Neuronal Conduction: Lectures 1-4

Question 1

Is electrical signalling passive or active?

Answer 1

BOTH

Active- K+ ATPase is required to maintain electrical gradients for Action Potential generation

Passive- electrotonic conduction (signal propagation)

Question 2

What is current?

Answer 2

Flow of electrons / charge due to a potential difference in voltage.

Amount of current flow is proportional to conductance of conductor and inversely proportional to resistance

Question 3

What is capacitance and what is a capacitor?

Answer 3

The storing and therefore build up of charge on a membrane. This causes slow changes in membrane potential.

A capacitor is when two conudctors are seperated by a small amount of insulation. The thinner the insulation the greater the attraction –> increased storage of charge.

Question 4

What affects capacitance

Answer 4

Surface area: increases surface area leads to increased storage potential

Thickness of the insulator: thicker = less attraction

Proportional to: Surface area / thickness.

Question 5

What passive electrical properties does a biological membrane posses?

Answer 5

• Membrane potential and membrane resistance are parralel
• An injection of current leads to an immediate response
  
  • A rise in voltage will be slower according to the time constant
• Voltage and current are proportional
• A significantly large stimuli –> an action potential

Question 6

What is the time constant (tau)?

Answer 6

Measurement of the time it takes for voltage to change by 63% of the maximum change.

Increase and decrease in voltage occurs over the same time course.

Question 7

What is cable theory?

Answer 7

• Decrease in response with distance from the stimuli due to passive propagation.
  
  • This is proportional to the length constant
  • The decrease is proportional to axial resistance (leakiness)
    
    • This inversly proportional to diameter

Question 8

What is the length constant

Answer 8

The distance it takes for a stimuli to decrease to 37% of the original amplitude due to passive propagation.

Question 9

What are the 3 forms of resistance

Answer 9

Axial resistance (increase with distance)

Extracellular resistance (increases with distance)

Membrane resistance

Question 10

Resistance across a membrane - Picture

Answer 10

Question 11

When is membrane conduction greatest

Answer 11

• Membrane resistance is high (leakiness is low)
  
  • Increased by myelination
• Axial resistance is low
  
  • Decreases with increases membrane diameter

Question 12

Properties of action potentials

Answer 12

• Self-propagating
• Cell length doesnt affect size or shape
• Ion channel populations greatly affect the AP
• There is an upper-limit on the frequency of action potentials
  
  • This is caused by a refractory period

Question 13

What is refractoriness

Answer 13

A reduction in the excitability of the membrane caused by sodium channel configuation.

The closer the two stimuli are together the lower the excitability.

There are two elements to refractoriness, relative and absolute refractory periods

Question 14

What is the relative refractory period

Answer 14

When most but not all sodium channels have returned to the resting / innactive state therefore a greater stimuli is required to trigger an action potential.

Question 15

What is the absolute refractory period

Answer 15

When no action potential can be generated no matter what stimulus applied, because the sodium channels are all closed.

Question 16

What are the different stages of sodium channel configuration - Picture

Answer 16

Question 17

Action potentials propagate by local current curcuit theory- how does this work

Answer 17

Current flows from positive to negative.

Depolarisation moves along membrane to resting areas. Prevented from flowing backwards by refractory period.

Increasing the length constant allows AP to flow further and faster.

Decreasing time constant:

• enhances rate of depolarisation
• Speeds up propagation - less time to reach threshold
• Increased difference between active and resting zones of the membrane

Question 18

Evidence for local current curcuit theory

Answer 18

Action Potentials can be prevented by cooling the membrane, but passive propagation will still occur.

Action potential conduction is affected by passive propagation.

Question 19

What is saltatory conduction?

Answer 19

• Myelination of the nerves leading to increased conduction speed due to increased resistance.
  
  • Myelinated by Schwann cells or oligodendrocytes
• In myelinated axons the Na+ channels are concentrated at the nodes of ranvier (gaps between myelin sheath).
  
  • AP therefore jump from node to nodes

Question 20

What are the benefits of myelination?

Answer 20

Increases the length constant

Decreases membrane capacitance

Allows for rapid depolarisation and rapid passive propagation

Huge positive difference in membrane resistance

Question 21

What is equilibrium potential?

How is it calculated?

Answer 21

The membrane potential at which concentration and electrical gradients are balanced.

Calculated using Ea= (RT/zF)ln([A]o/[A}i)

R= 8.3 JK-1mol-1

T= Temp in kelvin

F= faradays constant (96500 coulombs)

At room temperature RT/F = 25

Ea= 58/z log10 [A]o/[A}i

Question 22

What is the resting membrane potential?

Answer 22

Roughly -70mV

Close to Ek but membrane is also slective to other ions –> slightly more positive

Question 23

How does the permeability of the membrane change throughout an action potential

Answer 23

Changes in the membrane potential caused by the opening and closing of the ion channels as ions flow down the concentration gradient.

Rising phase: Na channels open –> depolarisation –> peak of membrane potential close to Ena

Falling phase: K+ channels open –> repolarisation –> slow to close –> hyperpolarisation

Question 24

Describe the voltage clamp technique

Answer 24

Allows the measuremtn of the movement of charge across a membrane down to a single charged ion

Uses a feedback amplifier to supply current in order to maintain a specific level –> this is measured to figure out change in membrane potential.

Question 25

Changes in conductance during an action potential

Answer 25

Conductance (G)= measure of the ability to pass current. Measured in siemens (s).

Gna= Ina / (Vm-Ena)

The section in brackets = driving force for Na movement

Increased depolarisation (I) leads to increased change in conductance –> changing shape of action potential.

Increased sodium conductance leads to increased potassium conductance

Question 26

How do you conduct single channel recordings?

What are the types of measurement

Answer 26

Fine glass micropippettes can be used to measure events 10^-12 – 10^-10

Types of measurement:

Cell attached - pinch cell –> gigaseal –> record single channels

Apply suction

Able to measure changes over whole cell as suction –> rupture

Pull off membrane

Inside out measurement

Can then change conditions (intracellular) and monitor the effects.

Pull on whole cell

Outside out measurement

Tube of membrane is extracted and then reseals - information about channel opening can be analysed

Question 27

Describe the properties of K+ channels

Answer 27

• Causes an upward deflection on a trace as the positive current flows out of the cell
• Slow to open and slow to close.
• Open during depolarisation, then close at hyperpolarisation

Question 28

Describe the properties of Na+ channels

Answer 28

• Supposed to be stochastic (random) in opening
• Produce a constant ampltidue with a single channel opening
• Compile information about each individual channel to create an average –> Probability of a channel being open

Question 29

Describe the conformations of the sodium channels

Answer 29

Opening and closing of the channel is controlled by two voltage dependent gates (activation and inactivation).

Resting –> stimulus (depolarisation) —> activation gate opens

Inactivation gate then closes on top after a period of time, so Na+ cannot permeate.

In order to return to resting the membrane must undergo repolarisation.

Question 30

How do the conformations of the sodium channels cause refractory periods

Answer 30

Inactivation gate closes over channel –> absolute refractory period

Slow recovery –> some channels resting –> relative refractory period (requires increased stimulus)

Question 31

The shape of the na channel and K channel conductance produces the final shape of the action potential.

Answer 31