Muscular System

Functions and Classification of Muscle Tissue

  • Primary Functions of Muscle Tissue:     * Produce Movement of Skeleton: Muscles pull on bones to facilitate movement.     * Maintain Posture and Body Position: Continuous muscle contractions keep the body upright or in specific positions.     * Support Soft Tissues: Muscle layers protect and support internal organs.     * Control Openings: Muscles form sphincters to regulate the passage of substances through internal and external openings.     * Homeostasis of Body Temperature: Muscle contractions generate heat as a byproduct, helping to maintain internal body temperature.

  • Three Basic Muscle Types:     1. Skeletal Muscle: Associated with the skeletal system.     2. Cardiac Muscle: Found exclusively in the heart.     3. Smooth Muscle: Found in the walls of hollow organs and tubes.

  • Terminology and Etymology:     * Muscle Cell: Often referred to as a muscle fiber.     * Prefixes:         * Myo-: Refers to muscle.         * Mys-: Refers to muscle.         * Sarco-: Refers to flesh.

Connective Tissue Organization

  • Connective Tissue Layers:     * Endomysium: The innermost layer that surrounds a single muscle fiber. It contains capillaries and axons (nerve fibers) to support and stimulate the individual muscle cell.     * Perimysium: The middle layer that surrounds a fascicle (a bundle of muscle fibers). This layer includes larger blood vessels and nerves.     * Epimysium: The outermost layer that surrounds the entire muscle. It is continuous with the external attachment structures.

  • Muscle Attachments:     * Tendon: A cord-like structure that connects muscle to bone.     * Aponeurosis: A broad, sheet-like structure that connects muscle to other tissues or bones.

Microscopic Anatomy of a Muscle Fiber

  • Cellular Characteristics:     * Multinucleate: Muscle cells are formed by the fusion of several myoblasts, resulting in multiple nuclei per cell.     * Nuclei Location: Nuclei are situated just beneath the sarcolemma, which is the plasma membrane of the muscle fiber. This layer is deep to the endomysium.     * Sarcoplasm: The cytoplasm of the muscle fiber.

  • Internal Organelles and Structures:     * Myofibrils: Specialized bundles of myofilaments (proteins) that fill the muscle fiber. They consist of repeating units called sarcomeres which are the functional units of contraction.     * Myofilaments: These include Thin filaments (actin) and Thick filaments (myosin).     * Sarcoplasmic Reticulum (SR): A specialized form of smooth endoplasmic reticulum that surrounds the myofibril. Its primary role is to store and release calcium ions (Ca+Ca^+).     * Mitochondria: Provide the necessary energy for contraction.     * T tubules (Transverse tubules): Invaginations of the sarcolemma that allow the action potential to reach deep into the muscle fiber.     * Terminal Cisterna: Expanded chambers of the sarcoplasmic reticulum located on either side of a T tubule.     * Triad: A functional unit consisting of one T tubule and the two terminal cisternae flanking it.

The Sarcomere Structure

  • Components of the Sarcomere:     * Z line: Marks the boundaries of a single sarcomere; thin filaments (actin) are anchored here.     * M line: The center of the sarcomere where thick filaments are held together.     * H band (or H zone): The "bare zone" in the center of the sarcomere that contains only thick filaments (at rest, there is no overlap here).     * I band: The region containing only thin filaments.     * Zone of Overlap: Where thick and thin filaments occupy the same area.     * Titin (Connectin filaments): Protein filaments that help align the thick and thin filaments.

  • Contraction Mechanism:     * During contraction, the length from Z line to Z line shortens.     * ATPATP (Adenosine Triphosphate) is utilized to facilitate the shortening of the sarcomere.

Physiology of Muscle Contraction

  • The Motor Unit:     * Skeletal muscles must be stimulated by a nerve to contract.     * A Motor Unit consists of one motor neuron and all the muscle cells it stimulates.

  • The Neuromuscular Junction (Myoneural Junction):     * Neuromuscular Synapse: The site where the synaptic terminal (axon) of a neuron meets the muscle fiber.     * Synaptic Cleft: The tiny gap between the nerve ending and the muscle sarcolemma.

  • The Contraction Sequence:     1. Release of Neurotransmitter: The axon's synaptic vesicles release acetylcholine (ACh).     2. Binding: Acetylcholine attaches to receptors on the sarcolemma.     3. Action Potential: An action potential is generated and spreads along the sarcolemma (similar to how a flame spreads along a twig).     4. Ions and Channels: Na+Na^+ diffuses into the cell.     5. Calcium Release: The action potential causes the sarcoplasmic reticulum to release Ca+Ca^+ ions.     6. Binding to Filaments: Calcium ions bind to the thin filaments to initiate contraction.

  • Ending a Contraction:     * Acetylcholine Removal: Acetylcholine is broken down and removed by the enzyme acetylcholinesterase.     * Ion Reuptake: Calcium ions are taken back up into the sarcoplasmic reticulum.     * Relaxation: Contraction ends.

Muscle Response and Energy Sources

  • Skeletal Muscle Dynamics:     * All or None Principle: An individual muscle fiber contraction is "all or none."     * Graded Responses: While individual fibers are all or none, the whole muscle can produce different levels of force through combinations of different fiber contractions.     * Muscle Force: Depends on the number of fibers stimulated; more contracting fibers result in greater force. Muscles contract until they exhaust their energy supply.

  • Muscle Fiber Types and Metabolism:     * Fast Fibers (White Fibers): Powerful but tire quickly. They have few mitochondria and low myoglobin levels. They function anaerobically (e.g., used for sprinting).     * Slow Fibers (Red Fibers): Slower to contract but possess high endurance. They are aerobic and rich in myoglobin (e.g., used for marathons).     * Intermediate Fibers: Characteristics reside between fast and slow fibers.     * Energy Source: Muscles primarily use ATPATP for energy.

Fascicle Arrangement and Muscle Function

  • Parallel Muscle: Fibers run parallel to the long axis (e.g., Biceps brachii). They can contract approximately 30%30\% of their length.

  • Convergent Muscle: Fibers converge on a single attachment point (e.g., Pectoralis muscles). They cover a broader area but are generally weaker than parallel muscles.

  • Pennate Muscle: Fibers are angled relative to a tendon. These are stronger than parallel muscles but have a shorter contraction distance.     * Unipennate: Fibers on one side of the tendon (e.g., Extensor digitorum muscle).     * Bipennate: Fibers on both sides of the tendon (e.g., Rectus femoris muscle).     * Multipennate: The tendon branches within the muscle (e.g., Deltoid muscle).

  • Circular Muscle (Sphincters): Fibers arranged in concentric circles to control entrances and exits (e.g., Orbicularis oris muscle).

Body Movements and Muscle Roles

  • Mechanics of Movement: Body movement occurs when a muscle moves an attached bone.

  • Attachment Points:     * Origin: The attachment to the bone that is less movable.     * Insertion: The attachment to the bone that is movable.

  • Functional Types of Muscles:     * Agonist (Prime Mover): The muscle primarily responsible for a specific movement.     * Antagonist: The muscle that opposes or reverses the action of the agonist.     * Synergist: Aids the prime mover and helps prevent unwanted rotation during movement.

Criteria for Naming Skeletal Muscles

  1. Direction of Muscle Fibers: e.g., Rectus (meaning straight).

  2. Relative Size: e.g., Maximus (meaning largest).

  3. Location: Named for the bones they are near, e.g., Temporalis.

  4. Number of Origins: e.g., Triceps (meaning three heads).

  5. Location of Origin and Insertion: e.g., Sterno (referring to the sternum).

  6. Shape: e.g., Deltoid (referring to a triangular shape).

  7. Action: e.g., Flexor and Extensor (describing flexing or extending a bone).

Levers and Pulleys in Anatomy

  • Lever Systems (Bones and Muscles):     * First-class Lever: Fulcrum is between the applied force (AFAF) and the resistance (e.g., scissors, seesaw).     * Second-class Lever: Resistance is between the fulcrum and the applied force (AFAF) (e.g., wheelbarrow).     * Third-class Lever: Applied force (AFAF) is between the fulcrum and the resistance (e.g., tweezers).

  • Anatomical Pulleys: These structures change the direction of a force. Examples include:     * The Patella (working with the quadriceps tendon and patellar tendon).     * The Lateral malleolus (associated with the Fibularis longus).

Muscular Disorders

  • Muscular Dystrophies: Sex-linked genetic disorders characterized by the progressive weakening of muscles.

  • Polio: A viral infection that attacks the part of the nervous system responsible for muscle control.

  • Tetanus: Caused by anaerobic bacteria that produce a toxin preventing muscles from relaxing.

  • Botulism: Caused by bacteria producing toxins that result in the paralysis of skeletal muscles.