Membrane Potentials and Action Potentials

Question 1

What is diffusion and what are its properties?

Answer 1

Diffusion is the movement of molecules down a concentration gradient until a dynamic equilibrium is achieved. This is useful for transport over a short distance and spontaneous so no energy input required.

Question 2

What is flux?

Answer 2

The number of molecules that cross a unit area per unit of time (number of particles).
i.e. molecules.m−2.s−1

Question 3

What are the conventions of membrane potential?

Answer 3

Electrode placed outside the cell taken as reference and hence known as zero-volt level. Potential measured inside with reference to this electrode. All cells have a membrane potential.

Question 4

Why are ion channels required and what selective ability does it have?

Answer 4

Lipid (hydrophobic) cell membrane is a barrier to ion movement and separates ionic environments. The cell membrane can selectively change its permeability to specific ions. Permeable pores in the membrane (ion channels) open and close depending on transmembrane voltage, presence of activating ligands or mechanical forces.

Question 5

What are the properties of ion channels?

Answer 5

Ion channels can be selective for different types of ion (K+, Na+, Cl-, Ca2+). Movement across the membrane will occur when the concentration of the ion is different on one side of the membrane and cease upon equilibration.

Question 6

What is electrochemical equilibrium?

Answer 6

Initial direction of flux determined by relative concentrations on different sides of the membrane. Electrochemical equilibrium is when electrical forces balance diffusion forces. A stable transmembrane potential is achieved as charges on both sides of the membrane are equal. Hence regardless of conc, rate of flux stabilises and no net movement.

Question 7

What is equilibrium potential?

Answer 7

The potential at which electrochemical equilibrium has been reached. It is the potential that prevents diffusion of the ion down its concentration gradient.

Question 8

What is the Nernst equation used for and what are its variables + constants?

Answer 8

Used to calculate equilibrium potential. Variables are temperature, charge on ion, intracellular ion conc and extracellular ion conc. Constants are gas constant and Faraday’s number.

Question 9

Why is the GHK equation more suitable?

Answer 9

Biological membranes are not uniquely selective for an ion. Membranes have mixed and variable permeability to all ions and each ion’s contribution to membrane potential is proportional to how permeable the membrane is to the ion at any time.

Question 10

Define depolarisation, repolarisation, overshoot and hyperpolarisation

Answer 10

Depolarisation: Membrane potential becomes more positive towards zero mV
Repolarisation: Membrane potential decreases towards resting potential
Overshoot: Membrane potential becomes positive
Hyperpolarisation: Membrane potential decreases beyond resting potential

Question 11

What are graded potentials?

Answer 11

A change in membrane potential happens in response to external stimulation or neurotransmitters. Change in membrane potential is graded in response to the type or strength of stimulation. Graded potentials produce the initial change in membrane potential that determines what happens next – they initiate or prevent action potentials.

Question 12

What is the mechanism of decremental spread of graded potentials?

Answer 12

Charge ‘leaks’ from axon and the size of the potential change decreases along the axon

Question 13

When is an action potential created?

Answer 13

Action potentials (AP) occur when a graded potential reaches a threshold for the activation (opening) of Na+ channels resulting in an “all-or-nothing” event. These occur in excitable cells (mainly neurons and muscle cells but also in some endocrine tissues). Play a central role in cell-to-cell communication and can be used to activate intracellular processes.

Question 14

Explain action potentials on an ionic basis

Answer 14

Permeability depends on conformational state of ion channels - Opened by membrane depolarisation, Inactivated by sustained depolarisation, Closed by membrane hyperpolarisation/repolarisation. When membrane permeability of an ion increases it crosses the membrane down its electrochemical gradient and movement changes the membrane potential toward the equilibrium potential for that ion. Changes in membrane potential during the action potential are not due to ion pumps.

Question 15

What are the five phases of an action potential?

Answer 15

Resting membrane potential, Depolarising stimulus, Upstroke, Repolarisation, After-hyperpolarisation.

Question 16

Describe what happens in phase 1 and 2?

Answer 16

Phase 1: Permeability for PK > PNa. Membrane potential nearer equilibrium potential for K+ (-90mV) than that for Na+ (+72mV)
Phase 2: The stimulus depolarises the membrane potential. Moves it in the positive direction towards threshold.

Question 17

What happens in phase 3?

Answer 17

Starts at threshold potential. Increase in PNa because voltage-gated Na+ channels open quickly [Na+ enters the cell down electrochemical gradient]. Increase in PK as the voltage-gated K+ channels start to open slowly [K+ leaves the cell down electrochemical gradient]. Less than Na+ entering. Membrane potential moves toward the Na+ equilibrium potential.

Question 18

What happens in phase 4?

Answer 18

Decrease in PNa because the voltage-gated Na+ channels close - Na+ entry stops. Increase in PK as more voltage-gated K+ channels open & remain open so K+ leaves the cell down its electrochemical gradient.
Membrane potential moves toward the K+ equilibrium potential. At start of repolarisation is the absolute refractory period Na channel activation gate is open and inactivation gate is closed. New action potential cannot be triggered even with very strong stimulus. Later, Activation AND Inactivation gates closed.

Question 19

What happens in phase 5?

Answer 19

At rest, voltage-gated K+ channels are still open
K+ continues to leave the cell down the electrochemical gradient. Membrane potential moves closer to the K+ equilibrium - some voltage-gated K+ channels then close
Membrane potential returns to the resting potential
This is also known as the relative refractory period. Some Na+ channels have recovered from inactivation – gate is open allowing depolarisation to occur but stornger than usual stimulus required.

Question 20

What kind of relationship do membrane permeability to sodium and membrane potential share?

Answer 20

The two share a regenerative relationship. Once threshold potential is reached an AP is triggered.
APs are “all-or-nothing” events. Once triggered a full-sized action potential occurs – positive feedback. Following an AP there is a refractory state where the membrane is unresponsive to threshold depolarisation until the voltage-gated Na+ channels recover from inactivation.

Question 21

What are the two types of propagation?

Answer 21

Passive propagation is when only resting K+ channels open. Internal (or axial) membrane resistance and diameter of axon alters propagation distance and velocity of signal. Active propagation occurs when local current flow depolarises adjacent region toward threshold.

Question 22

What affects conduction velocity?

Answer 22

Both axon diameter and myelination influence conduction velocity. Speed decreases with reduced axon diameter (i.e. re-growth after injury), reduced myelination (e.g. multiple sclerosis and diphtheria), cold, anoxia, compression, and drugs (some anesthetics).