Nerve impulses + synaptic transmission

Define stimulus

A change in environment

What is resting potential in a neurone?

-70mV

How is resting potential maintained?

1. The Na+/K+ pump actively transports 3Na+ ions out of the axon and 2K+ ions into the axon

2. The membrane is more permeable to K+ ions (as there are more K+ ion channels compared to Na+ ion channels) this means K+ ions diffuse out of the axon faster than Na+ ions diffuse back in. (Membrane is less permeable to Na+ ions)

3. There are negatively charged proteins (and Cl- ions) in the axon.

What happens when the axon is depolarised?

1. Na+ ions enter the axon due to a stimulus

2. Due to the influx of Na+ ions diffusing along the axon membrane the potential difference across the axon becomes more positive

3. If enough Na+ ions enter, the axons potential difference will reach threshold (-55mV) and exceed it

4. At -55mV Na+ voltage-gated channels will change tertiary structure and open

5. And so more Na+ ions move into the axon by facilitated diffusion (down their electrochemical gradient) until action potential is reached (+40mV)

What happens if threshold is not reached or exceeded?

Na+ ions will be pumped back out by the Na+/K+ pump and axon returns back to threshold

Define action potential

The momentary positive potential difference across the surface membrane of an neurone following its depolarisation (+40mV)

What happens when action potential is reached and axon returns to resting potential?

1. After it reaches action potential (+40mV), tertiary structure of the voltage-gated Na+ ion channels changes and they close.

2. And the voltage-gated K+ ion channels open

3. And so too many K+ ions move out of the axon so the axon becomes more negative than resting potential (hyperpolarisation)

4. (Na+/K+ still actively transports 3Na+ions out of axon and pumps in 2K+ ion into the axon)

5. The K+ voltage gated channels close

6. Axon returns to resting potential

Explain hyperpolarisation

Occurs when axon hyperpolarises to approx -80mV which is more negative than resting potential (-70mV)

Due to too many K+ ions leaving via the K+ voltage-gated channels

So the the voltage-gated K+ ions channels close and the potential difference slightly increases back to -70mV

Describe how an action potential is propagated in a myelinated axon

1. The Na+ ions enter the axon at the first Node of Raniver causing the axon to become more positive inside

2. The influx of positive Na+ ions repel the positive ions already present in the axon so they move towards the next Node of Ranvier

3. This causes threshold to be reached at the next Node of Ranvier, so the Na+ voltage gated channels open and move Na+ move in by facilitated diffusion

4. This process repeats itself at each node of Ranvier, down the axon

5. This is known as salutatory conduction, because action potential jumps from one node of Ranvier to the next (allowing conduction of nerve impulse to be faster)

What is meant by the all or nothing principle?

A threshold value must be reached in order for an action potential to occur

All action potentials are the same size/magnitude (+40mV)

How does the strength of a stimulus affect the frequency of action potentials?

The stronger the stimulus the higher the frequency of action potentials transmitted along a neuron.

However, above a certain strength of stimulus the frequency of action potentials cannot get any higher

(Frequency of action potentials is dependent on intensity of stimulus)

What is the refectory period?

The period of time during which a new action potential cannot be generated

What two factors contribute to the refractory period?

• The Na+ ion voltage gated channels become inactive immediately after they were opened. This means that for a short period of time they cannot open again

• The neurone is hyperpolarised so it is ‘further away’ from threshold level required to generate an action potential

How does the refectory period ensure impulses are kept discrete?

It prevents a new action potential being generated immediately after the previous one because the Na+ voltage-gated channels will not open (they are inactive)

How does the refractory period limit the frequency of impulses transmission

when and why does this matter?

It prevents a new action potential being generated immediately after the previous one because

This is relevant because if a stimulus strength increases, the frequency of impulses increases.

But because the frequency of impulses cannot exceed a certain level, it is not possible to differentiate between stimuli above a certain strength

How are nerve impulses an example of postive feedback ?

Because when Na+ ions enter the axon, it becomes slightly depolarised

Then the Na+ ion voltage-gated channels open axon becomes even more depolarised and so moves further away from resting potential

Describe and explain how myelination and saltatory conduction affect the speed of conductance

Speed of conduction is higher in myelinated neurones

This is because myelinated provides electrical insulation

So the depolarisation can jump from one node of Ranvier to other next, (salutatory conduction)

As ions only diffuse in and out of the axon at the Nodes of Ranvier

Whereas in non-myelinated axons, depolarisation concurs along the whole length of the axon.

Describe and explain how the axon diameter affects the speed of conductance

The bigger the diameter, the higher the speed of conductance

This is becaue a bigger diameter means there is less resistance to the movement of the action potential down the axon

There is also proportionally less leakage of K+ ions

Describe and explain how temperature affects speed of conductance

The higher the temperature, the higher the speed of conductance

This is because the ions have more kinetic energy so the rate of diffusion of ions into and out of the axon increases

Synaptic transmission

Describe what happens at a cholinergic synapse

1. The action potential reaches the end of the pre-synaptic neurone and is depolarised due to influx of Na+ ions

2. Voltage-gated Ca²⁺ ions channels open so Ca²⁺ ions move in by facilitated diffusion

3. The Ca²⁺ cause the synaptic vesicles containing neurotransmitters move towards the pre-synaptic membrane and fuses with it

4. The neurotransmitter (acetylcholine) is released via exocytosis

5. Acetylcholine diffuses across the synaptic cleft

6. Acetylcholine binds to specific, complementary receptors (on the ligand-gated Na+ ion channels) on the post-synaptic membrane

7. The ligand-gated Na+ ion channels open and Na+ ions move onto the post-synaptic neurone by facilitated diffusion, down their electrochemical gradient, causing it to depolarise

8. If threshold is exceeded, Na+ voltage-gated channels open, so more Na+ ions move in by facilitated diffusion and a new action potential is generated in the post-synaptic neurone

9. Acetylcholinesterase hydrolyses the acetylcholine to form ethanoic acid and choline

10. The ethanoic acid and choline are reabsorbed into the pre-synaptic neurone

11. ATP is used to regenerate acetylcholine using the ethanoic acid and choline

What is meant by ‘unidirectionality’

That the impulses only travel in one direction along axons and across synapses

Describe how a synapse ensures unidirectionality of impulses

Only the pre-synaptic neurone contains synaptic vesicles with neurotransmitter

Only the post-synaptic neurone has specific receptors which the neurotransmitters can bind to

What is an excitatory synapse

When Na+ ions move into the post synaptic neurone so it becomes more positive inside,

making it more likely that an action potential is generated in the post-synaptic neurone

What is an inhibitory synapse?

• When the post synaptic neurone becomes more negative

• Due to either K + ions moving out or Cl- ions moving in

• This means that more Na+ ions would be required in order for an action potential to be reached

• So it is less likely that an action potential is generated in the post-synaptic neurone

Summation?

The rapid build up of neurotransmitters in the synapse to help generate an action potential

Spatial summation?

When there is more than one pre-synaptic neurone synapsed to the same post-synaptic neurone

Describe how spatial summation makes it more possible for an action potential to occur in the post-synaptic neurone?

Neurotransmitter from both presynaptic neurones bind to the receptors (on the ligand-gated channels) on the post-synaptic neurone.

So sufficient Na+ ion ligand-gated channels open

So sufficient Na+ ions move into the post-synaptic neurone by facilitated diffusion

So threshold is exceeded and an action potential will be generated

Temporal summation?

When the pre-synaptic neurone has a high frequency of action potentials

Describe how temporal summation makes it more possible for an potential to occur in the post-synaptic neurone?

A large amount of neurotransmitter is released by the pre-synaptic neurone in a short space of time

So sufficient Na+ ion ligand-gated channels open

So sufficient Na+ ions move into the post-synaptic neurone by facilitated diffusion

So threshold is exceeded and an action potential will be generated

What is meant by agonist drugs? And how do they work?

An agonist drug has a similar shape to the neurotransmitter and will bind to the same receptor that the neurotransmitter binds to

An agonist causes the receptors to respond in the same way as if the neurotransmitter itself was bound

E.g. the neurotransmitter usually binds to a ligand-gated channel and causes the channel to open, agonist would bind to the channel and cause the channel to open, even in the absence of the neurotransmitter

What is meant by antagonist drugs and how do they work?

An antagonist drug has a similar shape to the neurotransmitter and will bind to the same receptor that the neurotransmitter binds to

An antagonist cause the receptor to respond in a different way to what would happen if the neurotransmitter itself was bound

E.g. neurotransmitter usually binds to a ligand-gated channel and cause the channel to open, the antagonist would bind to the channel (so the neurotransmitter cannot bind) and prevent the channel from opening.