USMLE Step 1 Physiology Review 53 09 Skeletal Muscle (1 of 3)

USMLE Step 1 Physiology Review 53 09 Skeletal Muscle (1 of 3)

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Begin 53 09 Skeletal Muscle (1 of 3) Transcription

Okay, now for some questions about skeletal muscle and action potentials in skeletal muscle.

What is the resting potential of muscle, typically?

  • Minus ninety millivolts (-90 mV).

What is the resting potential of neurons, typically?

  •  Minus seventy millivolts (-70mV).

Do the action potentials generated in muscle have a larger or smaller overall amplitude relative to neurons?

  • Larger.

How is the repolarization of skeletal muscle different than the repolarization of neurons?

  • Skeletal muscle repolarizes in two phases. 

After repolarizing to about minus sixty millivolts (-60 mV), there is a later phase.  Is the later phase slower or faster than the first phase of repolarization?

  • Slower.

Student Doctor, please pause the tape and summarize the information discussed about action potentials in skeletal muscle.

  • The resting potential of muscle is typically at minus ninety millivolts (-90 mV).  The resting potential of neurons typically is minus seventy millivolts (-70mV).  The action potentials generated in muscle have a larger overall amplitude relative to neurons.  Skeletal muscle repolarizes in two phases.  After repolarizing to about minus sixty millivolts (-60 mV), there is a later phase that is slower than the first phase of repolarization.

Okay, now for some questions about excitation contraction coupling in skeletal muscle.

Due to the structure of the muscle cell the spread of the action potential is more complicated in the muscle cell than it is in the nerve cell.  Plasma membrane of the muscle cell encapsulates a bundle of myofibrils.  The depolarizing effect of the action potential must reach each of the myofibrils and it must reach as much of the surface as possible on each myofibril.

There are special structures, invaginations, of the plasma membrane that allow the action potential to reach and to spread out over each of the myofibrils.  What are these structures called?

  • Transverse tubules, or T-tubules.

The t-tubules traverse the sarcoplasmic reticulum at regular intervals.  The action potential travels from the t-tubules into other structures.  The structures are part of the sarcoplasmic reticulum.  What are the structures called?

  • Lateral sacs.

Student Doctor, please pause the tape and summarize the information discussed this far on the excitation contraction coupling.

  • Plasma membrane of the muscle cell encapsulates a bundle of myofibrils.  The depolarizing effect of the action potential must reach each of the myofibrils and it must reach as much of the surface as possible on each myofibril.  Transverse tubules, or t-tubules, are special structures, invaginations, of the plasma membrane that allow the action potential to reach and to spread out over each of the myofibrils.  The t-tubules traverse the sarcoplasmic reticulum at regular intervals.  The action potential travels from the t-tubules into lateral sacs which are part of the sarcoplasmic reticulum.

Okay, now for some more questions about the excitation contraction coupling.

The spread of the depolarizing effect of the action potential into the lateral sacs triggers the release of something from the lateral sacs into the cytoplasm.  What is released?

  • Calcium.

What is triggered by the release of calcium from the lateral sacs into the cytoplasm?

  • Muscle contraction.

Muscle contraction takes place through the interaction of thin and thick filaments.  What are the thin filaments called?

  • Actin.

What are the thick filaments called?

  •  Myosin.

The myosin molecule can be divided into two portions according to molecular weight.  What is the term for the rod-like backbone of the myosin molecule?

  • Light meromyosin or LMM.

The heavier portion of the myosin molecule is called heavy meromyosin, or HMM, and it forms a head like structure on the end of the light meromyosin portion of the myosin molecule.  How does heavy meromyosin interact with actin?

  • The heavy meromyosin forms the cross bridge link for the actin molecule during contraction.

Student Doctor, please pause the tape and summarize the information discussed since the last summary on the excitation contraction coupling.  We started with the spread of the depolarization into the lateral sacs, triggering the release of calcium.

  • The spread of the depolarizing effect of the action potential into the lateral sacs triggers the release of calcium from the lateral sacs into the cytoplasm.  This release of calcium triggers muscle contraction.  Muscle contraction takes place through the interaction of thin filaments called actin and thick filaments called myosin.  The myosin molecule can be divided into two portions according to molecular weight.  The rod-like backbone of the myosin molecule is the light meromyosin, or LMM.  The heavy portion of the myosin molecule is called the heavy meromyosin, or HMM, and it forms a head like structure on the end of the light meromyosin portion of the myosin molecule.  The heavy meromyosin forms the cross bridge link with the actin molecule during contraction.

Regulatory proteins and calcium control the interaction between actin and myosin.

During the resting state of muscle, is calcium present or absent in the cytoplasm?

  • Absent.  At the end of contraction calcium is pumped back into the sarcoplasmic reticulum.

There is a rod like protein that takes part in regulation of muscle action.  What is the name of this regulatory protein?

  • Tropomyosin.

In the absence of calcium, where is the tropomyosin molecule situated?

  • It is situated in the f-actin groove of the actin molecule.

How does this positioning of tropomyosin inhibit muscle contraction?

  • It masks the myosin binding sites on the actin molecule.

Student Doctor, please pause the tape and summarize the information on excitation contraction coupling discussed since the last summary.  We started off with how regulatory proteins and calcium control the interaction between actin and myosin.

  • Regulatory proteins and calcium control the interaction between actin and myosin.  During the resting state of muscle, calcium is absent in the cytoplasm.  At the end of contraction calcium is pumped back into the sarcoplasmic reticulum.  The tropomyosin is a rod like protein that takes part in regulation of muscle action.  In the absence of calcium, the tropomyosin molecule sits in the f-actin groove of the actin molecule and masks the myosin binding sites on the actin molecule.

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