Human Anatomy and Physiology: Muscles and Muscle Tissue

Overview of Muscle Tissue

• Nearly half of body’s mass.
• Transforms chemical energy (ATP) into mechanical energy.
• Terminologies: Myo, mys, and sarco are prefixes for muscle (e.g., sarcoplasm).
• Three types: skeletal, cardiac, smooth.
• Only skeletal and smooth muscle cells are elongated and referred to as muscle fibers.

Types of Muscle Tissue

• Skeletal:
  • Attached to bones and skin.
  • Voluntary (consciously controlled).
  • Contracts rapidly, tires easily, powerful.
• Cardiac:
  • Found only in the heart.
  • Involuntary (cannot be consciously controlled).
• Smooth:
  • Found in walls of hollow organs.
  • Involuntary (cannot be consciously controlled).

Characteristics of Muscle Tissue

• Excitability (responsiveness): ability to receive and respond to stimuli.
• Contractility: ability to shorten forcibly when stimulated.
• Extensibility: ability to be stretched.
• Elasticity: ability to recoil to resting length.

Muscle Functions

• Produce movement: locomotion and manipulation (e.g., walking, digesting, pumping blood).
• Maintain posture and body position.
• Stabilize joints.
• Generate heat as they contract.

Skeletal Muscle Anatomy

• Organ made of different tissues with: nerve and blood supply, connective tissue sheaths, and attachments.
• Nerve and Blood Supply:
  • Each muscle receives a nerve, artery, and veins.
  • Requires oxygen and nutrients; waste removal.
• Connective Tissue Sheaths (external to internal):
  • Epimysium: surrounds entire muscle.
  • Perimysium: surrounds fascicles (groups of muscle fibers).
  • Endomysium: surrounds each muscle fiber.
• Attachments:
  • Span joints and attach to bones via insertion (movable bone) and origin (immovable bone).
  • Direct (fleshy): epimysium fused to periosteum/perichondrium.
  • Indirect: connective tissue extends as tendon or aponeurosis.

Muscle Fiber Microanatomy

• Skeletal muscle fibers are long, cylindrical cells with multiple nuclei.
• Sarcolemma: muscle fiber plasma membrane.
• Sarcoplasm: muscle fiber cytoplasm; contains glycosomes and myoglobin.
• Modified organelles: myofibrils, sarcoplasmic reticulum, T tubules.

Myofibrils

• Densely packed, rodlike elements (~80% of muscle cell volume).
• Features:
  • Striations: A bands (dark) with H zone and M line, I bands (light) with Z disc.
  • Sarcomere: Smallest contractile unit; area between Z discs.
  • Myofilaments: actin (thin) and myosin (thick).

Myofilaments

• Actin myofilaments:
  • Extend across I band and partway in A band.
  • Anchored to Z discs.
• Myosin myofilaments:
  • Extend length of A band.
  • Connected at M line
• Sarcomere cross section: hexagonal arrangement of one thick filament surrounded by six thin filaments

Sarcoplasmic Reticulum and T Tubules

• Sarcoplasmic reticulum (SR): network of smooth endoplasmic reticulum tubules.
  • Stores and releases Ca^{2+}.
  • Terminal cisterns at A–I band junction.
• T tubules:
  • Protrusions of sarcolemma into cell interior.
  • Triad: terminal cistern, T tubule, terminal cistern.

Sliding Filament Model of Contraction

• Contraction: activation of cross bridges to generate force.
• Thin filaments slide past thick filaments, increasing overlap.
• Neither thick nor thin filaments change length.
• Myosin heads bind to actin, forming cross bridges.
• Z discs are pulled toward M line.
• I bands shorten.
• H zones disappear.
• A bands move closer.

Muscle Fiber Contraction Overview

• Action potential (AP) travels from brain to motor neurons to muscle fibers.
• Neurons and muscle cells are excitable and capable of action potentials.
• AP crosses from neuron to muscle cell via acetylcholine (ACh).
  • Chemically gated ion channels: opened by chemical messengers such as neurotransmitters (e.g., ACh receptors).
  • Voltage-gated ion channels: open or close in response to voltage changes.
• Axon terminal and muscle fiber separated by synaptic cleft.
• Synaptic vesicles contain acetylcholine (ACh).
• Infoldings of sarcolemma, called junctional folds, contain millions of ACh receptors.
• NMJ consists of axon terminals, synaptic cleft, and junctional folds.

Events at the Neuromuscular Junction

• AP arrives at axon terminal.
• Voltage-gated calcium channels open; calcium enters motor neuron.
• Calcium entry releases ACh into synaptic cleft.
• ACh diffuses to ACh receptors (Na+ chemical gates) on sarcolemma.
• ACh binding opens gates, allowing Na+ to enter, resulting in end plate potential.
• Acetylcholinesterase degrades ACh.

Action Potential Across the Sarcolemma

• Resting sarcolemma is polarized (inside negative).
• Action potential is caused by changes in electrical charges.
• Occurs in three steps:
  • Generation of end plate potential.
  • Depolarization.
  • Repolarization.
• End plate potential:
  • ACh released from motor neuron binds to ACh receptors on sarcolemma.
• Causes chemically gated ion channels (ligands) on sarcolemma to open.
  • Na+ diffuses into muscle fiber.
• Some K+ diffuses outward, but not much.
• Because Na+ diffuses in, interior of sarcolemma becomes less negative (more positive).
• Results in local depolarization called end plate potential.
• Depolarization:
  • Generation and propagation of an action potential (AP).
  • If end plate potential causes enough change in membrane voltage to reach critical level called threshold, voltage-gated Na+ channels in membrane will open.
  • Large influx of Na+ through channels into cell triggers AP that is unstoppable and will lead to muscle fiber contraction.
  • AP spreads across sarcolemma from one voltage-gated Na+ channel to next one in adjacent areas, causing that area to depolarize
• Repolarization:
  • Na+ voltage-gated channels close, and voltage-gated K+ channels open.
  • K+ efflux out of cell rapidly brings cell back to initial resting membrane voltage.
  • Refractory period: muscle fiber cannot be stimulated for a specific amount of time until repolarization is complete.

Excitation-Contraction (E-C) Coupling

• Sequence of events by which transmission of an action potential along the sarcolemma leads to the sliding of myofilaments

Muscle Fiber Contraction: Cross Bridge Cycling

• At low intracellular Ca^{2+} concentration:
  • Tropomyosin blocks active sites on actin.
  • Myosin heads cannot attach to actin.
  • Muscle fiber remains relaxed
• Voltage-sensitive proteins in T tubules change shape, causing sarcoplasmic reticulum (SR) to release Ca^{2+} to cytosol
• At higher intracellular Ca^{2+} concentrations, Ca^{2+} binds to troponin
  • Troponin changes shape and moves tropomyosin away from myosin-binding sites.
  • Myosin heads is then allowed to bind to actin, forming cross bridge
  • Cycling is initiated, causing sarcomere shortening and muscle contraction
  • When nervous stimulation ceases, Ca^{2+} is pumped back into SR, and contraction ends
• Four steps of the cross bridge cycle
  • Cross bridge formation: high-energy myosin head attaches to actin thin filament active site
  • Working (power) stroke: myosin head pivots and pulls thin filament toward M line
  • Cross bridge detachment: ATP attaches to myosin head, causing cross bridge to detach
  • Cocking of myosin head: energy from hydrolysis of ATP “cocks” myosin head into high-energy state
    • This energy will be used for power stroke in next cross bridge cycle

Graded Muscle Response

• Muscle response to changes in stimulus strength.
• Recruitment (or multiple motor unit summation): stimulus is sent to more muscle fibers, leading to more precise control
  • Subthreshold stimulus: stimulus not strong enough, so no contractions seen
  • Threshold stimulus: stimulus is strong enough to cause first observable contraction
  • Maximal stimulus: strongest stimulus that increases maximum contractile force
• All motor units have been recruited
• Recruitment works on size principle
  • Motor units with smallest muscle fibers are recruited first
  • Motor units with larger and larger fibers are recruited as stimulus intensity increases
  • Largest motor units are activated only for most powerful contractions
  • Motor units in muscle usually contract asynchronously

Muscle Tone

• Constant, slightly contracted state of all muscles.
• Due to spinal reflexes.
• Keeps muscles firm, healthy, and ready to respond.

Isotonic and Isometric Contractions

• Isotonic contractions: muscle changes in length and moves load
  • Concentric contractions: muscle shortens and does work
• Example: biceps contract to pick up a book
  • Eccentric contractions: muscle lengthens and generates force
• Example: laying a book down causes biceps to lengthen while generating a force
• Isometric contractions
• Load is greater than the maximum tension muscle can generate, so muscle neither shortens nor lengthens

Energy for Contraction and ATP

• ATP supplies the energy needed for the muscle fiber to:
  • Move and detach cross bridges
  • Pump calcium back into SR
  • Pump Na+ out of and K+ back into cell after excitation-contraction coupling
• Anaerobic pathway: Glycolysis and lactic acid formation
  • Yields only 5% as much ATP as aerobic respiration, but produces ATP 2½ times faster

Muscle Fatigue

• Fatigue is the physiological inability to contract despite continued stimulation
• Possible causes include:
  • Ionic imbalances can cause fatigue
• Levels of K+, Na+ and Ca2+ can change disrupting membrane potential of muscle cell
  *Decreased ATP and increased magnesium
• Lack of ATP is rarely a reason for fatigue, except in severely stressed muscles

Factors of Muscle Contraction

• Force of contraction depends on number of cross bridges attached.
  • Number of muscle fibers stimulated: the more motor units recruited, the greater the force.
  • Relative size of fibers: the bulkier the muscle, the more tension it can develop
    *Muscle cells can increase in size (hypertrophy) with regular exercise
  • Frequency of stimulation: the higher the frequency, the greater the force
    *Stimuli are added together
  • Degree of muscle stretch: muscle fibers with sarcomeres that are 80–120% their normal resting length generate more force
    *If sarcomere is less than 80% resting length, filaments overlap too much, and force decreases
    *If sarcomere is greater than 120% of resting length, filaments do not overlap enough so force decreases

Velocity and Duration of Contraction

• How fast a muscle contracts and how long it can stay contracted is influenced by:
  • Muscle fiber type
  • Load
  • Recruitment
    *Classified according to two characteristics
• Speed of contraction: slow or fast fibers according to
  *Speed at which myosin ATPases split ATP
  *Pattern of electrical activity of motor neurons

Smooth Muscle

• Found in walls of most hollow organs
  • Respiratory, digestive, urinary, reproductive, circulatory (except in smallest of blood vessels) except heart

Contraction of Smooth Muscle

• Mechanism of contraction *Cells electrically coupled by gap junctions *Action potentials transmitted from fiber to fiber
  • Final trigger is increased intracellular Ca2+ level
    *ATP energizes sliding process
    *Contraction stops when Ca2+ is no longer available
    *Stopping smooth muscle contraction requires more steps than skeletal muscle