Chapter 10 - Muscle Tissue and Organization

Overview and Fundamental Concepts of Myology

• Definition of Myology: Derived from Greek roots where $\text{myo} = \text{muscle}$ and $\text{logy} = \text{study of}$. It is the scientific study of muscle tissue and the muscular system.

• The Muscular System: Composed of over $700$ individually named skeletal muscles working in coordination with the skeletal and nervous systems.

• General Functions of Muscle Tissue:

  • Skeletal Movement: Producing movements of the skeleton, including walking, running, moving arms, fingers, lifting, picking up objects, or blinking eyelids.

  • Internal Fluid and Material Transport: Pumping blood throughout the cardiovascular system and propelling food through the gastrointestinal (GI) tract.

  • Communication and Posture: Facilitating vocalization and creating words used in conversation, as well as maintaining continuous posture and body position against gravity.

  • Protection and Support: Encasing deep abdominal and pelvic organs within protective muscular walls.

  • Sphincteric Control: Regulating the passage of substances through digestive, urinary, and excretory passages.

  • Thermogenesis: Generating heat during contraction to maintain homeostatic body temperature.

Comparison of Muscle Tissue Types

Skeletal Muscle

• Location: Associated primarily with the skeletal system, attached to bones via tendons.

• Microscopic Appearance: Highly striated, featuring alternating lighter and darker protein bands across each fiber.

• Control Mechanism: Voluntary control under conscious regulation by the somatic nervous system.

• Cell Morphology: Elongated, cylindrical, non-branching fibers that contain multiple peripherally located nuclei per cell.

Cardiac Muscle

• Location: Confined exclusively to the muscular wall of the heart (myocardium).

• Microscopic Appearance: Striated appearance due to organized sarcomeres.

• Control Mechanism: Involuntary control operating autorhythmically, modulated by the autonomic nervous system (ANS).

• Cell Morphology: Short, branched ($\text{Y}$-shaped) cells containing $1$ or $2$ centrally located nuclei, joined end-to-end by specialized intercalated discs.

Smooth Muscle

• Location: Located within the walls of hollow internal organs and tubular structures, including blood vessels, stomach, intestines, urinary bladder, and respiratory airways.

• Microscopic Appearance: Non-striated (smooth) appearance due to the absence of organized sarcomeres.

• Control Mechanism: Involuntary control managed by the autonomic nervous system (ANS) and local endocrine/paracrine signals.

• Cell Morphology: Short, spindle-shaped (fusiform) cells with a single centrally located nucleus.

Unique Properties and General Functions of Muscle Tissue

Unique Functional Properties

1. Excitability: The capacity to receive and respond to stimuli (such as neurotransmitters, hormones, local pH changes, mechanical stretch, or direct electrical signals).

2. Conductivity: The capability to propagate an electrical signal (action potential) along the length of the cell membrane (sarcolemma).

3. Contractility: The specialized ability to forcefully shorten and generate mechanical tension or force upon stimulation.

4. Extensibility: The ability to be stretched or lengthened beyond resting state without sustaining structural damage.

5. Elasticity: The capacity to recoil back to original resting length after being stretched or contracted.

Core Functions of Skeletal Muscle Tissue

• Body Movement: Contraction generates force transmitted via tendons to pull on bones, causing leverage and physical displacement.

• Postural Maintenance: Continuous, low-level isometric contractions (muscle tone) stabilize joints and hold the head and trunk upright.

• Protection and Support: Layers of muscle tissue form abdominal and pelvic floors that support visceral organs and protect them from external trauma.

• Regulating Material Elimination: Circular muscle rings called sphincters control passage through the GI and urinary tracts. Sphincter contraction constricts or closes the lumen/orifice; relaxation expands the lumen to allow transit.

• Thermogenesis: Energy liberated during muscle metabolic activity and force generation releases heat as a metabolic byproduct, essential for shivering and thermal homeostasis.

Gross Anatomy and Connective Tissue Organization of Skeletal Muscle

Structural Hierarchy Sequence

$\text{Whole Muscle} \rightarrow \text{Fascicle} \rightarrow \text{Muscle Fiber (Cell)} \rightarrow \text{Myofibril} \rightarrow \text{Myofilaments (Thick and Thin)}$

Deep Fascia and Connective Tissue Wrappings

• Epimysium: A sheath of dense irregular connective tissue surrounding the outer perimeter of an entire named muscle. Connects to deep fascia.

• Perimysium: A layer of dense irregular connective tissue surrounding individual fascicles (bundles of muscle fibers). Contains extensive blood vessels and nerve branches.

• Endomysium: An innermost layer of delicate areolar connective tissue with reticular fibers surrounding each individual muscle fiber. Provides electrical insulation between adjacent fibers and houses capillaries and nerve terminals.

• Tendons and Aponeuroses: At muscle ends, epimysium, perimysium, and endomysium converge into cord-like tendons or broad flat sheets called aponeuroses, anchoring muscle to bone periosteum, cartilage, or fascia.

Development and Microscopic Anatomy of Skeletal Muscle

Embryonic Development and Satellite Cells

• Myoblast Fusion: During embryonic development, numerous embryonic progenitor cells called myoblasts fuse together end-to-end to form a single, long, multinucleated skeletal muscle fiber.

• Multinucleate Nature: Because hundreds of myoblasts fuse, mature muscle fibers contain hundreds of peripherally situated nuclei located just underneath the cell membrane.

• Lifespan: Formed prior to birth, these mature muscle fibers normally persist for a lifetime.

• Satellite Cells: Unfused myoblasts that persist postnatally within the endomysium surrounding skeletal muscle fibers. They act as adult stem cells capable of proliferating and fusing with injured fibers to facilitate muscle repair.

Microscopic Structural Components

• Muscle Fiber Size: Cylindrical cells with diameters ranging from $10\,\mu\text{m}$ to $100\,\mu\text{m}$ and lengths often spanning several inches or the entire length of a muscle bundle.

• Sarcolemma: The specialized plasma membrane surrounding the sarcoplasm of a skeletal muscle cell. Conducts electrical action potentials.

• Sarcoplasm: The cytoplasm of the muscle fiber, filled with cellular organelles, glycogen granules (for energy storage), myoglobin (oxygen-binding protein), and enzymes.

• Transverse Tubules (T-tubules): Deep, invaginated tubular extensions of the sarcolemma that penetrate straight into the sarcoplasm perpendicular to the long axis. Function to conduct electrical action potentials rapidly into the innermost regions of the muscle fiber.

• Sarcoplasmic Reticulum (SR): An elaborate network of smooth endoplasmic reticulum tubules surrounding each myofibril. Functions as a storage reservoir for intracellular calcium ions ($\text{Ca}^{2+}$).

• Terminal Cisternae: Enlarged, blind-ended sacs at the ends of the sarcoplasmic reticulum segments that directly flank each T-tubule. Store and release calcium ions ($\text{Ca}^{2+}$).

• Triad: A microscopic structural association consisting of one central T-tubule flanked on either side by two terminal cisternae of the sarcoplasmic reticulum.

Detailed Architecture of Myofibrils, Sarcomeres, and Myofilaments

Myofibril Architecture

• Myofibrils: Cylindrical contractile bundles running parallel along the entire length of the muscle fiber, packed densely within the sarcoplasm.

• Myofilaments: Microscopic protein threads housed inside myofibrils, divided into thick filaments and thin filaments.

Functional Regions of the Sarcomere

• Sarcomere: The basic functional and repetitive contractile unit of a myofibril, defined as the region extending from one $Z\text{ disc}$ to the adjacent $Z\text{ disc}$.

• $A\text{ band}$: The central dark-staining region covering the full length of the thick filaments. Contains overlapping thin filaments at its lateral ends. Does not shorten during contraction.

• $I\text{ band}$: The light-staining band containing thin filaments and structural connectin (titin) proteins, but no thick filaments. Centered on the $Z\text{ disc}$. Shortens during contraction.

• $H\text{ zone}$: The central lighter zone within the $A\text{ band}$ consisting exclusively of thick filaments, without overlapping thin filaments. Shortens and disappears during maximum contraction.

• $M\text{ line}$: A thin transverse protein meshwork located in the exact center of the $H\ zone$. Anchors adjacent thick filaments together.

• $Z\text{ disc}$ (Z line): A dark transverse protein structure in the middle of the $I\ band$ composed of connectin/alpha-actinin proteins that anchor thin filaments.

Molecular Structure of Myofilaments

Thick Filaments

• Diameter and Length: Approximately $11\,\text{nm}$ in diameter and $1\text{--}2\,\mu\text{m}$ long.

• Myosin Protein: Assembled from hundreds of bundled myosin molecules. Each myosin protein is a motor protein shaped like a double-headed golf club, featuring a flexible fibrous tail and two globular heads.

• Binding Sites: Globular myosin heads contain two functional active sites:

  1. Actin-binding site: Attaches to the active site on actin to form a crossbridge.

  2. ATP-binding site: Binds ATP and functions as an ATPase enzyme to hydrolyze ATP into $\text{ADP} + \text{P}_i$ for energy liberation.

Thin Filaments

• Diameter and Length: Approximately $5\text{--}6\,\text{nm}$ in diameter and $1\text{--}2\,\mu\text{m}$ long.

• Actin Protein: Composed of helical chains of globular actin molecules ($\text{G-actin}$) polymerized into filamentous actin ($\text{F-actin}$). Each actin monomer possesses a specific myosin-binding site.

• Tropomyosin: A strand-like regulatory protein that winds spirally around the actin helix. In a resting muscle, tropomyosin blocks the myosin-binding sites on actin.

• Troponin: A globular regulatory protein complex attached to tropomyosin. Contains three subunits, including a subunit that specifically binds calcium ions ($\text{Ca}^{2+}$). When $\text{Ca}^{2+}$ binds troponin, it undergoes a conformational change that pulls tropomyosin away from the myosin-binding sites on actin.

Structural Proteins

• Maintain structural alignment, stabilization, elasticity, and extensibility of myofibrils.

• Include titin (connectin), myomesin, nebulin, dystrophin, and $Z\text{ disc}$ proteins.

Physiology of Skeletal Muscle Contraction and Neuromuscular Junction

Neuromuscular Junction (NMJ) Structure

• Neuromuscular Junction: The specialized chemical synapse connecting the axon terminal of a somatic motor neuron to the motor end plate of a skeletal muscle fiber.

• Axon Terminal / Synaptic End Bulb: The swollen distal tip of the motor neuron nerve fiber.

• Synaptic Vesicles: Membrane-bound sacs inside the axon terminal filled with the neurotransmitter acetylcholine (ACh).

• Synaptic Cleft: The narrow fluid-filled gap separating the pre-synaptic axon terminal from the post-synaptic motor end plate.

• Motor End Plate: A specialized pocket of the sarcolemma underneath the synaptic terminal featuring deep junctional folds lined with transmembrane acetylcholine (ACh) receptors.

Excitation-Contraction Coupling Sequence

1. Action Potential Arrival: An electrical nerve impulse travels down the somatic motor neuron axon to the axon terminal.

2. Neurotransmitter Release: Nerve impulse triggers exocytosis of synaptic vesicles, releasing acetylcholine (ACh) into the synaptic cleft.

3. Binding to Receptor: ACh diffuses across the synaptic cleft and binds to ligand-gated ACh receptors on the motor end plate.

4. Propagation Along Sarcolemma: Binding opens ion channels, triggering an action potential that spreads along the sarcolemma and down the T-tubules into the cell interior.

5. Calcium Release: Action potential passing through T-tubules causes conformational change in membrane voltage sensors, opening $\text{Ca}^{2+}$ release channels in the terminal cisternae of the sarcoplasmic reticulum.

6. Calcium Activation: Calcium ions ($\text{Ca}^{2+}$) flood into the sarcoplasm and bind to troponin.

7. Tropomyosin Shift: Calcium-bound troponin changes shape, pulling tropomyosin away from the myosin-binding sites on actin monomers.

8. Crossbridge Cycle and Sliding Filament Mechanism: Myosin heads bind actin, undergo power strokes, hydrolyze ATP, and pull thin filaments inward toward the $M\ line$. As thin filaments slide over thick filaments, sarcomeres shorten, causing whole muscle fiber contraction.

Structural Organization and Patterns of Fascicle Arrangement

Muscle Architecture Patterns

• Circular Arrangement:

  • Description: Fascicles arranged concentrically around an opening or orifice; acts as a sphincter to open or close passageways.

  • Examples: Orbicularis oris (mouth), Orbicularis oculi (eyes), external anal sphincter.

• Parallel Arrangement:

  • Description: Fascicles run parallel to the long axis of the muscle. Contraction increases central muscle diameter; offers high endurance but modest overall power output.

  • Examples: Rectus abdominis, Sartorius.

• Fusiform Arrangement:

  • Description: Spindle-shaped muscles with a wide central belly and tapering tendon ends.

  • Example: Biceps brachii.

• Convergent Arrangement:

  • Description: Triangular/fan-shaped layout where broad fascicles converge to a single, narrow attachment point. Allows change in direction of force pull, but produces less overall force than equal-sized parallel muscles.

  • Example: Pectoralis major.

• Pennate Arrangement:

  • Description: Feather-like structure where fascicles attach obliquely to one or more central tendons running the length of the muscle. Contains more muscle fibers per unit area, pulling harder than parallel muscles of equal volume.

  • Unipennate: All fascicles situated on the same side of a single tendon (e.g., Extensor digitorum, Digital extensor).

  • Bipennate: Fascicles arranged on both sides of a central tendon (e.g., Rectus femoris).

  • Multipennate: Tendon branches extensively within the body of the muscle (e.g., Deltoid / Deltoideus).

Skeletal Muscle Actions, Functional Groups, and Naming Conventions

Attachments and Muscle Actions

• Origin: The attachment site of a muscle's tendon to the stationary or less movable bone/structure during contraction.

• Insertion: The attachment site of a muscle's tendon to the movable bone/structure pulled during contraction.

• Action: The main joint or anatomical movement produced during contraction (e.g., flexion, extension, abduction, adduction).

Opposing and Functional Muscle Groups

• Agonist (Prime Mover): The primary muscle responsible for producing a specific intended movement (e.g., Biceps brachii during elbow flexion).

• Antagonist: A muscle whose action directly opposes that of the agonist (e.g., Triceps brachii opposes Biceps brachii during elbow flexion).

• Synergist: A muscle that assists the agonist by generating extra force or eliminating unwanted movement at intermediate joints (e.g., Brachialis and Brachioradialis aiding Biceps brachii during forearm flexion).

Criteria for Naming Skeletal Muscles

1. Muscle Action: Indicates primary motion produced.

   • Examples: Adductor magnus (adducts thigh), Abductor pollicis longus (abducts thumb), Flexor carpi radialis (flexes wrist), Extensor hallucis longus (extends great toe).

2. Specific Body Regions: Indicates target body region or landmark.

   • Examples: Oris (mouth; Orbicularis oris), Cervicis (neck; Semispinalis cervicis), Brachial (arm; Biceps brachii), Carpi (wrist; Flexor carpi ulnaris), Pollicis (thumb; Opponens pollicis), Gluteal (buttock; Gluteus medius), Femoris (thigh; Quadratus femoris), Hallucis (great toe; Extensor hallucis longus), Anterior (front; Tibialis anterior), Posterior/Dorsal (back; Tibialis posterior, Latissimus dorsi), Superior (Serratus posterior superior), Inferior (Serratus posterior inferior), Superficialis (superficial; Flexor digitorum superficialis), Profundus (deep; Flexor digitorum profundus).

3. Muscle Attachments: Named after origin and insertion bone landmarks.

   • Examples: Sternocleidomastoid (sternum, clavicle, mastoid process), Intercostal (between ribs), Subscapularis (subscapular fossa), Fibularis longus (fibula), Zygomaticus major (zygomatic bone).

4. Orientation of Muscle Fibers: Direction relative to midline or body axis.

   • Examples: Rectus (straight along longitudinal axis; Rectus abdominis, Rectus femoris), Oblique (angled; External oblique), Orbicularis (circular; Orbicularis oculi).

5. Muscle Shape and Size: Geometric shape or relative dimension.

   • Examples: Deltoid (triangular), Quadratus (rectangular; Pronator quadratus), Trapezius (trapezoidal), Longus (long; Abductor pollicis longus), Brevis (short; Abductor pollicis brevis), Major (larger of two; Pectoralis major), Minor (smaller of two; Pectoralis minor), Maximus (largest; Gluteus maximus), Medius (medium; Gluteus medius), Minimus (smallest; Gluteus minimus).

6. Muscle Heads / Proximal Tendons: Number of origins/heads.

   • Examples: Biceps (two heads; Biceps brachii, Biceps femoris), Triceps (three heads; Triceps brachii), Quadriceps (four heads; Quadriceps femoris).

Specialized Anatomy of Cardiac and Smooth Muscle Tissue

Cardiac Muscle Cytology and Physiology

• Structural Organization: Striated branching cells joined end-to-end at specialized structural junctions called intercalated discs.

• Intercalated Disc Components:

  1. Gap Junctions: Low-resistance protein channels that allow direct passage of ions and electrical impulses from cell to cell, enabling functional syncytium.

  2. Desmosomes: Mechanical cell junctions that anchor adjacent cardiocytes together, preventing tissue pulling apart during forceful contractions.

• Metabolic Profile: High mitochondrial density ($25\text{--}30\%$ of cell volume) designed for continuous, non-fatiguing aerobic cellular respiration.

• Autorhythmicity: Specialized pacemaker cells generate periodic action potentials spontaneously without direct external nervous input.

• Autonomic Modulation: Involuntary neural control; the autonomic nervous system modulates cardiac contraction rate and force.

Smooth Muscle Cytology and Physiology

• Cell Structure: Non-striated, spindle-shaped (fusiform) cells with a single centrally located nucleus.

• Lack of Sarcomeres and T-tubules: Smooth muscle cells lack T-tubules and organized sarcomere striations. Sarcoplasmic reticulum is sparse.

• Dense Bodies and Intermediate Filaments: Thin myofilaments attach to protein structures called dense bodies distributed throughout the sarcoplasm and anchored to dense plaques along the sarcolemma. Dense bodies are interconnected by intermediate filaments.

• Calcium Regulation via Calmodulin:

  • Skeletal Muscle: Extracellular/SR $\text{Ca}^{2+}$ binds to troponin to move tropomyosin.

  • Smooth Muscle: Lacks troponin. Extracellular $\text{Ca}^{2+}$ enters cell through voltage-gated or ligand-gated channels and binds to calmodulin.

  • Calcium-calmodulin complex activates an enzyme called myosin light-chain kinase (MLCK).

  • MLCK phosphorylates myosin heads, enabling them to attach to actin filaments and execute sliding filament contraction.

• Contraction Characteristics: Slow, wave-like, energy-efficient, long-lasting, and highly fatigue-resistant contractions.

Comprehensive Practice Questions and Detailed Answer Key

Practice Questions

1. A muscle fiber is stimulated, but the electrical signal cannot travel deep into the cell. Which structure is most likely not functioning properly, and why?

2. A skeletal muscle cell is unable to release calcium from its internal storage site. Which structure is affected, and what would happen to muscle contraction?

3. Put the following structures in order from largest to smallest: Sarcomere, Muscle, Myofibril, Muscle fiber, Fascicle.

4. A microscope image shows a muscle cell with striations, one central nucleus, and branching cells connected by intercalated discs. Which type of muscle tissue is this? Give two features that support your answer.

5. A muscle cell is spindle-shaped, has one central nucleus, and no visible striations. Which type of muscle tissue is it?

6. A muscle surrounds an opening and closes it when it contracts. Which fascicle arrangement does it most likely have?

7. A muscle has fibers arranged at an angle to a tendon and can produce relatively high force. Which fascicle arrangement is this?

8. A muscle name includes the word “rectus.” What information does this name give you about the muscle?

9. A muscle name includes the word “magnus.” What does this tell you about the muscle?

10. A smooth muscle cell is exposed to calcium. Which regulatory protein does calcium bind to in smooth muscle, and how is this different from skeletal muscle?

11. During muscle contraction, which two proteins directly interact to produce movement? Which one forms the thick filament and which one forms the thin filament?

12. A sarcomere is examined under a microscope. Identify the region that contains:     a. Only thin filaments     b. Only thick filaments     c. The full length of the thick filament

13. A student says, “Actin and myosin become shorter during contraction.” Is this correct? Explain what actually happens.

14. A tendon attaches a muscle to two bones. One attachment is on the more stationary bone and the other is on the more movable bone. Identify the origin and insertion. During elbow flexion, the biceps brachii produces the main movement and the triceps brachii opposes it. Identify the:     a. Agonist     b. Antagonist

15. A drug prevents acetylcholine from acting at the motor end plate. Based on the neuromuscular junction, predict what would happen to skeletal muscle contraction.

16. Explain this sequence in one sentence: Motor neuron $\rightarrow$ ACh $\rightarrow$ muscle fiber $\rightarrow$ calcium release $\rightarrow$ actin–myosin interaction $\rightarrow$ contraction.

Comprehensive Answer Key

1. T-tubule (Transverse tubule): T-tubules carry the electrical signal (action potential) from the surface sarcolemma deep into the sarcoplasm of the muscle fiber cell.

2. Sarcoplasmic reticulum (SR), especially the terminal cisternae: The SR stores calcium, and terminal cisternae release calcium into the sarcoplasm upon electrical excitation. Without calcium release, troponin cannot shift tropomyosin, preventing crossbridge formation and muscle contraction.

3. Order from largest to smallest: Muscle $\rightarrow$ Fascicle $\rightarrow$ Muscle fiber $\rightarrow$ Myofibril $\rightarrow$ Sarcomere.

4. Cardiac muscle: Two supporting features are:

   • It is striated in appearance.

   • Cells are branched ($\text{Y}$-shaped) and joined by intercalated discs.

5. Smooth muscle: Smooth muscle cells are spindle-shaped (fusiform), possess a single centrally located nucleus, and lack microscopic striations.

6. Circular fascicle arrangement: Circular muscle arrangements encircle body openings and act as sphincters.

7. Pennate fascicle arrangement: Pennate muscles feature fascicles arranged at an angle to a central tendon, allowing more muscle fibers per unit area to generate high force.

8. Rectus means “straight”: Describes an orientation of muscle fibers running parallel to the longitudinal body axis (e.g., Rectus abdominis).

9. Magnus means “large”: Indicates that the named muscle is large in relative size (e.g., Adductor magnus).

10. Calmodulin: In smooth muscle, calcium binds to calmodulin, which then activates myosin light-chain kinase. In skeletal muscle, calcium binds to troponin.

11. Actin and Myosin:

    • Myosin forms the thick filament.

    • Actin forms the thin filament.     These contractile proteins physically interact via crossbridges to pull thin filaments during muscle contraction.

12. Sarcomere Regions:

    • a. $I\ band$: Contains thin filaments only.

    • b. $H\ zone$: Contains thick filaments only.

    • c. $A\ band$: Contains the full length of the thick filament.

13. Incorrect: Actin and myosin filaments themselves do not shrink or shorten. Instead, the sliding filament mechanism occurs: thin filaments slide past thick filaments toward the $M\ line$, shortening the sarcomere and muscle fiber.

14. Muscle Attachments and Group Roles:

    • Origin: Attachment to the stationary or less movable bone.

    • Insertion: Attachment to the movable bone.

    • a. Agonist: Biceps brachii.

    • b. Antagonist: Triceps brachii.

15. Skeletal muscle contraction would be prevented or reduced: Blocking acetylcholine from binding to motor end plate receptors prevents muscle membrane depolarization, stopping T-tubule action potentials and preventing calcium release from the sarcoplasmic reticulum.

16. Summary Sequence Sentence: A somatic motor neuron releases acetylcholine (ACh) to stimulate a muscle fiber action potential, inducing sarcoplasmic calcium release that allows actin and myosin to interact and contract the muscle.