U1-KA4-COMMUNICATION AND SIGNALLING 3)nerve impulse transmission

Question 1

what is the resting membrane potential of neurons

Answer 1

• the resting membrane potential of a neuron is a state in which there is no nervous impulse being transmitted ( a state where there is no net flow of ions across the membrane )

Question 2

all cells have an _________ _________ difference (voltage ) / __________ ________ across their plasma membrane.

Answer 2

all cells have an electrical potential difference (voltage) across their plasma membrane
- this voltage is called the membrane potential

Question 3

in neurons what is the the membrane potential typically between when the cell is not transmitting signals

Answer 3

in neurons, the membrane potential is typically between -60 and -80mV (millivolts) when the cell is not transmitting signals

Question 4

the inside of the cell is ________ relative to the outside

Answer 4

the inside of the cell is negative relative to the outside

Question 5

How is resting potential set up / membrane potential maintained

Answer 5

In order to maintain the cell membrane potential, cells keep a low concentration of sodium ions and high levels of potassium ions within the cell (intracellular).

Question 6

Explain setting up resting potential in terms of sodium potassium pump

Answer 6

• the sodium potassium pump moves three positively charged Na+ ions out of a neuron cell and two positively charged K+ ions into the neuron cell
• the neuron membrane also has potassium channels which let some of the K+ ions leak back out of the cell , and this results in a positive charge outside the cell compared to the inside of the cell

Question 7

What does transmission of a nerve impulse along a neuron require

Answer 7

changes in the membrane potential of the neuron’s plasma membrane

Question 8

Nerve transmission is…

Answer 8

Nerve transmission is a wave of depolarisation of the resting potential of a neuron

Question 9

What is depolarisation

Answer 9

Depolarisation is a change in the membrane potential to a less negative value inside. (When the interior voltage becomes less negative- and this is involved in neuron signalling.)

Question 10

Depolarisation is a result of?

- how is it brought about

Answer 10

Depolarisation results from a wave of electrical excitation along a neurons plasma membrane - this is called an action potential
- depolarisation of the resting potential can be triggered by neurotransmitter molecules (eg acetyl choline) at a synapse)

Question 11

What is an action potential

Answer 11

An action potential is a wave of electrical excitation along a neurons plasma membrane

Question 12

What does the action potential initiate

Answer 12

The action potential initiates the fusion of vesicles containing neurotransmitters with the presynaptic membrane

Question 13

What is a synapse

Answer 13

Junction between nerves or nerves and muscles

Question 14

Explain how a neurotransmitter triggers a nerve impulse

• what happens when the action potential reaches the end of the neuron ?

Answer 14

• neurotransmitterss initiate a response in neurons by binding to their receptors at a synapse
• neurotransmitter binds to a transmembrane receptor protein on the surface of the next neuron-the receptor is a ligand gated ion channel so the binding of neurotransmitter causes a change in conformation :making the channel open and letting Na+ (positive charge) ions diffuse through into neuron therefore causing depolarisation of the plasma membrane
• If sufficient ion movement occurs , then the voltage change across the membrane reaches a critical level and a patch of membrane is depolarised (becomes less negative). This voltage change causes the neighbouring voltage gated na+ channel to open , which depolarises neighbouring regions of the membrane and leads to a domino effect: triggering of one voltage gated channel depolarises the membrane , so triggering the adjacent voltage gated sodium channel and so on, allowing the action potential to travel rapidly along the neuron. membrane depolarisation leads to rapid and large changes in membrane potential.

When the action potential reaches the end of a neuron it causes vesicles containung neurotransmitters to fuse with the membrane , releasing neurotransmitters into the next synaptic cleft , which stimulates a response in the next connected cell

Question 15

Membrane depolarisation causes a ______ and ______ change in ________ ________.

Answer 15

Membrane depolarisation leads to a rapid and large change in membrane potential

Question 16

Summary of how a nerve impulse is triggered

Answer 16

• neurotransmitter binds to receptor protein (ligand gated channel as synapse )
• ligand gated channel opens , so Na+ ions diffuse into neuron
• na+ movement (sufficient ion movement) causes depolarisation of plasma membrane
• depolarisation reaches a critical threshold level
• voltage gated channel opens so na diffuse into neuron down electrochemical gradient
• This leads to rapid / large change in membrane potential causing action potential to be generated in this region of neuron
• this cause depolarisation of neighbouring voltage gated channels : ina. Domino effect

Question 17

Why is it crucial that a neuron goes back to its resting potential rapidly

Answer 17

• it is crucial that a neuron goes back to its resting potential rapidly. Restoration of the resting membrane potential allows the inactive voltage gated sodium channels to return to a confirmation that allows them to open up again in response to further signals , allowing the system to remain sensitive

Question 18

Explain how the resting potential is reset

Answer 18

• after the wave of depolarisation passes along the neuron , there has to be a process to re establish the edging potential for the next impulse..
• when the voltage reaches a critically high level , the voltage gated Na+ channels are inactivated (closed) and voltage gated K+ channels open.
• the K ions diffuse out the neuron , in opposite direction to the Na+ ions restoring the resting potential.
• once the resting potential is reached the K+ channel closes again and Na+ channels return to a conformation that allows them to respond again
• following re polarisation, ion concentration gradients are re established by the sodium potassium pump which actively transports excess ions in and out of the cell

Question 19

How resting potential is rest summary

Answer 19

• voltage gated channel opens so Na+ ions diffuse into the neuron
• voltage builds up so Na+ channel is inactivated and K+ channel opens
• k+ ions diffuse out of the neuron to reverse depolarisation / restore resting membrane potential
• resting potential is restored so K+ channels close again and N+ channels ready again
• sodium potassium pump rests the electrochemical gradient

Question 20

Overall summary of nerve transmission

Answer 20

• binding of a neurotransmitter triggers the opening of ligand gated ion channels at synapse
• ion movement occurs (na ions diffuse through) and there is depolarisation of membrane
• if sufficient ion movement occurs and membrane is depolarises beyond threshold value , the opening of voltage gated sodium channels is triggered and sodium ions enter the cell down their electrochemical gradient
• this leads to a rapid and large change in membrane potential ( and domino effect) Depolarisation of a patch of membrane causes neighbouring regions of membrane to depolarise and go through the same cycle, as adjacent voltage-gated sodium channels are opened
• a short time after opening the sodium channel becomes inactivated
• voltage gated potassium channel then opens to allow potassium ions to move out of the cell to restore the resting membrane potential
• ion concentration gradients are re established by the sodium potassium pump which actively transports excess ions in and out of the cell

Question 21

Explain travel of action potential

Answer 21

a) in response to a signal the soma end of the axon becomes depolarised
b) the depolarisation spreads down the axon, meanwhile the first part of the membrane repolarises. Because Na+ channels are inactivated and additional K+ channels have opened , the membrane cannot depolarise again
c) the action potential continues to travel down the axon

Question 22

The wave of depolarisation cannot spread directly from the membrane of one nerve to the next, what does it do instead

Answer 22

Instead, the wave of depolarisation triggers the release of hydrophilic neurotransmitters at the synapse which triggers the generation of a wave of depolarisation in the receiving cell.

Question 23

What happens when the action potential reaches the end of the neuron

Answer 23

• when the action potential reaches the end of a neuron it causes vesicles containing neurotransmitter to fuse with the membrane - this releases neurotransmitter into the synapse which stimulates a response in a connecting cell