NAS - Neuronal Excitability (Lecture 03)

What are “graded depolarisations”?

When neurones are activated and the membrane potential depolarises from RMP. The level of depolarisation will be proportional to the strength of stimulation applied.

What is an action potential?

When the membrane is sufficiently depolarised to a certain critical level of membrane potential. It’s an all-or-none event.

What is the “threshold level”?

The level of membrane potential at which graded depolarisations become an action potential

In general, when do most neurones produce action potentials?

When their membrane potential reaches -65mV

Which factor determines the movement of ions during the RMP?

The concentration gradient

What are the compositions of ions that are inside and outside of the cell?

Inside: Large concentration of K+ - 150mM

Outside: Large concentration of Na+ - 150mM

What happens to the movement of ions when the nerve cell is at rest? What happens to the membrane potential?

• Na+ diffuses down a concentration gradient in favour of them diffusing intracellularly

• K+ diffuses down a concentration gradient in favour of them diffusing extracellularly

• However, negatively charged particles prevent diffusion of K+ to the outside

• Overall, there is no net movement of ions

• The membrane potential hardly changes, remaining at -70mV

In the Nernst Equation, what does E represent?

Equilibrium potential of the ion

In the Nernst Equation, what does R represent?

The Gas Constant

In the Nernst Equation, what does T represent?

Temperature in Kelvin

In the Nernst Equation, what does z represent?

The valency (charge) of the ion

In the Nernst Equation, what does F represent?

Faraday’s number

How long do nerve action potentials usually last?

1-2 milliseconds

What events are happening during the RMP?

• Slow leak of Na+ ions into the cell and K+ ions out of the cell through leak channels

• The Na/K ATPase maintains the ionic concentration gradient over time

• The Na/K ATPase is electrogenic but contributes only 5-10 mV towards the RMP

• Amount of Na+ entering is the same as amount of K+ leaving

• RMP is much closer to Ek (equilibrium of K+ ion) as permeability to K+ is much greater

• ECl (equilibrium of Cl- ion) is very close to the resting potential and there is relatively little Cl- movement

  
  

What is the GHK equation?

The Goldman Hodgkin Katz equation which is a modification of the Nernst equation to take into account the relative permeabilities of different ions.

What events are happening when a neurone discharges from its RMP?

1. Initial entry of Na+ ions into the cell leads to depolarisation of the cell from the RMP

2. If the cell is sufficiently depolarised to reach the threshold level/potential, the cell will produce an unstoppable, all-or-none event called an action potential

3. Steep depolarisation that brings about even further depolarisation of the membrane of the cell

Is depolarisation an example of negative or positive feedback? Why that answer?

Positive. Once depolarisation occurs after Na+ ions enter in, more Na+ channels open and cause further depolarisation.

What events are happening during the repolarisation of the neurone?

1. K+ channels are opened and Na+ channels are deactivated

2. K+ efflux occurs and Na+ influx is stopped

3. Repolarisation occurs and there is a refractory period

Na/K pump is NOT involved in repolarisation

What event happens with the hyperpolarisation phase?

Na+ channels are activated and Na+ influx occurs to depolarise the membrane of the cell to the RMP.

What is the difference between propagation of action potentials between myelinated and unmyelinated axons?

Unmyelinated Axon - Continuous Conduction

Myelinated Axon - Saltatory Conduction

Which proteins set up the RMP?

Leak channels

What are the two types of refractory periods?

Absolute and Relative Refractory Periods

What are the differences between absolute and relative refractory periods?

Absolute - impossible to depolarise

Relative - possible to depolarise