Nervous system pt. IV (RA)

Question 1

Define graded potential.

Answer 1

Graded potentials are temporary changes in membrane potential occurring at the dendrites and cell body before the axon hillock.

Question 2

State factors that cause graded potentials.

Answer 2

Ligands binding to receptors, changes in charge across membrane, mechanical stimulation, changes in the surroundings.

Question 3

State what can happen to graded potentials.

Answer 3

They rapidly decrease in magnitude as the graded potential spreads over the surface of the plasma membrane. The graded potentials can also summate at the axon hillock to initiate action potential.

Question 4

Describe how an action potential starts off.

Answer 4

The stimulus triggers the receptor to depolarize. This produces a graded potential. An action potential is only produced if the sum of graded potentials at the trigger zone (axon hillock) is higher than a threshold potential. The action potential is then propagated along the axon.

Question 5

State what is meant by the action potential being an all-or-none response.

Answer 5

Once triggered, the action potential’s magnitude is fixed and non-decremental. The action potential’s magnitude is independent of the strength of the stimulus.

Question 6

Describe the 5 phases in an action potential.

Answer 6

1. Resting state
   The voltage-gated Na+ and K+ channels are both closed. Na+ and K+ leak channels are opened, maintaining the resting potential at -70mV.
2. Depolarisation
   Arrival of depolarising stimulus opens some voltage-gated Na+ channels. Na+ inflow depolarises the membrane. Membrane potential increases towards the threshold potential, -50mV.
3. Initiation of action potential/rising phase
   Threshold potential is reached. Most voltage-gated Na+ channels open, rapidly depolarising the membrane to around +40mV.
4. Repolarization / Falling phase
   At +40mV, voltage-gated Na+ channels become inactivated, blocking Na+ inflow. Voltage-gated K+ channels are opened. K+ outflow repolarises the membrane. Membrane potential decreases towards resting potential.
5. Hyper-polarisation / Undershoot
   Voltage-gated K+ channels are slow to close. Some voltage-gated K+ channels remained opened at -70mV while voltage-gated Na+ channels return to closed state. Short period of excessive K+ outflow before all voltage-gated K+ channels are closed cause membrane to be hyperpolarised.

(6. Return to resting state
Sodium-potassium pump returns membrane potential to -70mV)

Question 7

Define refractory period.

Answer 7

The refractory period is when the nerve will not respond to a second stimulus even if depolarization potential is above the threshold value.

Question 8

Distinguish between absolute and relative refractory period.

Answer 8

For the absolute refractory period, voltage-gated Na+ channels are already opened or are inactivated, so second action potential absolutely cannot be initiated.

For the relative refractory period, membrane is hyperpolarized due to slow closing voltage-gated K+ channels. Action potentials are harder to initiate than when membrane is at resting potential due to more negative membrane potential.

Question 9

State the significance of the refractory period.

Answer 9

It ensures that there is no overstimulation of nerve and that the impulse is unidirectional.

Question 10

Outline the propagation of an action potential down a non-myelinated axon.

Answer 10

1. An action potential is generated as Na+ flows inward across the membrane / into axoplasm at a segment of axon membrane.
2. Depolarisation at one axon segment triggers opening of Na+ channels at the next segment due to diffusion of Na+ (local current flow) along axon membrane. This initiates a 2nd action potential. At the site of the 1st action potential, the membrane is repolarizing as K+ flows outward (in its refractory period).
3. A 3rd action potential follows in sequence, with repolarization in its wake. In this way, an action potential spreads along the axon as a “wave” of depolarisation in a single direction.

Question 11

Outline the propagation of action potential down a myelinated axon - saltatory conduction and state its benefit.

Answer 11

The ion current during an action potential at one Node of Ranvier spreads along the interior of the axon to the next node where it will trigger an action potential. The action potential thus jumps from node to node as it propagates along the axon. Action potential can be transmitted over infinite distances with no change in speed and amplitude, so it is non-decremental.