A Brief Overview of Ion Channels and Their Role in Neuron Communication
Ion Channels:
In order for neurons to communicate, they must do so with chemicals. These chemical messengers are called neurotransmitters. Each neuron uses the same neurotransmitters for all of its communications. Whereas we tend to think of communication as a two-way interaction, communication with neurons is in one direction only; the chemical message will go from the axon where the axon’s branches end (terminals) of the sending neuron across a small gap in between neurons (a synapse) to the receptive surfaces of the receiving neuron. The neuron sending the message is called the pre-synaptic neuron, and the neuron receiving the message is the post-synaptic neuron.
In order for the pre-synaptic neuron to send a message (fire) it has to receive certain neurotransmitters at synapses. The effect of such signals is to alter the electrical state inside the cell, making it a bit more positive than it is when it is not firing. This small shift toward the positive triggers gates on sodium and/or calcium ion channels to open. When this happens, the positive ions (Na+ and Ca 2+) on the outside of the cell rush in because they are pulled by the negative electrical force that still exists inside the neuron (although it is more positive than before). For a second, the influx of positive ions makes the inside of the cell turn from a negative to a positive charge. This positive state causes adjacent channels to open, which repeats the process over and over again. This reversal of the neuron’s interior electrical state from negative to positive is called an action potential, and in less than 1/1000 of a second, it travels down the length of an axon, sending the message with it. After a neuron fires, it goes back to its original resting state.
Neuron Communication:
The succession of action potentials down the axon makes sure that the signal maintains its strength no matter how long the axon is. When the signal reaches the axon terminals, it triggers the release of another neurotransmitter, which is released into the synapse via synaptic vesicles. These vesicles fuse briefly with the pre-synaptic axon terminal and open, releasing the neurotransmitters into the synapse. These transmitters travel across the synapse and bind to the specific receptor on the post-synaptic neuron. This binding begins another opening of ion gates and thus continues the signal down other axons, all within less than a second.
The first picture is taken from: (The Society of Neuroscience – Ion Channel. www.sfn.org . Retrieved December 9th, 2007).
The second picture is taken from: (Synapse Structure. http://www.coolschool.ca/lor/BI12/unit12/U12L04.htm. Retrieved December 9th, 2007.)