What is the correct order of events in the generation and propagation of action

Question

What is the correct order of events in the generation and propagation of action potentials (APs) and initiation of synaptic transmission?

AP Group Assignment: Put these steps in the generation and propagation of the AP, and synaptic transmission in order Discuss as groups today, but enter individually into Group Assignments on Blackboard by Tuesday 11:59PM. K exits, helping to return the cell to its resting membrane potential Depolarizing Na" currents travel passively along myelinated segments of axon (electrotonic conduction) to the next myelin gap (Node of Ranvier) where the action potential is actively regenerated by opening of VGSCs. VGSCs close. Absolute refractory period period of time in which the membrane is completely insensitive to further stimulation Eventually, Na* leaves and K' re-enters (via the active process of the ATP-dependent Na/K pump) The NT binds to receptors on the post-synaptic membrane to induce a postsynaptic response Relative refractory period period of time in which the membrane has reduced sensitivity to further stimulation Delayed rectifier K channels open. The action potential reaches the pre-synaptic terminal. In the pre-synaptic terminal, VGCCs open and Ca" rushes in. Na' ons flood the axon, creating an action potential that causes the membrane potential to reach+40mV The synaptic response is terminated by one or more mechanisms, including NT breakdown, postsynaptic desensitization, NT reuptake, presynaptic auto- receptor activation.. receptor deactivation/ IF threshold is met, someVGSCs open, which triggers the opening of more VGSCs in a positive feedback loop between membrane depolarization and permeability to Na Ca causes synaptic vesicles, filled with NT, to fuse with the pre-synaptic membrane, releasing neurotransmitter into the synaptic gap

Explanation / Answer

The correct order of events in generation of AP and initiation of synaptic transmission is--

1] Resting neuron-

* The plasma membrane at rest is polarised

* Fewer positive ions are inside the cell than outside the cell.

2] Depolarisation

* A stimulus depolarises the neuron's membrane

* The membrane is now permeable to sodium as sodium channels open

* A depolarised membrane allows Na^+ to flow inside the membrane

3] Action potential-

* The movement of ions initiates an action potential in the neuron due to the stimulus.

* A graded potential exists [ localised potential] where the inside of the membrane is more positive than the outside.

4] Nerve Impulses

* Propogation of the action potential--

If enough sodium ions enter the cell, the AP starts and is propagated over the entire axon.

Impulses travel faster if the nerve fibre has a myelin sheath.

* Repolarisation

K^+ rush out of the neuron after Na^+ ions rush in, which repolarises the membrane.

Repolarisation means restoring the inside of the membrane to a negative charge and the outer surface to a positive charge

Initial ionic conditions are restored using the sodium - potassium pump

This pump uses ATP and restores the original membrane configuration.

Three sodium ions are ejected from the cell while two potassium ions are returned to the cell.

4] Transmission of the Signal at the Synapse

* When the AP reaches the axon terminal, the electrical charge opens the calcium channels.

* Calcium, in turn, causes the tiny vescicles containing the neurotransmitter chemicals to fuse with the axon membrane.

* The entry of calcium into the axon terminal causes porelike openings to form, thus releasing the neurotransmitter.

* The neurotransmitter molecules diffuse across the and bind to receptors on the membrane of the next neuron.

  Finally, the synaptic response is terminated by one or more mechanisms, including neurotransmitter breakdown, post synaptic receptor deactivation/ desensitisation, neurotransmitter uptake or presynaptic autoreceptor activation .

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