Nerve cell potentials

Question 1

What is the value of the resting membrane potential?

Answer 1

-70mV

Question 2

What are cell membrane impermeable to?

Answer 2

Substances dissolved in body fluids (e.g. ions) - These must be transported via protein channels etc.

Question 3

What does the Na+/K+ ATPase pump do?

Answer 3

Creates concentration gradients across the membrane, this maintains the resting membrane potential

Hydrolyses ATP and uses this energy to move:

• 3Na+ out, 2 K+ in

Voltage gated ion channels controlling membrane potential have an activation and inactivation gate

Question 4

Why are K+ ions able to leak through the membrane? What does it mean for theoretical equilibrium?

Answer 4

Driven by their concentration gradient

The sequence of amino acids within the potassium channel protein allows for potassium only to leak through, but doesn’t allow other ions to do this. This is a ‘potassium leak channel’

This allows the potassium ions to leave the cell, and this drives the membrane potential towards a theoretical values of -95mV. This is not the actual resting potential though, as the membrane is not perfectly impermeable to other ions (mainly Na), and therefore, this extra positive charge brings the resting membrane potential up to approximately -70mV.

Question 5

What is the equilibrium potential? How can it be calculated?

Answer 5

Potential difference that would precisely balance the concentration gradient

Can be calculated using Nernst equation

Question 6

Why is the actual membrane potential different to the equilibrium potential?

Answer 6

Actual membrane potential is smaller than predicted value because small numbers of other ions are able to leak

The membrane isn’t perfectly impermeable to other ions, namely Na+, which is able to get through, and this increases the positivity inside the membrane, hence why the RMP tends to be -70mV

Question 7

What are signals generated by?

Answer 7

Changing membrane permeability

Question 8

Explain what happens if sodium permeability is increased. What are electrotonic potentials?

Answer 8

Depolarisation

Electrotonic potential - Graded potential, represents changes to neuron’s membrane potential that do not lead directly to the generation of new current by action potentials. The shape of an electrotonic potential encodes information.

VgNa channels open, allowing many sodium ions to rush in

• VgNa opens when neurotransmitter binds to the receptor binding site on the ligand-gated receptor
• When the neurotransmitter is removed from the receptor, repolarisation occurs, as the channel closes, so no more Na moves in, but potassium ions are able to leak out via the potassium leak channels

A patch of depolarisation will create an electrotonic current

Electrotonic currents spread the depolarisation along the membrane

Question 9

What is a post-synaptic potential?

Answer 9

Potential generated by a synapse

Question 10

What does it mean if electronic potentials are graded?

Answer 10

Graded potentials refer to changes in the conductance of a sensory receptor cell’s membrane, primarily caused by sensory input.

Electrotonic potentials can carry information in their size, shape and pattern

Question 11

How does a patch of depolarisation create an Electrotonic current?

Answer 11

When an AP reaches the VgNa, the channel opens, allowing Na to flow, causing local depolarisation, as along that region there’s now a less positive charge, and along that region inside the cell, the charge is now less negative

The current will flow towards that point to try and evenly distribute the potential, so the charge ‘shifts’ from a region of high to low potential

Question 12

How far can electrotonic potentials carry signals?

Answer 12

A few 100 microns. - This means its able to carry through the dendrites of a cell and down to the root of the axon, as this is about the distance that would be.

Question 13

Explain the limitations of electrotonic conduction and why nerve and muscle cells therefore need action potentials

Answer 13

Electronic potentials carry signals for short distances (approx 100 microns)

Further from site of potentials generation = smaller the electronic potentials

To travel to nerve and muscle cells they will need to travel further than this, and therefore action potentials are required to do this as Electrotonic Potentials themselves cannot be propagated

Question 14

What are the key differences between action potentials and Electrotonic Potentials?

Answer 14

Action potentials have a much larger and quicker depolarisation (100mV compared to 1-15 mV for Electrotonic Potentials). Action potentials also repolarise rapidly.

Electrotonic Potentials have slow decay and are graded, and have an increased permeability to K+, hence why potential stays negative. Every Electrotonic potential looks different, as they all carry different information

Action potentials carry an increased permeability to Na+ due to the depolarisation opening the VgNa

Question 15

What are action potentials triggered by?

Answer 15

Summation of Electrotonic Potentials

Longer neurones utilise Electrotonic Potentials to trigger action potentials

Question 16

Describe the VgNa and how it responds to depolarisation

Answer 16

• Voltage sensitive domains, attracted to -ve charge
  
  • ‘Activation gate’ = Held closed at rest
  • ‘Inactivation gate’ = Held open at rest
• An electrotonic potential depolarises the axon membrane
• Depolarisation around channel reaches threshold value
• Force holding activation gate closed becomes too weak, so activation gate opens

Question 17

How does positive feedback ensure that an action potential is ‘all or nothing’?

Answer 17

VgNa channels open when the membrane potential is depolarised

Positive feedback ensures rapid activation of all available channels (PNa increases X100)

PNa = permeability of Na

Question 18

How is Na+ flow stopped once depolarisation has occurred?

Answer 18

Activation gate cannot close

Inactivation gate is pulled close due to its positive charge being attracted to the now more negative outer membrane, preventing further Na+ flow

AND ‘delayed rectifier’ (VgK) channels open, allowing increased K+ efflux - This is repolarisation, they carry the excess positive charge back out of the cell, as they also have a gate with a positive charge that is pulled towards the outer membrane, opening it, and allowing potassium ions to exit the cell

Question 19

What is the absolute refractory period?

Answer 19

Interval of time during which a second action potential cannot be initiated

Question 20

What is the relative refractory period?

Answer 20

Interval of time during which a second action potential can be initiate, but initiation will require a greater stimulus than before

Question 21

What does it mean if action potentials propagate?

Why are action potentials only able to move in 1 direction?

Answer 21

They replicate themselves - They produce electrotonic currents that spread across part of an axon, and then that part of the axon reaches threshold, and propagates its action potential, and this will continue the length of the axon.

The action potential is forced to move in one direction, as each part of the axon will then move past the threshold value for an action potential, so the action potential keeps moving forwards.

Question 22

What factors increase the speed of propagation?

Answer 22

1. Distance
2. Diameter - This decreases resistance to the current
3. Myelination - This is the layers of glial cells (ogliodendrocytes) wrapped around the axon, creating an insulating layer. This prevents any leakage of ions out of the axon, meaning the maximum potential will produce an electrotonic current flow that spread for a longer distance (1 or more millimetres)

Question 23

Explain how the features of myelination increases propagation and the effects of damage to the myelin on APs

Answer 23

• Low permeability (good insulation) - Prevents leakage of ions out of axon membrane, meaning the maximum potential will produce an electrotonic current flow that spread for a longer distance (1 or more millimetres)
• Assembles VgNa channel clusters at discrete nodes (nodes of ranvier) along its length, allowing propagation of AP. There is no delay between the action potentials being generated along an axon, as an action potential near the start of the axon will still be depolarising when the node of ranvier is reaching the threshold value, so the action potentials can ‘jump’ between gaps. This is saltatory conduction.
• Damage to myelin sheath means AP transmission may be delayed or blocked completely

Question 24

What is an example of a disease that affects myelination?

Answer 24

Multiple sclerosis - loss of myelin

Plaques can occur anywhere in CNS white matter - produces sensory, motor, cognitive or behavioural deficits

Question 25

What threshold do vgNa channels open?

Answer 25

-50mV