Membrane and Action Potential (Physiology)

Question 1

Explain what the resting membrane potential is

Answer 1

• When at rest or electrically inactive, cells are negatively charged on the inside compared to the outside and are therefore referred to as being polarised.
• Approximately -70 millivolts in a resting neurone.

Question 2

Explain how the resting membrane potential is generated

Answer 2

• Results from the combined effect of three factors: the diffusion of down their concentration gradient through the membrane; selective permeability of the membrane; and the electrical attraction between cations and anions.
• Potassium ions have the greatest influence on the RMP because the membrane is most permeable to potassium ions.
• Concentration of potassium ions in the cell equals the anion concentration in the cell.
• Potassium ions therefore diffuse through leak channels, down their concentration gradient and into the extracellular fluid.
• As the ICF becomes more negative, it exerts a stronger attraction for the potassium ions and attracts some of them back into the cell (electrochemical gradient).
• Eventually an equilibrium is reached in which potassium is diffusing in and out of the cell at equal rates.
• The membrane is much less permeable to sodium ions but sodium still diffuses down its concentration gradient and into the cell, attracted by the negative charge in the ICF.
• Sodium leakage is small, but it is enough to reduce the voltage across the membrane.
• The sodium-potassium pump continually compensates for the leakage of sodium and potassium ions across the membrane by pumping three sodium ions out of the cell for every two potassium ions it brings in.

Question 3

Explain what the Nernst Equation calculates and what is the significance of this

Answer 3

• Mathematically determines the contribution of a particular ion to the resting membrane potential.
• Does this by calculating the equilibrium potential for a particular ion.
• This value indicates the membrane potential at which the chemical and electrical gradients for an ion balance each other out.
• By comparing the equilibrium potential of an ion to the resting membrane potential, it can be seen how much that ion contributes to the resting membrane potential value.

Question 4

Explain what the purpose of the action potential is

Answer 4

• It is a rapid and uniform electrical signal conducted along a cell membrane.
• Acts as a signal for cell to cell communication.
• Causes neurotransmitter to be released.

Question 5

Explain how local and action potentials differ

Answer 5

Local potentials

• Produced by gated channels on the dendrites and the soma.
• May be a depolarising or hyperpolarising voltage change.
• They are graded, meaning they can vary in magnitude according to the strength of the stimulus.
• They are reversible, meaning that the resting membrane potential can be returned to normal if stimulation ceases before the threshold potential is reached.
• Described as local because the effects are only carried a short distance from the point of origin.
• They are decremental, meaning the signal gets weaker as it spreads from the point of origin.

Action potentials

• Produced by voltage-gated channels on the trigger zone and axon.
• Always begins with depolarisation.
• All-or-none law meaning it either does not occur at all or exhibits the same peak voltage regardless of stimulus strength.
• Irreversible, meaning that if a neurone reaches the threshold potential the action potential goes to completion and cannot be stopped once it begins.
• Self-propagating and therefore exerts an effect at a greater distance from the point of origin.
• Non-decremental, meaning the signal maintains the same strength regardless of its distance from the point of origin.

Question 6

Explain the significance of the threshold potential

Answer 6

This is the critical voltage level needed by the local potential to produce an action potential. If the threshold potential is not reached by the local potential, an action potential will not be produced.

Question 7

Identify the phases of the action potential and explain how they occur

Answer 7

• The local potential arrives at the axon hillock and depolarises the membrane at that point.
• If the local potential reaches the threshold potential (approximately -55mV) sodium voltage-gated channels open quickly while potassium voltage-gated channels open more slowly.
• Initially, only a few sodium voltage-gated channels open, but as sodium ions enter the cell it further depolarises the membrane and stimulates more sodium voltage-gated channels to open. This allows more sodium ions to enter the cell, causing the membrane voltage to rise more rapidly.
• Sodium channels are inactivated when the rising potential passes 0 mV and therefore begin to close. By the time all channels close, the voltage peaks at approximately +35 mV. The membrane is now depolarised due to the reverse in polarity.
• By the time the voltage peaks the slow potassium channels are fully open and potassium ions flow out of the cell due to the caused by the positive ICF. Potassium ion outflow causes the membrane to become repolarised.
• Potassium channels stay open slightly longer than sodium channels causing hyperpolarisation of the membrane.
• During hyperpolarisation the membrane voltage gradually returns to the resting membrane potential because of sodium diffusion into the cell and the removal of extracellular potassium by astrocytes.

Question 8

Explain what the refractory period is and why it occurs

Answer 8

• During the action potential and for a short period after it is either impossible or difficult to stimulate the membrane to depolarise again. This is known as the refractory period.
• The absolute refractory period extends from the start of depolarisation until the resting membrane potential is reached again. It is impossible to stimulate the membrane during this period because sodium channels only stay open for a short period of time before they become inactivated.
• The relative refractory period lasts until hyperpolarisation is over. During this period sodium channels are open but potassium channels are also open so it requires a greater stimulus than normal to open enough sodium channels to overcome the opposing effects of potassium efflux.

Question 9

Describe the two ways in which the action potential is conducted throughout the neurone

Answer 9

In unmyelinated fibres

• When an action potential occurs at the trigger zone, sodium ions enter the axon and diffuse for a short distance just beneath the membrane.
• This depolarisation excites voltage-gated channels immediately distal to the action potential causing sodium and potassium channels to open and close, and produce a new action potential.
• The refractory period of the section of membrane immediately behind the action potential ensures that the signal travels unidirectionally.
• This chain reaction continues until it reaches the end of the axon.
• This mechanism is called continuous conduction due to the wave of uninterrupted electrical excitation produced.

In myelinated fibres

• The myelin sheath is not a continuous covering but is interrupted by gaps at Nodes of Ranvier.
• These nodes allow the action potential to jump along the axon by a process called saltatory conduction.
• Saltatory conduction propagates an action potential quicker than continuous conduction because depolarisation does not have to occur along every part of the membrane, only at the Nodes of Ranvier.

Question 10

Describe the factors affecting conduction velocity of the action potential

Answer 10

• Myelin speeds up signal conduction by increasing the nerve fibre’s resistance against leakage of sodium ions. This maintains a higher concentration of sodium ions inside the membrane which allows for a quicker transfer of energy.
• Myelin also creates a greater separation between the ICF and ECF meaning that cations and anions of the ICF and ECF are less attracted to each other. Sodium ions can therefore move more freely within the axon, transferring energy from one to the other.
• A greater axon diameter increases conduction velocity as there is proportionally less leakage of sodium ions from the axons.