Muscle Anatomy and Physiology Quiz

Muscle Tissue Overview

• Muscle tissue is specialized for shortening, enabling movement
• Composed of myocytes, elongated in one direction
• Muscle fibers are groups of myocytes
• Connective tissue invests muscle fibers, transmitting contraction force and supporting blood vessels/nerves

Learning Outcomes

• Differentiate skeletal, cardiac, and smooth muscles based on structure and function
• Identify muscle fiber components (myofibrils, sarcomeres, filaments)
• Understand the hierarchical structure of muscle tissue (tissue → fibers → fascicles → myofibrils)
• Classify skeletal muscles based on action, location, shape, fiber arrangement, and lever system roles (e.g., agonists, antagonists)

General Features of Muscle Tissue

• Muscle cells are specialized for shortening and generating movement
• Primarily composed of myocytes/muscle cells
• Connective tissue envelopes muscle fibers and facilitates force transmission to other structures
• Connective tissue provides pathways for blood vessels and nerves

Muscles - 3 Types

• Skeletal Muscle:
  • Attached to bones, responsible for voluntary movement
  • Characterized by visible striations
  • Controlled by somatic motor nerves
• Smooth Muscle:
  • Found in internal organs, involuntary movements
  • No visible striations
  • Controlled by autonomic nerves
• Cardiac Muscle:
  • Exclusively in the heart, involuntary and rhythmic
  • Striated
  • Controlled by autonomic nerves

Skeletal Muscle

• Composed of long, cylindrical fibers
• High variability in length and diameter
• Possesses transverse striations
• Multiple nuclei per fiber, placed peripherally
• Encased by a cell membrane (sarcolemma)

Skeletal Muscle - Cellular Components

• Cytoplasm (sarcoplasm) contains numerous myofibrils
• Organelles (mitochondria, glycogen) located near the nuclei
• Nucleus positioned peripherally beneath the cell membrane

Structural Components of Muscle

• Muscle fibers are composed of myofibrils
• Myofibrils contain repeating units called sarcomeres
• Sarcomeres consist of thick (myosin) and thin (actin) filaments
• Arrangement of filaments creates striations
• Supporting structures within include tendons, epimysium, perimysium and endomysium

Ultrastructure of Skeletal Muscle

• The cell membrane (sarcolemma) is covered by a basement membrane (external lamina)
• Cytoplasm (sarcoplasm) exhibits transverse (T) tubules for signaling
• Sarcoplasmic reticulum (SR) manipulates calcium for contraction
• Cell organelles (e.g., mitochondria) are aligned near the nuclei
• Cell contains large amounts of glycogen

Bands - Striations

• Skeletal muscle displays distinct, repeating dark and light bands (striations)
• Bands are associated with the arrangement of contractile proteins like actin and myosin
• I band (isotropic) reflects equally in all directions
• A band (anisotropic) cannot reflect light equally

Structure of a Myofibril

• Myofibrils are composed of actin and myosin filaments
• Actin filaments are thin and are found in the I band and part of the A band
• Myosin filaments are thick and occupy the A band
• Other proteins (e.g., titin, nebulin) provide structural support and elasticity

Structure of a Myofibril - Detailed

• I band – actin filaments
• A band – myosin filaments
• H band – has no actin filaments
• Z band – where actin filaments of adjoining sarcomeres meet
• M band – myosin filaments of adjacent myosin filaments

Neuro-Muscular Junction

• Motor end plates allow for fast phasic contraction
• Trail endings enable tonic contractions
• Neurotransmission involves neurotransmitters stored in vesicles
• Neurotransmitters are released into the synaptic cleft, initiating muscle contraction

Muscle Spindles

• Sensory receptor organs found between muscle fascicles
• Play role in stretch reflexes

Organization of Muscle Fibres

• Muscle fibers are organized into fascicles
• Different fiber arrangements result in varying strength and range of motion
• Examples are strap-like, fusiform, or pennate type

Classification of Muscles - Based on Colour

• Red muscles (deep) experience slower, sustained contractions with more myoglobin
• White muscles (superficial) experience faster, less sustained contractions with reduced myoglobin content

Classification of Muscles – Based on Arrangement of Muscle Fibres

• Muscle fiber arrangements influence their functions
• Parallel, Pennate, Unipennate, Bipennate, Multipennate, Circular or Convergent fibers exist

Classification of Muscles- Based on Action of Muscles

• Muscles produce actions by working together in groups (Spurt muscles or Shunt muscles)
• Prime movers, agonists, antagonists, and synergists are examples of muscle action classifications

Classification – Based on Action of Muscles - Detailed

• Prime movers generate intended movement
• Antagonists oppose intended movement
• Fixation muscles stabilize proximal joints
• Synergists coordinate actions to prevent undesired movements
• Skeletal muscle contraction involves individual muscle fibers where a portion shortens 50-55% during contraction

Nomenclature of Muscles

• Muscles are named based on various criteria (size, shape, location, and actions)

Connective Tissue Framework

• Endomysium surrounds individual muscle fibers
• Perimysium surrounds muscle bundles (fascicles)
• Epimysium surrounds the entire muscle

Bursa

• Small fluid-filled sacs, reducing friction between tendons and bones

Tendon Synovial Sheath

• Double-layered bursa encasing tendons in areas of high friction

Smooth Muscle

• Non-striated
• Spindle-shaped myocytes
• Central nucleus
• Endomysium invests smooth muscle cells
• Involuntary contractions
• Walls of hollow organs

Smooth Muscle - Innervation

• Controlled by the autonomic nervous system
• Multi-unit smooth muscle receives direct nerve input
• Unitary smooth muscle exhibits intrinsic rhythmic contractions

Smooth Muscle - Ultrastructure

• Composed of actin and myosin filaments
• Encased by a basement membrane (external lamina)
• Sarcoplasmic reticulum (SR) facilitates calcium management for contraction
• Other organelles, such as mitochondria, are also present

Cardiac Muscle

• Involuntary
• Specialized, branching myocytes with interconnected junctions (intercalated discs)
• Spontaneously contracts rhythmically (pacemaker function)
• Autonomic innervation (though not essential for contraction)

Individual Cardiac Muscle Fibers

• Branched, anastomosing (connecting) arrangement
• Uninucleated (one nucleus per cell)
• Central location of nucleus within cell
• Contains abundant sarcoplasm and mitochondria
• Intercalated discs

Similarities Between Cardiac and Smooth Muscle

• Elongated myocytes, with myofilaments and transverse striations (in cardiac)
• Similar connective tissue and capillary network
• Similar presence of organelles (sarcoplasmic reticulum, mitochondria, T-system)

Differences Cardiac vs Skeletal Muscle

• Skeletal: parallel fibers, no branching, no intercalated discs, voluntary control, somatic nerves
• Cardiac: branched fibers, intercalated discs, involuntary control, autonomic nerves

Differences Smooth vs Skeletal Muscle

• Smooth: no striations, spindle-shaped fibers, central nucleus, involuntary control, autonomic nerves
• Skeletal: striated, cylindrical fibers, peripheral nucleus, voluntary control, somatic nerves