Channel Kinetics and the Ionic Basis of the Spike · seed 1 · A4, ink-friendly. The answer key prints on its own page for grown-ups.

The spike in two currents

Science · The Human Body · ages 18-19
Name ______________________   Date ____________

  1. What drives the falling phase?

    • Sodium rushing back in
    • The membrane sealing shut
    • Potassium leaving through late channels

  2. What drives the rising phase of the spike?

    • Potassium leaving the cell
    • Sodium flooding into the cell
    • Pumps reversing direction

  3. Threshold is the point where sodium entry outruns potassium exit.

    Circle one:   True   False

  4. What does the refractory period come from?

    • Potassium running out
    • Sodium channel inactivation
    • The axon cooling down

  5. How does an inactivated sodium channel differ from a closed one?

    • It leaks more sodium
    • It stays open longer
    • It must reset before reopening

  6. What does holding the voltage fixed let an experimenter measure?

    • The exact number of channels
    • Each current alone, without voltage chasing it
    • The speed of the pumps

  7. A second nudge arrives during the refractory pause. What happens?

    • No new spike fires yet
    • A smaller graded spike fires
    • Sodium flows backward

  8. A drug blocks potassium channels only. Which part of the spike changes?

    • The rise grows taller
    • The fall slows or fails
    • Threshold disappears
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Answer key

For grown-ups. Fold this page away before handing over the rest.

The spike in two currents W1-mt_47-LDzgcKp-s1

  1. Potassium leaving through late channels · Late potassium channels open while sodium inactivates, so voltage falls.
  2. Sodium flooding into the cell · Fast sodium channels open and sodium entry drives voltage up.
  3. True · Past that point the rise feeds itself and the spike is all or none.
  4. Sodium channel inactivation · Inactivated sodium channels enforce a pause before the next spike.
  5. It must reset before reopening · Inactivated channels need a reset pause; closed ones can open at once.
  6. Each current alone, without voltage chasing it · Clamping voltage separates the fast inward and late outward currents.
  7. No new spike fires yet · Inactivated channels cannot reopen, so the nudge finds no response.
  8. The fall slows or fails · The fall belongs to potassium exit, so blocking it slows repolarization.
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