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Friday 21 October 2011

Medications: How does neuronal signal transmission work?

(This is part of a series of posts on the basics of psychopharmacology that I’m posting so that I can refer people back to it.)
Two neurons, not quite touching...

Remember from our last post that cells are made up of dendrites, one or more axons and, at the ends of the axons, terminal buttons that sit on the dendrites of adjacent neurons.

Electrical (or chemical, if you prefer) signals flow along one neuron, then jump the gap to the next neuron in line. If the signal has sufficient strength, an impulse is created that travels along the next neuron to its own terminal buttons, where it jumps to the next neurons in line. Neurons are usually thought of as binary switches: signals either reach the strength to produce an impulse, or they don’t - something like computers, which operate entirely using sequences of 0’s and 1’s.

Most of the attention paid to neuronal transmission focuses on what happens at the point of contact between one neuron and the next.
Neurotransmitter being released and received

The signal travels down the axon to the terminal buttons. Within the cell there are small sacs of chemicals called neurotransmitters. When a signal arrives, some of the neurotransmitter is exuded into the tiny gap between the button and the next cell’s dendrites. The molecules of neurotransmitter float across the gap, and some of them attach to chemical receptor sites on the receiving neuron. If neurotransmitter molecules dock with enough of the receptor sites, the next neuron will fire.

Once the sending cell has released its transmitter, it opens ports that gather some of the transmitter back again to be used the next time. This is the process called reuptake – which is important in discussions of the action of antidepressants and other medications.

Serotonin on the move?
When I taught this material at university many years ago I found it helpful to use analogies. Sometimes this can come across as juvenile, but sometimes it helped get an idea across without doing too much violence to the concept. So my apologies for what comes next.

I live in British Columbia, which apparently has the largest fleet of car ferries in the world, operating between closely-spaced islands and the mainland. Inevitably, when I think of neural transmission I think of ferries.

One station, let’s call it Victoria, builds ferries. When it receives a signal, it sends out a collection of boats in the direction of the other port, which we’ll call Vancouver. Vancouver’s station has to have docks that can accept the specific ferries that Victoria creates. If enough Victoria ferries arrive at Vancouver docks, the resulting vehicles can form a convoy and move through Vancouver to Horseshoe Bay, where more ferries are being built to travel to Gibsons Landing.

Once the ferries have been sent, Victoria can open its own docks again to gather back up any ferries that don’t make it across to Vancouver. Then it can recycle the ferries next time there is a signal.

This sounds simple, but lots of things can go wrong.

Sometimes Victoria doesn’t have the right parts to build ferries. The result is too few ferries in the fleet. Translation: Nerve cells need the building blocks from which the transmitter chemicals are made. If these are unavailable, there is a shortage of transmitter.

Vancouver may not have enough docks built. This makes it difficult to get enough vehicles across to create a convoy. Translation: Sometimes receiving cells lose their receptor sites or do not create enough to make transmission work easily.

Other ships may occupy Vancouver’s docks, preventing the real ferries from arriving. Translation:  Receptor sites are not completely exclusive to the neurotransmitter. Some chemicals can bind to the receptor sites without contributing to a signal. These effectively block transmission.

Victoria may open too many of its own docks for returning boats, and may do it too early. As a result, the recently launched ferries return to Victoria and there are not enough of them to make it across to Vancouver. Translation: Reuptake may be too enthusiastic, or if there is a shortage of transmitter the reuptake pumps may operate normally but still take back too much transmitter so that the next neuron doesn’t fire.

More can go wrong.  But that’s enough to get us started.

Next:  What is serotonin?

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