Topic 6—B: Nervous Coordination-1. Neurones

Question 1

What are nervous impulses?

Answer 1

• They are electrical charges transmitted along a neurone

Question 2

How are nervous impulses created?

Answer 2

• By the movement of sodium and potassium ions

Question 3

The resting membrane potential

Answer 3

• In a neurone’s resting state (when it’s not being stimulated) the outside of the membrane is positively charged compared to the inside.

Question 4

How is the outside of the membrane polarised?

Answer 4

• There are more positive ions outside the cell than the inside

Question 5

What does it mean when the membrane is polarised?

Answer 5

• There is a difference in charge (called a potential difference or voltage) across it.

Question 6

Resting potential

Answer 6

• Voltage across the membrane when it’s at rest

Question 7

How is the resting potential created and maintained?

Answer 7

• By the sodium-potassium pumps and potassium ion channels in a neurones membrane

Question 8

Sodium potassium pumps

Answer 8

• they use active transport to move three sodium ions out of the neurone for every two potassium ions moved in
• ATP is needed

Question 9

Potassium ion channels

Answer 9

• They allow facilitated diffusion of potassium ions out of the neurone down their concentration

Question 10

Sodium ion channels

Answer 10

• Diffusion of sodium ions into the neurone

Question 11

Movement of sodium and potassium ions across a resting cell membrane

Answer 11

• The sodium potassium pumps move sodium ions out of the neurone but the membra isn’t permeable to sodium ions so they can’t diffuse back in - this creates a sodium ion electro chemical gradient. ( A concentration gradient of ions) because there are more positive sodium ions outside the cell than inside
• The sodium potassium pumps also move potassium ions into the membrane
• When the cells at rest, most potassium ion channels are open. This means that the membrane is permeable to potassium ions, so some diffuse back out through potassium ion channels

Question 12

How is the outside of the cell positively charged compared to the inside?

Answer 12

• More positive ions move out of the cell than enter

Question 13

What happens when a neurone is stimulated?

Answer 13

• Other ion channels in the cell membrane called sodium ion channels open

Question 14

What happens if the stimulus is big enough?

Answer 14

• It will trigger a rapid change in the potential difference which causes the cell membrane to become depolarised (action potential)

Question 15

Stimulus

Answer 15

• This excites the neurone cell membrane
• This causes sodium ion channels to open
• The membrane becomes more permeable to sodium, so sodium ions diffuse into the neurone down their sodium ion electrochemical gradient.
• This makes the inside of the neurone less negative

Question 16

Resting potential

Answer 16

• There is a positive charge on the outside of the neurone due to the presence of sodium and potassium ions
• There is a negative charge in the inside of the neurone due to the presence of anions
• Gated sodium ion channels are closed so little diffusion of sodium into the neurone
• Gated potassium ion channels are open so potassium can diffuse out of neurone
• Sodium potassium pump returns the membrane to it’s resting potential by pumping 3 sodium ions out for every 2 potassium ions in
• Resting potential is maintained until the membranes excited by another stimulus

Question 17

Depolarisation

Answer 17

• The outside of the neurone gets more negative due to the loss of sodium and potassium ions
• The inside of the neurone is more positive due to the presence of sodium and potassium ions
• If potential a difference reaches threshold more gated sodium ion channels open increasing permeability to sodium ions
• 2 potassium ions are pumped in for every 3 sodium ions pumped out.
  -They diffuse down its concentration gradient into the neurone making potential difference more positive
• If there is enough sodium ions diffusing across membrane then threshold potential is reached and neurone membrane is depolarised.

Question 18

Generator potential

Answer 18

• The same as depolarisation but less sodium ion channels are open compared to action potential

Question 19

Repolarisation

Answer 19

• Outside the neurone is more positive due to the presence of sodium and potassium ions
• Inside of the neurone is negative
• Gated sodium ion channels close so less diffusion of sodium ions into the neurone
• Potassium ion channel open allow potassium ions to diffuse out of neurone
• Sodium-potassium pump removes sodium from neurone cytoplasm
• The membrane is more permeable to potassium so potassium ions diffuse out of neurone down the potassium ion concentration gradient
• This starts to get the membrane back to it’s resting potential

Question 20

Hyperpolarisation

Answer 20

• Potassium ion channels are slow to close so there’s a slight ‘overshoot’ where too many potassium ions diffuse out of the neurone
• The potential difference becomes more negative than the resting potential

Question 21

What happens to the membrane when sodium ion channels are open?

Answer 21

• The permeability of membrane to sodium ions increase

Question 22

What happens to the membrane when sodium ion channels are closed?

Answer 22

• The permeability of membrane to sodium ions decrease

Question 23

Refractory period

Answer 23

• The time taken before a new depolarisation (action potential) can occur (during Repolarisation and Hyperpolarisation)
• It acts as a time delay between one action potential and the next
• This makes sure that action potentials don’t overlap but pass along as discrete (separate) impulses.

Question 24

Waves of depolarisation

Answer 24

• When an action potential happens some of the sodium ions that enter the neurone diffuse sideways.
• This causes sodium ion channels in the next region of the neurone to open and sodium ions diffuse into that part.
• This causes waves of depolarisation to travel along the neurone.
• The wave moves way from the parts of the membrane in the refractory period because these parts can’t fire an action potential

Question 25

All or nothing principle

Answer 25

• Once the threshold is reached, an action potential will always fire with the same change in voltage, no matter how big the stimulus is
• If the threshold isn’t reached an action potential wont fire
• A bigger stimulus wont cause a bigger action potential, but it will cause them to fire more frequently

Question 26

What three factors affect the speed of conduction of action potentials?

Answer 26

1. Myelination
2. Axon diameter
3. Temperature