Skeletal Muscle Fibers

Exercise 11: Skeletal Muscle Fibers and the Neuromuscular Junction

Objectives

  • Describe the anatomy of the skeletal muscle fiber

  • Explain the components of a sarcomere

  • Describe the events that occur at the neuromuscular junction

Muscle Fibers

  • An entire muscle, such as the biceps brachii, is classified as an organ.

  • It is composed of three types of tissue:

    • Skeletal muscle tissue

    • Nervous tissue

    • Connective tissues

Characteristics of Skeletal Muscle Tissue

  • Skeletal muscle cells are referred to as muscle fibers due to their resemblance to long threads.

  • Understanding the mechanism of muscle contraction is facilitated by comprehending the anatomy of skeletal muscle cells.

Anatomy of the Skeletal Muscle Fiber

  • Shape and Structure:

    • Skeletal muscle cells are cylindrical in shape.

  • Plasma Membrane:

    • The outer membrane is called the sarcolemma.

  • Cytoplasm:

    • The cytoplasm within a skeletal muscle fiber is known as sarcoplasm.

    • At rest, the sarcoplasm has a low concentration of calcium ions.

    • The low calcium level is maintained by calcium pumps that transport calcium ions into the smooth endoplasmic reticulum (ER), which in muscle cells is termed sarcoplasmic reticulum.

Components Important for Muscle Contraction

  • T-tubules:

    • Invaginations of the sarcolemma that allow electrical impulses to travel into the interior of the muscle cell.

    • Located adjacent to the sarcoplasmic reticulum.

Myofibrils and Myofilaments

  • Myofibrils:

    • Groups of myofilaments present within the muscle cell.

  • Myofilaments:

    • Composed of proteins, primarily actin (thin filament) and myosin (thick filament).

    • The specific arrangement of myofilaments in myofibrils results in visible striations within the muscle cell.

Identifying Muscle Structures

  • Students are encouraged to use muscle cell models to identify and describe the following structures:

    • Sarcolemma

    • Sarcoplasm

    • T-tubules

    • Myofibrils

    • Myofilaments (Actin and Myosin)

  • Discussion among partners regarding the purpose of each structure is emphasized.

  • Light and Dark Bands:

    • The myofibrils manifest light and dark bands due to the arrangement of thick and thin myofilaments.

Sarcomere: The Basic Unit of Contraction

  • Definition:

    • A sarcomere is a unit of muscle contraction that comprises a dark band and half of a light band on either side.

  • Structure:

    • On models, myofibrils are shown to consist of many adjacent sarcomeres.

  • Note that laboratory activities will provide a deeper understanding of the sarcomere's structure by allowing students to construct two adjacent sarcomeres as a group exercise.