Muscle Tissue Types: Smooth, Cardiac, and Skeletal
Muscle Tissue Classification and Histogenesis
Histogenesis (Origin)
- Smooth muscle tissue: Mesenchyme
- Striated skeletal muscle and striated cardiac muscle: Mesoblast
- Myoepithelial cells (epithelial cells covering muscle tissue): Ectoblast
Classification
Muscle tissues are classified by innervation and/or striation:
By Innervation (Voluntary or Involuntary):
- Striated skeletal muscle tissue: Voluntary
- Smooth muscle tissue: Involuntary
- Striated cardiac muscle tissue: Involuntary
By Striation:
- Striated: Skeletal and Cardiac
- Smooth muscle tissue: Found in visceral organs (e.g., intestines), myo-epithelial cells in acini, and sphincters and dilators of the pupils of the eye.
Smooth Muscle Tissue
Origin: Mesenchyme, involuntary muscle
Structure:
- Cellular type of microscopic organization with centrally arranged nuclei.
- Non-striated muscle
- Thin, elongated muscle cells (fibers) pointed at their ends with a single, large, oval nucleus.
- Filled with sarcoplasm (specialized cytoplasm) and surrounded by sarcolemma (thin cell membrane).
- Each cell has many myofibrils, which lie parallel to one another.
- Smooth muscle fibers cross over to form layers of muscle tissue rather than bundles as in striated muscle.
Function:
- Located in the walls of hollow organs such as the digestive tract (lower part of the esophagus, stomach, and intestines), bladder, uterus, various ducts of glands, and blood vessels.
- Single-unit variety mostly, with either contraction or relaxation of the whole unit (e.g., blood vessels, urinary tract, digestive tract). Multiunit smooth muscles are also present (e.g., trachea, large elastic arteries, iris of the eye).
- Myosin works with actin to form a motor unit, causing muscle contraction using energy from ATP.
- Contractions move food through the digestive tract and push blood through blood vessels.
- The weakest of all muscle tissues, smooth muscle facilitates organ contraction to move substances.
Cardiac Striated Muscle Tissue
Origin: Mesoblasts, involuntary muscle
General Characteristics:
- Presence of intercalated discs organized in a step-like manner.
- Intercalated discs consist of gap junctions (nexus) and desmosomes, allowing cardiac muscles to stay attached under strain.
- Reduces the gap for nerve impulse passage, speeding up stimulation.
- Allows for rapid, wave-like contractions.
Types:
- Contractile: Allows the heart to beat as one unit, with each cell connected to four others through intercalated disks.
- Pacemakers: Conduct nerve impulses (sinoatrial node and atrioventricular node) and have fewer myofibrils than contractile tissue.
- Endocrine: (Details not provided in original text)
Structure:
- Single nucleus, striated with light and dark bands visible under a microscope.
- Dark bands represent myosin proteins.
- Light bands represent thin actin filaments.
- During contraction, myosin pulls actin filaments together, shrinking the muscle.
Skeletal Striated Muscle Tissue
Origin: Mesoblasts, innervated by the sympathetic nervous system (conscious control)
Structure:
- Actin and myosin work with troponin and tropomyosin for muscular contraction.
- Symplast type of microscopic organization
- The triad of myofibril consists of T system and 2L system.
- Long and cylindrical shape, with nuclei located at the edges (periphery) of the cell, beneath the sarcolemma.
- Striated appearance due to light and dark bands.
- Dark bands represent myosin proteins.
- Light bands represent thin actin filaments.
- During contraction, myosin pulls actin filaments together, shrinking the muscle cell.
- Each cell is filled with sarcoplasm and surrounded by sarcolemma.
- I bands are longer in skeletal muscle tissue than in cardiac muscle tissue.
Function:
- Movement of the skeleton under conscious control, including movement of limbs, fingers, and toes.