EMG and Muscle Contraction

LAB 7: EMG AND MUSCLE CONTRACTION

TYPES OF MUSCLE TISSUE

  • Cardiac Muscle

    • Features:

    • 1-2 nuclei in fibers.

    • Fibers are physically and electrically connected to each other.

    • Striated appearance.

    • Involuntary control.

  • Smooth Muscle

    • Features:

    • Single nucleus in fibers.

    • Non-striated appearance.

    • Involuntary control.

  • Skeletal Muscle

    • Features:

    • Multinucleated fibers.

    • Striated appearance.

    • Voluntary control.

HISTOLOGY OF SKELETAL MUSCLE

  • Longitudinal Section of Skeletal Muscle at 400x

    • Distinct striations should be visible.

  • Transverse Section of Skeletal Muscle at 400x

    • Peripheral Nuclei:

    • Numerous peripheral nuclei are visible (indicated by arrows).

    • Skeletal muscle cells possess hundreds of nuclei squeezed between the outer cell membrane (sarcolemma) and myofibrils (bundles of muscle cell proteins).

    • Example tissue from the tongue shows skeletal muscle cells cut in longitudinal and cross-section.

    • Peripheral location of nuclei is evident in cross-sectioned cells.

SKELETAL MUSCLE FIBER CHARACTERISTICS

  • Unique Properties of Skeletal Muscle Cells (Fibers):

    • Size: Much larger than other body cells.

    • Nuclei: Multinucleate (typically hundreds of nuclei).

    • Appearance: Striated (banded).

    • Components:

    • Sarcolemma = plasma membrane.

    • Sarcoplasm = cytoplasm containing myofibrils.

MYOFIBRIL FUNCTION

  • Role in Muscle Contraction:

    • Myofibrils cause muscle contraction through the process of shortening.

    • Structure: Myofibrils consist of bundles of protein filaments called myofilaments, organized into functional units known as sarcomeres.

SARCOMERE STRUCTURE

  • Definition:

    • A sarcomere is defined as the region between one Z-line and the next Z-line.

    • Components:

    • Thin filaments (actin) occupy the I band.

    • Thick filaments (myosin) occupy the A band.

    • Functional Unit:

    • The sarcomere is the functional unit of a skeletal muscle fiber.

SLIDING FILAMENT THEORY

  • Mechanism of Contraction:

    • During a contraction:

    • The A band remains the same width.

    • Z-lines move closer together.

    • I band decreases in width.

    • When myofibril ends are free to move, the sarcomeres shorten together, pulling towards the center of the myofibril.

MOTOR UNIT

  • Definition:

    • A motor unit consists of a motor neuron and the muscle fibers it innervates.

    • Muscle fibers are dispersed throughout the muscle.

    • Recruitment Process:

    • Small motor units are recruited first in response to weaker stimuli.

    • The force of muscle contraction is determined by the number of motor units activated and the force generated by individual muscle fibers.

EXCITATION-CONTRACTION COUPLING

  • Process Overview:

    • Excitation:

    • Generation of an action potential in the sarcolemma by the motor neuron.

    • Contraction Triggering:

    • Calcium release from the sarcoplasmic reticulum is triggered by the action potential, leading to muscle contraction.

CALCIUM AND MUSCLE CONTRACTION

  • **Calcium Role:

    • Calcium ions bind to troponin, causing a conformational change in troponin.

    • This change moves tropomyosin away from active sites on thin filaments (actin).

    • Myosin heads can then bind to these active sites, forming cross-bridges.

CROSS-BRIDGE CYCLE

  • Key Events in the Cycle:

    • Cross-Bridge Formation:

    • Myosin heads bind to actin to form a cross-bridge.

    • Power Stroke:

    • The splitting of ATP causes the myosin head to change shape and pull against actin.

    • Cross-Bridge Detachment:

    • A new ATP molecule binds to myosin, causing it to detach from actin.

    • Myosin Reactivation:

    • ATP is hydrolyzed, resetting the myosin to its high-energy state for another cycle.

MUSCLE TWITCH PHASES

  • Phases Defined:

    • Latent Period:

    • Occurs between the action potential in the muscle fiber and the onset of contraction.

    • Contraction Period:

    • Begins when the muscle generates force and continues until peak tension is reached.

    • Relaxation Period:

    • The longest phase during which Ca extsuperscript{2+} reuptake into the sarcoplasmic reticulum (ATP-dependent) exceeds its release.

    • Tension diminishes until the fiber returns to its resting state.

SUMMATION AND TETANUS

  • Concept of Recruitment:

    • Recruitment increases due to frequency of stimulation, known as summation.

    • If a second stimulus is applied before relaxation from the first, greater force production occurs due to residual calcium ions in the cytoplasm.

    • Incomplete Tetanus:

    • Individual twitches are observable, but tension is higher than a single twitch with frequent stimulation.

    • Complete Tetanus:

    • No relaxation occurs, leading to maximum tension.

TYPES OF MUSCLE FIBERS

  • Fast Glycolytic Fibers:

    • Last to be recruited.

    • Break down glucose anaerobically.

    • Generate the most force but fatigue quickly.

    • Fast cycling due to fast ATPase activity.

  • Fast Oxidative Fibers:

    • Second to be recruited.

    • Use oxygen to aerobically break down glucose, generating more ATP.

    • Most resistant to fatigue.

    • Fast cycling due to fast ATPase activity.

  • Slow Oxidative Fibers:

    • First to be recruited.

    • Use oxygen to aerobically break down glucose, generating more ATP.

    • Most enduring fibers, but generate the least force.

    • Slow cycling due to slow ATPase activity.

MUSCLE FATIGUE

  • Definition:

    • A decrease in tension production following frequent stimulation.

  • Potential Causes Include:

    • Depletion of calcium.

    • Depletion of ATP.

    • Depletion of electrolytes (e.g., sodium).

    • Loss of neuronal signaling.

    • Build-up of metabolites disrupting signaling.

  • High-Intensity Activities:

    • Recruit fast glycolytic fibers, which rely on glycolysis for ATP, producing lactic acid that alters pH and disrupts contractile function.

  • Neuromuscular Fatigue:

    • Occurs when somatic motor neurons deplete their neurotransmitter supply due to intense exercise.

  • Low-Intensity, Long-Duration Exercises:

    • Primarily fatigue slow oxidative and fast oxidative fibers due to fuel source depletion.

ELECTROMYOGRAPHY (EMG)

  • Definition:

    • EMG measures electrical activity of muscles.

    • A healthy muscle will exhibit no electrical activity during rest.

  • Contractile Response:

    • As muscle contraction increases in force, more muscle fibers are activated, resulting in increased action potentials.

    • The change in mV (ΔV) increases with increased motor unit activation.

MUSCLE FATIGUE AND CENTRAL DRIVE

  • Definition of Central Drive:

    • Central drive refers to the neural signal from the brain (central nervous system) to the muscles.

  • Counteracting Fatigue:

    • The brain can increase central drive through:

    • Increased Motor Unit Recruitment: Activating more motor units to generate increased force.

    • Increased Firing Rate: More frequent signals sent to the muscles from the brain.