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Question about Synaptic Transmission

It would be great if you could have a look at my notes on synpatic transmission below and answer my questions...

  • A nerve impulse reaches the end of the pre-synaptic neurone:
  • When the axon terminal is depolarized (through Na moving in), voltage-gated calcium channels open, and calcium ions rush into the axon terminal.
  • Inflow of calcium ions, makes vesicles of neurotransmitter move to membrane and release neurotransmitter (=Exocytose)
  • Energy from Mitochondrion (re)form vesicles
  • Calcium is pumped out –
  • Neurotransmitter is broken down in the cleft (see below) and reabsorbed into vesicles
  • Neurotransmitter diffuses across synaptic cleft and binds to receptors – when they fit, sodium ion channels open, through which sodium ions enter and cause depolarisation. If the threshold level is reached, an action potential is established (so another nerve impulse starts)

  1. Is my summary correct?
  2. I'm confused about "Calcium being pumped out" - is this "true" and does THIS break down the neurotransmitter in the cleft? And if yes, then just some of them, coz some bind to the receptors!?
  3. If the synapse is inhibitory, do the neurotransmitter cause sodium ion channels to remain closed? or how does it work??
  4. Where do neurotransmitter come from??

Thank you sooo much!!!

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Okay, your summary is not incorrect but it is a bit incomplete.

Messages are passed across the synapse (junction between two neurons) in the form of chemicals called neurotransmitters. They pass always in the same direction: from the pre-synaptic neuron to the post-synaptic neuron.

1) A nerve impulse arrives the pre-synaptic knob.

2) Depolarization of the pre-synaptic membrane causes voltage-gated calcium channels to open. Calcium ions (Ca2+) will now diffuse into the pre-synaptic neuron.

3) The inflow of calcium ions causes vesicles of the neurotransmitter to move to the pre-synaptic membrane and fuse with it, releasing the neurotransmitter into the synaptic cleft by exocytosis.

4) The neurotransmitter diffuses across the synaptic cleft and binds to receptors in the post-synaptic membrane.

5) The receptors are transmitter-gated ion channels, which open when neurotransmitter binds. Sodium and other positively charged ions diffuse into the post-synaptic neuron. This causes depolarization of the post-synaptic membrane.

6) The depolarization passes on down the post-synaptic neuron as an action potential.

7) Neurotransmitter in the synaptic cleft is rapidly broken down, to prevent continuous synaptic transmission. (neurotransmitters are degraded by enzymes to avoid perpetuation).

The neurotransmitters are produced in the cyton or soma (the body of the nerve cell containing the nucleus).

The inhibitory synapse works as following: these synapses have neuroreceptors that are chloride channels. When the channels open, negative ions flow in causing a local hyperpolarisation and making an action potential less likely. So with these synapses an impulse in one neurone can inhibit an impulse in the next.

I hope that my information clarifies your doubt. If you still dont understand something just tell me!


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4) Neurotransmitters are made in the neuron

3) If it's inhibitory, then the neurotransmitter that's released into the synaptic cleft might cause K+ to diffuse out of the next neuron, which would hyperpolarize the cell so that no action potential would be propagated. Does that make sense?

2) I'm not sure if calcium is pumped out. I guess it is or else there'd be a build up of calcium ions within the synaptic knob. Anyways, calcium ions do not break down the neurotransmitters. Enzymes do. The most common neurotransmitter [in relation to skeletal muscle, especially] is acetylcholine, and the enzyme that breaks it down is acetylcholinesterase. The enzyme breaks down all of the neurotransmitter, I assume. When sarin, a nerve gas, is inhaled, it inhibits the actions of acetylcholinesterase, so that the acetylcholine is allowing the facilitated diffusion of sodium ions into the postsynaptic neuron. Basically, the impulse is constantly being fired and exhaustion of the neurons lead to paralysis of the muscle, from what I understand.

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Kind regards,
IB Survival Staff

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