MUSCULAR SYSTEM NOTES

MUSCULAR SYSTEM

UNDERSTANDING THE STRUCTURE AND FUNCTION OF MUSCLES

PART 1

INTRODUCTION

  • Muscle Definition: Muscle is a contractile tissue of the body.

  • Origin: Muscles derive from the mesodermal layer of embryonic germ cells.

  • Body Weight: Muscles account for approximately 40% of total body weight.

  • Functionality: Muscles are responsible for creating movement within certain organs and across the body, functioning as the "motors of the body."

DEVELOPMENT OF MUSCLE TISSUES

  • Mesodermal Derivation: All muscles are derived from the mesoderm.

  • Exceptions:

    • Muscles of the iris

    • Arrectores pilorum

    • Myoepithelial cells of glands

    • Note: These exceptions develop from the ectoderm rather than the mesoderm.

TYPES OF MUSCLE

  • Skeletal Muscle (Voluntary):

    • Attached to the skeleton.

    • Responsible for body movements.

    • Noticeable during dissection of cadavers.

  • Smooth Muscle (Involuntary):

    • Forms the walls of internal organs and viscera.

  • Cardiac Muscle (Involuntary):

    • Comprised of the myocardium (heart wall).

  • Other Contractile Cells or Tissues:

    • Myoepithelial Cells: Spindle-shaped, containing actin filaments that contract to aid secretion expulsion.

    • Myo-fibroblasts: Involved in the wound healing process.

BASIC PROPERTIES OF MUSCLE

  • Irritability: Muscles are sensitive to stimuli.

  • Contractility: Upon stimulation, muscle fibers contract, leading to shortening.

  • Extensibility: When stimuli are removed, muscle fibers return to their original length.

  • Elasticity: Muscles can assume a desired shape regardless of how stretched.

SKELETAL MUSCLES

  • Characteristics:

    • Located superficially and attached to the skeleton.

    • Commonly paralyzed and injured (especially in athletes).

  • Intramuscular Injection Sites: Common sites for intramuscular injections include:

    • Deltoid Muscle: Located in the shoulder region.

    • Gluteus Medius: Located in the gluteal region.

    • Vastus Lateralis: Located in the thigh region.

Main Functions of Skeletal Muscles

  • Motion: Includes walking and running.

  • Heat Production: Metabolism within muscles produces heat, which increases with strenuous activity.

  • Posture and Body Support: Muscles maintain body posture and offer support.

MAJOR SUPERFICIAL MUSCLES OF THE BODY

  • Major superficial muscles identified include:

    • Sternocleidomastoid

    • Pectoralis Major

    • Biceps Brachii

    • Rectus Abdominis

    • Sartorius

    • Temporalis

    • Trapezius

    • Vastus Lateralis

    • Vastus Medialis

    • Gastrocnemius

    • Tibialis Anterior

    • And others visible upon examination.

MICROSCOPIC STRUCTURE OF MUSCLES

  • Components of Skeletal Muscle:

    • Muscle fibers

    • Supporting connective tissues:

    • Endomysium: Surrounds individual muscle fibers.

    • Perimysium: Surrounds bundles of muscle fibers (fascicles).

    • Epimysium: Encases the entire muscle.

MUSCLE FIBERS

  • Description: Muscle fibers (myocytes) are longer than smooth muscle fibers, measuring up to 30 cm in length and 10 to 100 μm in diameter.

  • Characteristics:

    • Each muscle fiber is multinucleated, cross-striated, and cylindrical.

  • Cytoplasm: The cytoplasm of muscle fibers is called sarcoplasm, containing bundles known as myofibrils.

MYOFIBRILS

  • Composition: Myofibrils are constructed of contractile proteins termed myofilaments.

  • Types of Myofilaments:

    • Thin Myofilaments: Composed mainly of actin and tropomyosin.

    • Thick Myofilaments: Composed mainly of myosin.

ARRANGEMENT OF MYOFILAMENTS

  • The structural arrangement creates a cross-striated appearance with alternating dark and light bands:

    • A Band: Dark band representing thick myofilaments (myosin).

    • I Band: Light band representing thin myofilaments (actin).

    • H Line: Central line within the A band indicating the center of thick filaments.

    • Z Line: Thin dark line dissects the I band, indicating boundaries of the sarcomere.

SARCOMERE FUNCTIONAL UNIT

  • Definition: The sarcomere is the basic contractile unit in muscle, defined as the segment between two Z lines.

  • Overlap: In the A band, thick and thin myofilaments partially overlap.

  • Composition:

    • Thick Filaments: Composed of myosin.

    • Thin Filaments: Composed of actin and tropomyosin.

MECHANISM OF CONTRACTIONS

  • Process: Myosin takes up the A band, while actin, tropomyosin, and troponin constitute the thin filament.

  • Contraction Dynamics: During contraction, thin actin filaments slide between thick myosin filaments toward the center of the sarcomere, resulting in:

    • Shortening of the muscle fibers.

    • Narrowing of I bands.

    • Disappearance of the H zone.

  • Note: Individual thick and thin filaments do not shorten during contraction — they merely slide past each other.

  • Sliding Filament Model: This theory describes the contraction process and is influenced by calcium and ATP.

TYPES OF SKELETAL MUSCLE FIBERS

  • Red Type I Fibers:

    • Slow twitch muscles.

    • Examples: Postural muscles; anti-gravity muscles beneficial for long-distance running.

  • White Type II Fibers:

    • Fast twitch muscles that contract quickly but fatigue easily.

    • Example: Extraocular muscles of the eyeballs.

CLINICAL CORRELATION

  • Rigor Mortis:

    • A condition that occurs post-mortem, indicated by stiffening of voluntary and involuntary muscles.

    • Timeline: Appears 2 to 4 hours after death and resolves within 2 to 4 days.

    • Causes: Results from loss of ATP, causing myosin to remain attached to actin, preventing muscle relaxation.

    • Utility: Can assist in determining time of death.

FASCICULAR ARCHITECTURE

  • Definition: The arrangement of muscle fibers within individual muscles, influenced by muscle function.

  • Types of Fasciculi:

    1. Parallel Fasciculi: Muscle fibers run parallel to the line of muscle pull, enabling maximum range of motion (e.g., Thyrohyoid muscle, Pronator quadratus).

    2. Strap Muscles: Example includes the rectus abdominis.

    3. Fusiform Muscles: Example—biceps.

    4. Convergent Muscles: Muscle fibers converge at an insertion point (e.g., Adductor longus, Temporalis).

    5. Spiral Muscles: Fibers are twisted and include temporalis and latissimus dorsi.

    6. Cruciate Muscles: Features fibers in both superficial and deep planes crossed in an X formation (e.g., Sternocleidomastoid, Masseter).

    7. Pennate Fasciculi: Muscles resemble feathers and provide strength.

    • Unipennate examples: Extensor digitorum longus, Flexor pollicis longus.

    • Bipennate examples: Flexor hallucis longus, Peroneus tertius.

NAMING OF MUSCLES

  • Muscles are typically named based on several attributes:

    • Location: E.g., Tibialis anterior, located at the anterior aspect of the tibia.

    • Shape: E.g., Trapezius (trapezoid shape), Quadratus (quadrangular shape).

    • Function: E.g., Extensor carpi ulnaris (wrist extension), Flexor carpi radialis (wrist flexion).

    • Number of Heads:

    • Biceps Brachii: Two-headed muscle.

    • Triceps Brachii: Three-headed muscle.

    • Quadriceps Femoris: Four-headed muscle composed of Rectus femoris, Vastus medialis, Vastus lateralis, and Vastus intermedius.

    • Attachments: E.g., Sternocleidomastoid connects to the sternum, clavicle, and mastoid process of the skull.

    • Size: Examples include Gluteus maximus, Gluteus medius, and Gluteus minimus.

GROSS FEATURES OF SKELETAL MUSCLE

  • Structure: Comprises two ends (origin and insertion) and two parts (belly and tendon).

    • Origin: The more stationary attachment point.

    • Insertion: The moveable end of the muscle.

  • Reversal of Muscle Action: May occur where the origin and insertion points can switch based on the action being performed.

  • Belly: Refers to the thickened, contractile portion, highly vascular but not able to withstand pressure or friction.

  • Tendon: Connects muscle to bone, transferring contraction forces.

    • Composition of Tendon:

      • Aponeurosis: Flattened tendon forming a myotendinous junction (dove-tail arrangement).

      • Muscle fibers of the belly remain contiguous but not continuous with tendon fibers.

TENDONS

  • Function: Tendons are cord-like structures connecting muscle belly to periosteum of bone.

  • Force Transfer: They facilitate the transfer of muscle contraction force across joints to the bones being moved.

  • Blood Supply: Derived from both the muscle’s own vessels and periosteal vessels from the attached bones.

  • Strength: Tendons are strong and supported by sensory nerve endings.

  • Synovial Sheaths: Tendons are covered to prevent friction through synovial sheaths:

    • Visceral Layer: Covers the tendon.

    • Parietal Layer: Forms the outer layer of the sheath.

TENDON INJURIES AND REGENERATIVE ISSUES

  • Rupture: A ruptured tendon renders the muscle ineffective.

  • Causes of Rupture: Can occur due to trauma or be weakened by adjacent fractures.

  • Commonly Ruptured Tendons: Notable examples include the biceps brachii and achilles tendons.

  • Blood Supply Limitation: Tendons have a poor blood supply, affecting their healing capacity.

SYNOVIAL BURSA AND SHEATH

  • Definition: Synovial bursa (also termed synovial sheath) is similar in structure to synovial membranes.

  • Functionality: Contains slippery fluid (bursa) that lubricates joints to diminish friction and enable smooth movement.

  • Formation: Bursae develop wherever tendons rub against bones or cartilage.

  • Bursitis: Condition characterized by inflammation of the synovial bursa leading to swelling and pain.

  • Adventitious Bursa: Formed due to continuous pressure and friction on soft tissues, such as in "porter's shoulder" or "tailor's ankle."

IMAGES AND DIAGRAMS

  • Major superficial muscles are illustrated in included images, providing details of both anterior and posterior views for anatomical reference.