Muscle Histology & Physiology Study Notes

Muscle Histology & Physiology

Overview of Muscle Tissue

  • Three Types of Muscle Tissue:

    1. Skeletal Muscle (attached to bone, voluntary):

    • Cylindrical fibers with cross stripes or striations (termed striated muscles)

    • Features many peripheral nuclei

    1. Cardiac Muscle (heart muscle, involuntary):

    • Cylindrical, branching fibers with cross stripes or striations (also striated muscles)

    • Cells are interconnected by dark bands known as intercalated disks

    • Typically contains one central nucleus

    1. Smooth Muscle (found in blood vessels & internal organs, involuntary):

    • Comprised of spindle-shaped cells that taper at both ends with no cross striations (termed non-striated muscles)

    • Contains a single central nucleus

Detailed Structure of Muscle Tissue

  • Skeletal Muscle (Striated Voluntary Muscle):

    • Components:

    • Sarcolemma: Outer membrane of a muscle fiber

    • Myofibrils: Striated components of muscle fibers that facilitate contraction

    • Anisotropic Substance: Refers to the part of myofibrils that has different properties in different directions (A bands)

    • Isotropic Substance: Has the same properties in all directions (I bands)

    • Intermediate Line: Line within the myofibrils

Structure of Skeletal Muscles

  • Skeletal Muscle Anatomy:

    • Belly (Gaster): The fleshy part of the muscle

    • Origin: End of the muscle attached to a stable bone

    • Insertion: End of the muscle attached to a movable bone

    • Connective Tissue Structure:

    • Each muscle is encompassed by CT (epimysium)

    • Contains bundles of muscle fibers (fasciculus or fascicle) surrounded by CT (perimysium)

    • Individual muscle fibers surrounded by CT (endomysium)

Muscle Fiber Composition

  • Muscle Fiber Proteins:

    • Contains two types of proteins: Actin and Myosin

    • Actin filaments: Thin (light)

    • Myosin filaments: Thick (dark)

Muscle Fibers under Microscope

  • Observable structures include alternating light bands (isotropic, I bands) and dark bands (anisotropic, A bands)

    • I Bands: Contain only actin filaments, light in appearance

    • Z Line: A dark line bisecting the I band

    • A Band: Extends the entire length of myosin filaments

    • H Zone (H Band): Central zone containing only myosin filaments

    • M Line: Found in the middle of the myosin filaments

    • Sarcomere: The functional unit of myofibrils, extends from one Z line to the next

Muscle Action and Roles

  • Prime Mover (Agonist): The muscle responsible for the primary action

  • Synergist: A muscle that assists or supports the prime mover in action

  • Antagonist: A muscle that opposes the action of the prime mover

Mechanism of Muscle Contraction

  • Initiation:

    • Begins with stimulation by nerve cells, classified as motor neurons

    • Each neuron supplies a group of muscle fibers collectively referred to as a motor unit

    • The location where a motor neuron meets a muscle fiber is called the neuromuscular junction (NMJ), consisting of:

    • Synaptic Knob

    • Synaptic Cleft

    • Motor End Plate

    • Neuromuscular Junction:

    • Contains synaptic vesicles that house neurotransmitters, e.g., acetylcholine (ACh)

Nerve Stimulation Process

  • Upon stimulation, acetylcholine is released and binds to receptors on the sarcolemma (muscle fiber membrane)

  • The impulse propagates through the sarcolemma into T-tubules which triggers the release of calcium ions from the sarcoplasmic reticulum

  • Calcium ions change the configuration of binding sites on actin filaments and breakdown ATP

  • Cross Bridge Cycle:

    • Myosin heads attach to actin, forming cross bridges that rotate in a “power stroke,” sliding actin over myosin

    • Actin is anchored to the Z lines which move closer together as the sarcomere shortens (muscle contraction)

Eye Muscles - Extrinsic Eye Muscles Functions

  • Types of Eye Muscles and Functions:

    1. Lateral Rectus: Rotates eyeball outwards

    2. Medial Rectus: Rotates eyeball inwards

    3. Superior Rectus: Rotates eyeball upwards

    4. Inferior Rectus: Rotates eyeball downwards

    5. Inferior Oblique: Rotates eyeball upwards & outwards

    6. Superior Oblique: Rotates eyeball downwards & outwards
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