USMLE Step 1 Physiology Review 53 03 Electrophysiology (2 of 2)

USMLE Step 1 Physiology Review 53 03 Electrophysiology (2 of 2)

 

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Play USMLE Audio MP3 53 03 Electrophysiology (2 of 2) Below

Begin 53 03 Electrophysiology (2 of 2) Transcription

Okay, now for some questions about agents that change the responses of excitable membranes.

Agents that change the responses of excitable membranes are classified as either membrane excitants or membrane depressants.

A condition or agent that lowers threshold, destabilizes resting membrane potential, and may provoke spontaneous spike initiation describes a membrane depressant or a membrane excitant? Which one?

  • A membrane excitant.

What is the term for this condition or agent that acts like a membrane excitant by lowering threshold, destabilizing resting membrane potential and possibly provoking spontaneous spike initiation?

  • Hypocalcemia.

What is hypocalcemia?

  • Abnormally low levels of calcium in the circulating blood.

Which membrane excitant is an anti-hypertensive alkaloid that prolongs active sodium conductance time, thus promoting repetitive active potentials?

  • Veratrum.

Which membrane excitant depolarizes the membrane and moves resting potential toward threshold?

  • Hyperkalemia.

What is hyperkalemia?

  • Abnormally high concentration of potassium ions in the circulating blood.

Student Doctor, please the tape and summarize the information about membrane excitants.

  • List the excitant and describe its effect on the membrane.  Membrane excitants include hypocalcemia, veratrum and hyperkalemia.  Hypocalcemia destabilizes resting membrane potential, lowers the threshold for initiation of action potential, and may provoke spontaneous spike initiation  Veratrum is an anti-hypertensive alkaloid that prolongs active sodium conductance time, and promoting repetitive active potentials.  Hyperkalemia depolarizes the membrane and moves the resting potential toward threshold.

Okay, now for some questions about membrane depressants.

Would hypoxia be a membrane depressant or a membrane excitant?

  • A membrane depressant.

Would abnormally high levels of calcium circulating in the blood be a membrane depressant or a membrane excitant?

  • A membrane depressant.

What is the term for abnormally high levels of calcium circulating in the blood?

  • Hypercalcemia.

What is the membrane depressing effect of hypercalcemia?

  • It raises the threshold by increasing membrane stabilization.

Why does abnormally high level of calcium in the circulating blood stabilize membrane potential?

  • Calcium loosely binds to the excitable membrane and screens sodium from the sodium conduction channels.

If an agent or condition hyperpolarizes the resting potential, what effect would that have on the threshold?

  • It would raise the threshold.

Would raising the threshold have an exciting effect or a depressing effect on the membrane?

  • Depressing.

What effect would an abnormally low level of potassium in the circulating blood have on the resting potential?

  • It would hyperpolarize the resting potential, and therefore raise the threshold.

What is the term for an abnormally low level of potassium in the circulating blood?

  • Hypokalemia.

What effect to local anesthetics like procaine, have on the membrane?

  • They block activation of sodium conductance.

What effect is inversely proportional to axon diameter?

  • The effect of procaine is inversely proportional to axon diameter.

Would procaine affect small fibers first or large fibers first?

  • Small fibers first

Does hypoxia act as a membrane excitant or a membrane depressant?

  • Membrane depressant.

What effect does hypothermia have on excitable membranes?

  • It slows membrane processes and acts as a local anesthetic.

Student Doctor, please pause the tape and summarize the information discussed about membrane depressants.  List the agent and the effect it has on the membrane.

  • Membrane depressants include hypercalcemia, hypokalemia, procaine, hypoxia and hypothermia.  Hypercalcemia raises the threshold by stabilizing the membrane potential.  Calcium loosely binds to the membrane and screens sodium from the sodium conduction channels.  Hypokalemia hyperpolarizes the resting potential and raises the threshold for action potential initiation.  Procaine blocks activation of sodium conductance and its effect is inversely proportional to axon diameter.  Hypoxia acts as a membrane depressant.  Hypothermia slows membrane processes and acts as a local anesthetic.

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