Topic 8: Grey Matter

Question 1

Resting Membrane Potential of a neuron

Answer 1

• The outside of the membrane is positively charged compared to the inside because there are more sodium ions outside the cell than there are on the inside.
• This means the membrane is polarised (has a potential difference/voltage). The voltage of the membrane in about -70.
• The resting membrane potential is primarily established by a sodium-potassium pump which works by active transport (uses ATP) to move three sodium ions (3Na+) outside of the neuron for every 2 potassium ions (2K+) which move into the neurons.

This process sets up 2 concentration gradients:
1. There is an inward concentration gradient for sodium - high conc. outside, low conc. inside. Sodium ion channels are primarily shut at this stage, which maintains this gradient.
2. There is an outward concentration gradient for potassium - high con. on inside, low conc. on outside. Potassium ion channels are mainly shut a this stage, which maintains this gradient.

Question 2

Describe what happens when an action potential reaches a neuron

Answer 2

1. A stimulus to the neuron triggers the opening of sodium ion channels. If the stimulus is above the threshold for that neuron then it will trigger more sodium ion channels to open (positive feedback).

Depolarisation:
When the sodium ion channels open, sodium ions diffuse into the cell down their electrochemical gradient (+ve to -ve). and concentration (high to low) gradients. This makes the inside of the neuron less negative.

Repolarisation:
At a potential difference of around +40mV*, the voltage-gated sodium ion channels close and voltage-gated potassium channels open. Potassium will now diffuse out of the neuron down the electrochemical ion concentration gradient. The inside of the cell becomes more negative again.

Hyperpolarisation
Potassium ion channels are slow to close so there’s a slight ‘overshoot’, in which the neuron becomes more negative than the resting potential because too many K+ ions leave the cell.

Refractory Stage
During repolaristaion and hyperpolarisaton the neuron cannot receive another stimulus (sodium ion channels cannot open again until the resting potential has been re-established)

The resting membrane potential & concentration gradients of Na+ & K+ are re-established using the sodium-potassium pump.

Question 3

What is an action potential

Answer 3

An action potential is a wave of positive charge which moves along the axon. It is an all-or-nothing response because it is only one size (+40mV). If the body wants to communicate a stronger message then it need t either:
- Stimulate more neurons
- Send the message more **frequently

Question 4

Explain how an action potential moves along an axon

Question 5

Why is the refractory stage important