Neuro 2 - Resting and action potentials

Question 1

Explain why the resting membrane potential of most cells is around -70 mV.

Answer 1

The equilibrium potential of potassium is around -80 mV and the membrane is very permeable to potassium so the resting membrane potential is around the same as the equilibrium potential of potassium.

Question 2

What does the Nernst equation show?

Answer 2

It allows you to calculate the equilibrium potential of a particular ion (dependent on temperature and charge).
E=equilibrium potential = the membrane potential when dynamic equillibrium has been reached for the ion; i.e. no net movement of the ion across the membrane

Question 3

What does the Goldman-Hodkin-Katz equation show?

Answer 3

Takes into account the permeability of the membrane to different ions so you can figure out the resting membrane potential as multiple ions contribute to the resting potential.

Question 4

What is electrochemical equilibrium?

Answer 4

The point at which the concentration gradient balances the electrical gradient.

Question 5

What is equilibrium potential?

Answer 5

The potential that prevents diffusion of ions down its concentration gradient as charge (electrical) comes into account too.
= the membrane potential when dynamic equillibrium has been reached for the ion; i.e. no net movement of the ion across the membrane

Question 6

What is the difference between graded potentials and action potentials?

Answer 6

Action potentials are an all-or-nothing event - they have the same amplitude every time
Graded potentials can vary in amplitude and the amplitude is affected by the strength of the stimulus. Graded potentials can be positive or negative and they decrease in altitude as they travel away from the point of origin.

Question 7

Where do graded potentials occur?

Answer 7

Synapses and sensory receptors

Question 8

What is the role of graded potentials?

Answer 8

Generate or prevent action potentials

Question 9

What are the two gates in voltage gated ion channels?

Answer 9

Activation gate - opened by conformational changes

Inactivation gate - ball and chain

Question 10

What happens during depolarisation?

Answer 10

Depolarisation makes the voltage gated sodium channels open so there is a large increase in the permeability to sodium. The membrane potential drives towards the equilibrium potential of sodium. The voltage gated potassium channels are much slower to activate.

Question 11

What happens during the start of repolarisation? What makes this the absolute refractory period?

Answer 11

The inactivation gate of the VGSCs close meaning that regardless of the stimulus strength, another action potential cannot be generated - absolute refractory period.
Then activation gate closes too

Question 12

What is the relative refractory period?

Answer 12

The activation gate closes and the inactivation gate opens so an action potential can be generated but only with a stimulus of greater than normal strength.

Question 13

How long does a normal action potential last? What is the size?

Answer 13

2 ms

from -70mV to +30mV

Question 14

What factors affect conduction velocity?

Answer 14

Myelination and axon diameter (wider = less resistance so faster)

Question 15

What do voltage-gated channels open in response to?

Answer 15

Transmembrane voltage = Voltage-gated ion channels e.g. Na+, K+, Ca2+
Mechanical force e.g. in Pacinian corpuscle
Activated by ligands

NOTE
Voltage gated channels =NO ACTIVE TRANSPORT, this is facilitated diffusion - PASSIVE

Question 16

What is the IC and EC conc. of Na+ and K+?

Answer 16

Na+: IC - 150 mM
EC - 10
K+: IC - 5
EC - 150

Resting membrane potential generally maintained in cells by Na+/K+ pump and flow out of K+ through open channels

Question 17

What are the stages of an AP?

Answer 17

1. Resting membrane potential
2. Depolarizing stimulus -has to reach threshold potential
3. Upstroke - depolarisation- starts at threshold potential. VGSC opened quickly
   Membrane potential moves towards the Na+ equilibrium potential.
4. Repolarization
   membrane potential moves towars the K+ equilibrium potential. VGKC opened sp PK increased, PNa decreases as Na+ channels are closed
5. Hyperpolarization

Question 18

What makes a membrane selectively permeable?

Answer 18

Ion channels

Question 19

What does “regenerative nature” of the AP mean?

Answer 19

Once the threshold is reached, the cycle continues - this is like postitive feedback - activated VGSC –> increased Na+ permeability –> Na+ flow into cell –> depolarization –> more VGSC open etc. This ends when inactivation of VGSC occurs and this leads to absolute refractory period and then Relative Refractory period

Question 20

What is positive feedback in terms of ion channels?

Answer 20

Depolarisation of the membrane stiumulates VGSC to open, which leads to Na+ ion influx into the cell, causing more depolarisation and hence more VGSC open = cycle
This cycle continues UNTIL the inactivation gate closes