Session 4 Flashcards
What is an action potential?
A change in voltage across a membrane.
What does an action potential depend on?
On ionic gradients and the relative permeability of the membrane.
What is the sodium hypothesis?
Once the membrane reaches a threshold voltage the Na+ channels open. Allows Na+ in. This depolarises the membrane, has a snowball effect.
What happens to Na+ channels during repolarisation?
They close by a mechanism called inactivation.
What other channels open during repolarisation?
Voltage gated K+ channels, so there is a K+ efflux.
Why does K+ flow out of the cell during repolarisation?
Because it wants to move towards its equilibrium potential.
Hiw do Na+ channels ensure an all or nothing response?
They are voltage gated with positive feedback. Therefore once some are activated they cause Na+ influx so it becomes more positive which causes more to open, causing more depolarisation.
What is the Absolute refractory period?
All Na+ channels are in the inactivated state, excitability is at 0.
What is the Relative refractory period?
Na+ channels are recovering from inactivation. Excitability returns to normal as the number of channels in the inactivated state decreases.
What is accommodation?
The longer a stimulus is, the larger the depolarisation necessary to initiate an action potential. This is because Na+ channels become inactivated.
What is the molecular similarity between voltage gated Na+ and Ca2+ channels?
The main pore is one peptide consisting of four homologous repeats. Each repeat has 6 transmembrane domains (one is voltage sensored) Function requires one subunit.
What is the molecular structure of K+ channels?
It has 4 peptides and 6 transmembrane domains. One is voltage sensitive and function requires 4 subunits.
How do local anaesthetics work?
They bind and block the Na+ channel (when it is open) so no action potential can be generated. Also have a high affinity for the inactivated state of the Na+ channel.
What is the order that local anaesthetics block conduction in nerve fibres?
Small myelinated axons
Non myelinated axons
Large myelinated axons
(Means they tend to effect sensory before motor)
How can conduction velocity be calculated?
Recording changes between the stimulating and recording electrodes on an axon and using an equation. (Velocity = Distance / Time)
What is the local circuit theory of propagation?
Depolarisation of a small region causes transmembrane currents in neighboring regions. Because Na+ channels are voltage gated, more open casing propagation of the action potential. The further the current spreads, the faster the conduction velocity of the axon.
What properties of an axon lead to high conduction velocity?
High membrane resistance
High axon diameter
Low cytoplasmic resistance
Low membrane capacitance.
Why would the conduction velocity be higher with a high membrane resistance?
Ohms law states that higher resistance of the membrane means higher potential difference so more voltage across the membrane means more voltage gated Na+ channels are open.
Why would conduction velocity be increased with a large axon diameter?
Ohms law states that the lower resistance, the larger the current so the action potential will travel further.
Why would conduction velocity be increased with a low membrane capacitance?
Because capacitance is the ability to store charge, a low one means less time will be needed to charge it.
What increases the conduction velocity?
Myelination of axons.
What does myelination do?
Reduces capacitance
Increases membrane resistance
Therefore increases conduction velocity
What is saltatory conduction?
Where the action potential jumps between Nodes of Ranvier.
How does saltatory conduction work?
Because the myelin sheath is a good insulator and causes a local circuit current to depolarise to the next node above the threshold and generate an action potential.
How does the distribution of sodium channels vary along a myelinated axon?
There are many sodium channels in the Nodes of Ranvier and very few in the myelinated areas.
What cells form the myelin in peripheral axons?
Schwann cells.
What cells form the myelin in CNS axons?
Oligodendrocytes.
What disease destroys the CNS axon’s myelin?
Multiple Sclerosis - Autoimmune disease
What does Multiple Sclerosis cause?
The action potentials cannot be conducted properly because the myelin has been broken down. There is decreased conduction velocity, complete blockage or partial blockage.
