Describe the molecular events (i.e., channels, ion movements, changes in voltage

Question

Describe the molecular events (i.e., channels, ion movements, changes in voltage, etc.) that occur during an action potential. 2)S Since an AP is supposed to be "all or none" and unchanging in magnitude amplitude once it begins G.e., "without decrement) why does the frog sciatic nerve appear to have a graded response (increasing AP peak magnitudes) to increasing stimulus voltages? 3)Explain the effects of axon diameter (small vs large) and myelination (myelinated vs unmyelinated) on AP conduction speed down an axon 4Why is the sciatic nerve "refractory" to additional stimuli when the stimuli are close in time? What is the cause of the relative refractory and absolute reftractory period? (explain channels, voltage-gates and ions involved)

Explanation / Answer

Ans.

At resting potential of -70mv, the inside of a cell is more negative as compared to outside. The different concentration of Na+ and K+ ions inside and outside of cell causes resting potential. Sodium potassium pump present on cell membrane brings two K+ ions inside the cell and sends three Na+ ions outside the cell due to which inside of cell is less positive than outside of cell in other words inside of cell is more negative at resting potential and -70mv is the resting membrane potential.

Impulses travel down the axon, when impulses reach pre synaptic terminal then Ca2+ ions enter the pre synaptic terminal due to which neurotransmitters are released from synaptic vesicles. This neurotransmitter from pre synaptic neuron comes to the synapse and attaches itself to the receptors present on dendrites of post synaptic neurons. At this point voltage gated channels open up and positive ions enters inside the cell and causes depolarization of the membrane to the threshold potential of -55mv, at this point Na+ channels at axon hillock opens. This starts the action potential. The entry of Na+ causes complete depolarization till the membrane potential reaches +40mv. The action potential then travels the lentgh of axon and it travels to next neuron by the same process as discussed above. After depolarization the previous membrane goes back to resting potential and it repolarises. Axon is covered by myelin sheath. Nodes of Ranvier is present on axon at frequent gaps. Nodes of Ranvier are unmyelinated and have Na+ and K+ voltage gated channels. Flow of Na+ inside the cells at nodes of ranvier give rise to action potential again and again at each nodes. Action potential jump from one node to other.

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