Receptor & Action Potential

Question 1

what causes the Na+ gated ion channels of the input area of the cell to open?

Answer 1

a mechanical, electric, or chemical stimulus

Question 2

what happens when Na+ gated ion chanels of the input zone open?

Answer 2

• influx of Na+ causes voltage gated ion channels to open
• receptor or synaptic potential is generated

Question 3

What is generated when voltage gated ion channels in the input zone open?

Answer 3

Receptor or synaptic potential is generated

Question 4

What is a receptor/synaptic potential?

Answer 4

A graded local signal whose amplitude and duration depends on the intensity of the stimulus

Question 5

Where does the receptor/generator potential occur?

Answer 5

At the input zone of the first cell in a sensory pathway

Question 6

Where does a synaptic potential occur?

Answer 6

In the input zone of a post synaptic neuron (neuron receives NT)

Question 7

Why doesn’t the input zone generate its own action potential?

Answer 7

Input zone doesn’t have enough voltage gated channels

Question 8

Where is it determined if threshold is reached?

Answer 8

Trigger zone (spike initiation zone)

Question 9

What happens if the receptor potential reaches the trigger zone?

Answer 9

• Depolarized to threshold
• “all or none” AP generated (-55mV)

Question 10

Where is the highest density of voltage gated Na+ channels?

Answer 10

Trigger zone

Question 11

Why does the trigger zone have the lowest threshold for generating an AP?

Answer 11

Has the highest density of voltage gated Na+ channels -> spike is un-recoverable

Question 12

What channels are always open?

Answer 12

Leak channels (K>Na)
Na/K pumps

Question 13

Action Potential steps

Answer 13

• input zone activation activates voltage gated Na+ and K+ channels
• Rising phase: V-Na+ channels open first, Na+ rushes into cell
  K+ leak channels oppose Na+ equilibrium potential (+55mV)
• Falling phase: V-Na+ channels close & lock, membrane potential drops
• Undershoot: At 1ms, V-K+ channels open, K+ rushes out of cell
  Reaches K+ equilibrium potential until exhausted - V-K+ channels close
• Na/K pumps & leak channels restore RMP

Question 14

Rising phase of AP

Answer 14

• Voltage gated Na channels open first, Na rushes into cell & depolarizes membrane (+40mV)
• K+ leak channels oppose Na+ equilibrium potential (+55mV)

Question 15

Falling phase of action potential

Answer 15

• Voltage gated Na+ channels close & lock
• Membrane potential decreases

Question 16

Undershoot of action potential

Answer 16

• At 1ms, voltage gated K+ channels open, K+ rushes out of cell
• Membrane potential reaches K+ equilibrium potential until exhausted
• Voltage gated K+ channels close

Question 17

How is RMP restored

Answer 17

After voltage gated K+ channels close, Na/K pumps and leak channels restore RMP

Question 18

What is the absolute refractory period?

Answer 18

• Period of time (frequency of stimuli) during an AP when it is impossible to initiate another AP (falling phase)
• Voltage gated Na+ channels go from inactive state back to resting state (V-Na channels locked, Na can’t enter to depolarize)

Question 19

what is the relative refractory period?

Answer 19

• period of time during an AP when the threshold required to generate another AP is elevated (undershoot - below RMP)
• volatge gated K+ channels cause hyperpolarization until closed (at -75mV, Na channels able to open now, but stimulus needs to be stronger to reach threshold)