Lecture 4 NS - Resting and Action Potentials

Question 1

How do nerve cells function to generate the behaviour of the organism?

Answer 1

Produce sophisticated electrical and chemical signals

Question 2

What is flux?

Answer 2

The rate of transfer of molecules, so number of molecules that cross a unit area per unit of time

Question 3

What is voltage?

Answer 3

Generated by ions to produce a charge gradient

Question 4

What is current?

Answer 4

Movement of ions due to a potential difference

Question 5

What is resistance?

Answer 5

Barrier that prevents the movements of ions

Question 6

What is the zero volt level?

Answer 6

The reference electrode outside the cell

Question 7

Why are ion channels needed?

Answer 7

The lipid cell membrane is a barrier to ion movements, separating ionic environments -> to allow ions to move across the channel

Question 8

What is the nerve cells potential difference value when at rest?

Answer 8

-70mV

Question 9

What type of stimuli do ion channels respond to when they need to open/close?

Answer 9

Trans-membrane voltage, presence of activating ligands or mechanical forces

Question 10

To what can ion channels be selective for?

Answer 10

Different ions -> Na, K, Ca, Cl

Question 11

When does movement across a membrane through an ion channel occur?

Answer 11

When there is a difference in concentration of the permanent species on one side of the membrane

Question 12

When is electrochemical equilibrium achieved?

Answer 12

When electrical force prevents further diffusion across the membrane

Question 13

What is the Nernst equation?

Answer 13

Question 14

What is the composition of the main fluid compartments?

Answer 14

NB: Na and K are the most important ions for resting potential of neurons

Question 15

What is a disadvantage of the Nernst equation?

Answer 15

Cells are not uniquely permeable to one thing, so we need to use the GHK equation

Question 16

What is the GHK equation?

Answer 16

Describes the resting membrane potential, where P is permeability/channel open probability

Question 17

What does increasing the permeability for Na do in the GHK eqn?

Answer 17

Makes the cell more positive

Question 18

What are some key terms that are needed for membrane potential changes?

Answer 18

Question 19

How is the resting potential achieved?

Answer 19

Through mainly K diffusion from inside to outside of cell

Question 20

What is depolarisation?

Answer 20

Positive increase of the membrane potential

Question 21

What are the characteristics of graded potentials?

Answer 21

Changes in membrane potential in response to stimulation

Question 22

What is the mechanism of decremental spread of graded potentials?

Answer 22

Question 23

Where do graded potentials occur and what is their function?

Answer 23

Occur at synapses and sensory receptors and contribute to initiating or preventing action potentials

Question 24

Where do AP occur?

Answer 24

In excitable cells -> mainly neurons and muscle cells but also endocrine tissue

Question 25

What is the function of AP?

Answer 25

Allow transmission of information reliably and quickly over long distances -\> central role in cell-cell communication and can activate intracellular processes

Question 26

What does the permeability of the nerve cell depend on?

Answer 26

Conformational state of the ion channel -\> could be opened by membrane depolarization/inactivated by sustained depolarisation/closed by membrane hyper/repolarisation

Question 27

What happens when membrane permeability of an ion increases?

Answer 27

It crosses its membrane in direction dictated by electrochemical gradient -\> changes membrane potential toward the equilibrium potential for that ion

Question 28

What does the action potential look like?

Answer 28

Question 29

What are the 5 phases of action potential?

Answer 29

1) Resting membrane potential, 2) depolarising stimulus, 3) upstroke, 4) repolarisation, 5) after-hyperpolarisation

Question 30

What occurs in the first phase of action potential - resting membrane potential?

Answer 30

Membrane permeability for potassium is a lot higher than Na, lying nearer to K electrochemical equilibrium than Na

Question 31

What occurs in the second phase of action potential - depolarising stimulus?

Answer 31

The stimulus depolarises the membrane potential and moves it in the positive direction towards the threshold

Question 32

What occurs in the third phase of action potential - upstroke?

Answer 32

Starts at threshold potential, with massive increases in Na permeability (because of VGSC opening) so membrane potential moves towards Na equilibrium potential, so Na moves into the cell down via the VGSC and K moves out of the cell (but less K than Na leaving)

Question 33

What occurs in the fourth phase of action potential - repolarisation?

Answer 33

VGSC closes (conformational change), so Na entry stops, but increased K as more VGKC open and remain open, so K leaves the cell down electrochemical gradient, so membrane potential moves toward K equilibrium potential

Question 34

What occurs in the fifth phase of action potential - after-hyperpolarisation?

Answer 34

At rest VGKC are still open, so K continues leaving the cell, moving closer to the K equilibrium -\> some VGKC then close and membrane potential returns to resting potential

Question 35

What happens at the start of repolarisation?

Answer 35

Na channels inactivate as portion of structure senses the opening and blocks the channel -\> called an absolute refractory period as the inactivation gate is closed and new action potential cannot be triggered even with very strong stimulus NB: K+ channels still open so cell begins to repolarise

Question 36

What happens at the end of repolarisation?

Answer 36

It enters the absolute refractory period, and both inactivation and activation gate of the Na channel is closed

Question 37

What happens during the after-hyperpolarisation?

Answer 37

Relative refractory period -\> Inactivation gate (VGSC) is open, so if the stimulus is stronger than normal stimulus another AP can be evoked

Question 38

What are the changes in permeability that occur during the changes in membrane potential in an AP?

Answer 38

Question 39

What happens once the threshold potential is reached?

Answer 39

An all-or-nothing response occurs, which once triggered cannot be stopped -\> only in the refractory state which is unresponsive to threshold depolarisation

Question 40

What is the loop of membrane potential?

Answer 40

Question 41

How long does the cycle of action potential increase for?

Answer 41

Continues until VGSC inactivate where the membrane remains in a refractory state until VGSC recover from inactivation

Question 42

How are ion pumps involved in action potentials?

Answer 42

They ARE NOT involved DURING the AP! To restore the electrochemical equilibrium after the AP, K and Na move through non-VGC and some ions are exchanged through pumps

Question 43

How does the AP propagate?

Answer 43

If there is a site of depolarisation, there is a decay in polarisation due to K ions moving out of the cell -\> internal/membrane resistance alters propagation distance and velocity

Question 44

How is the action potential actively propagated?

Answer 44

Local current flow due to Na entering and depolarising -\> the adjacent area is depolarised (may be enough to cause AP)

Question 45

How does the neuron prevent back flow of AP?

Answer 45

At this point the axon is refractory so there is no back flow of impulse

Question 46

How does the myelin sheath cause saltatory conduction?

Answer 46

It allows a more prolonged graded events of the AP until it reaches the next NoR

Question 47

What changes the conduction velocity in neurons?

Answer 47

Axon diameter and myelination (increasing both increases velocity)

Question 48

How does myelination change velocity of conduction?

Answer 48

Myelination increases speed, but reduced myelination decreases velocity

Question 49

What factors slow down conduction velocity?

Answer 49

Reduced axon diameter, reduced myelination, cold, anoxia, compression, drugs