The Action Potential Flashcards
What is an action potential? How long does it usually take?
A rapid change in the membrane potential
0.5ms in an axon
What is meant by the ‘all or nothing’ rule of action potentials?
Only occurs if a threshold level is reached
If sufficient depolarisation takes place and action potential will be initiated. Where do this occur?
At the axon hillock
If the conductance of an ion is increased, the membrane potential will move ________ the equilibrium potential for that ion
Towards
What causes an action potential to start?
A large increase in permeability to Na+ ions
What two channels are involved in an action potential?
Voltage dependent Na+ channels
Voltage dependent K+ channel
What does depolarisation do to voltage gated channels?
Open them
What does hyperpolarisation do to voltage dependent channels?
Close them
What happens to Na+ channels after they open?
They inactivate
Do K+ channels inactivate?
No
K+ channels are ______ in their closing causing ______
Delayed
Hyperpolarisation
How many ions need to flow to cause an action potential?
A small amount
What can be used to measure membrane currents at a set membrane potential?
Voltage clamps
Depolarisation is caused by…
An influx of sodium ions
Repolarisation is caused by….
Inactivation of sodium channels
(Slower) opening of K+ channels
Hyperpolarisation is caused by…
Increased K+ conductance
Delayed closing of K+ channels
What are the two periods of recovery for Na+ channels after an action potential? How long does each period last?
ARP - Absolute refractory period (1ms)
RRP - Relative refractory period (4ms)
What happens during the ARP?
Na+ channels inactivated
0 membrane excitability
No further action potentials can be sent
What happens during RRP?
Na+ channels recovering from inactivation
Recover once mp= -ve
Strong stimulus may result in action potential
What forms the functional part of a voltage gated Na+ channel?
1 peptide —-> 1 alpha subunit
What forms the functional part of a voltage gated K+ channel?
4 alpha subunits
How many repeats are found in a voltage gated Na+ channel?
4
How many repeats are found in a voltage gated K+ channel?
1
The 4th transmembrane region of each repeat on both voltage gated sodium and potassium channels contains…
Many positives amino acids, acts as a voltage sensor
What does a voltage gated Na+ channel contain to allow it to become inactivated?
An inactivation particle between repeats 3 and 4
Which region on both voltage gated sodium and potassium channels allows ions through?
The pore region
Many local anaesthetics work by blocking the action of which channels? Give an example?
Na+ channels
Procaine
Local anaesthetics block different axons in which order?
First… small myelinated axons —> unmyelinated axons —-> large myelinated axons
What is the local current theory?
Injection of a current to one part of axon, will result in a spread of charge and therefore immediate changes and depolarisation of adjacent parts of the axon
What is the length constant?
The distance it takes for the potential (of an injection of current) to drop to 37% of its original value
_______ membrane resistance = increased conduction velocity/spread of charge
Increased
________ membrane capacitance = increased conduction velocity/spread of charge
Decreased
What is capacitance?
A property of the lipid bilayer (ability to store charge)
What does resistance of a membrane depend upon?
The number of open channels
E.g. High resistance = less channels open
Why does high capacitance result in decreased conduction velocity?
Voltage changes more slowly in response to current injection
What affect does high resistance have on the spread of charge?
Spreads further along the axon
What is responsible for the propagation of the action potential?
Local currents
How does myelination increase the conduction velocity?
Increases membrane resistance
Decreases membrane capacitance
The myelin sheath is produced by which cells in the PNS?
Schwann Cells
The myelin sheath is produced by which cells in the CNS?
Oligodendricytes
There is a high density of _______ at the nodes of Ranvier of myelinated axons
Na+ channels
How are Na+ channels distributed along an unmyelinated axon?
Evenly
What does saltatory conduction describe? What does it result in?
The jumping of the local circuit current from node to node
Increased conduction velocity
In a myelinated axon, the diameter is ______ to the conduction velocity
Proportional
In an unmyelinated axon, the conduction velocity is proportional to ______ of the diameter
Square root
What is an example of a demyelinating disease? Which nerves does it affect? What does it result in?
Multiple sclerosis
Affects all cns nerves
Poorer transmission of action potentials (doesn’t reach threshold)
What is a neuromuscular junction?
The synapse between a nerve and skeletal muscle cell (fibre)
What channels are present at NERVE TERMINALS?
Voltage gated Na+ channels
Voltage gated K+ channels
A high density of voltage gated Ca2+ channels
What does the opening of voltage gated calcium channels at nerve terminals result in?
Calcium into the cell
Release of neurotransmitter
What does an increased frequency of action potentials result in?
Increases calcium entry at nerve terminals
More transmitter released
Bigger response
What is the structure of a voltage gated Ca2+ channel?
Similar to structure of Na+ channel
1 alpha subunit makes a function channel
What effect does nifedipine have on calcium channels? What is used to treat?
Blocks L type calcium channels
High blood pressure
A pore forming alpha subunit is necessary for a functional channel, what is the function of other subunits?
They regulate the activity of the channel through phosphorylation/glycosylation of parts of the sub unit
How do calcium channels activate/inactivate in comparison to sodium channels?
More slowly
At more positive membrane potentials
Inactivate more slowly as well
What is voltage gated calcium channel inactivation dependent on?
The intracellular concentration of calcium
During neurotransmitter release what does calcium bind to after its entry into the cell? What does this result in?
Synaptotagmin
Vesicle is brought close to the membrane?
What happens for a vesicle of neurotransmitter to be released at a membrane?
Snare complex makes a fusion pore at the membrane
Transmitter released through the pore
How are nicotinic acetylcholine receptors activated?
Binding of two molecules of ACh —> conformational change
Channels opens to Na+ and K+ ions
How does crurare cause paralysis?
Blocks transmission between nerve and muscle
Nicotinic ACh receptors can be blocked by which two methods, give an example of a drug that acts by each method?
Competitive blocker (tubocurarine)
Depolarising blocker (succinylcholine)
How does competitive blocking of nicotinic ACh receptors by drugs such as tubocurarine work? How can it be overcome?
Blocking of ligand binding sites —> channel cannot open
Increasing [ACh]
How does depolarising blocking of nicotinic ACh receptors by drugs such as succinylcholine work?
Binds to ligand binding sites activating the receptors and causing depolarisation
Stays bound, maintained depolarisation will not activate Na+ channels as they are inactivated
What is mayasthenia gravis? What are its symptoms?
Autoimmune disease targeting nAChR
Muscle weakness and fatigue
