out of the neuron. These effects are responsible for the changes in membrane potential that you have just explored.
Before they are released, neurotransmitters are stored in vesicles within the presynaptic terminal. When they are released neurotransmitters travel across the synapse and bind with receptors located on the postsynaptic terminal. Synapses are the very small gaps between the axon of one neuron and the dendrites of its neighbor. The synapse is so small that it can only be seen under an electron microscope. Electrical impulses cannot normally jump across the synapse. Neurotransmitters are the way in which information from one neuron is carried to neighboring neurons. These relationships are summarized in this table
Presynaptic terminal | Synapse | Postsynaptic terminal |
neurotransmitters stored in vesicles | gap between neurons | contains receptor sites |
Neurotransmitters and the action potential
So far we have studied the way in which neurotransmitters set in train
a series of changes in the movement of ions across the nerve membrane
that can trigger an action potential. The effect of an action potential
passing down an axon is to cause the release of neurotransmitters from
the axon's presynaptic terminal. Activate the VRML model by clicking on
the red box - representing an excitatory neurotransmitter. The movement
of sodium ions into the neuron in the middle of the picture triggers an
action potential. Now focus on the neuron on the right hand side of the
VRML model. Notice how it releases several blue (inhibitory)
neurotransmitters from its axon terminal.
It is important to appreciate that action potentials are not always
produced when neurotransmitters bind to a neuron's receptor sites.
An action potential is produced:
Before we discuss these issues, identify the following parts of this neuron in the model:
My VRML representation of a neuron is a crude simplification of reality. For comparison, here is a picture of a neuron that appears in one of the recommended texts for this course (Green, Principles of Biopsychology, Lawrence Erlbaum, 1994).
I need to make you aware that all these representations are vast oversimplifications of reality. For example, here is an electron micrograph showing a cell body and many dendrites; there is no axon in this picture so I have indicated where it might be located. Even a cursory examination of this picture is enough to convince you of the tremendous amount of opportunity there exists for subtle interactions between neurons in our brains.
In the VRML model you will have seen these two 'squiggles' move across
the dendrite, cell body and down the axon
This tracing represents the action potential passing down the axon. Notice how its progress is rather 'jerky'. This represents the action potential jumping between the Nodes of Ranvier.
Notice the smaller peak in this tracing which represents the postsynaptic
potential as it moves down the dendrite across the cell body to end
at the axon hillock.
Differences between action potentials and postsynaptic potentials
Types of postsynaptic potentials
There are two types of postsynaptic potential: