briefly describe how the structure of synapse ensure that the signals can only pass through it in only one direction
Answers
There are a few reasons why nerve impulses only travel in one direction, the most important being synaptic transport.
In order for a depolarization wave, or "nerve impulse" to pass from cell to cell, there are what we call synaptic junctions. This means that the nerve cells are lined up head to tail all the way down a nerve track, and are not connected, but have tiny gaps between them and the next cell. These tiny gaps are called synapse.
When you get a nerve firing, you have probably heard that it is an electrical impulse that carries the signal. And this is true, but it is not electrical the way your wall outlet works. This is electrochemical energy. In a nutshell, a nerve firing results in a chain reaction down the nerve cell's axon, or stemlike section. Na+ ions flow in, K+ ions flow out, and we get an electrochemical gradient flowing down the length of the cell. You can think of it as a line of gunpowder that someone lit, with the flame traveling down the length of it. Electricity in your wall is more like a hose full of water, and when you put pressure on one end, the water shoots out the other.
Anyways, when this depolarization wave hits voltage gated Ca+ channels near the end of the nerve cell, extracellular calcium flows in. The calcium ions hit storage vesicles in the cell, which contain neurotransmitters, signaling the vesicle to dump them into the synapse. Neurotransmitters are molecules that fit like a lock and key into a specific receptor. The receptor is located on the next cell in the line. When the neurotransmitter hits the receptor on the next cell in line, it signals that cell to begin a depolarization wave as well. This will continue all the way down the length of the nerve track.
So, you can see, nerve impulses cannot travel in the opposite direction, because nerve cells only have neurotransmitter storage vesicles going one way, and receptors in one place.
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