Lecture 3: Cellular Neurophysiology

Question 1

Define chemical gradient (concentration gradient)

the _______ provided by the difference in ________ across the _____ ________

Answer 1

the energy provided by the difference in concentration across the plasma membrane

Question 2

Define electrical gradient (potential difference)

Answer 2

the energy associated with moving charged molecules across the membrane, when a membrane potential exists

Question 3

What is the charge difference between the two sides of the membrane (resting membrane potential)?

Answer 3

-70mV

Question 4

There is a high concentration of Na+ ions inside or outside the cell?

Answer 4

outside

Question 5

There is a high concentration of K+ ions inside or outside the cell?

Answer 5

inside

Question 6

There is a high concentration of Cl- ions inside or outside the cell?

Answer 6

outside

Question 7

The cell membrane is most permeable to which ion?

Answer 7

K+

Question 8

What is the electrical gradient of K+ ions and why?

Answer 8

Since K+ ions are positively charged and the inside of the cell is negatively charged (-70mV), the electrical gradient drives K+ ions into the cell

Question 9

What is the electrical gradient of Na+ ions and why?

Answer 9

Since Na+ ions are positively charged and the inside of the cell is negatively charged (-70mV), the electrical gradient drives Na+ into the cell

Question 10

What is the chemical gradient of K+ ions and why?

Answer 10

There is a higher concentration of K+ ions inside the cell than outside the cell. As ions flow down their concentration gradient, the chemical gradient drives K+ outside the cell

Question 11

What is the chemical gradient of Na+ ions and why?

Answer 11

There is a higher concentration of Na+ ions outside the cell than inside the cell. As ions flow down their concentration gradient, the chemical gradient drives Na+ inside the cell

Question 12

What are electrochemical gradients set up by?

Answer 12

the ionic distribution across the membrane

Question 13

What is the electrochemical gradient used for?

Answer 13

It is used to quantitate the driving force acting on a molecule to cause it to move across a membrane

Question 14

What is the electrochemical gradient a measure of?

Answer 14

the free energy available to carry out the useful work of transporting the molecule across the membrane

Question 15

What are the two components of the electrochemical gradient and what do these represent?

Answer 15

1. the chemical gradient which represents the energy in the concentration gradient for an ion across the membrane
2. the electrical (potential) gradient which represents the energy associated with moving charged molecules across the membrane when a membrane potential exists

Question 16

What are the three definitions for equilibrium potential (reversal potential)?

Answer 16

• the energy of a concentration gradient of an ion
• the electrical potential that exactly balances the concentration gradient of an ion
• the charge pushing an ion in one direction balancing the concentration gradient pushing it in the other direction

Question 17

What is the Nernst equation and what does it calculate?

Answer 17

E(ion) = (60/Z)log(Co/Ci)

This calculates the equilibrium potential for the ion

Question 18

What does the Z in the Nernst equation represent?

Answer 18

the valence (charge of the ion)

Question 19

What is the base for the log in the Nernst equation?

Answer 19

base 10

Question 20

What does the Co in the Nernst equation represent?

Answer 20

the concentration of the ion outside the cell

Question 21

What does the Ci in the Nernst equation represent?

Answer 21

the concentration of the ion inside the cell

Question 22

How is the resting membrane potential of -70mV maintained?

Answer 22

by the movement of K+ and Na+ ions against their concentration gradient via ATPase

Question 23

What are local potentials?

Answer 23

Small ion fluxes (channel openings) across the membrane which change the membrane potential in one area that cannot spread far from the site of stimulation

Question 24

Define depolarisation

Answer 24

becoming more postive

Question 25

Define overshoot

Answer 25

becoming more positive than zero

Question 26

Define repolarisation

Answer 26

becoming more negative but still above RMP

Question 27

Define hyperpolarisation

Answer 27

move negative than RMP

Question 28

Describe a local potential (5)

• it is graded in ____
• the _______ decreases over _______
• it is _____ in ____ and ____
• can ________ and ________
• influences the generation of an _______ ______

Answer 28

• it is graded in size
• the amplitude decreases over distance
• summated in time and space
• can depolarise and hyperpolarise
• influences the generation of an action potential

Question 29

What is the difference between an action potential and a local potential?

Answer 29

Local potentials are passive changes in membrane potential by flow of current across the membrane by opening of channels when synapses are active. Local potentials decrease in amplitude from the point of origin. This differs from an action potential which are an actively propagated depolarisation that spreads from the point of origin and typically down the axon to trigger communication with the next neuron in the sequence

Question 30

What is the length constant?

Answer 30

the distance over which the voltage in a local potential decays to 37%

Question 31

What is the axial resistance?

Answer 31

the resistance along the axis

Question 32

What happens when the resistance is low?

Answer 32

the voltage decays much quicker

Question 33

What are some features of action potentials?

4

Answer 33

• they are actively propagated from the point of origin
• a local potential is generated in that region which pushes it to threshold
• all or nothing so once threshold is reached, an action potential occurs
• regenerative

Question 34

Describe the process of an action potential

Answer 34

• A local potential lifts the membrane potential to threshold and then an action potential is generated
• this overshoots beyond zero towards the equilibrium potential for sodium
• then repolarisation and afterhyperpolarisation

Question 35

An action potential is regenerative. What does this mean?

Answer 35

it is propagated without decreasing amplitude