The smallest contractile unit of a muscle is a: fiber myofibril sarcomere myofilament

Break Down the Problem

1. Identify the smallest contractile unit of muscle tissue from the given options: fiber, myofibril, sarcomere, myofilament.

Relevant Concepts

2. Understand the definitions of each term related to muscle anatomy:
   • Muscle Fiber: A single muscle cell, which can contain multiple myofibrils.
   • Myofibril: A long, thread-like structure composed of repeating segments called sarcomeres.
   • Sarcomere: The basic contractile unit within a myofibril, defined by the region between two Z-discs.
   • Myofilament: The individual protein filaments (actin and myosin) that make up a sarcomere.

Analysis and Detail

3. Among the options provided:

   • Muscle fibers are made of myofibrils.
   • Myofibrils are composed of repeating units called sarcomeres.
   • Sarcomeres contain myofilaments.

   Therefore, the smallest contractile unit specifically refers to the sarcomere, since it is the structure that actively contracts during muscle contraction.

Verify and Summarize

4. Since the sarcomere is the structure that directly engages in muscle contraction and is the smallest functional unit, the correct answer is confirmed.

Final Answer

The smallest contractile unit of a muscle is a: sarcomere.