Action Potential

Question 1

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?

Answer 1

+40mV

Question 2

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

Answer 2

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

Question 3

What is the resting membrane potential?

Answer 3

-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+).

Question 4

What is the poison from pufferfish?

Answer 4

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

Question 5

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

Answer 5

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

Question 6

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

Answer 6

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).

Question 7

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

Answer 7

Velocity is proportional to diameter.

Question 8

How does a nerve impulse travel down a myelinated axon?

Answer 8

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

Question 9

Why do synapses allow only unidirectional transmission?

Answer 9

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

Question 10

Where are the neurotransmitter molecules synthesised?

Answer 10

Cytosol of the neurone.

Question 11

What is the effect of botulinum toxin?

Answer 11

Prevents docking of synaptic vesicles onto presynaptic membrane.

Question 12

What is the effect of organophosphate?

Answer 12

Irreversibly inhibit acetylcholinesterase.

Question 13

What is spatial summation?

Answer 13

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

Question 14

What is temporal summation?

Answer 14

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

Question 15

What is a neuromuscular junction?

Answer 15

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

Question 16

What is a motor end plate?

Answer 16

The post synaptic portion of the neuromuscular junction.

Question 17

How does suxamethonium work?

Answer 17

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

Question 18

What causes myasthenia gravis?

Answer 18

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

Question 19

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

Answer 19

-55mV

Question 20

What is an action potential?

Answer 20

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

Question 21

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

Answer 21

Axon hillock

Question 22

What is another name for graded potentials?

Answer 22

Electrotonic potentials

Question 23

What is summation?

Answer 23

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.

Question 24

What is meant by “graded potentials are decremental”?

Answer 24

They decay over time, unlike action potentials.

Question 25

What is meant by temporal summation of graded potentials?

Answer 25

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

Question 26

What is the Graded Response of a neurone?

Answer 26

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

Question 27

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

Answer 27

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

Question 28

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

Answer 28

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).

Question 29

Why can injecting potassium chloride stop the heart?

Answer 29

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.

Question 30

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

Answer 30

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.

Question 31

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

Answer 31

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

Question 32

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

Answer 32

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

K+ leak channels are open.

Question 33

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

Answer 33

Na+ voltage gated ion channels

Question 34

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

Answer 34

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.

Question 35

What is the relative refractory period?

Answer 35

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.

Question 36

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

Answer 36

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

Question 37

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

Answer 37

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.

Question 38

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

Answer 38

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

Question 39

How is carbamazapine used to treat epilepsy?

Answer 39

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.

Question 40

What is tDCS and what is it used to treat?

Answer 40

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.

Question 41

What is multiple sclerosis (MS)?

Answer 41

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.

Question 42

What are the two types of local anaesthetic?

Answer 42

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

Question 43

How can the effect of local anaesthetics be prolonged?

Answer 43

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