Skeletal Muscle Tissue

• Skeletal muscle histology focuses on differentiating between skeletal, cardiac, and smooth muscle.
• At the end of this lecture, students should be able to describe the traits of skeletal muscle and at light/micro levels, and be capable to differentiate between skeletal, cardiac and smooth muscle.

Connective Tissue Components

• The epimysium is a dense, irregular collagenous connective tissue that covers the entire muscle.
• The perimysium surrounds each bundle (fascicle) of muscle, is a less dense type of collagenous connective tissue.
• The endomysium surrounds each muscle fiber and is composed mainly by external lamina and reticular fibers.
• Blood vessels penetrate the muscle within the connective tissue septa, forming networks.
• The capillaries are the continuous type
• Lymphatic vessels can be found the connective tissue.

Skeletal Muscle Fibers

• Skeletal muscle consists of muscle fibers arranged in bundles.
• These cells are 40 mm long cylindrical shaped.
• They have a 10-100 μm diameter range.
• Their nuclei are multiple, oval and peripheral.
• The cytoplasm is acidophilic striated.
• Longitudinal sections show the sarcoplasm that each muscle fibers is filled by long filamentous bundles called myofibrils.
• Cross sections show bundles of myofibrils that appear as Cohenheim’s areas.

Light Microscopy

• Skeletal muscle fibers show characteristic striations.
• Dark bands are called A bands.
• Light bands are called I bands.
• The center of the A band is occupied by a pale area called H bands.
• H bands this are bisected by a thin M line.
• Each I band is bisected by a line called Z line or disk.
• Dark band A (anisotropic) causes double refraction to polarized light.
• Light band I (isotropic) causes single refraction to polarized light.

Unique Terminology

• Cell membrane = Sarcolemma
• Cytoplasm = Sarcoplasm
• Smooth endoplasmic reticulum = Sarcoplasmic reticulum
• Mitochondria = Sarcosomes

Electron Microscopy

• Sarcoplasm contains myofibrils and myofilaments, Golgi complex, ribosomes, and mitochondria.
• The sarcoplasmic reticulum is well developed to form the structure of triads.
• Myofilaments are bundles of protein filaments.
• Myofibrils run parallel with the long axis of the muscle fibers.
• Each myofibril includes a large number of myofilaments that are of 2 types
• Myofilaments include thin (actin) filaments.
• Myofilaments include thick (myosin) filaments
• The thick myofilaments (myosin) are present in the dark (A) bands.
• The thin myofilaments (actin) are present in the light (I) bands and extend for a short distance into the dark bands.
• Each dark band (A band) is divided into two equal parts by Hensen's disc or H band.
• A fine dark middle strip or M line may be inside each A band.
• The major protein of the M line creatine kinase.
• A light band (I band) divides into two equal parts by a dark membrane called Z-line.
• Z lines divide myofibrils into contractile units called sarcomeres.
• The region of the sarcomere with both actin and myosin filaments appears darker than the region with actin filaments only.
• The H band appears slightly lighter because it has only myosin filaments.

Sarcomeres

• The sarcomere is the unit of contraction of the muscle fiber.
• The boundary of a sarcomere includes two successive Z lines.

Transverse Tubules

• Myofibrils are surrounded by a network of sarcoplasmic reticulum.
• Regions of A-I junctions that include sacs are called terminal cisternae.
• Between the neighboring terminal cisternae there is a transverse tubule, or T-tubule.
• It is formed by the invagination of the sarcolemma.
• It extends to encircle the sarcomeres like collars at the A-I junction.
• Two terminal cisternae with a T-tubule in between are together called the triad.

Skeletal Muscle Types/Fibers

• Type I fibers (red fibers) are small in diameter.
• Contain a large amount of myoglobin responsible for the red color.
• They sustain contraction for a long time without fatigue.
• Type II fibers (white fibers) have a large diameter with a small amount of myoglobin.
• White fibers contract faster but become fatigued rapidly.

Cardiac Muscle (LM)

• Cardiac muscle is formed by joined short cylindrical cells that creates cardiac muscle fibers.
• Cardiac muscle branch and anastomose forming a network.
• Have length of 80-100 um in length.
• Each cell has one or two central nuclei.
• The cytoplasm is acidophilic.

Cardiac Muscle (EM)

• Has no triads, only diads
• T tubules are short and wide
• Sarcoplasmic reticulum are not well developed
• Aerobic cells include high myoglobin content
• High mitochondria account for 40% of the cell
• Intercalated discs are transverse lines between joining fibers

Smooth Muscle (LM)

• Smooth muscle fibers are non-striated.
• Found in blood vessels, and the viscera of systems.
• They are fusiform in shape with a single central nucleus.
• Contain the smallest diameter of all muscle fibers (3 µm).
• Lengths vary by location from 30 to 500µm.
• Homogeneous acidophilic cytoplasm.
• Arrangement can be in different layers (circular, longitudinal, oblique, or spiral)
• Contractile filaments are actin, myosin, and intermediate filaments (vimentin and desmin).
• Dense bodies located in both the cytoplasm and sarcolemma may be similar to Z lines .
• Alpha actinin form the intermediate filaments which are inserted to it.
• Sarcolemmal vesicles (caveoli) are termed caveoli.
• They are invaginations of the cell membrane.
• They function by the intake and release of calcium ions.
• A sparse smooth endoplasmic reticulum closely associates with sarcolemmal vesicles.
• A Gap Junction helps excitation spread between smooth muscle fibers.
• Glycogen granules, but no satellite cell within.

Explore the structure and function of skeletal muscle tissue. Learn about connective tissue layers, capillary networks, fiber length, and sarcoplasmic structures. Understand striations, nuclei, Z lines, the sarcolemma, and sarcomere organization for muscle contraction.