What are the properties of an action potential?
An AP is a change in voltage across a membrane
Depends on ionic gradients and relative permeability of the membrane
Only occurs if a threshold level is reached
All or nothing - all action potentials are the same
Propagated (conducted along the axon) without loss of amplitude
Draw the Action Potential of An Axon including duration
Draw the Action Potential of Skeletal Muscle including duration
Draw the Action Potential of a Sinoatrial Node including duration
Draw the Action Potential of a Cardiac Ventricle including duration
What is the Sodium Hypothesis?
Influx of Na+ ions drives the rise (upward stroke) of the action potential in axons; APs are generated by an increase in permeability to Na+ bringing the membrane potential closer to the E(Na).
Once the membrane has been depolarised to the threshold voltage,
Positive Feedback occurs as Na+ channels open allowing Na+ influx as Na+ ions attempt to move to their equilibrium potential of +61mV
Influx depolarises the membrane further, causing more voltage-gated Na+ channels to open and even more depolarisation.
Describe what is happening during Repolarization (downstroke of action potential)
During maintained depolarisation, Na+ channels are closed by a mechanism called inactivation.
Voltage gated K+ channels are also opened (activated) by depolarisation causing a K+ ion efflux as K+ attempts to move towards it's own equilibrium potential.
A combination of these two events causes the membrane to repolarise. NOTE THAT Sodium Pump is not involved in the repolarization of the action potential.
Describe the change in ion concentration gradient needed for generating an action potential
SMALL Change in concentration = mol/volume
What are the ways of investigating the mechanism of Action Potential generation?
Voltage-clamping controls the membrane potential - allows measurement of ionic (membrane) currents at a constant voltage (set membrane potential)
Using different ionic concentrations the contribution of various ions can be assessed,
Patch-clamping enables currents flowing through individual ion channels to be measured.
Where is the action potential initiated?
Depolarization to threshold initiates an action potential at the axon hillock
Describe the All or Nothing basis of an AP
The Na+ channels that open to cause depolarisation are voltage-gated.
This means that as the membrane potential becomes more positive, positive feedback means that more channels will open until they all are.
The depolarisation cannot stop halfway as this voltage will be the point at which more channels open, thus causing more depolarisation.
What happens to the Na+ channels after an action potential has been generated?
Most of the Na+ channels have been inactivated - as soon as Na+ channels are open they are susceptible to becoming inactivated very rapidly.
Na+ channels only recover during Hyperpolarization (become closed).
What is the ARP?
Absolute Refractory Period
Nearly all Na+ channels are in the inactivated state. The period is the duration of the action potential
Excitability is 0 - not able to generate an Action Potential (Excitability 1= cell is ready to generate an AP)
What is the RRP?
Relative Refractory Period
Na+ channels are recovering from inactivation. The excitability returns towards normal as the number of channels in the inactivated state decreases
A stronger stimulus is needed to generate an AP (threshold potential is likely to be positive at this point)
What is accommodation?
The longer the stimulus, the larger the depolarization necessary to initiate an action potential - the threshold becomes more positive due to the accumulation of inactivated sodium channels which can only recover during Hyperpolarization.
Stimuli of slowly increasing intensity causes the threshold potential to become more positive until the threshold is no longer reached as too many Na+ channels are inactivated so an AP can't be generated - even if membrane potential surpasses the original threshold.
Describe the basic structure of voltage gated Na+ and Ca2+ channels
They are similar in structure.
Their main pore-forming alpha subunit is one peptide consisting of 4 homologous repeats.
Each repeat consists of 6 transmembrane spanning domains with one of those domains being voltage-sensitive (able to detect the voltage field across the membrane)
A functional channel requires 1 subunit.
There is also an inactivation particle attached to the subunit - 'ball polypeptide' - acts as a plug, entering the pore and binds, preventing flow of ions.
Describe the structure of voltage gated K+ channels.
Similar in structure to Na+ but EACH REPEAT is a SEPARATE SUBUNIT.
Each subunit still has six transmembrane domains, one of which is voltage sensitive
A functional channel requires 4 alpha SUBUNITS.
The S4 region on each subunit has positive amino acid residues which sits within the membrane so when the voltage changes, it undergoes a conformational change leading to opening or closing of the the ion channel. The P (or H5) region contributes to pore selectivity.
Describe the action of local anaesthetics
Local anaesthetics such as Procaine act mainly by binding to and blocking Na+ channels, thereby stopping AP generation.
They block conduction in nerve fibres in the following order:
1. Small myelinated axons
2. Non-myelinated axons
3. Large myelinated axons Because of this they tend to effect sensory before motor neurons.
Describe the properties of local anaesthetics
They are weak bases and cross the membrane in their unionised form.
They block Na+ channels when the channel is open and also have a higher affinity to the inactivated state of the Na+ channel - the more Na+ channels are open, the more they are blocked.
Describe Extracellular Recording and how can it be used to measure Conduction Velocity?
Different classes of axons have different sized diameters and different conduction velocities.
Electrical stimulation: electrodes are used to raise the membrane potential to threshold to generate an action potential.
By recording changes in between the stimulating cathode (-ve) and recording anode (+ve) electrodes along an axon, conduction velocity can be calculated using the equation conduction velocity = Distance / time
How is the action potential calculated?
Measure the distance between the stimulating electrode and the recording electrode
Measure the time gap between the stimulus and the action potential being registered by the recording electrode.
Conduction Velocity = Distance / Time
Explain the Local Current Theory of Propragation
Happens more or less instantaneously.
Injection of current (depolarisation) into the of the membrane of an axon will cause the resulting charge (influx of Na+) to spread along the axon and cause an immediate local change (depolarisation) in the membrane potential.
This is electrotonic/passive membrane potential spread but NOT AN ACTION POTENTIAL.
How is an Action Potential conducted along an axon?
A change in membrane potential in one part can spread to adjacent areas of the axon because of local current spread.
The depolarisation of a small region of membrane produces transmembrane currents in neighbouring regions.
As Na+ channels are voltage gated, this opens more channels, causing the propragation of the action potential.
The further the local current spreads, the faster the conduction velocity of the axon.
What are the properties that lead to a high conduction velocity?
A high membrane resistance
A low membrane capacitance
A large axon diameter (this leads to low cytoplasmic resistance)
Explain how high membrane resistance leads to a high conduction velocity?
Ohm's Law: V=IR, states that the higher the resistance of the membrane, the higher the potential difference across it.
More voltage across the membrane means more voltage gated Na+ channels are open; making it easier to reach the threshold to fire an AP. Conduction velocity is therefore increased.
How does Large Axon Diameter increase Conduction Velocity?
Ohm's Law: I = V / R states that the lower the resistance (in this case low cytoplasmic resistance is due to large axon diameter), the larger the current, therefore the action potential will travel further. Conduction velocity is therefore increased.
How does low membrane capacitance lead to high conduction velocity?
Capacitance is the ability to store charge and is a property of the bilayer.
Therefore a membrane with a high capacitance takes more current to charge (or a longer time for a given current) and can cause a decrease in spread of the local current, especially with brief current pulses.
Similarly for a given current, a membrane with a low capacitance will take less time to charge, increasing conduction velocity.
Describe the effect of myelination on conduction velocity?
Conduction velocity is increased considerably by myelination of axons.
Large diameter axon such as motoneurones are myelinated.
Smaller ones such as C-fibres (sensory neurones) are not.
How does myelination increase conduction velocity?
Increases membrane resistance of the axon
How is myelin formed by special cells?
Schwann cells myelinate peripheral axons
Oligodendrocytes myelinate axons in the CNS
They both envelop axons in the plasmalemma.