Neuronal Conduction: Lectures 1-4 Flashcards
Is electrical signalling passive or active?
BOTH
Active- K+ ATPase is required to maintain electrical gradients for Action Potential generation
Passive- electrotonic conduction (signal propagation)
What is current?
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
What is capacitance and what is a capacitor?
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.
What affects capacitance
Surface area: increases surface area leads to increased storage potential
Thickness of the insulator: thicker = less attraction
Proportional to: Surface area / thickness.
What passive electrical properties does a biological membrane posses?
- 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
What is the time constant (tau)?
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.
What is cable theory?
- 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
What is the length constant
The distance it takes for a stimuli to decrease to 37% of the original amplitude due to passive propagation.
What are the 3 forms of resistance
Axial resistance (increase with distance)
Extracellular resistance (increases with distance)
Membrane resistance
Resistance across a membrane - Picture
When is membrane conduction greatest
- Membrane resistance is high (leakiness is low)
- Increased by myelination
- Axial resistance is low
- Decreases with increases membrane diameter
Properties of action potentials
- 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
What is refractoriness
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
What is the relative refractory period
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.
What is the absolute refractory period
When no action potential can be generated no matter what stimulus applied, because the sodium channels are all closed.
What are the different stages of sodium channel configuration - Picture
Action potentials propagate by local current curcuit theory- how does this work
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
Evidence for local current curcuit theory
Action Potentials can be prevented by cooling the membrane, but passive propagation will still occur.
Action potential conduction is affected by passive propagation.
What is saltatory conduction?
- 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
What are the benefits of myelination?
Increases the length constant
Decreases membrane capacitance
Allows for rapid depolarisation and rapid passive propagation
Huge positive difference in membrane resistance
What is equilibrium potential?
How is it calculated?
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
What is the resting membrane potential?
Roughly -70mV
Close to Ek but membrane is also slective to other ions –> slightly more positive
How does the permeability of the membrane change throughout an action potential
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
Describe the voltage clamp technique
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.
Changes in conductance during an action potential
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
How do you conduct single channel recordings?
What are the types of measurement
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
Describe the properties of K+ channels
- 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
Describe the properties of Na+ channels
- 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
Describe the conformations of the sodium channels
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.
How do the conformations of the sodium channels cause refractory periods
Inactivation gate closes over channel –> absolute refractory period
Slow recovery –> some channels resting –> relative refractory period (requires increased stimulus)
The shape of the na channel and K channel conductance produces the final shape of the action potential.
