Question

Given below are two statements, one is labelled as Assertion (A) and other one labelled as Reason (R).
Assertion (A) : Triad is present in skeletal muscle.
Reason (R) : T tubule and terminal cisternae of sarcoplasmic reticulum form triad.
In light of the above statements, choose the correct answer from the options given below.

• A. Both (A) and (R) are true and (R) is the correct explanation of (A).
• B. Both (A) and (R) are true and (R) is NOT the correct explanation of (A).
• C. (A) is true but (R) is false.
• D. (A) is false but (R) is true.

Hint:

Remember the structural differences in muscle types:
- Skeletal muscle has triads (1 T-tubule + 2 terminal cisternae).
- Cardiac muscle has diads (1 T-tubule + 1 terminal cisterna).

Answer 1

The Correct Option is A

Step 1: Understanding the Concept:
Muscle contraction relies on excitation-contraction coupling, which converts an electrical stimulus into a mechanical action.
In skeletal muscle, this process is facilitated by a specialized membrane structure called a triad.

Step 2: Detailed Explanation:
Let us analyze the structural elements of skeletal muscle fibers:
- Triad Structure: A triad is a specific three-part membrane structure found in skeletal muscle fibers.
It consists of one central transverse tubule (T-tubule) flanked by two terminal cisternae of the sarcoplasmic reticulum on either side.
- T-tubules: These are deep invaginations of the sarcolemma (plasma membrane) that allow electrical depolarization to penetrate deep into the muscle fiber.
- Terminal Cisternae: These are enlarged chambers of the sarcoplasmic reticulum that store calcium ions (\(\text{Ca}^{2+}\)).
- Functional Role: When an action potential travels down the T-tubule, it triggers the release of calcium from the adjacent terminal cisternae into the sarcoplasm, initiating myofibril contraction.
Therefore, the assertion is correct, the reason is correct, and the reason directly describes the structure and components of the triad in skeletal muscle.

Step 3: Final Answer:
Both (A) and (R) are true, and (R) is the correct explanation of (A). This corresponds to option (A).