Monday, October 8, 2012

Neurons

Conduction of an impulse down one neuron to the next.
(Click to enlarge)
Encyclopedia Britannica


Channels, Channels, Channels!
There are 9 different kinds of gates/channels/ pumps in neurons.  I will group them here by their function which can help you remember them.
So, just to be clear this is how I categorized the channels by what they do.

Maintain membrane potential:
Na+ diffusion channel (Sodium leak channel)
K+ diffusion channel (Potassium leak channel)
Sodium-potassium pump

Involved in action potentials:
Voltage-gated sodium channels (allow sodium to rush in, resulting in depolarization)
Voltage-gated potassium channels (allow potassium to rush out, resulting in repolarization)
Voltage-gated calcium channels (are triggered at the end of the axon, and trigger release of neurotransmitters)

Involved in post-synaptic reception:
Chemically-gated Na+/K+ channel (Ligand-gated sodium-potassium channel)
Chemically-gated K+ channel (Ligand-gated potassium channel)
Chemically-gated Cl- channel  (Ligand-gated chloride channel)

Here is a diagram I made, grouping the channels by function (each function has the same shape).

Notice that the 3 responsible for membrane potential (triangles) are all over the whole cell membrane.  The 3 responsible for action potentials (stars) are only on the axon (from axon hillock to axon terminal) because that is where the action potentials happen.  The 3 responsible for post-synaptic reception are the circles and are on the cell body.  I put them only on the post-synaptic neuron for simplicity.  But obviously the cell body of the pre-synaptic neuron would also have them.

The influx of Calcium triggers the neurotransmitters to exocytose and that is the end of the message for the pre-synaptic neuron.


Example of chemically-gated channels with Chloride channel being activated (notice the red triangles represent neurotransmitter and they do not enter the cell)
Neurotransmitter is not allowed into the postsynaptic neuron.  Rather, the neurotransmitter binds to the channel receptor, changing its shape and allowing a particular ion or ions to diffuse through, down their concentration gradient.  Each gate lets through what the name says it does.  For instance, Chemically-gated Chloride Channels let chloride through.  Here's a good diagram:
Source

Here's a fantastic little animation which has a dynamic way of illustrating the action potential moving down the axon and how the ions are moving to make that happen!  (Thanks David and Scott!)

Neurons are fascinating.  Please comment to let me know what you found useful in this post, or if you have suggestions for improvement.  Also, see the below post for more info:
Click here to learn about action potential
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