Review of Muscle Characteristics

Key Properties of Muscle Tissue

• Excitability: The ability of muscle cells to receive and respond to stimuli, typically from nerve impulses.

• Contractility: The capacity of muscle fibers to shorten and thicken upon stimulation, generating force.

• Extensibility: Muscle fibers can be stretched or extended beyond their resting length.

• Elasticity: After contraction or extension, muscle fibers can return to their original shape.

Functions of Muscle

Major Roles of Muscle Tissue

• Motion: Muscles produce movement of the body and its parts.

• Movement of Substances: Muscles move substances through hollow organs (e.g., food through the GI tract, blood via the heart).

• Posture: Muscles maintain body position, such as the 'muscle tone' that keeps us upright.

• Joint Stabilization: Muscles help stabilize joints.

• Heat Production: Muscle contraction generates heat, which helps maintain body temperature.

Skeletal Muscle

Structure and Organization

• Organ of the Muscular System: Skeletal muscle is a major organ composed of muscle tissue, blood vessels, nerves, and connective tissue.

• Characteristics: Skeletal muscle is multinucleated, striated, and under voluntary control.

• Attachment: Muscles attach to bones via tendons, allowing movement at joints.

• Origin: The fixed attachment point of the muscle.

• Insertion: The movable attachment point; contraction pulls the insertion toward the origin.

Development of Muscle

Formation and Growth

• Myoblasts: Embryonic muscle cells that fuse to form multinucleated myotubes, which mature into muscle fibers.

• Satellite Cells: Stem cells that aid in muscle growth and repair.

• Hypertrophy: Increase in muscle fiber size due to exercise or growth.

• Atrophy: Decrease in muscle fiber size due to inactivity or disease.

Structure of Skeletal Myocytes (Muscle Fibers)

Microscopic Anatomy

• Sarcolemma: The plasma membrane of a muscle fiber.

• Sarcoplasm: The cytoplasm of a muscle fiber, rich in mitochondria.

• Sarcoplasmic Reticulum: Specialized endoplasmic reticulum that stores calcium ions ().

• Transverse (T) Tubules: Invaginations of the sarcolemma that conduct impulses deep into the muscle fiber.

• Myofibrils: Cylindrical structures within muscle fibers composed of repeating units called sarcomeres.

Connective Tissue Components

Organization and Support

• Tendon: Dense regular connective tissue connecting muscle to bone.

• Aponeurosis: Broad, flat sheet of connective tissue attaching muscles to other structures.

• Epimysium: Outer layer of connective tissue surrounding the entire muscle.

• Perimysium: Connective tissue surrounding bundles of muscle fibers (fascicles).

• Endomysium: Thin connective tissue layer surrounding individual muscle fibers.

Myofibril Structure

Sarcomere Organization

• Myofilaments: The contractile proteins within myofibrils, including actin (thin filaments) and myosin (thick filaments).

• Z line: Defines the boundary of each sarcomere.

• A band: Dark region containing thick filaments (myosin).

• I band: Light region containing thin filaments (actin).

• H zone: Central region of the A band where only myosin is present.

• M line: Center of the sarcomere, anchoring myosin filaments.

Muscle Proteins

Contractile and Regulatory Proteins

• Actin: Main protein of thin filaments; contains myosin binding sites.

• Myosin: Main protein of thick filaments; has ATPase activity and binding sites for actin.

• Tropomyosin: Blocks myosin binding sites on actin in resting muscle.

• Troponin: Binds calcium ions (), causing tropomyosin to move and expose myosin binding sites.

• Titin: Elastic protein that helps muscle recoil after contraction.

• Dystrophin: Links cytoskeleton of muscle fiber to extracellular matrix; mutations cause Duchenne Muscular Dystrophy.

Neuromuscular Junction

Communication Between Nerve and Muscle

• Motor Neuron: Transmits impulses from the central nervous system to muscle fibers.

• Motor End Plate: Specialized region of the sarcolemma at the neuromuscular junction.

• Synaptic Cleft: Space between the motor neuron and muscle fiber where neurotransmitters are released.

• Acetylcholine (ACh): The neurotransmitter that initiates muscle contraction.

Excitation Coupling / Sliding Filament Mechanism

Steps of Muscle Contraction

1. Nerve impulse triggers release of ACh at the neuromuscular junction.

2. ACh binds to receptors on the sarcolemma, generating an action potential.

3. Action potential travels along the sarcolemma and down T-tubules.

4. Sarcoplasmic reticulum releases into the sarcoplasm.

5. binds to troponin, causing tropomyosin to move and expose myosin binding sites on actin.

6. Myosin heads bind to actin, forming cross-bridges and initiating contraction.

7. ATP is required for myosin to detach from actin and reset for another contraction cycle.

Equation:

Energy for Contraction

ATP and Creatine Phosphate

• ATP: Immediate source of energy for muscle contraction; must be continually regenerated.

• Creatine Phosphate (CP): Stores high-energy phosphate groups; regenerates ATP during short bursts of activity.

Reaction during relaxation:

Reaction during contraction:

Clinical Application: Rigor Mortis

Postmortem Muscle Stiffness

• Occurs when ATP is depleted after death, preventing muscle relaxation.

• Muscles remain contracted until proteins degrade.

Review of the Basics of Cellular Respiration

Energy Production in Muscle

• Aerobic Respiration: Requires oxygen; produces most ATP per glucose molecule.

• Anaerobic Respiration: Does not require oxygen; produces less ATP and generates lactic acid.

• Equation for Aerobic Respiration:

• Equation for Anaerobic Respiration:

Source of Oxygen

Hemoglobin and Myoglobin

• Hemoglobin: Protein in red blood cells that carries oxygen from lungs to tissues.

• Myoglobin: Oxygen-binding protein in muscle cells; stores oxygen for use during contraction.

Consequences of Oxygen Depletion

Effects on Muscle Function

• Aerobic respiration cannot proceed; muscle relies on anaerobic pathways.

• Lactic acid accumulates, lowering pH and causing muscle fatigue.

• Lactic acid can be transported to the liver and converted to glucose via gluconeogenesis.

Muscle Fatigue

Causes and Effects

• Loss of ability to contract due to depletion of ATP, accumulation of lactic acid, and disturbances in availability.

• Electrolyte imbalances, especially and , contribute to fatigue.

Cramps

Spasmodic Contractions

• Involuntary, sustained muscle contractions often due to lack of ATP or ion imbalances.

• Commonly known as "charley horse".

Types of Muscle Contractions

Isotonic vs. Isometric

• Isotonic: Muscle changes length, producing movement (e.g., lifting a weight).

• Isometric: Muscle does not change length; tension increases (e.g., holding a book steady).

• Eccentric: Muscle lengthens while contracting (e.g., lowering a weight).

• Concentric: Muscle shortens while contracting (e.g., picking up a weight).

All or None Principle

Muscle Fiber Response

• When stimulated above threshold, a muscle fiber contracts to its fullest extent.

• Entire muscle may show varying degrees of contraction depending on the number of fibers activated.

Motor Units

Organization and Control

• Motor Unit: A motor neuron and all the muscle fibers it innervates.

• Fine control muscles (e.g., eye muscles) have fewer fibers per neuron; gross control muscles (e.g., postural muscles) have more fibers per neuron.

Myograms

Recording Muscle Contraction

• Graphical representation of muscle contraction over time.

• Twitch: Single, brief contraction in response to a stimulus.

• Latent Period: Time between stimulus and contraction onset.

• Contraction Period: Time during which muscle tension increases.

• Relaxation Period: Time during which muscle tension decreases and muscle returns to resting state.

Effects of Frequency of Contraction

Summation and Tetanus

• Wave Summation: Increased frequency of stimulation leads to greater contraction force.

• Treppe (Staircase Effect): Gradual increase in contraction strength with repeated stimulation.

• Tetanus: Sustained contraction due to rapid, repeated stimulation.

• Incomplete Tetanus: Partial relaxation between contractions.

• Complete Tetanus: No relaxation between contractions; maximal sustained contraction.

Skeletal Muscle Fiber Types

Classification and Properties

Muscle Disorders and Myopathies

Common Conditions

• Muscular Dystrophy: Genetic disorder causing progressive muscle weakness due to defective dystrophin.

• Myopathies: Diseases affecting muscle tissue, leading to weakness or dysfunction.

• Charley Horse: Sudden, involuntary muscle cramp.

Additional info: Some explanations and definitions have been expanded for clarity and completeness. Equations and tables have been formatted for academic study purposes.