USMLE Step 1 Physiology Review 53 02 Electrophysiology (1 of 2)

USMLE Step 1 Physiology Review 53 02 Electrophysiology (1 of 2)

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Play USMLE Audio MP3 53 02 Electrophysiology (1 of 2) Below

Begin 53 02 Electrophysiology (1 of 2) Transcription

Okay, now for some questions about propagation.

The active locus moves down the length of the nerve axon as an action potential propagates along the length of the nerve axon.  What is the active locus?

  • It is the region of reversed membrane potential; i.e. positive inside.

What is created by the influx of sodium at the active locus?

  • A local circuit current.

What effect does the local circuit current have on the membrane?

  • It depolarizes the resting membrane ahead of the active locus. 

The local circuit current brings the membrane adjacent to the active locus to threshold, and the active locus moves smoothly down the unmyelinated axon.  In myelinated axons, what happens to the length of the local circuits?

  • They are greatly increased.

Student Doctor, please pause the tape and summarize the information discussed this far on propagation.

  • The active locus moves down the length of the nerve axon as an action potential propagates down the length of the nerve axon.
  • The active locus is the region of reversed membrane potential; i.e. positive inside.  A local circuit current is created by the influx of sodium at the active locus.  The local circuit current depolarizes the resting membrane ahead of the active locus.  The local circuit current brings the membrane adjacent to the active locus to threshold, and the active locus moves smoothly down the unmyelinated axon.

Where is the only place that the local circuits can cross the membrane to create active loci?

  • Local circuits can cross the membrane only at Nodes of Ranvier.

What are Nodes of Ranvier?

  • They are points of juncture where the end of one myelin sheath meets the beginning of the next myelin sheath. 

The active locus in a myelinated axon does not progress smoothly down the axon.  How does it move down the axon?

  • It jumps from node to node.

Therefore, propagation of the action potential is discontinuous, jumping from node to node.  What is this process called?

  • Saltatory Conduction.

What effect does saltatory conduction have on the velocity of action potentials?

  • It significantly increases the velocity.

What effect would saltatory conduction have on the energy required for the sodium potassium pump?

  • It greatly decreases the energy required.

Student Doctor, please pause the tape and summarize the information discussed this far on myelinated axons and saltatory conduction.

  • In myelinated axons the length of the local circuits greatly increases.  The local circuits can cross the membrane to create active loci only at Nodes of Ranvier.  Nodes of Ranvier are points of juncture where the end of one myelin sheath meets the beginning of the next myelin sheath.  The active locus in a myelinated axon does not progress smoothly down the axon, but jumps from node to node.  Therefore, propagation of the action potential is discontinuous, jumping from node to node.  This process is called saltatory conduction.  Saltatory conduction significantly increases the velocity of conduction of action potentials.  Saltatory conduction greatly decreases the energy required for the sodium potassium pump.

Okay, now for some more questions.

Axon excitability and conduction velocity are directly proportional to what measurement of the axon?

  • The square of the axon radius.

Since axon size and excitability are directly proportional, as axon diameter increases, what happens to the threshold of action potential initiation?

  • It decreases.

As axon diameter increases, what happens to conduction velocity?

  • It increases.

What is the term for the extracellular summation of thousands of action potentials recorded from nerve trunks?

  • The compound action potential.

What does the Doctor stimulate in order to measure conduction velocity in patients?

  • A large subcutaneous nerve.

What does the Doctor record after stimulating a large subcutaneous nerve?

  • The latency of the compound action potential in one or more locations of the same nerve.

Is the measurement taken at a point proximal or distal to the point of stimulation?

  • Either proximal or distal.

If the Doctor wants to measure conduction velocity, why does he record latency of the compound action potential?

  • The latency of the compound action potential is an indirect measure of conduction velocity.

Student Doctor, please pause the tape and summarize the information discussed since the last summary on propagation.

  • Axon excitability and conduction velocity are directly proportional to the square of the axon radius.  As axon diameter increases, the threshold of action potential initiation decreases.  As axon diameter increases, conduction velocity increases.  The compound action potential is the extracellular summation of thousands of action potentials recorded from nerve trunks.  In order to measure conduction velocity in patients, the Doctor stimulates a large subcutaneous nerve and records the latency of the compound action potential on one or more locations of the same nerve.  The measurement can be taken at a point either proximal or distal to the point of stimulation.  The latency of the compound action potential is an indirect measure of conduction velocity.

Okay, now for a few more questions on propagation.

The latency of a muscle action potential after stimulating a motor nerve is inaccurate.  Why?

  • Because the latency includes neuromuscular delay.

How can the Doctor measure the conduction velocity more accurately so that the neuromuscular delay cancels out?  Please pause the tape.

  • Stimulate a nerve near the muscle and then stimulate it again farther away from the muscle and the conduction velocity is calculated as the difference in distance divided by the difference in latency.

Student Doctor, please pause the tape and summarize the information discussed since the last summary on propagation.  We started with the latency of the neuromuscular action potential.

  • The latency of muscle action potential after stimulating a motor nerve is inaccurate because the latency includes neuromuscular delay.  In order to cancel out the neuromuscular delay and obtain a more accurate measure of conduction velocity, stimulate a nerve near the muscle and then farther away from the muscle and the conduction velocity is calculated as the difference in distance divided by the difference in latency.

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