Muscle Tissue and Physiology

Overview of Muscle Tissue

Muscle tissue is specialized for contraction and is essential for movement, posture, and many physiological processes. There are three main types of muscle tissue: skeletal, cardiac, and smooth, each with unique structures and functions.

• Major Functions of Muscle Tissue:

  • Producing body movements (locomotion and manipulation)

  • Maintaining posture and body position

  • Stabilizing joints

  • Generating heat (thermogenesis)

  • Moving substances within the body (e.g., blood, food, urine)

• Structural Elements Common to All Muscle Cells:

  • Elongated cells called muscle fibers

  • Presence of contractile proteins (actin and myosin)

  • Excitability, contractility, extensibility, and elasticity

• Comparison of Muscle Types:

  • Skeletal Muscle: Voluntary, striated, multinucleated, attached to bones

  • Cardiac Muscle: Involuntary, striated, branched, single nucleus, found in heart

  • Smooth Muscle: Involuntary, non-striated, spindle-shaped, single nucleus, found in walls of hollow organs

Structure and Function of Skeletal Muscle Fibers

Structural Properties and Components

Skeletal muscle fibers are long, cylindrical cells containing multiple nuclei and specialized organelles for contraction.

• Myofibrils: Bundles of contractile proteins within muscle fibers

• Sarcolemma: Plasma membrane of muscle fiber

• Sarcoplasm: Cytoplasm of muscle fiber, contains glycogen and myoglobin

• Sarcoplasmic Reticulum (SR): Stores calcium ions necessary for contraction

Organization of a Myofibril

• Composed of repeating units called sarcomeres

• Sarcomeres are the functional contractile units of muscle

Filament Structure

• Thick Filaments: Made of myosin

• Thin Filaments: Made of actin, tropomyosin, and troponin

• Elastic Filaments: Composed of titin, provide elasticity and stability

Protein Components of a Sarcomere

• Contractile Proteins: Actin (thin) and myosin (thick)

• Regulatory Proteins: Troponin and tropomyosin (regulate contraction)

• Structural Proteins: Titin, nebulin, dystrophin (maintain structure and alignment)

Sliding-Filament Mechanism

• Muscle contraction occurs as thin filaments slide past thick filaments, shortening the sarcomere

• ATP and calcium ions are required for this process

Skeletal Muscle Fibers as Electrically Excitable Cells

Ion Concentrations and Gradients

• Sodium (Na+): Higher concentration outside the cell

• Potassium (K+): Higher concentration inside the cell

• Concentration Gradient: Difference in concentration across the membrane

• Electrochemical Gradient: Combined effect of concentration and electrical charge differences

Resting Membrane Potential

• Generated by the unequal distribution of ions and selective permeability of the membrane

• Maintained by the sodium-potassium pump

• Typical value:

Action Potential Sequence

• Depolarization: Na+ influx

• Repolarization: K+ efflux

• Restoration of resting potential

The Process of Skeletal Muscle Contraction and Relaxation

Neuromuscular Junction (NMJ)

• Synapse between a motor neuron and a muscle fiber

• Components: axon terminal, synaptic cleft, motor end plate

Events at the NMJ

• Release of acetylcholine (ACh) from neuron

• ACh binds to receptors on muscle fiber, triggering action potential

Excitation-Contraction Coupling

• Action potential travels along sarcolemma and T-tubules

• Triggers release of Ca2+ from SR

• Ca2+ binds to troponin, initiating contraction

Contraction Cycle

• Cross-bridge formation (myosin binds actin)

• Power stroke (myosin pulls actin)

• Detachment (ATP binds myosin, releasing actin)

• Resetting of myosin head (ATP hydrolysis)

Muscle Relaxation

• ACh is broken down by acetylcholinesterase

• Ca2+ is pumped back into SR

• Muscle fiber returns to resting state

Energy Sources for Skeletal Muscle

Immediate Energy Sources

• Stored ATP

• Creatine phosphate (CP) donates phosphate to ADP to form ATP

• Equation:

Glycolytic and Oxidative Mechanisms

• Glycolysis: Anaerobic, produces ATP quickly, yields lactic acid

• Oxidative phosphorylation: Aerobic, occurs in mitochondria, produces more ATP

Duration of ATP Sources

• Immediate (CP): ~10 seconds

• Glycolytic: ~1 minute

• Oxidative: Several minutes to hours

Muscle Tension at the Fiber Level

Twitch Contraction

• Single contraction in response to a single stimulus

• Stages: latent period, contraction phase, relaxation phase

• Frequency of stimulation affects tension (summation, tetanus)

Sarcomere Length and Tension

• Optimal sarcomere length produces maximal tension

• Too short or too long reduces tension

Muscle Fiber Types

Muscle Tension at the Organ Level

Motor Units

• One motor neuron and all the muscle fibers it innervates

• Small motor units: fine control; large motor units: gross movements

Muscle Tone

• Continuous, passive partial contraction of muscles

• Maintains posture and readiness

Types of Contractions

Skeletal Muscle Performance

Effects of Physical Conditioning

• Endurance Training: Increases mitochondria, capillaries, and fatigue resistance

• Resistance Training: Increases muscle fiber size (hypertrophy) and strength

Muscular Fatigue

• Caused by depletion of ATP, accumulation of lactic acid, ionic imbalances

Recovery Period

• Restores metabolic conditions

• Oxygen debt is repaid

• Removal of lactic acid

Smooth and Cardiac Muscle

Structure, Location, and Function

Contraction Process

• Smooth Muscle: Involuntary, slow, can sustain contractions, regulated by autonomic nervous system and hormones

• Cardiac Muscle: Involuntary, rhythmic, self-excitable, regulated by pacemaker cells

• Contrast with Skeletal Muscle: Skeletal muscle is voluntary, contracts rapidly, and fatigues more quickly