Chapter 5-Membrane/Action Potentials

Question 1

What is the diffusional potential?

Answer 1

 Potential (ion concentration) difference between the inside and outside caused by selective ion diffusion

Question 2

What causes the large negative potential in the cell?

Answer 2

 Because three sodium is being pumped out for two potassium via Na/K-ATP pump, this creates negative charge inside the cell.

Question 3

What is the nerst equation?

Answer 3

Ex = (-61/Z) log ([X]in/[X]out)

Question 4

What are the 2 factors that give rise to the negative membrane potential?

Answer 4

K consistently leaving the cell towards its equilibrium potential of -86mV, and the 3 Na out & 2 K in giving about -4mV. Total is -90mV

Question 5

What occurs during the depolarization phase?

Answer 5

Na conductance is high because the cell’s RMP is negative and sodium is positive. Activation of Na Channel and Na rushes into the cell making the cell more positive. Very fast opening of Na gates

Question 6

What occurs during the overshoot?

Answer 6

The great excess of positive sodium ions moving to the inside causes the membrane potential to overshoot beyond zero level and become somewhat positive. Doesn’t happen with CNS and most of small fibers. Slow closing of Na inactivation channel and slow opening of potassium channel

Question 7

What occurs during the repolarization phase?

Answer 7

Sodium channel is still open but closing slowly. Potassium channel is open slowly. Potassium moves out. The cell is slowly becoming more negative. Re-establishment of the normal negative resting membrane potential

Question 8

What occurs during the hyperpolarization phase?

Answer 8

Membrane potential is more negative than at rest. Potassium channel is slow to close (close to potassium equilibrium)
• Sodium inactivation gate will not reopen until the membrane potential returns to or near the original resting membrane potential.

Question 9

What is the Na activation (m) gate?

Answer 9

When the membrane potential become less negative (depolarization) and reaches between -70 and -50 millivolts this causes a sudden conformation change in the activation gate, flipping it all the way to the open position. This called the activated state and allows sodium ions to pour into the cell. This increases sodium’s permeability to 500-to 5000 fold.

Question 10

What is the Na inactivation (h) gate?

Answer 10

The same increase that opened the activation also closed the inactivation gate. The inactivation gate however, closes a few 10,000ths of a second later after the activation channel opens. This is caused by the conformation change to close the inactivation gate taking longer than the change to open the activation gate. The closing of the inactivation gates stops the in pouring of sodium into the cell and the cells begins to recover back towards the resting membrane potential (repolarization).

Question 11

How does the K gate change during depolarization?

Answer 11

during the resting state the gate of the potassium channel is closed and no potassium ions can pass into the cell. When resting membrane potential raised from -90 millivolts toward zero the voltage change causes a conformational opening of the gate and allows increased potassium to diffusion out of the cell.

Question 12

What are the 3 action potential imitators?

Answer 12

o Mechanical disturbance-mechanical pressure to excite sensory nerve endings
o Chemical effects on membrane-chemical neurotransmitters to transmit signals from one neuron to the next in the brain
o Passage of electricity- using electric currents to transmit signals between successive muscle cells in the heart and intestine.

Question 13

What is the threshold of a membrane?

Answer 13

An action potential will not occur until the initial rise in membrane potential is great enough to create the positive feedback mechanism. This happens when the amount of sodium entering the cell is greater than the number of potassium leaving. Sudden rise in membrane potential of 15 to 30 millivolts is required. Going to from -90 to -65 millivolts is said to the the threshold for stimulation

Question 14

Why are action potentials considered “all or none?”

Answer 14

 When the threshold to start an action potential is reached the resultant action potential will proceed and cannot be stopped. Action potentials either fully occur or do not occur at all.

Question 15

What is the absolute refractory period?

Answer 15

the period during which another action potential cannot be elicited no matter how large the stimulus. Caused by the depolarization that takes place which closes the inactivation gates of sodium channels. They remain closed until repolarization of the cell.

Question 16

What is the relative refractory period?

Answer 16

begins at the end of the absolute refractory period and continues until the membrane potential returns to resting level. Action potential can be elicited during this time but with the need for a larger stimulus. The cell becomes hyperpolarized by the increase in potassium leaving the cell to repolarize the cell and so a larger stimulus would be needed to cause an action potential. In relation to the sodium channels the membrane reaches its resting membrane so they would be open to function to cause an action potential.

Question 17

What are the 2 things that increase the action potential propogation?

Answer 17

increasing the diameter and myelination of a neuron

Question 18

What is salutatory conduction?

Answer 18

 Salutary conduction is the conduction of action potential from node to node. This is accomplished by the electrical current flowing through the surrounding extracellular fluid outside the myelin sheath as well as through the axonplasm in the axon, existing successive nodes along the way. The nerve impulse jumps along the fiber.

Question 19

What are the 2 purposes of salutatory conduction?

Answer 19

o By causing the depolarization process to jump long intervals along the axis if the nerve fiber increases the velocity of nerve transmission 5-50 fold.
o Conserves energy for the axon because only nodes depolarize allowing perhaps 100 times less loss of ions. This allows the axon to not have to reestablish the sodium and potassium concentration difference across the membrane.