USMLE Step 1 Physiology Review 56 08 Cardiac muscle vs. skeletal muscle

USMLE Step 1 Physiology Review 56 08 Cardiac muscle vs. skeletal muscle

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Begin 56 08 Cardiac muscle vs. skeletal muscle Transcription

Okay now for some question on cardiac muscle and skeletal muscle contrasted. What aspect of the membrane potential trace is present in cardiac muscle and absent in skeletal muscle?

  • The phase two plateau

What structure which connects cells electrically is present in myocyte but absent in skeletal muscle cells?

  • Gap junctions.

Finally, what do cells in the cardiac nodes do that muscle cells cannot?

  • Spontaneously depolarize

Student doctor please pause the tape and summarize the information on Cardiac and skeletal muscle contrasted.

  • The phase two plateau of the membrane potential trace is present in cardiac muscle and absent in skeletal muscle. Gap junctions which connects cells electrically are present in myocytes but absent in skeletal muscle cells. Cells in the cardiac nodes spontaneously depolarize but muscle cells cannot.

Now for some questions about alteration of heart beat. The heart beat is normally automatically regulated by events in what structure?

  • SA node

However, heart rate can be regulated for several physiologic reasons. The next several questions related to ways in which different nerve pathways and transmitters can alter heart rate. What does the right vagus nerve do?

  • It reduces heart rate.

How does the right vagus nerve accomplish this?

  • It reduces heart rate via reduced frequency by slowing down the SA node.

What does the left Vagus nerve do?

  • It also reduces heart rate

The left Vagus nerve slows heart rate in two ways, what are they?

  • It reduces heart rate by reducing conduction of the AV node as well as reducing the force of myocytic contraction in the atria.

What three ways does the sympathetic nerve system affect the heart beat?

  • First, it increases frequency of heart beat;  second, increases conduction of action potential; and third increases contractile force

What three ways does epinephrine affect heart beat?

  • First, increases frequency of the heart beat; second, increases conduction of action potential; and third increases contractive force

What three ways does norepinephrine affect the heart beat?

  • First, it decreases frequency of heart beat via the baroreceptor reflex; second, it increases conduction of action potentials; third, it increases contractive force.

Student doctor please pause the tape and summarize the information on the alteration of heart beat.

  •  The heart beat is normally automatically regulated by events in SA node. However, heart rate can be regulated for several physiologic reasons. The right vagus nerve reduces heart rate via reduced frequency by slowing down the SA node. The left Vagus nerve also reduces heart rate by reducing conduction of the AV node as well as reducing the force of myocytic contraction in the atria. The sympathetic nerve system and epinephrine affect the heart beat in the same way by increasing frequency of heart beat, increasing the conduction of action potential; and increasing contractive force. Norepinephrine decreases frequency of heart beat via the baroreceptor reflex but increases conduction of action potentials and increases contractive force.

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