Lecture 21 - Nerve cells and Excitability - Action potentials

Question 1

How do neurons communicate? How do they receive and process signals?

Answer 1

Neurons can communicate with each other via their dendrites and axons.

Incoming signals are received at the dendrites, the cell body processes the information and the axon carries the information along to the synaptic terminals, which transmit the information

Question 2

The signals sent from the cell body/ soma to the synaptic terminals along the axon is called….

Answer 2

action potentials

Question 3

Define an action potential?

Answer 3

a change in the voltage across the membrane of the neuron due to the flow of certain ions into and out of the neuron.

Question 4

How are action potentials initiated?

Answer 4

• The neuron is stimulated at the dendrites. Excitatory signals at dendrites open ligand-gated sodium ion channels and allow sodium to flow into the cell, neutralising some of the negstive charge inside of the cell so the membrane becomes less negative. This is depolarisation, the cell membrane becomes less polarised.
• The influx of sodium diffuses inside the neuron and produces a current which travels towards the axon hillock- the trigger zone where action potentials start because they contain lots of voltage gated ion channels

Question 5

If stimulation is strong then the signal…

Answer 5

is transmitted along the entire axon in an action potential

Question 6

Two properties of action potentials

Answer 6

• very large
• rapid/transient

Question 7

What is the ‘all or none’ law?

Answer 7

• If the neuron depolarisation exceeds the threshold potential of -55mv, action potential will fire
• The size of these action potentials is always the same size
• The neuron either does not reach the threshold and there is no action potential fired OR it does reach the threshold and an action potential is fired

Question 8

What controls the opening/close of voltage gated ion channels?

Answer 8

voltage dependent - they only open when the membrane potential reaches a certain value

Question 9

If threshold is reached, which voltage gated ion channels open? What is the result of this? When do they close?

Answer 9

sodium ion channels.
sodium ions move down concentration gradient into the cell making the membrane less negative.
eventually they close because of the change in charge across the membrane.

Question 10

When membranes become +33 mV, which voltage gated ion channels open?

Answer 10

potassium voltage gated ion channels open wide and K+ ions move down the concentration gradient and out of the cell.

Question 11

Name the 5 stages of the action potential

Answer 11

hyperpolarisation
depolarisation
overshoot
repolarisation
hyperpolarisation/undershoot

Question 12

1. Hyperpolarisation

Answer 12

the initial increase of the membrane potential to the value of the threshold, from -70mV to -55mV

Question 13

2. Depolarisation

Answer 13

• Once threshold is reached, -55mV, the VGNC open very quickly
• the channels open wide to allow lots of Na+ ions to enter cell membrane by diffusion
• the large influx of Na+ ions causes the cell membrane become positively charged - the membrane is depolarised. This means that the membrane potential moves to less negative values as the inside of the cell membrane is electropositive
• as the membrane becomes more positive, more VGNC open- positive feedback.
• when the voltage is +33mV, the voltage gated potassium ion channels open. These open slowly during depolarisation and remain open during this stage
• the VGNC will begin to become inactivated

Question 14

3. Overshoot

Answer 14

The inside of the cell keeps getting more electropositive until the potential gets closer to the equilibrium of sodium ions which is +61mV. The peak of action potential is about +40mV.

Question 15

4. Repolarisation

Answer 15

The membrane potential moves back to RMP. Sodium channels now begin to close. By this time, the slow VGKC should be fully opened. The K+ ions rush out of the cell down the concentration gradient

The voltage returns to its original RMP. The VGKC will slowly begin to close during this stage

Question 16

5. Hyperpolarisation

Answer 16

Because the potassium ion channels are slow to close, the potassium ions leave the cell for a little too long, resulting in negative overshooting called hyperpolarisation/undershoot.
The potential moves away from the RMP in a negative direction

Question 17

How do we go from hyperpolarisation to the RMP?

Answer 17

the VGKC finally close and the potassium leak channels and sodium/potassium ion pumps restore the RMP

Question 18

What is the refractory period?

Answer 18

• During and shortly after an action potential is generated, it is impossible/ difficult to stimulate that part of the membrane to fire again. This is the refractory period.

Question 19

What are the two types of refractory period?

Answer 19

absolute and relative refractory periods.

Question 20

What is the absolute refractory period?

Answer 20

lasts from the start of an action potential to the point the voltage first returns to the resting membrane value. The voltage gated sodium ion channels are inactivated during this time and cannot be activated again until the membrane is repolarised and RMP restored.

Question 21

What is the relative refractory period?

Answer 21

the membrane potential is hyperpolarised by voltage gated potassium ion channels. An action potential can be generated if the stimulus is strong enough to overcome the hyperpolarisation and reach the threshold value of -55mv.
- The relative refractory period lasts until the end of hyperpolarisation

Question 22

It is hard to depolarise the membrane during the relative refractory period. Why?

Answer 22

During this time some of the potassium channels are still open and the membrane is much more negatively charged than the RMP.
so a strong signal is required to induce another AP.

Question 23

What is action potential propagation?

Answer 23

the movement of an action potential along the axon.

Question 24

How does propagation work?

Answer 24

• During AP, there is sodium ion influx at one point of the membrane in the axon.
• Sodium ions diffuse along the axon membrane.
• The adjacent membrane therefore becomes more positively charged - depolarised and an action potential is generated at this point of the membrane.

Question 25

Why does propagation only move in one direction?

Answer 25

• Sodium ions can diffuse in both directions along the axon, however, because of refractory periods, propagation only occurs in ONE DIRECTION. The part of the membrane which has just fired is inactive.

Question 26

An action potential generated at the axon hillock travels down the axon to the nerve terminal and not towards the cell body. Why?

Answer 26

there are higher concentration of voltage gated ion channels in the axon than in the cell body.

Question 27

The velocity of propagation is dependent upon two things…

Answer 27

large axon diameter and myelin

Question 28

How does a large axon diameter increase velocity of propagation?

Answer 28

• large axons = less resistance to local current because the ions have more space to travel so it is more likely that they will keep moving in the right direction – remember axons are part of the cell so contain proteins, vesicles, etc which can get in the way of ions. Therefore, larger the diameter, more space, less chance they will bump into these proteins and vesicles and things so the faster the ions move so the faster the propagation

Question 29

Why does myelin sheath increase the velocity of propagation?

Answer 29

electrically insulates areas of axon and so allows for saltatory conduction to occur.

Question 30

What is saltatory conduction?

Answer 30

The propagation of action potentials along myelinated axons from one node of Ranvier to another, increasing the conduction velocity of action potentials
it decreases time spent passing messages and also conserves energy.

Question 31

Explain how saltatory conduction works

Answer 31

• an action potential is initiated at node A.
• The action potential cannot propagate actively because of the myelin which obstructs the voltage gated ion channels.
• Instead, the action potential at one node spreads along the internodal region of the axon passively.
• As the potential spreads, it gets smaller.
• Eventually the potential will reach then next node, node B
• At this node, a new action potential is initiated as the current opens up voltage gated sodium ions at the node.
• The node acts as a relay station that renews the weak signal flowing passively

Question 32

Give two demyelination diseases.

Answer 32

• Guillain barre syndrome = destruction of Schwann cells in the PNS
• Multiple sclerosis = loss of oligodendrocytes in the brain and spinal column.

Question 33

If the myelin of sensory nerves/ afferent fibres degenerate, what is the effect?

Answer 33

cause numbness or tingling- sensations are not travelling the way that they should

Question 34

When efferent motor nerves are demyelinated, what is the effect?

Answer 34

this can cause weakness because the brain is spending a lot of energy but it still cannot move the affected limbs

Question 35

Why are libs especially effected by the demyelination of nerves

Answer 35

because they have the longest nerves. The longer the nerve, the more myelin that can be destroyed

Question 36

What is a graded potential?

Answer 36

changes in the membrane potential that are confined to a small region of the membrane

usually produced when some specific change in the cells environment acts on specialised region of the membrane.

Question 37

Why are they called graded potentials?

Answer 37

the magnitude of the potential can vary with the strength of stimulus.

Question 38

If the initiating event is not strong enough to induce depolarisation of the membrane to reach the threshold then what happens to the graded potential?

Answer 38

graded potential dies out and no action potential is generated.

Question 39

The local current of graded potentials is decremental. What does this mean?

Answer 39

the flow of charge decreases as distance from site of origin of the graded potential increases.

Question 40

Graded potentials can be depolarising or hyperpolarising. Explain

Answer 40

some stimuli are not excitatory and do not take membrane towards the threshold. Instead, they can take the membrane further from the threshold- inhibitory postsynaptic potential. The membrane is hyperpolarised.

Question 41

What is temporal summation?

Answer 41

graded potentials decay with time and distance. Because they decay with time, they may be separated by enough time to have no effect on each other. But, if they happen close to each other, their effects may be additive- temporal summation.

Question 42

What is spatial summation?

Answer 42

Because graded potentials decay with distance they may be separated by enough distance to have no effect on each other. If they occur near the same part of the membrane their effects may be additive – we call this spatial summation.

Question 43

If two depolarisations of the same size occur at the same time and place then…

Answer 43

then a depolarisation twice the size will happen. The same happens for hyperpolarisations.

Question 44

What happens if excitatory and inhibitory potentials of the same size occur at same time and place?

Answer 44

They may cancel each other out and the membrane potential may not change.