Anatomy Unit: Muscle Tissue Overview

Anatomy of Muscle Tissue

• Muscle tissue consists of muscle fibers grouped together, enabling contractility due to myofibrils in the cytoplasm.
• The muscle cell membrane is referred to as the sarcolemma, while the cytoplasm is called sarcoplasm.
• Muscle fibers can be categorized as striated or non-striated, and as voluntary or involuntary.

Types of Muscle Fibers

• Skeletal Muscle Fibers:

  • Striated with transverse dark and light bands.
  • Primarily voluntary, except in the upper third of the esophagus, pharynx, and cremaster muscle.
  • Generally do not branch, with exceptions in the tongue and face.
  • Attached to bones in the skeleton, and found in the eye and tongue.

• Cardiac Muscle Fibers:

  • Distinctive striations but branched; involuntary control.

• Smooth Muscle Fibers:

  • Non-striated, involuntary, and spindle-shaped.

Structure of Skeletal Muscle Fibers

• Composed of individual muscle cells (fibers) bundled by connective tissue.

• Connective Tissue Components:

  • Epimysium: Dense connective tissue encapsulating the entire muscle.
  • Perimysium: Dense connective tissue that divides the muscle into bundles.
  • Endomysium: Loose connective tissue separating individual muscle fibers.

• Microscopic Structure:

  • Longitudinal section: Skeletal muscle fibers are elongated, multinucleated cells (syncytium) with flattened oval nuclei located peripherally.
  • Regular transverse striations are visible; sarcoplasm is acidophilic and contains glycogen granules and myoglobin.
  • Transverse section: Polyhedral shape with possible visibility of nuclei; dark areas (Cohnheim’s areas) indicate myofibril grouping.

Myofibrils (Sarcostyles)

• Contractile elements arranged longitudinally within the sarcoplasm of skeletal muscle fibers.

• Structure:

  • Display alternating light and dark bands: the light band (I-band) is pale, while the dark band (A-band) is prominent.
  • Adjacent myofibrils’ dark bands align to create cross-striation observable under a light microscope.

• Band Structure:

  • Each dark band is bisected by a light disc termed the H-zone.
  • Each light band is divided by a dark line known as the Z-line, indicating the boundaries of sarcomeres.