General anatomy and histology: skeletal muscle ultrastructure: practice question

A Anatomy question in the style of the NEET PG paper. Answer it first, then read why the right option is right and why each of the others is wrong.

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Anatomy · General anatomy and histology: skeletal muscle ultrastructure

Sample item

An electron micrograph of mammalian skeletal muscle shows, at the junction of each A band and I band, a central transverse tubule closely flanked on either side by a dilated sac of sarcoplasmic reticulum. Which description names this arrangement correctly?

Original practice item, not a recalled exam question. Nothing here is recorded.

The answer and the reasoning

B. Triad: a T tubule between two terminal cisternae

Mammalian skeletal muscle has two triads per sarcomere, each at an A-I junction, formed by one T tubule flanked by two terminal cisternae of sarcoplasmic reticulum. Depolarisation of the T tubule is sensed by the dihydropyridine receptor, which opens the ryanodine receptor and releases calcium. Cardiac muscle instead has diads at the Z line.

Option by option

  • A. Triad: two T tubules around one terminal cistern

    The arrangement is reversed; the tubule is the central element, not the cistern.

  • B. Triad: a T tubule between two terminal cisternae (correct)

    Correct. One T tubule plus two terminal cisternae is the triad at the A-I junction.

  • C. Diad: a T tubule beside a single terminal cistern at the Z line

    This describes the cardiac diad at the Z line, not skeletal muscle.

  • D. Dyad of the sarcolemma: two caveolae flanking one mitochondrion

    Caveolae and mitochondria do not form the excitation-contraction coupling unit.

Source note: Original NextPrep sample item — pending expert review. This item is original practice material awaiting independent expert review, so confirm important facts in a standard textbook.

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Questions about this Anatomy question

What is the answer to this Anatomy question?

The best answer is B, Triad: a T tubule between two terminal cisternae. Mammalian skeletal muscle has two triads per sarcomere, each at an A-I junction, formed by one T tubule flanked by two terminal cisternae of sarcoplasmic reticulum.

Why is "Triad: two T tubules around one terminal cistern" not the answer?

The arrangement is reversed; the tubule is the central element, not the cistern.

Why is "Diad: a T tubule beside a single terminal cistern at the Z line" not the answer?

This describes the cardiac diad at the Z line, not skeletal muscle.

Why is "Dyad of the sarcolemma: two caveolae flanking one mitochondrion" not the answer?

Caveolae and mitochondria do not form the excitation-contraction coupling unit.

What concept does this question test?

It tests General anatomy and histology: skeletal muscle ultrastructure in Anatomy. Depolarisation of the T tubule is sensed by the dihydropyridine receptor, which opens the ryanodine receptor and releases calcium. Cardiac muscle instead has diads at the Z line.

How hard is this question?

We tag it as an application question: it asks you to use a fact on a patient or a finding. The tag is our own editorial judgement, not an official difficulty rating, so read the reason for every option, not only the answer.

How should I study General anatomy and histology: skeletal muscle ultrastructure?

Learn every nerve by its route, its muscles and what a lesion looks like at the bedside. That one habit answers a large share of questions. Then use the flashcards and the other Anatomy questions on this site to test what stuck.

Is this a real NEET PG question?

No. It is an original practice item (editorial status: sample, version 1). NextPrep does not publish recalled or leaked questions, and independent faculty review of this item has not been completed yet, so confirm important facts in a standard textbook.