Muscle Tissue Overview

• Muscle tissue is specialized for contraction/movement

Types of Muscles

• Skeletal muscle
• Cardiac muscle
• Smooth muscle

Muscle Cell Organelles

• Cytoplasm is also known as sarcoplasm
• Plasma membrane also known as sarcolemma (plasmalemma)
• Smooth endoplasmic reticulum also known as sarcoplasmic reticulum
• Mitochondrion are called sarcosome

Skeletal, Cardiac, and Smooth Muscle Characteristics

• Cell shape for skeletal muscle is long elongated, cardiac muscle arranged in irregular parallel, and smooth muscle is cylindrical
• Nucleus for skeletal is multinucleated and peripheral; cardiac is mononuclear; and smooth muscle has only one nucleus
• Striations are present/due to sarcomere in skeletal muscle; cardiac muscle has present due to binding to troponin; smooth muscle is absent

Contraction

• Skeletal muscle is voluntary and controlled via somatic nervous system and innervation
• Requires Ca+ to initiate contraction
• Muscle contains motor neuron

Skeletal Muscle Details

• Skeletal muscle fibers/cells/myocytes are formed by the fusion of multiple myoblasts.
• A single muscle fiber contains hundreds of nuclei.
• Myofilaments: Actin (thin) and Myosin (thick).

Connective tissue framework

• Epimysium: dense connective tissue surrounding the entire muscle.
• Perimysium: surrounds bundles of muscle fibers (fascicles); loose connective tissue.
• Endomysium: basal lamina of connective tissue surrounding individual muscle fibers.

Diagram of Muscle fibers

• Muscle fiber contains myofibrils, contractile thread-like structures
• Cross striations result from sarcomeres arranged end to end.
• Sarcoplasm contains myofibrils.
• Sarcomere is a functional unit of contraction.
• Myofibrils contain thick and thin filaments arranged into repeating units called sarcomeres.

Sarcomere Bands

• Contains both actin & myosin.
• A-band (anisotropic band) is the dark band.
• I-band (isotropic band, light band) contains only thin filaments (actin, troponin, tropomyosin).
• Z-line (Z-disc) is the boundary between 2 sarcomeres
• H-zone (Henson's zone) is the central region of A-band; contains only myosin thick filaments.
• M-line is located in the center of Sarcomere.
• Contraction occurs at sites along overlapping filaments of sarcomere past one another.

Thick Filaments - Myosin

• Tail
• Heads with Actin-binding site, ATP & ATPase binding site

Sarcomere

• Myosin
• F-actin
• G-actin

Formation of skeletal muscles (myogenesis)

• Differentiation & fusion of muscle precursor cells (myocytes) /myoblasts into mature muscle fibers (myocytes).
• Origin: paraxial mesoderm from somites (blocks of mesoderm along neural tube).
• Somites differentiate into myotomes.
• Myoblasts align & adhere to each other leading to fusion and form myotubes.
• Myotubes elongate & synthesize myofilaments (actin, myosin).
• Mature muscle fibers (myofibers)

Activation of Satellite Cells

• A portion of myoblast does not fuse so they do not differentiate; these are satellite cells stem
• These cells remain around the muscle fibers for future repair/regeneration
• Muscle injury/mechanical stress activates satellite cells to re-enter cell cycles
• Satellite cells proliferate to produce myoblasts
• Some fuse damaged myofibers to repair
• Others remain as satellite cells for future regeneration
• Muscle fibers grow by hypertrophy (increase in size not number)
• Satellite cells reside around sarcolemma basal lamina of muscle fibers