3.6.2.1 Nerve impulses

Question 1

Explain how a resting potential is established and maintained

Answer 1

• Na+\K+ pump actively transports 3Na+ out of neurone and 2K+ in
• Na+ cannot diffuse down its electrochemical gradient back into the neurone because the membrane is not permeable to Na+ (channels closed)
• Some K+ can diffuse out of the axon because the membrane is slightly more permeable to K+ (channels open)
• The membrane is polarised as there are more positive ions outside the membrane than inside (-70mV inside)
• Negatively charged proteins in the cytoplasm of the neurone contribute to the more negative charge inside the cell

Question 2

What is the resting potential?

Answer 2

• when neurone is not being stimulated
• outside of membrane is more positive than inside (more + ions outside)
• membrane is polarised
• resting potential value is -70mV

Question 3

What is differential membrane permeability?

Answer 3

membrane is slightly more permeable to potassium ions, but not permeable to sodium ions

Question 4

Explain how an action potential is generated

Answer 4

• A stimulus causes some Na+ channels to open. Membrane more permeable to Na+, Na+ diffuses in, inside of neurone becomes less negative
• Depolarisation - threshold reached, more Na+ channels open, Na+ rapidly diffuse into neurone (more positive)
• Repolarisation - at peak of action potential, Na+ channels close (impermeable), K+ channels open (more permeable so K+ diffuses out of neurone). Inside of neurone becomes more negative
• Hyperpolarisation - K+ channels slow to close so too many K+ diffuse out (more K+ out than Na+ stay in). Potential difference more negative than resting potential. Means the membrane can’t be excited again until threshold reached (refractory period)
• Resting potential re-established - Na+/K+ pump return membrane to -70mV

Question 5

What is the advantage of the refractory period?

Answer 5

Ion channels cannot open meaning:

• Action potentials are unidirectional
• Action potentials are discrete and don’t overlap - help with conduction of impulse

Question 6

What is the all or nothing principle?

Answer 6

• If threshold not reached, no depolarisation and action potential not generated
• Bigger stimulus will result in an increased FREQUENCY of action potentials

Question 7

State 3 factors which affect speed of transmission

Answer 7

• Myelination - saltatory conduction
• Increase in diameter of axon - less resistance to flow of ions in cytoplasm
• Higher body temperature - rate of ion diffusion os faster as ions have more kinetic energy