Muscle Tissue and Contraction Flashcards

Muscle Tissue Types

• Three types: skeletal, cardiac, and smooth.
  • Skeletal Muscle
    • Location: Attached to bones.
    • Microscopic Appearance: Striated (striped).
    • Control: Voluntary.
  • Cardiac Muscle
    • Location: Heart.
    • Microscopic Appearance: Striated, has intercalated discs.
    • Control: Involuntary.
  • Smooth Muscle
    • Location: Walls of hollow organs (e.g., digestive tract, blood vessels).
    • Microscopic Appearance: Non-striated.
    • Control: Involuntary.

Functions of Skeletal Muscle

• Movement: Produce skeletal movement.
• Posture: Maintain body posture and position.
• Support: Support soft tissues.
• Guard Openings: Guard entrances and exits of the body (sphincters).
• Maintain Body Temperature: Generate heat (thermogenesis).
• Nutrient Reserves: Store nutrient reserves.

Characteristics of Skeletal Muscle Fibers

• Excitability: Ability to respond to stimuli.
• Contractility: Ability to shorten and exert tension.
• Extensibility: Ability to be stretched.
• Elasticity: Ability to return to original length.

Skeletal Muscle Cell Structure

• Sarcolemma: The cell membrane of a muscle fiber.
• Sarcoplasm: The cytoplasm of a muscle fiber.
• Sarcoplasmic Reticulum (SR): Specialized endoplasmic reticulum that stores and releases calcium ions (Ca^{2+}).

Contents of Muscles

• Blood vessels: Provide oxygen and nutrients, remove waste.
• Nerves: Control muscle contraction.
• Connective tissues: Support and organize muscle fibers.

Names for Skeletal Muscle Cells

• Muscle fiber
• Myocyte

Organization of Skeletal Muscle Tissue

• Muscle: Organ composed of many muscle fibers.
• Fascicle: Bundle of muscle fibers.
• Muscle fiber (cell): Single muscle cell.
• Myofibril: Contractile unit within a muscle fiber.
• Myofilaments: Protein filaments (actin and myosin) that make up myofibrils.
• Connective Tissues:
  • Epimysium: Surrounds the entire muscle.
  • Perimysium: Surrounds fascicles.
  • Endomysium: Surrounds individual muscle fibers.

Fascicles

• Bundles of muscle fibers within a muscle.

Relationship of Myofiber, Myofibril, and Myofilaments

• Myofiber (muscle fiber) contains myofibrils.
• Myofibrils are made up of myofilaments.
• Myofilaments (actin and myosin) are the proteins responsible for muscle contraction.

Sarcomere

• The basic functional unit of a muscle fiber.
• Segment of a myofibril between two Z-discs.

Myofilaments

• Actin (Thin Filament)
  • Composed primarily of the protein actin.
  • Associated with troponin and tropomyosin.
• Myosin (Thick Filament)
  • Composed primarily of the protein myosin.
  • Has myosin heads that bind to actin.

Proteins Associated with the Sarcomere

• Troponin:
  • Binds to actin, tropomyosin, and calcium ions (Ca^{2+}).
  • Regulates muscle contraction by controlling the position of tropomyosin.
• Tropomyosin:
  • Covers the myosin-binding sites on actin when the muscle is at rest.
  • Prevents myosin from binding to actin.
• Titin:
  • Large protein that anchors myosin to the Z-disc.
  • Contributes to the elasticity of muscle.
  • Stabilizes the position of the thick filaments (myosin).

Striations in Muscle

• Striations: The striped appearance of skeletal and cardiac muscle due to the arrangement of actin and myosin.
• Skeletal and cardiac muscle are striated.

Regions of a Sarcomere

• Z-discs: Boundaries of the sarcomere.
• M-line: Center of the A band; helps anchor thick filaments.
• A band: Contains the entire length of the thick filaments (myosin).
• H zone: Contains only thick filaments (myosin).
• I bands: Contain only thin filaments (actin).
• Changes During Contraction:
  • I bands narrow.
  • H zone narrows.
  • A band remains the same width.
  • Z-discs move closer together.

Organization of T-Tubules and Sarcoplasmic Reticulum

• T-tubules: Transverse tubules that penetrate into the cell's interior to transmit the action potential.
• Sarcoplasmic Reticulum (SR): Stores and releases calcium ions (Ca^{2+}).
• Myofibrils: Contractile units within the muscle fiber; surrounded by T-tubules and SR.

Myosin Head

• Part of the myosin protein that binds to actin during muscle contraction.
• Contains ATPase, which hydrolyzes ATP to provide energy for contraction.

Role of Myosin Head in Muscle Contraction

• Binds to actin, forming cross-bridges.
• Pivots, pulling the actin filament toward the center of the sarcomere (power stroke).
• Detaches from actin and reattaches further along the actin filament, repeating the cycle.

Myosin Binding Sites

• Located on the actin filaments.
• Covered by tropomyosin when the muscle is at rest.

Tropomyosin

• A regulatory protein that covers the myosin-binding sites on actin when the muscle is at rest.
• Prevents myosin from binding to actin.

Sarcoplasmic Reticulum (SR)

• Specialized endoplasmic reticulum that stores and releases calcium ions (Ca^{2+}).
• Terminal Cisternae: Enlarged areas of the SR surrounding the T-tubules; store and release calcium.
• Significance: Calcium release from the SR triggers muscle contraction.

Troponin

• A regulatory protein that binds to actin, tropomyosin, and calcium ions (Ca^{2+}).
• When calcium binds to troponin, it changes shape, causing tropomyosin to move away from the myosin-binding sites on actin.
• This allows myosin to bind to actin and initiate muscle contraction.

Synapse

• Synapse: Junction between two neurons, or between a neuron and a muscle cell or gland.
• Neuromuscular Junction: Synapse between a motor neuron and a muscle cell.
• Synapse with a Gland: Neuroglandular junction

Neuromuscular Junction

• Synapse between a motor neuron and a muscle fiber.
• Equivalent terms: Motor end plate, myoneural junction.

Events at the Neuromuscular Junction

1. Action potential arrives at the axon terminal of the motor neuron.
2. Voltage-gated calcium channels open, and calcium (Ca^{2+}) enters the axon terminal.
3. Calcium entry causes synaptic vesicles to release acetylcholine (ACh) via exocytosis.
4. Acetylcholine diffuses across the synaptic cleft and binds to ACh receptors on the sarcolemma.
5. ACh binding opens ligand-gated ion channels, allowing sodium (Na^{+}$) to enter the muscle fiber and potassium (K^{+}$) to exit.
6. The influx of sodium (Na^{+}$) causes depolarization of the sarcolemma, leading to an action potential in the muscle fiber.

Membrane Potential Changes

• Resting membrane potential: -70mV.
• Depolarization: Influx of sodium (Na^{+}$) makes the membrane potential more positive.
• Repolarization: Outflow of potassium (K^{+}$) restores the negative membrane potential.

Excitation-Contraction Coupling

• The sequence of events by which an action potential in the sarcolemma leads to muscle contraction.

Travel of Action Potential

• The action potential travels along the sarcolemma and into the T-tubules.

T-Tubules

• Transverse tubules that penetrate into the cell's interior.
• Function: Transmit the action potential deep into the muscle fiber, ensuring that all myofibrils contract simultaneously.

Sarcoplasmic Reticulum (SR) Role in Muscle Contraction

• Stores and releases calcium ions (Ca^{2+}).
• Action potential triggers the release of calcium from the terminal cisternae of the SR.

Calcium Release and Muscle Contraction

• Calcium ions (Ca^{2+}) bind to troponin, causing it to change shape.
• This moves tropomyosin away from the myosin-binding sites on actin.
• Myosin heads can now bind to actin, initiating muscle contraction.

Steps of Skeletal Muscle Contraction

1. Cross-bridge formation: Myosin head attaches to the actin filament.
2. The power (working) stroke: The myosin head pivots and pulls the actin filament toward the M line.
3. Cross-bridge detachment: ATP binds to the myosin head, causing the cross-bridge to detach.
4. Cocking of the myosin head: Hydrolysis of ATP cocks the myosin head back to its high-energy configuration.
5. Calcium binding: Calcium ions (Ca^{2+}) bind to troponin, initiating the cycle.

Factors Determining Duration of Muscle Contraction

1. Frequency of stimulation.
2. Calcium ion (Ca^{2+}) availability.
3. ATP availability.

Acetylcholine Removal from Synaptic Cleft

• Enzymatic degradation: Acetylcholinesterase (AChE) breaks down ACh.
• Reuptake: ACh is transported back into the axon terminal.

Regulation of Calcium Levels in Resting Muscle

• Calcium is actively transported back into the sarcoplasmic reticulum (SR) by the SERCA pump (Sarcoplasmic/endoplasmic reticulum calcium ATPase).

ATP-Dependent Processes in Muscle Contraction and Relaxation

• Muscle contraction: ATP is required for the myosin head to detach from actin and recock.
• Muscle relaxation: ATP is required for the transport of calcium ions (Ca^{2+}$$) back into the sarcoplasmic reticulum (SERCA pump).

Skeletal Muscle Motor Unit

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

Motor Units for Fine vs. Gross Motor Control

• Fine control: Small motor units (few muscle fibers per neuron).
• Gross motor skills: Large motor units (many muscle fibers per neuron).

Innervation of Muscle Fibers

• A single skeletal muscle fiber is innervated by only one motor neuron.

Ways to Increase Muscle Tension

1. Increase the frequency of stimulation (wave summation).
2. Recruit more motor units (motor unit summation).
3. Increase the size of muscle fibers (hypertrophy).

Types of Muscle Fibers

• Slow Oxidative (Type I)
  • High myoglobin content (dark meat).
  • High endurance, fatigue resistant.
  • Use aerobic metabolism.
• Fast Oxidative (Type IIa)
  • Intermediate myoglobin content.
  • Moderately fatigue resistant.
  • Use both aerobic and anaerobic metabolism.
• Fast Glycolytic (Type IIb or IIx)
  • Low myoglobin content (white meat).
  • Fatigue quickly.
  • Use anaerobic metabolism.

Dark Meat

• Predominantly slow oxidative (Type I) fibers.
• Darker color due to higher myoglobin content, which binds oxygen.

Twitch

• A single, brief contraction of a muscle fiber in response to a single action potential.

Phases of a Muscle Twitch

• Latent period: Time between stimulation and the start of contraction; excitation-contraction coupling is occurring.
• Period of contraction: Muscle fibers shorten; cross-bridges form and cycle.
• Period of relaxation: Muscle tension declines; calcium is transported back into the SR, and cross-bridges detach.

Wave Summation

• Increasing the frequency of stimulation, so that the muscle does not fully relax between stimuli.
• This increases overall muscle tension.

Tetanus

• Incomplete/Unfused Tetanus: Muscle fibers partially relax between stimuli.
• Complete/Fused Tetanus: Muscle fibers do not relax between stimuli; generates more tension than unfused tetanus.

Rigor Mortis

• Stiffness of muscles that occurs after death.
• Cause: Lack of ATP prevents myosin from detaching from actin.

Skeletal Muscle Tone

• A state of sustained, partial contraction of muscles.
• Function: Maintains posture and readiness for action.

Sarcomere Length and Muscle Tension

• Optimal sarcomere length: Maximizes the number of cross-bridges that can form, resulting in maximal tension.
• Excessively shortened or stretched sarcomeres: Reduce tension because fewer cross-bridges can form.

Isotonic vs. Isometric Contraction

• Isotonic contraction: Muscle changes length while tension remains constant.
  • iso- = same
  • -tonic = tension
• Isometric contraction: Muscle does not change length while tension increases.
  • iso- = same
  • -metric = length

Types of Isotonic Contractions

• Concentric contraction: Muscle shortens while generating force (e.g., lifting a weight).
• Eccentric contraction: Muscle lengthens while generating force (e.g., lowering a weight).