Question

Explain in terms of channel opening/closing/inactivation and ion movements what is going on at the indicated points of the action potential. (2 points each)

A. Threshold

B. Rising phase

C. Declining phase

D. Undershoot

E. Return to baseline

Please no cursive handwriting, I can't read cursive.

Answer 1

Threshold - Threshold is the voltage that is necessary to trigger an action potential.The threshlod potential is the critical level to which a membrane potential must be depolarized to initiate an action potential.Threshold potentials are necessary to regulate and propagate signaling in both central nervous system and peripheral nervous system.Suprathreshold stimuli also produce an action potential,but their strength is higher than the threshold stimuli.So,an action potential is generated when a stimulus changes the membrane potential to the values of threshold potential.Treshold potential is usually around -50 to -55 mV.If the membrane potential reaches the threshold potential,the voltage regulated sodium channels all open,Sodium ions rapidly diffuse inward and depolarization occur.

Rising phase - The rising phase of the action potential is caused by an influx of Na+,ie an increase in ionic conductance in the membrane of the axon results in an increase in the action potential.Due to depolarization,it opens both the sodium and pottassium channels in the membrane.If the depolarization is small,the outward pottassium current overhelms the inward sodium current and the membrane repolarizes back and if depolarization is large enough the inward sodium current increases,and Vm also increases.The increasing voltage inturn causes more sodium channels to open.The sharp rise in Vm and sodium permeability causes the rising phase of the action potential.

Declining phase - An increase in ionic conductance in the membrane of the axon results in an increase in the action potential.The falling phase of the action potential is caused by a later increase permeability to K+.It usually refers to the change in the membrane potential to a negative value.It returns the membrane potential back to the resting membrane potential.The efflux of pottassium ions causes this and this ions pass through the selectivity filter of the K+ channel pore.This causes a decrease of voltage.Protein transport molecules are responsible for the Na+ out of the cell and K+ into the cell to restore the original resting ion concentrations.

Undershoot - It is a phenomenon in which the membrane potential briefly dips lower or more negative than its resting potential.An action potential begins when a depolarization increases the membrane voltage so that it crosses a threshold value.This results in a phenomenon called undershoot.During undershoot the open pottassium channels add to the normal resting membrane permeability to pottassium,and drives the membrane potential close to the equillibrium potential for pottassium,and thus hyperpolarize the membrane.

Return to base line - It is called the resting state.The neuron goes from a polarized state at the resting potential,when the neuron is more negatively charged inside than outside the membrane to a depolarized state during the action potential with the cell positively charged on the outside.Resting membrane potential is called the voltage of the resting membrane during resting state.It is caused by the differences in the concentration of ions inside and outside the cell.