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Hi Lina. This is quite a complex question to answer for a quick response like this, but is quite simple when you get your head around it! So firstly, a synapse is a connection between 2 neurones or nerves, between a muscle and a nerve, or between a nerve and the brain. They are a physical space between these things, and the signal down the nerves, known as an Action Potential, is carried on through this physical space/gaps by chemicals, known as Neurotransmitters. The neurotransmitters are released from the first neurone, diffuse across the synapse gap, and bind a receptor. These receptor trigger the signal to continue down the next nerve/muscle/brain tissue. To understand your question, you have to understand how an Action Potential is fired down a nerve in the first place. A normal neurone has a resting potential of -70mV. This means that overall, there is a relatively negative charge in the cell, and a positive one outside the cell. When an AP starts, a positive charge flows into the neurone. This triggers an action potential in the next segment, so that it is positively charged. This happens by the positively charged signal reaching the new section of neurone, causing its membrane potential to become positive, triggering Voltage Gated Sodium channels to open in This region. Complicated I know! It is not as complex as it sounds. These are channels that let sodium in, but they are only opened between membrane potential slightly more positive than -70mV and +30mV. Na+ flows in, causing the membrane voltage to increase as it is positively charged, to +30mV, and then the Na+ channels close so that no more + ions flow in. Now when this point is reached, another voltage gated channel is opened at +30mV. This is the potassium channel. Potassium is naturally found in the nerve, and is positively charged. So to balance charges, the potassium diffuse out of the nerve, to reduce charge in the neurone and make it negative again. By this point the signal has already started triggering action potentials further down. The reason the action potential cannot flow the other way is that these channels are completely blocked, or inactivated for a certain amount of time after they have been activated. They become active once the action potential has already travelled too far down the nerve to trigger that segment again. This is the refractory period. The same principal applies for a synapse. As the neurotransmitter is released from the first nerve to the second, the section a the tip of the first nerve is in the “refractory period”. This means that it’s voltage gated sodium channels are blocked, and so if the neurotransmitter binds here, the channels will not be open. Another more obvious reason is that the neurotransmitter receptors are only found on the second neurone/muscle/brain cells. I just wanted to clear up why an AP is unidirectional first! I hope this clears some things up. Don’t worry if you’re confused, I would certainly recommend looking at diagrams in a textbook/in Google! Please message me if you’re still confused I think I could teach in a far more clear way via diagrams and messages!!! Thanks, Ben
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