Muscular System Overview

Introduction to Muscle Tissue

The muscular system comprises nearly half of the body's mass and is essential for movement, posture, joint stability, and heat generation. Muscle tissue transforms chemical energy (ATP) into mechanical energy, enabling forceful contractions.

• Prefixes: myo-, mys-, sarco- (e.g., sarcoplasm = muscle cell cytoplasm).

• Muscle fibers: Only skeletal and cardiac muscle cells are elongated and called fibers.

Types of Muscle Tissue

Characteristics and Functions of Muscle

• Excitability: Responds to stimuli.

• Contractility: Shortens forcibly when stimulated.

• Extensibility: Can be stretched.

• Elasticity: Recoils to resting length.

• Functions: Movement, posture, joint stabilization, heat generation.

Skeletal Muscle Anatomy

Structure and Organization

Skeletal muscle is an organ composed of muscle fibers, connective tissue, blood vessels, and nerves. It is organized in hierarchical levels:

• Epimysium: Dense irregular CT, surrounds entire muscle.

• Perimysium: Fibrous CT, surrounds fascicles.

• Endomysium: Areolar CT, surrounds each muscle fiber.

Attachments: Muscles attach to bones via origins (less movable) and insertions (more movable), either directly (epimysium fused to periosteum) or indirectly (via tendons or aponeuroses).

Muscle Fiber Microanatomy

• Sarcolemma: Plasma membrane of muscle fiber.

• Sarcoplasm: Cytoplasm, rich in glycogen (glycosomes) and myoglobin (O2 storage).

• Myofibrils: Rodlike structures, ~80% of cell volume, contain contractile elements.

Striations and Sarcomeres

• A bands: Dark regions; contain thick filaments (myosin).

• I bands: Light regions; contain thin filaments (actin).

• Z disc: Protein sheet in the middle of I band; boundary of sarcomere.

• Sarcomere: Smallest contractile unit, from Z disc to Z disc.

Myofilaments

• Myosin: Has heads (bind actin, form cross bridges) and tails.

• Actin: G actin subunits with myosin-binding sites; F actin is a double helix of G actin.

• Regulatory proteins: Tropomyosin (blocks binding sites), troponin (binds Ca2+).

• Elastic filament (titin): Maintains alignment, elasticity.

• Dystrophin: Links thin filaments to sarcolemma; defective in Duchenne muscular dystrophy.

Sarcoplasmic Reticulum (SR) and T Tubules

• SR: Stores and releases Ca2+ for contraction.

• T tubules: Invaginations of sarcolemma; transmit action potentials deep into fiber.

• Triad: T tubule flanked by two terminal cisterns of SR; coordinates Ca2+ release.

Muscle Contraction: Mechanisms and Steps

Sliding Filament Model

Muscle contraction occurs when thin filaments slide past thick filaments, increasing overlap. Neither filament shortens; the sarcomere shortens as Z discs move closer.

• Key changes during contraction:

  • Z discs: move closer together

  • I bands: shorten

  • H zones: disappear

  • A bands: remain same length

Steps of Skeletal Muscle Contraction

1. Events at the Neuromuscular Junction (NMJ):

   • Action potential (AP) arrives at axon terminal.

   • Voltage-gated Ca2+ channels open; Ca2+ enters neuron.

   • Acetylcholine (ACh) released into synaptic cleft.

   • ACh binds receptors on sarcolemma, opening Na+ channels (end plate potential).

   • ACh is degraded by acetylcholinesterase.

2. Muscle Fiber Excitation:

   • End plate potential triggers AP across sarcolemma.

   • Depolarization: Na+ influx; if threshold reached, AP propagates.

   • Repolarization: K+ efflux restores resting potential.

   • Na+-K+ pump restores ionic balance.

3. Excitation-Contraction (E-C) Coupling:

   • AP travels down T tubules.

   • SR releases Ca2+ into cytosol.

   • Ca2+ binds troponin, moving tropomyosin and exposing actin sites.

   • Contraction occurs; when AP ends, Ca2+ is pumped back into SR, leading to relaxation.

4. Cross Bridge Cycling:

   • Myosin head attaches to actin (cross bridge formation).

   • Power stroke: myosin head pivots, pulling actin.

   • ATP binds myosin, causing detachment.

   • ATP hydrolysis "cocks" myosin head for next cycle.

ATP is required for detachment of myosin from actin.

Clinical Note: Rigor Mortis

• After death, ATP production ceases; Ca2+ accumulates, cross bridges form but cannot detach, causing muscle stiffness until proteins degrade.

Whole Muscle Contraction

Motor Units and Muscle Twitch

• Motor unit: One motor neuron and all muscle fibers it innervates.

• Muscle twitch: Response of a muscle fiber to a single AP; recorded as a myogram.

Graded Muscle Responses

• Temporal (wave) summation: Increased frequency of stimulation increases force.

• Recruitment (multiple motor unit summation): Increased stimulus strength recruits more motor units.

• Size principle: Smallest motor units recruited first; largest last.

Types of Contractions

Muscle Tone

• Constant, slight contraction due to spinal reflexes; keeps muscles ready to respond.

Muscle Metabolism and Fatigue

ATP Regeneration Pathways

• Creatine phosphate: Immediate ATP source for short bursts.

• Anaerobic glycolysis: Fast, low yield, produces lactic acid.

• Aerobic respiration: Slow, high yield, uses glucose and fatty acids.

Muscle Fatigue and EPOC

• Fatigue: Inability to contract despite stimulation; due to ionic imbalances, Pi accumulation, decreased ATP/glycogen, increased Mg2+.

• Excess postexercise oxygen consumption (EPOC): Oxygen needed to restore muscle to pre-exercise state (replenish O2, convert lactic acid, restore ATP/CP, replace glycogen).

Muscle Actions, Naming, and Major Groups

Muscle Actions and Interactions

• Prime mover (agonist): Main muscle for a movement.

• Antagonist: Opposes/reverses movement.

• Synergist: Assists prime mover, adds force, reduces unwanted movement.

• Fixator: Stabilizes origin of prime mover.

Naming Skeletal Muscles

• Location: e.g., temporalis (temporal bone)

• Shape: deltoid (triangle)

• Size: maximus (largest), minimus (smallest)

• Fiber direction: rectus (straight), oblique (angled)

• Number of origins: biceps (2), triceps (3)

• Attachment sites: sternocleidomastoid (sternum, clavicle, mastoid)

• Action: flexor, extensor

Major Muscle Groups and Actions

Muscles of Facial Expression and Mastication

Neck and Vertebral Column Muscles

• Sternocleidomastoid: Flexes, rotates head.

• Scalenes: Elevate ribs, flex/rotate neck.

• Splenius: Extend/hyperextend head.

• Erector spinae: Prime mover of back extension.

• Quadratus lumborum: Lateral flexion, lumbar extension.

Thorax, Abdomen, and Pelvic Floor

• Diaphragm: Prime mover of inspiration.

• Intercostals: External (inspiration), internal (forced expiration).

• Rectus abdominis, obliques, transversus abdominis: Trunk flexion, rotation, compression of abdominal contents.

• Pelvic diaphragm (levator ani, coccygeus): Supports pelvic organs.

Muscles of the Shoulder and Arm

• Rotator cuff (SITS): Supraspinatus (abduction), Infraspinatus (lateral rotation), Teres minor (lateral rotation), Subscapularis (medial rotation).

• Deltoid: Prime mover of arm abduction.

• Pectoralis major, latissimus dorsi: Arm flexion/adduction/extension.

• Biceps brachii, brachialis, brachioradialis: Forearm flexion.

• Triceps brachii: Forearm extension.

Forearm and Hand Muscles

• Flexor group (anterior): Flex wrist/fingers.

• Extensor group (posterior): Extend wrist/fingers.

• Pronator teres/quadratus: Pronate forearm.

• Supinator: Supinates forearm.

• Intrinsic hand muscles: Fine movements (thenar, hypothenar, lumbricals, interossei).

Hip, Thigh, Leg, and Foot Muscles

• Iliopsoas: Prime mover of thigh flexion.

• Quadriceps femoris: Knee extension (rectus femoris also flexes hip).

• Hamstrings (biceps femoris, semitendinosus, semimembranosus): Thigh extension, knee flexion.

• Gluteus maximus: Thigh extension.

• Adductors (magnus, longus, brevis): Thigh adduction.

• Tibialis anterior: Dorsiflexion of foot.

• Gastrocnemius, soleus: Plantar flexion.

• Intrinsic foot muscles: Toe movement, arch support.

Summary Tables: Major Actions and Muscles

Key Equations and Concepts

• ATP hydrolysis:

• Creatine phosphate reaction:

• Aerobic respiration (glucose):

Clinical Correlations

• Duchenne muscular dystrophy: Genetic defect in dystrophin; progressive muscle weakness.

• Bell's palsy: Facial nerve paralysis; affects muscles of facial expression.

• Rigor mortis: Postmortem muscle stiffening due to lack of ATP.

Additional Info

• Muscle groupings (e.g., hamstrings, quadriceps, rotator cuff) are essential for coordinated movement and are often tested in clinical and academic settings.

• Superficial vs. deep, anterior vs. posterior muscle locations are important for understanding function and for clinical examination.