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Flashcards in Action Potential Deck (43)

What is the membrane potential at which the membrane is depolarised and the inactivation gates on the Na+ channels close to begin the absolute refractory period?



What is the difference between the absolute and relative refractory period?

Absolute = the inactivation gates of the Na+ channels are shut.
Relative = the membrane is hyperpolarised due to an overshoot of K+.


What is the resting membrane potential?

-70mV, the balance between the electrochemical gradients of all the ions (closest to the equilibrium potential of K+ because the membrane is so permeable to K+).


What is the poison from pufferfish?

Tetrodotoxin (TTX) which irreversibly blocks Na+ ion channels, and causes victims to die by respiratory paralysis.


How do local anaesthetics (e.g lidocaine) work?

They block Na+ ion channels, so the inward Na+ current is prevented and no action potentials are generated.


What is the relationship between velocity and diameter in an unmyelinated fibre?

Velocity is directly proportional to the square root of diameter.
(At very small diameters, the velocity of nerve transmission is actually faster in unmyelinated than myelinated fibres).


What is the relationship between velocity and diameter in a myelinated fibre?

Velocity is proportional to diameter.


How does a nerve impulse travel down a myelinated axon?

By saltatory conduction, jumping from one node of Ranvier to the next.


Why do synapses allow only unidirectional transmission?

The vesicles containing neurotransmitter are only present in the presynaptic cell, and the receptor molecules are only present on the post synaptic membrane.


Where are the neurotransmitter molecules synthesised?

Cytosol of the neurone.


What is the effect of botulinum toxin?

Prevents docking of synaptic vesicles onto presynaptic membrane.


What is the effect of organophosphate?

Irreversibly inhibit acetylcholinesterase.


What is spatial summation?

The additive effect of more than one synaptic potential arriving at a given region of a neurone.


What is temporal summation?

The additive effect of more than one synaptic potential arriving at a neurone in a short time span.


What is a neuromuscular junction?

A synapse between an alpha motor neurone and a skeletal muscle membrane.


What is a motor end plate?

The post synaptic portion of the neuromuscular junction.


How does suxamethonium work?

Initially activates the person receptor, then causes a prolonged depolarisation of the motor end plate which inactivates the fast Na+ channels, leading to relaxation.


What causes myasthenia gravis?

Antibodies binding to nicotinic receptors and causing a decrease in nerve conduction leading to muscle weakness.


What is the threshold that the sum of a graded response must exceed to generate an action potential in the axon hillock?



What is an action potential?

A rapid change in the electrical potential across the plasma membrane of a cell.


Where in the neurone do graded potentials that reach the threshold become an action potential?

Axon hillock


What is another name for graded potentials?

Electrotonic potentials


What is summation?

Inhibitory and excitatory graded potentials are summed up to produce either a net positive or net negative graded potential. If there is a net positive charge after summation, the graded potential moves onto the axon hillock to initiate an action potential.


What is meant by "graded potentials are decremental"?

They decay over time, unlike action potentials.


What is meant by temporal summation of graded potentials?

Two depolarisations at the same time add together to make a bigger depolarisation.


What is the Graded Response of a neurone?

The passive spreading of electrotonic potentials from the dendrites to the soma, and the sum of the electrotonic potentials reaching the axon hillock.


What are the three things which set up the resting membrane potential of the neurone.

1) Na+/K+ ATPase pumps out 3Na+ for every 2K+ pumped in
2) K+ leak channels allow K+ to move down its concentration gradient out of the cell
3) The membrane has low permeability to proteins so negatively charged proteins are trapped in the cytosol


What is meant by the (Nernst) equilibrium potential for one type of ion?

The voltage at which the ion's net flow across the membrane is zero (to calculate it you need to know the valence/charge of the ion and the extracellular and intracellular concentrations).


Why can injecting potassium chloride stop the heart?

It increases the extracellular concentration of K+ so that the equilibrium potential of K+ is increased so the resting membrane potential can't be achieved and an action potential can't be generated.


What is the difference between the Nernst equation and the Goldman equation for equilibrium potentials?

The Nernst equation only takes into account one type of ion, the Goldman equation takes into account the valence, concentration and permeability of multiple ions. The resting membrane potential can be calculated by balancing all these factors.


Why is the resting membrane potential close to the equilibrium potential of K+?

The membrane is highly permeable to K+, but it's to exactly the same because there's a few open Na+ channels.


When the membrane potential is at resting (-70mV) which channels are open and which are shut?

Both K+ and Na+ voltage gated ion channels are closed.
K+ leak channels are open.


When the membrane potential exceeds the threshold (-55mV) which channels open to depolarise the membrane?

Na+ voltage gated ion channels


When the membrane has been depolarised to +40mV, which channels shut and which open?

The inactivation gate shuts on the voltage gated Na+ channels so the absolute refractory period is in place.
The voltage gated K+ channels open to depolarise the membrane.


What is the relative refractory period?

When an overshoot of K+ out of the cell hyperpolarises the membrane to -80mV (as K+ tries to reach its equilibrium potential). The resting membrane potential is restored by Na+/K+ ATPase.


What is the difference between the effect of a bigger stimulus when the depolarisation is below -55mV, and when the depolarisation is above -55mV?

Below -55mV a bigger stimulus means a bigger graded potential.
Above -55mV a bigger stimulus means an increased frequency of action potentials.


How is an action potential unidirectionally propagated down an unmyelinated axon?

When Na+ enters at the axon hillock and depolarises the membrane, the Na+ inside the axon will increase the membrane potential of the neighbouring region of membrane so that the Na+ voltage gated ion channels are opened and the action potential is propagated. The absolute refractory period behind the action potential means is moves only one way down the axon.


How is an action potential unidirectionally propagated down an myelinated axon?

Jumping from one node of Ranvier to the next by saltatory conduction.


How is carbamazapine used to treat epilepsy?

It increases the duration of the inactivation gate of the Na+ channels being closed, so the absolute refractory period is increased and the frequency of action potentials is decreased.


What is tDCS and what is it used to treat?

Transcranial Direct Current Stimulation - stimulates areas of the brain that may have lost stimulation due to a problem. Used to treat depression, heal stroke victims, maintain the vision centre of the brain while the eye is treated for macula degeneration.


What is multiple sclerosis (MS)?

An autoimmune disease where T cells attack the myelin sheath leading to the demyelination of multiple tissues and the scarring of neural tissue (sclerosis).
The symptoms include double vision, muscle weakness, fatigue, double incontinence, cognitive impairment.


What are the two types of local anaesthetic?

Esters - (previously used) names contain one "i" e.g cocaine, procaine, less stable and risk of severe allergic reaction and addiction
Amides - (now used) names contain two "i"s e.g lidocaine, bupiracaine, ropivacaine


How can the effect of local anaesthetics be prolonged?

Using a vasoconstrictive agent (e.g noradrenaline) to reduce the rate at which the blood washes away the Na+ channel antagonist.