Untitled Quiz

Human Physiology BIOL3205 - Muscle System

• Course instructor: Prof. Chi Bun Chan
• Contact information: [email protected], 39173822
• Location: 5N10 Kadoorie Biological Sciences Building

Lecture Outline

• Muscle types and their structural characteristics
• Mechanism of muscle contraction
• Muscle contraction mechanics
• Control of muscle contractions
• Muscle metabolism and fatigue
• Muscle diseases

Classification of Muscles

• Structural: Striated vs. smooth (unstriated)
• Control Methods: Voluntary or involuntary
• Locations: Skeletal muscle, heart (cardiac), hollow organs (e.g., intestinal smooth)
• Metabolic Classification: Fast-twitch and slow-twitch muscle

Characteristics of Muscle Types

• Skeletal Muscle: Striated, voluntary, multinucleated, long, cylindrical cells. Located attached to bones. Function: body movement.
• Cardiac Muscle: Striated, involuntary, uninucleated, branching cells. Located in the heart. Function: pumping blood.
• Smooth Muscle: Unstriated, involuntary, uninucleated, spindle-shaped cells. Located in hollow organs. Function: movement of contents within organs.

Properties of Muscle

• Excitability: Responds to stimuli, generating action potentials.
• Contractility: Shortens in response to stimulation.
• Extensibility: Stretches without damage.
• Elasticity: Returns to original length after stretching.

Muscular System Functions

• Gesture maintenance
• Locomotion (movement)
• Organ movement (e.g., eye movement)
• Propulsion of contents through hollow organs
• Breathing
• Accomplishing work (pushing, pulling)

Shapes of Muscle

• Fusiform, parallel, convergent, unipennate, bipennate, multipennate, circular

Skeletal Muscle Structure

• Connective Tissues: Epimysium, perimysium, endomysium
• Blood Vessels/Nerves: Supply muscle fibers with nutrients and impulses.
• Muscle Fibers: Bundles of myofibrils.
• Myofibrils: Protein structures causing muscle contractions.
• Sarcomere: Functional unit of myofibrils.

Myofibril Structure

• Thick Filaments: Composed primarily of myosin.
• Thin Filaments: Primarily composed of actin.
• Sarcomere: The repeating unit with A-band, I-band, Z-line, M-line.

Microfilaments Organization

• Thick Filaments: Myosin molecules with heads for cross-bridges.
• Thin Filaments: Actin filaments, plus tropomyosin and troponin involved in Ca2+ and myosin interaction.
• Cross-bridges form between thick and thin filaments initiating muscle contractions.

Sliding Filament Mechanism

• Muscle contraction occurs by thin filaments sliding over thick filaments.
• Sarcomere shortening results in muscle contraction.

Mechanisms of Skeletal Muscle Contraction

• Alpha Motor Neurons: Nerves controlling skeletal muscle.
• Motor Unit: Motor neuron and all the muscle fibers it stimulates.
• Neuromuscular Junction (NMJ): Where motor neuron communicates with muscle fiber.
• Excitation-Contraction Coupling: The sequence turning nerve signal into muscle contraction.

Mechanism of Skeletal Muscle Contraction—Cross-bridge Cycle

• Attachment: Myosin head attaches to actin.
• Power Stroke: Myosin head bends, moving actin.
• Detachment: ATP binds, releasing myosin from actin.
• Energization: ATP hydrolysis "resets" the myosin head.

Rigor Mortis

• Postmortem muscle stiffening due to ATP depletion.
• Muscles remain contracted.

Type of Skeletal Muscle Contraction

• Isometric: Muscle tension increases, but muscle length stays the same.
• Isotonic: Muscle length changes, but tension remains constant.
• Concentric: Muscle shortens when tension exceeds load.
• Eccentric: Muscle lengthens when load exceeds tension.
• Cross-bridge cycle occurs in all type of skeletal muscle contractions.

Sarcomere Length-Tension Relationship

• Optimal sarcomere length provides maximum tension based of overlap and cross-bridge formation between actin and myosin.
• Decreased Tension: If length deviates from optimal, tension decreases.

Control of Muscle Tension

• Motor Unit Recruitment: Increasing the number of motor units being stimulated to increase muscle tension. Weak contraction -> use less motor units. Stronger contraction -> recruit more motor units.
• Number of Myofibers per Motor Unit: Eyes need fine coordinated movements which require small motor units. Legs use larger motor units to perform larger movements.

Muscle Twitch

• Mechanical response of a muscle fiber to a single action potential.
• Phases: Latent period, contraction period, relaxation period
• Fast-twitch vs. slow-twitch fibers: Different myosin types, ATPase (actin-myosin interactions) activity and metabolic properties.
• Factors affecting twitch duration include myosin type, cytosolic calcium concentration.

Frequency-Tension Relation

• Twitch Summation: Increasing tension by stimulating muscle fiber before complete relaxation.
• Tetanus: Continued, sustained contraction caused by rapid stimulation of fiber
  • Frequency-tension relationship: Rate of stimulation -> sustained muscle contraction -> muscle fatigue.

Muscle Fatigue

• Inability to maintain muscle tension.
• Depletion of ATP, leakage of Ca+ in the sarcoplasmic reticulum, centrally induced (in the CNS)
• Coordinated asynchronous recruitment of motor units and recruitment of metabolically different fibers prevent fatigue for prolonged periods.

Energy Source During Exercise

• ATP and creatine phosphate (short bursts)
• Anaerobic glycolysis (medium duration)
• Aerobic oxidation (long duration)

Muscle Fiber Types

• Slow-oxidative: Myoglobin rich, slow speed of contraction, high fatigue resistance.
• Fast-oxidative: Intermediate speed of contraction, intermediate fatigue resistance
• Fast-glycolytic: Fast speed of contraction, low fatigue resistance.

Coordination of Muscle Type Usage

• Most muscles have a mixture of fiber types.
• Different muscle groups (e.g., erector spinae vs. biceps brachii) have different proportions of fiber types.
• Fiber type composition can potentially depend on genetics or adaptation.

Muscle Cramping

• Involuntary muscle contractions, often caused by electrolyte imbalances or neuromuscular issues.

Tetanus Infection

• Caused by bacterial toxin affecting inhibitory neurons.
• Results in muscle stiffness, breathing problems, and spasms.

Skeletal Muscle and Smooth Muscle Differences

• Cell Shape: Skeletal = cylindrical, smooth = spindle-shaped.
• Nucleus: Skeletal = multinucleated, smooth = single nucleus.
• Functional Units: Skeletal is sarcomeres; smooth is not.
• Microfilament Arrangement: Skeletal is parallel bundles; smooth are often diagonally arranged.
• Contractile Response: Skeletal is quick; smooth is slower.
• Neuronal Control: Skeletal via NMJ; smooth is diffuse via neurotransmitters

Neuronal control of smooth muscle

• Contractile activity controlled by neurotransmitters from autonomic neurons.
• Varicosities are swollen regions of the autonomic neurons that release neurotransmitters.
• Same neurotransmitter may cause different responses depending on the specific receptor and tissue.

Types of Smooth Muscles

• Single-unit smooth muscle - cells contract as a single unit coordinated via gap junctions
• Multiunit smooth muscle- each cell contracts autonomously and is controlled by its own nerve supply