Question #148

Which of the following is true of the transverse tubular system in cardiac muscle?

transverse tubules are not seen in atrial muscle cells

transverse tubules do not play a role in action potential propagation along the length of the muscle fibre

the ryanodine receptor is present in the transverse tubular membrane

when modeling the myocyte as an electrical circuit, the transverse tubules may be considered to be equivalent to the surface membrane

it is important that the transverse tubular membrane comes into physical contract with ryanodine receptors in the sarcoplasmic reticular membrane

The transverse tubules are membrane invaginations found in both skeletal and cardiac muscle cells. They are notably absent, however, in atrial muscle. The transverse tubules permit propagation of the depolarizing action potential wave into the interior of the cell.

Ca2+ influx through L-type Ca2+ channels present in the tubular membranes are key in intiating Ca2+ induced Ca2+ release (CICR) via the ryanodine receptor ion channel, present on the sarcoplasmic reticular membrane. Evidence suggests that the spread of depolarization from transverse tubule to transverse tubules may be important in action potential propagation. These structures cannot therefore be considered to be equivalent to the surface membrane.

See A Thinking Approach to Physiology, pages 45 - 50.

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