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Questions and Answers
Which type of muscle tissue is characterized as voluntary and striated?
What is the term used for the thin connective tissue layer surrounding individual muscle fibers?
Which connective tissue covers the entire muscle?
How are skeletal muscle fibers arranged?
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Which type of muscle tissue lacks cross striations?
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What is the primary origin of muscular tissue?
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Which feature distinguishes striated muscle from non-striated muscle?
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Which of the following components is NOT found in the connective tissue of skeletal muscle?
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What is the primary role of connective tissue associated with skeletal muscle fibers?
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Which characteristic is NOT true regarding skeletal muscle fibers?
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What is the role of satellite cells in skeletal muscle?
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Which of the following accurately describes the myofibrils in skeletal muscle fibers?
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Which structure serves as the store for calcium ions in skeletal muscle fibers?
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What component is abundantly found in the cytoplasm of skeletal muscle fibers, aiding in oxygen binding?
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Which of the following statements about mitochondria in skeletal muscle fibers is incorrect?
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Skeletal muscle fibers' sarcoplasm can be described as:
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What are the main components of thin filaments in muscle fibers?
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What is the function of the triad tubular system in skeletal muscle?
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What occurs after a mild injury to skeletal muscle fibers?
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Which area of the sarcomere is exclusively made up of thick filaments?
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How do skeletal muscle fibers primarily increase in mass?
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What is the role of the Z line in a sarcomere?
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What happens when the electrical impulse to the muscle fiber ceases?
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Which structure between the A and I bands contains the T-tubules?
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Study Notes
Learning Objectives
- Students should be able to identify the types of muscle tissue.
- Students should be able to describe the structure of skeletal muscle as viewed by light and electron microscopes.
- Students should be able to explain the regeneration of skeletal muscle.
- Students should be able to explain the growth processes in skeletal muscle.
Types of Muscle
- There are three types of muscle: skeletal, cardiac, and smooth.
General Characteristics of Muscle
- Muscle is one of the four basic tissues of the body.
- All muscle types are mesodermal in origin.
- Muscle is characterized by its contractility, which is the ability to shorten and generate force.
Skeletal Muscle
- Skeletal muscle is striated and voluntary.
- Skeletal muscle fibers are elongated, cylindrical cells called myofibers.
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Connective Tissue: Connective tissue surrounds and supports the muscle fibers.
- The epimysium is a dense irregular connective tissue sheath that surrounds the entire muscle.
- The perimysium is a thinner layer of connective tissue that surrounds bundles of muscle fibers called fascicles.
- The endomysium is a delicate layer of reticular fibers and scattered fibroblasts that surrounds each individual muscle fiber.
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Functions of Connective Tissue:
- Connective tissue carries blood vessels and nerves to the muscle.
- Connective tissue helps transmit the force of muscular contraction by binding muscle units together.
Skeletal Muscle Fibers
- Site: Skeletal muscles are found in skeletal muscles, tongue, and pharynx.
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Light Microscopy (LM):
- Shape: Each muscle fiber is an elongated cell measuring 10-100 um in diameter and 1-3 cm in length.
- Sarcolemma: The cell membrane is called the sarcolemma.
- Nucleus: Muscle fibers are multinucleated with peripherally located nuclei.
- Sarcoplasm/Cytoplasm: Sarcoplasm, the cytoplasm of a muscle fiber, is acidophilic and has clear transverse striations.
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Electron Microscopy (EM):
- Muscle fibers contain a variety of organelles including membranous (sarcoplasmic reticulum, mitochondria) and non-membranous (myofibrils) organelles.
- Sarcoplasmic Reticulum (SR): A specialized smooth endoplasmic reticulum responsible for storing calcium ions.
- Mitochondria: Located in rows between the myofibrils, mitochondria are responsible for energy production.
- Myofibrils: Specialized contractile units that run the length of the muscle fiber.
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Inclusions:
- Myoglobin: An oxygen-binding protein that stores oxygen in the muscle fiber.
- Glycogen: A storage form of glucose used as an energy source during muscle contraction.
Myofibrils
- Myofibrils are bundles of myofilaments responsible for muscle contraction.
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Myofilaments are the protein filaments composing the myofibrils.
- Thick Filaments: Composed of the protein myosin. Each filament has a thickened mid portion, tapering ends, and occupies the central portion of the sarcomere.
- Thin Filaments: Composed primarily of the protein actin and the regulatory proteins troponin and tropomyosin.
- Sarcomere: The functional unit of a muscle fiber. The sarcomere is the distance between two successive Z lines.
Striations in Skeletal Muscles
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Dark Bands (A Band): The A band consists of both thick and thin myofilaments.
- The center of the A band is the H zone, composed only of thick filaments (myosin).
- The M line is a dark membrane that divides the H zone.
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Light Bands (I Band): The I band consists of thin myofilaments (actin).
- The Z line is a dark membrane that divides the I band into two equal portions. The Z line anchors the actin filaments.
Triad or Tubular System of Skeletal Muscle
- The triad is a complex of structures located at the junction of the A and I bands.
- The sarcoplasmic reticulum forms a pair of transverse dilated sarcotubules called terminal cisternae.
- The sarcolemma, the plasma membrane of the muscle fiber, forms transverse invaginations called T-tubules. T-tubules form collars around the myofibrils.
- The triad is composed of one T-tubule and two terminal cisternae.
Functions of the Triad
- The triad transmits impulses from the exterior of the muscle fiber to all of the myofibrils, allowing for coordinated contraction.
- Electrical impulses traveling down the T-tubules trigger the release of calcium ions from the sarcoplasmic reticulum, initiating muscle contraction.
- The sarcoplasmic reticulum actively pumps calcium ions back into the terminal cisternae when the impulse ends, allowing the muscle to relax.
Regeneration of Skeletal Muscle Fibers
- Skeletal muscle fibers have a limited capacity for regeneration.
- Satellite Cells: Small spindle-shaped precursor cells located beneath the external lamina of a muscle fiber.
- In Mild Injury: Necrotic areas are removed by macrophages, and satellite cells replace the degenerated portion.
- In Severe Lesion: Fibrous scar tissue will form.
Growth of Skeletal Muscle Fibers
- Skeletal muscle fibers cannot divide.
- Increase in muscle mass (hypertrophy) is achieved by:
- Increase in width: New myofibrils are synthesized, increasing the width of the muscle fiber.
- Increase in length: Activation of satellite cells leads to the formation of new myoblasts that fuse with existing muscle fibers.
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