Sesh 5: Action Potential And The NMJ Flashcards
Depolarisation to threshold initiates an action potential at what part of the axon?
The axon hillock.
Why does the Na+ current diminish if the voltage is held at +20mV during a voltage clamp experiment?
Voltage-gated Na+ channels inactivate
Why does the action potential have an ‘undershoot’ phase?
Voltage-gated K+ channels don’t inactivate, therefore don’t close immediately when resting potential is reached, meaning they still drive the m.p. Towards Ek, resulting in hyperpolarisation.
What happens during the absolute refractory period?
Voltage-gated Na+ channels are inactivated due to blockage of their pore via their inactivation particle.
What is the relative refractory period?
Occurs for about 4ms after the absolute refractory period. Voltage-gated Na+ channels are now closed and recovering from inactivation. A strong stimulus may cause them to open and initiate an action potential.
What makes up the functional part of voltage-gated Na+ channels?
An alpha subunit with 4 homologous repeats- each with 6 transmembrane domains.
What has to occur for the inactivation particle to unblock the voltage-gated Na+ channel pore?
Membrane hyperpolarisation.
What does a voltage-gated K+ channel need to be functional, and how is this different from a voltage-gated Na+ channel?
Needs 4 alpha subunits together, whereas Na channel only needs 1 alpha subunit to be functional.
How do local anaesthetics, such as procaine, work?
By blocking voltage-gated Na+ channels when they are in the open or inactivated state, thereby blocking the action potential in pain fibres.
Which axon class has the highest conduction velocity, and where are these axons found?
- A alpha
- Motoneurones to skeletal muscle, and sensory fibres to muscle spindle
A nerve fibre is composed of several ______.
Axons
What is the local current theory?
Injection of current into an axon will cause an immediate local change in membrane potential due to Na+ influx. However, this local current dissipates.
Is conduction velocity higher in smaller or larger diameter axons?
Larger.
What is the length constant of an axon?
The distance along the axon it takes for the initial depolarisation to fall to 37% of its original value.
What is capacitance?
The ability of a membrane to store charge. Lower capacitance= higher conduction velocity.
What is membrane resistance determined by?
The number of open ion channels in the membrane. High resistance= few ion channels open= greater conduction velocity.
How does myelination change the membrane capacitance and resistance?
It insulates the axon, so lowers capacitance and increases resistance, to increase membrane conduction velocity.
Why can the action potential only travel in one direction?
Because the region of axon behind the action potential is refractory- v-gated Na channels are inactivated
In a myelinated neurone, how is conduction velocity related to fibre diameter?
Linearly
In unmyelinated neurones, how is conduction velocity related to fibre diameter?
Conduction velocity is proportional to the square root of fibre diameter.
What is the most common demyelinating disease of the CNS?
Multiple Sclerosis
How can a stronger stimulus be encoded?
Increased action potential frequency
What type of calcium channels do dihydropyridines e.g. Nifedipine block?
L-type
Compared to voltage-gated Na+ channels, do voltage-gated Ca2+ channels activate/ inactivate more slowly or quickly?
More slowly.
If extracellular Ca2+ is reduced, what will happen to the motor end plate potential?
The amplitude will be reduced.
How is the action potential at the neuromuscular junction terminated?
AChesterase degrades ACh in the cleft.
What is tubocurarine, and how does it work?
A neuromuscular blocker that works by blocking nicotinic ACh receptors, via binding to the ACh binding site.
A competitive nAChR blocker.
What is succinylcholine, and how does it work?
A neuromuscular blocker that blocks nicotinic ACh receptors.
Acts as a depolarising blocker, by binding to ACh receptors, and prolonging depolarisation, thereby inactivating adjacent Na+ channels.
What is Myasthenia Gravis?
An autoimmune disease of which sufferers produce auto-antibodies to nicotinic ACh receptors on skeletal muscle.
NAChRs are then degraded, reducing motor endplate potentials, leading to muscle weakness and fatigue.
What are muscarinic AChR’s and where are they found?
GPCR’s that bind ACh on target tissue innervated by the parasympathetic NS.
