USMLE Step 1 Physiology Review 53 11 Skeletal Muscle (3 of 3)

USMLE Step 1 Physiology Review 53 11 Skeletal Muscle (3 of 3)

 

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Play USMLE Audio MP3 53 11 Skeletal Muscle (3 of 3) Below

Begin 53 11 Skeletal Muscle (3 of 3) Transcription

Okay, now for some questions about mechanics.

A twitch is a response of a muscle fiber to what?

  • A single action potential.

What constitutes a motor unit?

  • A motor unit is a single motor neuron and all the muscle fibers that the motor neuron innervates.

The strength of contraction depends on the number of motor units to a muscle that are activated.  What happens if a second stimulus arrives before the muscle is relaxed from the initial stimulus?

  • The contraction from the second stimulus adds to the first contraction.

Stimuli applied rapidly produces a summation of twitches which is referred to as what?

  •  A tetanus.

How many times greater is the tension produced by tetanus than the tension of a single twitch?

  • Three or four times greater.

This greater tension is related to ion concentration.  In terms of ion concentration, why tetanic tension greater than twitch tension?

  • The greater tetanic tension is due to a higher concentration of intracellular calcium.

Student Doctor, please pause the tape and summarize the information on mechanics from twitches to tetanus.

  • A twitch is a response of a muscle fiber to a single action potential.  A motor unit is a single motor neuron and all the muscle fibers that the motor neuron innervates.  The strength of contraction depends on the number of motor units to a muscle that are activated.  If a second stimulus arrives before the muscle is relaxed from the initial stimulus, the contraction from the second stimulus adds to the first contraction.  Stimuli applied rapidly produces a summation of twitches which is referred to as a tetanus.  The tension produced by tetanus is three to four times greater than the tension of a single twitch.  The greater tetanic tension is due to a higher concentration of intracellular calcium.

Now for some more questions.

Stretching a muscle with various weights develops what kind of tension?

  • Passive tension.

What is the relationship between the passive tension of a muscle and the actin-myosin complex?

  • There is none.

Passive tension and the elasticity of a muscle are due to three structures of the muscle fiber.  List them from exterior to interior.

  • First, the sarcolemma.  Second, the connective tissue between myofibrils.  Third, the sarcoplasmic reticulum.

Student Doctor, please pause the tape and summarize the information discussed this far on the mechanics of muscle dealing with passive tension.

  • Stretching a muscle with various weights develops passive tension.  There is no relationship between the passive tension of a muscle and the actin-myosin complex.  Passive tension and the elasticity of a muscle are due to the sarcolemma, the connective tissue between myofibrils and the sarcoplasmic reticulum.

Okay, now back to some more questions about muscle mechanics.

The force developed by the muscle is referred to as what kind of tension?

  • Active tension.

What does active tension vary with?

  • It varies with the initial length of the muscle.

How does one derive the total tension developed by a muscle?

  • It is the sum of the passive tension and the active tension.

What is active tension due to?

  • It is due to the interaction of the cross bridges linking the myosin molecule to the actin molecule.

During isometric contraction, what happens to the external muscle length while the developed tension in the muscle increases?

  • The external muscle length remains constant.

During an isotonic contraction, what happens to the variables of tension and muscle length?

  • The tension is constant while the muscle shortens.

When a person strains against the weight of the barbell, and then lifts the barbell, both isometric and isotonic contraction occur.

When she strains against the barbell, but she is not yet lifting the barbell, what kind of contraction takes place?

  • Isometric. 

Explain why.

  • She increases tension in her muscle, trying to lift the barbell, but her external muscle length remains unchanged because she is not yet lifting the barbell.

Student Doctor, please pause the tape and summarize the information on mechanics dealing with active tension and isometric contractions.

  • The force developed by the muscle is referred to as active tension.  Active tension varies with the initial length of the muscle.  The total tension developed by a muscle is the sum of the passive tension and the active tension.  Active tension is due to the interaction of the cross bridges linking the myosin molecule to the actin molecule.  During isometric contraction, the external muscle length is held constant while the developed tension in the muscle increases.  During an isotonic contraction, the tension is constant while the muscle shortens.  When a person strains against the weight of a barbell, and then lifts the barbell, both isometric and isotonic contraction occur.  When she strains against the barbell, but she is not yet lifting the barbell, isometric contraction takes place.  She increases tension in her muscle, trying to lift the barbell, but her external muscle length remains unchanged because she is not yet lifting the barbell.

Okay, now back to some more questions.

The instant she starts lifting the barbell, what kind of contraction occurs?

  • Isotonic.

Explain why.

  • As soon as the tension in her muscle produces the force needed to lift the barbell, her muscle shortens while the tension in her muscle remains constant.  This scenario is a description of afterload contractions.  Almost all movements by humans are afterloaded contractions.

With regard to the configuration of actin and myosin, when is the development of active tension the greatest?

  • The development of active tension is greatest when the configuration of the actin and myosin molecules is such that there is maximum interaction of myosin cross bridges with the actin.

Is the development of active tension at its greatest when the muscle is maximally contracted?

  • No.

What is it about the configuration of the actin myosin molecules when a muscle is maximally contracted that precludes the maximum development of active tension?

  • The decline in tension is due to overlap of thin filaments from adjacent sarcomeres causing interference in cross bridging.

In terms of percent of L sub zero (L0), when is there a maximum development of active tension?

  • When L sub zero (L0) is at one hundred percent.  So one hundred percent L sub zero (L0).

Student Doctor, please pause the tape and summarize the information on mechanics discussed since the last summary. The first question in this section that I want you to summarize was about isotonic contractions.

  • The instant she starts lifting the barbell, isotonic contraction occurs.  As soon as the tension in her muscle produces the force needed to lift the barbell, her muscle shortens while the tension in her muscle remains constant.  This scenario, where the woman strains to lift and then lifts the barbell, is a description of afterload contractions.  Almost all movements by humans are afterloaded contractions.  The development of active tension is the greatest when the configuration of the actin and myosin molecules is such that there is maximum interaction of myosin cross bridges with the actin.  When the muscle is maximally contracted, active tension cannot develop to its maximum.  The decline in tension is due to overlap of thin filaments from adjacent sarcomeres causing interference in cross bridging.  There is maximum development of active tension at one hundred percent L sub zero (L0).

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