Component 3.5 - The Nervous Impulse

Question 1

What is the resting potential?

Answer 1

The potential difference across the membrane of a cell when no nervous impulse is being conducted

Question 2

How is a neurone different to other cells?

Answer 2

It can change the potential difference across its cell membrane

Question 3

Describe what is happening in the cell membrane when establishing the resting potential?

Answer 3

• The inside of the cell has a higher conc of K+ and a lower conc of Na+
• Channels allowing K+ to leak out are mostly open whereas those allowing Na+ in are closed
• Sodium-Potassium pumps are a trans-membrane protein with ATPase activity that maintain conc and uneven distribution across membrane
• 3 Na+ pumped out and 2 K+ pumped in for every ATP molecule hydrolysed

Question 4

What initiates an action potential?

Answer 4

Energy of a stimulus

Question 5

What is an action potential?

Answer 5

The rapid rise and fall of the electrical potential across a nerve cell membrane as a nervous impulse passes

Question 6

Describe what happens during depolarisation

Answer 6

• Stimulus causes voltage-gated sodium channels in axon membrane to open
• Na+ rapidly diffuse in by facilitated diffusion into axon down conc gradient
• This depolarises the axon membrane to about +40mV (this is the action potential)

Question 7

Describe what happens during repolarisation

Answer 7

• Occurs as Na+ channels close
• Potassium channels open and K+ diffuses out down conc gradient
• Reduces potential difference across the axon membrane and it is repolarised

Question 8

Describe what happens during hyperpolarisation

Answer 8

• More K+ ions diffuse out than Na+ ions diffuse in
• Potential difference across the membrane becomes even more negative than the resting potential
• Membrane becomes hyperpolarised

Question 9

Why is the resting potential negative ?

Answer 9

Due to the negative ions of large proteins, organic phosphates remaining in the cytoplasm

Question 10

What is the absolute refractory period and what does it ensure?

Answer 10

• When the sodium channels are inactivated and concs of K+ and Na+ ions are restored to that of resting potential
• Ensures that no new action potential may be initiated by axon and this ensures that the nervous impulses travels in one direction

Question 11

What is the all or nothing law?

Answer 11

• As long as the stimulus exceeds the value of the the threshold potential an action potential is generated
• action potential is always the same size

Question 12

What does a stronger stimulus result in?

Answer 12

A greater frequency of action potentials as the intensity of stimulation increases but the size of the impulses is always the same

Question 13

What does the “all or nothing law” allow?

Answer 13

It allows the action potential to act as a filter so that minor stimuli do not set up nervous impulses, so the brain is not overloaded with information

Question 14

How does temperature effect the speed of a nervous impulse?

Answer 14

• Higher temperature = higher speed

- More kinetic energy of ions

Question 15

How does the diameter of the axon effect the speed of conduction?

Answer 15

• Greater the diameter, faster the speed

- More sodium ions can flow through axon (greater volume in relation to area)

Question 16

How does myelination affect the speed of conduction?

Answer 16

The voltage- gated ion channels only occur at the nodes of Ranvier and the myelin sheath between them acts as a good insulator - action potential can jump large distances - 1mm. This allows Saltatory conduction

• unmyelinated is 1 m/s and myelinated is 100 m/s

Question 17

Why do myelinated axons use less ATP than non-myelinated axons of equal diameter?

Answer 17

Because active transport of sodium ions occurs only at the nodes, rather than along the whole length of the axon

Question 18

How does an action potential move down an axon?

Answer 18

1) Na+ ions move into the axon
2) The reversal of potential (action potential)sets up local currents as Na+ ions move down the axon (positive attracted to negative)
3) These depolarise the adjacent section of the membrane