Muscle Tissue: Structure and Properties

Basic Properties of Muscle Tissue

Muscle tissue exhibits several unique physiological properties that enable movement and force generation in the body.

• Excitability (Responsiveness/Irritability): The ability of muscle cells to receive and respond to stimuli, typically from the nervous system.

• Contractility: The capacity of muscle cells to shorten forcefully when stimulated.

• Extensibility: The ability of muscle tissue to be stretched or extended.

• Elasticity: The ability of muscle to recoil to its resting length after being stretched.

Types of Muscle Tissue

• Skeletal Muscle: Voluntary, striated muscle attached to bones; responsible for body movement.

• Cardiac Muscle: Involuntary, striated muscle found only in the heart; responsible for pumping blood.

• Smooth Muscle: Involuntary, non-striated muscle found in walls of hollow organs (e.g., intestines, blood vessels).

Skeletal Muscle Structure

Connective Tissue Organization

• Epimysium: Dense collagenous layer surrounding the entire muscle.

• Perimysium: Divides muscle into compartments called fascicles (bundles of muscle fibers).

• Endomysium: Surrounds individual muscle fibers (cells).

• Tendon: At each end of the muscle, collagen fibers from all three layers merge to form a tendon (cord) or aponeurosis (broad sheet) that attaches muscle to bone.

Cellular Components

• Muscle Fiber: A single, multinucleated skeletal muscle cell formed by fusion of myoblasts.

• Myosatellite Cells: Stem cells involved in muscle repair.

• Sarcolemma: Plasma membrane of a muscle fiber.

• Sarcoplasm: Cytoplasm of a muscle fiber.

• Transverse (T) Tubules: Invaginations of the sarcolemma that conduct action potentials deep into the muscle fiber.

• Sarcoplasmic Reticulum (SR): Specialized endoplasmic reticulum that stores and releases calcium ions.

• Terminal Cisternae: Enlarged regions of the SR adjacent to T tubules; part of the triad (T tubule + 2 terminal cisternae).

Myofibrils and Sarcomeres

• Myofibrils: Cylindrical structures within muscle fibers, composed of repeating units called sarcomeres.

• Myofilaments: Protein filaments (thick and thin) that make up myofibrils.

• Sarcomere: The functional contractile unit of muscle, defined by Z lines.

Sarcomere Structure

• A Band: Dark band; contains thick filaments (myosin) and overlapping thin filaments (actin).

• I Band: Light band; contains only thin filaments.

• M Line: Center of the A band; stabilizes thick filaments.

• H Band: Lighter region on either side of the M line; contains only thick filaments.

• Zone of Overlap: Area where thick and thin filaments overlap.

• Z Line: Boundary of each sarcomere; anchors thin filaments and titin.

• Titin: Elastic protein that connects thick filaments to Z line, providing elasticity and stability.

Myofilament Proteins

• Thin Filaments: Composed mainly of actin (F-actin: two twisted rows of G-actin monomers), tropomyosin (blocks active sites), troponin (binds calcium), and nebulin (stabilizes actin).

• Thick Filaments: Composed of myosin molecules (motor proteins) with heads that bind to actin during contraction.

Muscle Contraction: Physiology

Sliding Filament Theory

The sliding filament theory explains muscle contraction as the sliding of thin (actin) and thick (myosin) filaments past each other, shortening the sarcomere without changing the length of the filaments themselves.

Excitable Membranes and Action Potentials

• Excitable Membranes: Allow rapid communication via electrical signals.

• Action Potential: A brief, propagating change in membrane potential that triggers muscle contraction.

Neuromuscular Junction (NMJ)

The NMJ is the synapse where a motor neuron communicates with a muscle fiber, initiating contraction.

1. Acetylcholine (ACh) is released from the axon terminal into the synaptic cleft.

2. ACh binds to receptors on the motor end plate, increasing sarcolemma permeability to sodium ions.

3. Sodium influx generates an action potential in the sarcolemma.

4. ACh is broken down by acetylcholinesterase (AChE), terminating the signal.

Excitation-Contraction Coupling

1. Neural Control: Action potential arrives at NMJ.

2. Excitation: ACh release leads to action potential in sarcolemma.

3. Calcium Release: Action potential travels down T tubules, triggering Ca2+ release from SR.

4. Contraction Cycle Begins: Ca2+ binds troponin, exposing actin active sites.

5. Sarcomere Shortening: Cross-bridge cycling shortens sarcomere.

6. Muscle Tension: Muscle shortens, generating tension.

Contraction Cycle and Cross-Bridge Formation

1. Ca2+ binds troponin, exposing actin active sites.

2. Myosin heads bind to actin, forming cross-bridges.

3. Power stroke: Myosin head pivots, pulling actin toward M line (ADP and Pi released).

4. ATP binds myosin, causing detachment from actin.

5. ATP hydrolysis re-cocks myosin head.

6. Cycle repeats as long as Ca2+ and ATP are available.

Muscle Contraction and Relaxation: Steps

• Initiation: ACh release, action potential propagation, Ca2+ release, cross-bridge formation, contraction cycle.

• Termination: ACh breakdown, Ca2+ reabsorption, active sites covered, contraction ends, muscle relaxes.

Muscle Performance and Types of Contraction

Length-Tension Relationship

The force a muscle fiber produces depends on the length of its sarcomeres at the time of stimulation.

Twitch and Myogram

• Twitch: A single contraction-relaxation cycle in a muscle fiber.

• Myogram: Graphical recording of muscle tension over time.

• Latent Period: Time between stimulus and contraction onset.

• Contraction Phase: Tension rises to peak.

• Relaxation Phase: Tension falls as Ca2+ is reabsorbed.

Summation and Tetanus

• Treppe: Gradual increase in contraction strength with repeated stimulation.

• Wave Summation: Second stimulus before relaxation increases tension.

• Incomplete Tetanus: Rapid cycles of contraction and relaxation, almost maximal tension.

• Complete Tetanus: No relaxation phase; maximal, sustained tension.

Motor Units and Recruitment

• Motor Unit: A motor neuron and all the muscle fibers it controls.

• Recruitment: Increasing the number of active motor units to increase tension.

• Muscle Tone: Resting tension in a muscle.

Types of Muscle Contraction

• Isotonic Contraction: Muscle changes length; tension exceeds load (concentric) or is less than load (eccentric).

• Isometric Contraction: Muscle length does not change; tension does not exceed load.

Muscle Metabolism and Fiber Types

Energy Sources

• Creatine Phosphate (CP): Stores high-energy phosphate to rapidly regenerate ATP.

• Creatine Kinase: Enzyme that transfers phosphate from CP to ADP to form ATP.

• Aerobic Metabolism: Uses oxygen to generate ATP from fatty acids (dominant at rest).

• Anaerobic Process: ATP generated without oxygen (glycolysis); less efficient, produces lactic acid.

Muscle Fatigue and Recovery

• Fatigue: Inability to maintain required activity level.

• Recovery Period: Time needed to restore muscle to pre-exertion state.

• Oxygen Debt (EPOC): Extra oxygen required after exercise to restore metabolic conditions.

Muscle Fiber Types

Muscle Adaptation

• Hypertrophy: Increase in muscle size due to enlargement of fibers.

• Atrophy: Decrease in muscle size due to inactivity or disease.

• Fibrosis: Replacement of muscle tissue with fibrous connective tissue (common with aging).

Cardiac and Smooth Muscle

Cardiac Muscle

• Cardiac Muscle Cells (Cardiomyocytes): Striated, branched, involuntary, connected by intercalated discs (gap junctions and desmosomes).

• Automaticity: Ability to contract without neural stimulation; pacemaker cells set rhythm.

Smooth Muscle

• Smooth Muscle Tissue: Non-striated, involuntary, found in walls of hollow organs.

• Dense Bodies: Anchor thin filaments; analogous to Z lines.

• Calmodulin: Calcium-binding protein that activates myosin light chain kinase for contraction.

• Plasticity: Ability to adapt to stretching and maintain contractility.

• Multiunit Smooth Muscle: Each cell innervated individually.

• Visceral (Single-Unit) Smooth Muscle: Cells contract together as a sheet; found in digestive tract, uterus, blood vessels.

Major Skeletal Muscles and Movements

Facial and Head Muscles

• Frontalis: Raises eyebrows, wrinkles forehead.

• Buccinator: Compresses cheek, draws mouth laterally.

• Zygomaticus: Raises angle of mouth.

• Orbicularis Oculi: Closes eyelids.

• Orbicularis Oris: Closes and protrudes lips.

• Masseter, Temporalis: Elevate mandible (close mouth).

• Sternocleidomastoid: Flexes and rotates head.

• Splenius Capitis: Extends and rotates head.

Trunk and Shoulder Muscles

• Trapezius: Elevates, depresses, adducts scapula; extends head.

• Serratus Anterior: Abducts and rotates scapula; raises shoulder.

• Pectoralis Major: Flexes, adducts, medially rotates humerus.

• Latissimus Dorsi: Extends, adducts, medially rotates arm.

• Deltoid: Abducts, flexes, extends arm.

• Supraspinatus, Infraspinatus, Subscapularis, Teres Minor: Rotator cuff muscles; stabilize and rotate shoulder.

Arm and Forearm Muscles

• Biceps Brachii: Flexes and supinates forearm; flexes arm.

• Triceps Brachii: Extends forearm and arm.

Abdominal and Back Muscles

• External/Internal Abdominal Oblique, Transversus Abdominis, Rectus Abdominis: Compress abdomen, flex and laterally flex vertebral column.

• Erector Spinae (Iliocostalis, Longissimus, Spinalis): Extend vertebral column.

Hip and Leg Muscles

• Psoas Major, Iliacus: Flex hip and thigh.

• Gluteus Maximus: Extends thigh.

• Gluteus Medius: Abducts thigh.

• Adductors, Gracilis: Adduct thigh.

• Rectus Femoris, Vastus Lateralis/Medialis/Intermedius: Extend leg (quadriceps group).

• Biceps Femoris, Semitendinosus, Semimembranosus: Flex leg, extend thigh (hamstrings).

• Sartorius: Flexes thigh and leg, rotates thigh.

• Gastrocnemius, Soleus: Plantar flexion of foot.

• Tibialis Anterior: Dorsiflexion, inverts foot.

Movements

• Flexion: Decreases angle between two parts.

• Extension: Increases angle between two parts.

• Abduction: Movement away from midline.

• Adduction: Movement toward midline.

• Dorsiflexion: Bending foot upward.

• Plantar Flexion: Pointing toes downward.

Sample Questions and Key Facts

• Most human muscles have both slow and fast fibers.

• Slow fibers are abundant in back muscles, not in hand or eye muscles.

• At rest, muscle generates most ATP by aerobic metabolism of fatty acids.

• Acetylcholine binding at the motor end plate increases sarcolemma permeability to sodium ions.

• The I band of the sarcomere always contains thin filaments.

• Slow fibers are best adapted for prolonged contraction.

• The Z line anchors thin filaments and marks the boundary of a sarcomere.

*Additional info: Some explanations and context have been expanded for clarity and completeness based on standard Anatomy & Physiology textbooks.*