Electrical Excitabilty (the AP And Properties)

Question 1

What are features of an Action Potential?

Answer 1

A change in the voltage across membrane 
Depend on ionic gradients and relative permeability of membrane 
Only occur when threshold is reached
All or nothing 
Propagated without loss of amplitude

Question 2

How does depolarisation imitate an action potential?

Answer 2

Single Action potential at each synapse releases nertotransmittter which binds to the axon hillock and acts to depolarise it.
AP’s from separate synapses all release neurotransmitter so has summarise effect that allows threshold to be reached.

This hen opens voltage gated ion channels in axon.

Question 3

What happens if conductance (membrane permeability) to an io is increased?

Answer 3

The membrane potential will move closer t the equilibrium potential for that ion.

Question 4

What is membrane permeability to an ion dependant on?

Answer 4

The number of channels for that ion that are open

Question 5

How do we know which direction an ion will travel in?

Answer 5

It will travel dependant on the direction and size of its electrochemical gradient.

Na+ moves in (to depolarise)
K+ moves it (to hyperpolarise)

Question 6

How can it be shown experimentally that Na+ is responsible for AP depolarisation?

Answer 6

As external (Na+) was reduced the E(Na+) reduced in a straight line.

The peak in action potential changed in a manner parallel to the E(Na+) change.

Supports idea that upstroke in action potential is due to a large increase in permeability to Na+ ions

Question 7

How much ion movement is needed to fire an AP?

Answer 7

A very small amount, a few mM.

Even lower in wider axons

Question 8

What is not involved in the repolarization of the AP?

Answer 8

The sodium potassium pump

Question 9

Where is there an example of positive feedback in an AP?

Answer 9

The Na influx (it causes the membrane to depolarise which causes more Na channels o open and more Na to influx)

It’s switched of when the k channels often and the Na channels close

Question 10

How does the membrane repolarised after an action potential?

Answer 10

Na influx stops and channels are inactivated

K+ efflux begins

Question 11

What is an absolute refractory period?

Answer 11

When nearly all Na+ channels are in the inactivated state.

Relative excitability is 0

Question 12

What is a relative refractory period?

Answer 12

Na+ channels are recovering from inactivation, xcitabilty returns towards normal as number of inactive channels decreases and the number of K close.

A very strong stimulus is required to make an AP

Question 13

What is the basic structure of a voltage-gated Na+ channel?

Answer 13

One long peptide chain with 4 subunits
Pore region
An inactivation particle that can plug the pore
Has a voltage sensor (S4)

Question 14

What is the role of the VOltage sensor (S4) in a voltage-gated Na+ channel?

Answer 14

It is positively charged AA’s in the voltage field of the channel.
When the voltage of the membrane changes their charge changes, this causes a conformational change to the protein opening its pore to allow Na+ through into the cell.

Question 15

What is the basic structure of a voltage-gated K+ ion channel?

Answer 15

4 individual subunit peptide chains
Pore region contributes to selectivity
Voltage sensing S4 region

Question 16

How does the opening of channels allow us to see membrane potential change?

Answer 16

Channels open and close in a random manner

The potential is an average of the channels

Question 17

Give an example of a local anaesthetic and explain it two forms and their relevance.

Answer 17

Lidocaine is an anaesthetic (also treat arrhythmia)
It has an in-protonated form and a protonated form. The unprotonated form is membrane permeable to can get into cells. The protonated form is membrane impermeable,

At the body’s pH of 7.2 most molecules are in protonated (insoluble) form.

Question 18

How do local anaesthetics work?

Answer 18

The work in 2 ways depending if in un-protonated or protonated form.

Un-protonated=soluble, moves through membrane and blocks a closed channel by filling the pore from the other side. This can block the channel in an inactivated state. This is most effective.

Protonated=can block an open channel by using pore.

Both blocks stop Na+ getting in so stop AP’s being created so stop pain sensing fibres receiving info

Question 19

In what order to local anaesthetics block axons?

Answer 19

First= small myelinated axons
Then= in-myelinated axons
Last= large myelinated axons

Question 20

Why do diff nerves have diff conductance velocity?

Answer 20

As large (diameter) axons conduct faster.
Small (diameter) axons conduct slower.

Question 21

Explain the local current theory.

Answer 21

Injection of a current into an axon wil cause the resulting potential to spread along the axon in either direction and cause an immediate local change in membrane potential.

This doesn’t use channels only local charges

Question 22

What is capacitance?

Answer 22

The ability to store charge

If capacitance is high, the change in voltage spreads further along the axon so more channels open

Question 23

What does membrane resistance depend on?

Answer 23

Depends on the number of ion channels open. The more channels open the lower the resistance.

If resistance is high the collage changes more slowly in response to current injection and doesn’t travel as far

Question 24

How can we make AP’s travel further along the axon?

Answer 24

By opening more channels (increase resistance)
By increasing the diameter of the axon

If the voltage travels further, more of the axon is initiated per AP, so conduction velocity is increased

Question 25

How does refractory period effect the direction of an impulse?

Answer 25

It prevents it travelling backward along the axon after current injection.

It’s sped up by hyperpolarisation

Question 26

Hw does myelination effect a nerve?

Answer 26

There are nodes of Ranvier between myelin sheaths. This is where all the Na+ channels are concentrated into.

It’s acts as a good insulation so local circuit currents depolarise the next node above threshold and create an action poteniton.

In this way AP’s jump between nodes in Saltatory conduction. This is faster than it passing along an axon.

Question 27

State some diseases of the central nervous system. What cases them?

Answer 27

Multiple SClerosis - all CNS nerves
d=Devic’s disease- optic + spinal nerve only

Result from breakdown/damage to myelin sheaths.
Symptoms depend on where myelin is damaged

Question 28

State some disease of the peripheral nervous system

Answer 28

Landry-Gillian-Barre syndrome

Charcot-Marie-Tooth disease

Question 29

What happens in demyelination

Answer 29

In regions of demyelination the density of AP is reduced because of resistive and capacitive shunting. This means the next node of ranvier does not reach threshold value and AP stops