Skeletal Muscle Tissue

• There are three types of muscular tissue: skeletal, cardiac, and smooth muscle

Skeletal Muscle General Characteristics

• Skeletal muscle is attached to the skeleton
• Skeletal muscle is found in the tongue and eye muscles
• Skeletal muscle is located in the pharynx and larynx
• Skeletal muscle is formed of a group of muscle fibers
• The cell membrane of skeletal muscle is called the sarcolemma
• The cytoplasm of skeletal muscle is called the sarcoplasm
• The cytoplasm contains myofibrils, which are contractile elements
• The cytoplasm has regular transverse striations and is striated
• Skeletal muscle is voluntary

Skeletal Muscle Organization

• Whole muscle is surrounded by a dense connective tissue called the epimysium
• Skeletal muscle is organized in bundles called fascicles
• Fascicles are separated by dense connective tissue called perimysium
• Each muscle fiber is enclosed by loose connective tissue called endomysium

Skeletal Muscle Fibers LM

• Muscle fibers are formed of single elongated cells
• Muscle fibers have no branching
• Muscle fiber length measures 1-40mm
• Muscle fiber diameter measures 10-100 um
• Muscle fiber cells are multinucleated, with flattened and peripheral nuclei
• The sarcoplasm is acidophilic with regular striations, that are regular myofibrils

Skeletal muscle fibers EM

• The cytoplasmic contents include: sarcoplasmic reticulum (SER), numerous mitochondria, myoglobin, Beta glycogen granules, and myofibriles (sarcostyles)
• Longitudinal contractile elements in the sarcoplasm
• Myofibrils consist of light and dark bands
• Light band (I-band) contains thin filaments only
• Dark band (A-band) contains thin and thick filaments
• The dark band of one myofibril is present beside those of adjacent myofibrils, this creates cross striations seen by LM
• Z-lines are dark lines in the center of the I bands
• A sarcomere begins from Z-line to Z-line
• Each myofibril consists of successive sarcomeres
• A sarcomere is an entire A-band plus 1/2 I band on either side
• The H-zone is a light area in the middle of the A-band
• Thin filaments are actin and thick filaments are myosin
• Actin extends from the Z-line to the beginning of the H-zone while myosin is in the A-band only
• One end of actin is free and the other is attached to the Z-line, both ends of myosin are free
• Actin mainly contains actin, tropomyosin, and troponin while myosin only contains myosin
• Actin has a diameter of 7nm and myosin has a diameter of 15nm
• The conducting system contains one T-tubule
• The T-tubule arises from the sarcolemma at the A-I junction
• The conducting system contains two sarcoplasmic tubules
• One T-tubule and two sarcoplasmic tubules form a triad

Cardiac Muscle

• Cardiac muscle is located in the wall of the heart and is involuntary
• Each fiber is formed of chains of muscle cells linked by intercalated discs
• Cardiac muscle has branching anastomosing fibers
• Its length is variable
• The diameter is smaller than skeletal muscle fibers
• Cells have one or two central nuclei
• The cytoplasm is acidophilic with irregular striations
• Cardiac tissue has numerous mitochondria
• Cell inclusions include Glycogen, Lipid & lipochrome pigments
• Myofibrils: branching and anastomosing, Irregular giving irregular striations
• The conducting system (diads): consists of one T-tubule and one sarcoplasmic cisterna
• Intercalated discs connect adjacent cells

Smooth Muscle

• Smooth muscle is located in wall of viscera (GIT and urinary organs) and blood vessels
• Smooth muscle is involuntary
• Smooth muscle is formed of a single cell
• Smooth muscle is fusiform (spindle-shaped)
• Cell diameter is small
• Length is variable, measures 30 µm in blood vessels and 500 µm in pregnant uterus
• Cells have a single, oval nucleus, centrally positioned
• The cytoplasm is acidophilic and non-striated

LM Differences in Muscular Tissue

• Skeletal muscle fiber is formed of one cell, cardiac is formed of more than one cell, and smooth muscle is formed of one cell
• Skeletal and smooth muscle have no branching while cardiac muscle has branching and anastomosing fibers
• Skeletal has very long length and cardiac and smooth muscle have variable lengths
• Smooth muscle has the smallest diameter, cardiac muscle has less than that of skeletal muscle, and skeletal muscle has the largest diameter
• Skeletal muscle is multinucleated and has flattened and peripheral nuclei. Cardiac muscle has one or two central nuclei. Smooth muscle has a single oval central nucleus.
• All three have acidophilic cytoplasm
• Cytoplasm is regular striated in skeletal, irregular striated in cardiac, and non-striated in smooth muscle