Nervous Transmission 13.4

Question 1

Potential Difference

Answer 1

The difference in charge between the inside and outside of the axon

Question 2

Resting Potential

Answer 2

This is when the receptors are not detecting any stimuli. The membrane is polarised because there is a potential difference across it, there are more positively charged ions on the outside of the membrane than inside.

Question 3

Resting Potential Difference

Answer 3

-70mv

Question 4

Why does the resting potential occur?

Answer 4

This is because of the movement of sodium and potassium ions across the membrane of the axon. They cannot diffuse normally due to the phospholipid bilayer rejecting them, so they move in through facilitated diffusion which means they can only enter if the channel proteins are open. Some of the channels are gated and some open all the time.

Question 5

Creating a resting potential

Answer 5

• Sodium ions are actively transported out of the axon and potassium ions are actively transported into the cytoplasm of the axon by an intrinsic protein called a sodium potassium pump. Their movement is not equal, for every 3 Na+ pumped out, 2 K+ are pumped in.
• There are more sodium ions on the outside than inside so they will diffuse back into the axon down an electrochemical gradient whereas potassium ions will want to diffuse out.
• The sodium ion channels are gated so will stay closed and sodium will not be able to diffuse into the axon, but potassium ion channels are open and so K+ ions will diffuse out
• This creates a much bigger potential difference of more positively charged ions outside the cell compared to inside. This will be -70mv and is the induced resting potential.

Question 6

Creating an action potential

Answer 6

When a stimulus is detected by a sensory receptor, the energy of the stimulus will reverse the charged on the axon membrane. The potential difference will rapidly change to +40mv which is called depolarisation. This is a change in the potential difference from negative to positive. As the impulse passes repolarisation will occur where the potential difference will be changed back to negative where it will return to its resting potential.

Question 7

What do voltage gated ion channels do?

Answer 7

It creates an action potential by changing the shape of the channels when there is a change in voltage across the membrane. The channel can open or close.

Question 8

Describe the graph of action potential

Answer 8

1. (-65mv) This is at resting potential. The potassium ion channels are open and the sodium voltage gated ion channels are closed.
   2-4 . (-65mv -> 40mv) This is depolarisation. The energy of the stimulus will cause some voltage gated sodium ion channels to open, Na+ ions will diffuse down their electrochemical gradient, into the axon and so the charge inside the cytoplasm will become more positive, the potential difference will decrease. As the sodium ions enter the axon, more sodium ion channels will open so more sodium ions will be able to enter. This will continue until it reaches +40mv and then the sodium ion channels will close. Potassium ion voltage gated channels will open.
2. Potassium ions will diffuse out of the axon which will result in the inside becoming more negative than the outside even than its resting state. This is called hyperpolarisation.
3. The voltage gated ion channels are now closed and the sodium pump will actively pump sodium ions out of the axon and potassium ions in. The axon becomes repolarised because it returns to its resting potential.

Question 9

How does an action potential travel along a non-myelinated neurone?

Answer 9

Once a stimulus has been detected and an action potential has been triggered. The first region of the axon will be depolarised so sodium ions will diffuse in. This will act as a stimulus for depolarisation of the next region which will form a wave of depolarisation across the membrane of the axon. Once the sodium ions are inside the axon, they are attracted by negative charge further along the axon so will diffuse further along inside the axon.

Question 10

Propagation steps

Answer 10

1. In resting potential, there are more sodium ions outside than inside and more potassium ions inside than outside. This means there amore more positive charges on the outside relative to the inside. The membrane is polarised
2. A stimuli will cause a sudden influx of sodium ions which will reverse the charges on the membrane and it becomes depolarised so now the Na+ channels will stay open until it reaches 40mv
3. The local depolarisation will cause sodium ion channels in the next region along to open as well so that region of the axon will also depolarise. This is the wave of depolarisation. Behind this new region, the sodium voltage gated channels will close and potassium ones will open so K+ ions will leave.
4. The outward movement of the K+ will continue until the potential difference is the same as in resting state. This means it has been repolarised. It can then make a new action potential if a stimulus appears.

Question 11

Refractory Period

Answer 11

The period of time where the axon cannot be excited again after an action potential has passed. This is when the voltage gated sodium ion channels will remain closed so depolarisation cannot occur.

Question 12

Why is the refractory period important?

Answer 12

This is important because it prevents the propagation of an action potential backwards along the axon. It also ensures the action potentials don’t overlap and occur as discrete impulses.

Question 13

Why is myelinated axon transfer faster than non-myelinated?

Answer 13

It is faster as the depolarisation can only occur at the nodes of Ranvier where there are no myelin sheaths present.

Question 14

Saltatory conduction

Answer 14

This is when the depolarisation of the membrane only occurs at the node of Ranvier and the action potential will jump from node to node. This takes up a lot less time as there are less channels to open and it also is energy efficient as it needs less ATP to get the channels open.

Question 15

What else affects the speed of transmission?

Answer 15

Axon diameter, temperature

Question 16

Axon diameter

Answer 16

This affects the speed, because the bigger the axon and the larger the diameter, the faster the impulse as the more ions can flow through the cytoplasm at the same time and there is less resistance compared to the smaller cytoplasms.

Question 17

Temperature

Answer 17

The higher the temperature, the faster the transmission because the ions will have more kinetic energy. This will be the case up to 40 degrees and then the proteins will start to denature ( the proton pump and channel proteins) so it will no longer work.

Question 18

All or Nothing Principle

Answer 18

Nerve impulses will only trigger a response if a minimum level of stimulus called the threshold value has been reached. It doesn’t matter how large the stimulus is, it will create the same sized action potential, but if the stimulus doesn’t reach the threshold value, there will be no action potential produced. The stronger the stimuli, the more frequent the action potentials will be, but they will always be the same size.