USMLE Step 1 Physiology Review 54 03 Smooth Muscle (1_3)

USMLE Step 1 Physiology Review 54 03 Smooth Muscle (1_3)

 

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Let’s start our USMLE Review with Anatomy from the Gold Standard USMLE Step 1 Audio Review program.

Play USMLE Audio MP3 54 03 Smooth Muscle (1_3) Below

Begin 54 03 Smooth Muscle (1_3) Transcription

Okay, now we are going to talk about the electrophysiology of smooth muscle.

  There are many variations of smooth muscle. For example, action potentials elicit responses in some kinds of smooth muscle, while other kinds of smooth muscle respond to a different kind of change in membrane potential. What kind of change would that be?

  • A graded change.

Do all kinds of smooth muscle require at least some kind of change in membrane potential as a minimum requirement for contraction?

  • No, some kinds of smooth muscle contract in the absence of change of membrane potential.

 Can the major source of calcium for contraction and smooth muscle be exclusively limited to either extracellular or intercellular locations?

  • No, both locations are pertinent depending on the kind of smooth muscle.

 Can smooth muscle contraction be exclusively characterized as either strong or weak?

  • No, contractions of some smooth muscle are weak, while other smooth muscles contract with a force equal to or greater than skeletal muscle.

 Student Doctor, please pause the tape and summarize the information discussed thus far on smooth muscle.

  • There are many variations of smooth muscle. For example, action potentials elicit responses in some kinds of smooth muscle, while other kinds of smooth muscle respond to graded changes in membrane potential. Some kinds of smooth muscle contract in the absence of change of membrane potential. The major source of calcium for contraction and smooth muscle can be either extracellular or intercellular. Contractions of some smooth muscle are weak, while other smooth muscles contract with a force equal to or greater than skeletal muscle.

 What are the two broad classifications for smooth muscle?

  • Multi-unit and single unit.

 Which kind of smooth muscle has more communication between individual muscle cells?

  • Single unit smooth muscle.

  There is little intercellular communication in multi-unit tissue.  Individual muscle cells operate more or less independently from one another.

 What is the structure through which individual muscle cells in single unit tissue communicate?

  • Gap junctions.  This allows action potentials to travel from cell to cell so contraction can take place more or less simultaneously in all of the muscles cells in a single unit tissue.

 Are the muscles of the iris single unit or multi-unit smooth muscle tissue?

  • Multi-unit.

 Are the muscles in the uterus, gastrointestinal tract and the smaller blood vessels multi-unit or single unit tissue?

  • Single unit.

 Student Doctor, please pause the tape and summarize the information on the information discussed thus far on the classification of smooth muscle.

  • The two broad classifications for smooth muscle are multi-unit and single unit. Single unit smooth muscle has more communication between individual muscle cells than multi-unit smooth muscle does. There is little intercellular communication in multi-unit tissue and individual muscle cells operate more or less independently from one another. Muscle cells in single unit tissue however, communicate via gap junctions; this allows action potentials to travel from cell to cell so contraction can take place more or less simultaneously in all of the muscles cells in single unit tissue. The muscle cells of the iris are multi-unit smooth tissue. The muscles in the uterus, gastrointestinal tract and the smaller blood vessels are single unit tissue.

 

 Okay, now let’s talk about the excitation contraction coupling in smooth muscle.

 Smooth muscle cells in some tissues exhibit rhythmic oscillations in membrane potential. What is the term for these oscillations?

  • Slow wave depolarization.

 What are the two mechanisms for excitation contraction coupling in smooth muscle?

  • Electro-mechanical and pharmaco-mechanical coupling.

 What two things happen when the cell is excited in the electro mechanical coupling?

  • There is a partial depolarization of the membrane and an increase in cytosolic calcium.

 What happens to the membrane potential in pharmaco-mechanical coupling?

  • There is no significant increase in membrane potential.

 Student Doctor, please pause the tape and summarize the information discussed thus far on slow wave depolarizations and the two wave mechanism for excitation contraction coupling and smooth muscle. 

 Smooth muscle cells in some tissues exhibit rhythmic oscillations in membrane potential which are called slow wave depolarizations. The two mechanisms for excitation contraction coupling in smooth muscle are electro-mechanical and pharmaco-mechanical coupling. When the cell is excited in the electro-mechanical coupling there is a partial depolarization of the membrane and an increase in cytosolic calcium. In pharmaco-mechanical coupling there is no significant change in membrane potential.

 

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54-03 Smooth Muscle

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