Human Anatomy and Physiology - Chapter 09: Muscles and Muscle Tissue

Human Anatomy and Physiology

Chapter 09: Muscles and Muscle Tissue

9.1 Overview of Muscle Tissue

• Muscle tissue comprises nearly half of the body’s mass.
• It can transform chemical energy stored in ATP into directed mechanical energy capable of exerting force.
• Key areas of investigation into muscle include:
  • Types of muscle tissue
  • Characteristics of muscle tissue
  • Muscle functions
• Terminology associated with muscle includes the prefixes myo, mys, and sarco.
  • Example: sarcoplasm refers to the cytoplasm of muscle cells.
• There are three types of muscle tissue:
  • Skeletal muscle
  • Cardiac muscle
  • Smooth muscle
• Only skeletal and smooth muscles are elongated, and they are referred to as muscle fibers.

Types of Muscle Tissue

Skeletal Muscle

• Skeletal muscle tissue is organized into skeletal muscles, which are attached to bones and skin.
• Characteristics of skeletal muscle fibers include:
  • They are the longest muscle fibers and have striations (striped appearance).
  • They are classified as voluntary muscles, meaning they can be consciously controlled.
  • They contract rapidly, yet they tire easily and are inherently powerful.
• Key words associated with skeletal muscle: skeletal, striated, and voluntary.

Cardiac Muscle

• Cardiac muscle tissue is found exclusively in the heart.
• It constitutes the bulk of the heart's walls and is characterized by being:
  • Striated
  • Involuntary, meaning it cannot be consciously controlled.
  • The heart contracts at a steady rate due to intrinsic pacemaker cells, but the nervous system can influence the rate of contraction.
• Key terms for cardiac muscle: cardiac, striated, and involuntary.

Smooth Muscle

• Smooth muscle tissue is located in the walls of hollow organs.
• Examples of organs with smooth muscle include the stomach, urinary bladder, and airways.
• Characteristics of smooth muscle include:
  • It is non-striated.
  • Involuntary control, meaning it cannot be consciously controlled.
• Key terms for smooth muscle include: visceral, nonstriated, and involuntary.

Comparison of Skeletal, Cardiac, and Smooth Muscle

Characteristics of Muscle Tissue

All muscles share four primary characteristics:

1. Excitability (Responsiveness): The ability to receive and respond to stimuli.
2. Contractility: The capability to forcibly shorten when stimulated.
3. Extensibility: The ability to stretch.
4. Elasticity: The ability to recoil to resting length.

Muscle Functions

Muscle tissue serves four essential functions:

1. Produce movement: Muscles are responsible for all locomotion and manipulation (Example: walking, digesting food, and pumping blood).
2. Maintain posture and body position.
3. Stabilize joints.
4. Generate heat during contraction.

Additional Functions

• Skeletal muscle acts as an organ comprising different tissue types, characterized by three features:
  1. Nerve and blood supply: Each muscle receives nerve, artery, and veins. Voluntary skeletal muscle has nerves supplying every fiber for functional control.
  2. Contracting muscle fibers require substantial amounts of oxygen and nutrients and need quick removal of metabolic waste products.

Connective Tissue Sheaths

• Each skeletal muscle and fiber is enveloped in connective tissue.
• These sheaths ensure support and reinforce the entire muscle structure.
• Connective sheath organization from external to internal:
  1. Epimysium: Dense irregular connective tissue surrounding the entire muscle; may blend with fascia.
  2. Perimysium: Fibrous connective tissue surrounding fascicles (groups of muscle fibers).
  3. Endomysium: Fine areolar connective tissue surrounding each muscle fiber.

Attachments

• Muscles span joints and attach to bones in at least two locations:
  • Insertion: Attachment to the movable bone.
  • Origin: Attachment to an immovable or less movable bone.
• Attachments may be direct or indirect:
  • Direct (fleshy): Epimysium fused to the periosteum of the bone or perichondrium of cartilage.
  • Indirect: Connective tissue wrappings extend beyond the muscle as ropelike tendons or sheetlike aponeurosis.

9.3 Muscle Fiber Microanatomy and Sliding Filament Model

• Skeletal muscle fibers are long, cylindrical cells containing multiple nuclei.
• Sarcoplasm: Muscle fiber cytoplasm.
• Contains numerous glycosomes for glycogen storage and myoglobin for oxygen storage.
• Key components include modified organelles:
  • Myofibrils: Play a critical role in muscle contraction and occupy approximately 80% of muscle cell volume.
  • Sarcoplasmic reticulum: Smooth endoplasmic reticulum associated with calcium storage and release.
  • T tubules: Deep invaginations of the sarcolemma that enhance fiber surface area.

Myofibrils

• Myofibrils are densely packed, rod-like structures, with a single muscle fiber capable of containing thousands.
• Striations: Stripes along myofibrils formed from a repeating series of dark (A bands) and light (I bands) bands.
  • A bands: Dark regions.
  • H zone: Lighter region in the middle of each A band.
  • M line: Line of protein (myomesin) bisecting the H zone.
  • I bands: Lighter areas appearing on either side of the A bands.
  • Z discs: Coin-shaped proteins marking the midline of each I band.

Sarcomere

• Sarcomere: The smallest contractile unit (functional unit) of muscle fiber.
  • It contains an A band and half of an I band at each end and consists of the area between Z discs.
  • Individual sarcomeres are aligned end to end along the myofibril, akin to boxcars in a train.

Myofilaments

• The orderly arrangement of actin and myosin myofilaments defines the sarcomere.
  • Actin myofilaments: Thin filaments extending across the I band and partway into the A band, anchored to Z discs.
  • Myosin myofilaments: Thick filaments extending the entire length of the A band, connected at the M line.
• Cross-section of the sarcomere reveals a hexagonal arrangement of one thick filament surrounded by six thin filaments.

Molecular Composition of Myofilaments

Thick Filaments

• Composed of the protein myosin which contains two heavy and four light polypeptide chains.
• Heavy chains intertwine to form a myosin tail, while light chains form the myosin globular heads.
• The heads play a critical role during contraction, linking thick and thin filaments to form cross bridges.
• The myosin heads are offset from each other, resulting in a staggered arrangement along the thick filament.

Thin Filaments

• Composed of the fibrous protein actin, which is a polypeptide made up of kidney-shaped G-actin (globular actin) subunits.
• G-actin subunits bear active sites for myosin head attachment during contraction.
• G-actin subunits link together to form long fibrous F-actin (filamentous actin), which twists to form thin filaments.
• Tropomyosin and troponin are regulatory proteins associated with actin.

Other Structural Proteins of Myofibrils

• Elastic filaments (titin) hold thick filaments in place, facilitate recoil after stretch, and resist excessive stretching.
• Dystrophin connects thin filaments to proteins of the sarcolemma.
• Nebulin, myomesin, and C proteins bind filaments or sarcomeres together, maintaining sarcomere alignment.

Clinical – Homeostatic Imbalance

Duchenne Muscular Dystrophy (DMD)

• DMD is the most common and severe form of muscular dystrophy, typically manifesting in childhood.
• It is inherited as a sex-linked recessive disease, primarily affecting males (1 in 3600 births).
• Symptoms appear between ages 2 and 7 when affected boys begin to fall and display clumsiness.
• The disease progresses from the extremities upward, ultimately affecting head and chest muscles and cardiac muscle.
• With supportive care, individuals with DMD often live into their 30s or beyond.
• It is caused by a defective gene for dystrophin, which stabilizes the sarcolemma and links thin filaments to the extracellular matrix.
• Sarcolemma of DMD patients is fragile, allowing excessive calcium to enter, damaging contractile fibers and leading to increased apoptosis of muscle cells.

Sarcoplasmic Reticulum and T Tubules

• The sarcoplasmic reticulum (SR) is a network of smooth endoplasmic reticulum tubules surrounding each myofibril, mainly running longitudinally.
• Terminal cisterns form perpendicular cross channels at the A–I band junction.
• The SR regulates intracellular Ca²⁺ levels and functions in calcium storage and release.

T Tubules

• T tubules (transverse tubules) are formed by protrusions of the sarcolemma deep into the cell interior, significantly increasing muscle fiber surface area.
• Their lumen is continuous with the extracellular space, allowing electrical nerve transmission to reach deep into each muscle fiber interior.
• Each T tubule penetrates the cell’s interior at each A–I band junction between terminal cisterns, forming a triad with the adjacent terminal cisterns.

Triad Relationships

• T tubules possess integral membrane proteins acting as voltage sensors that change shape in response to electrical currents.
• SR cistern membranes also contain integral proteins that protrude into intermembrane space and control the opening of calcium channels in SR.
• When an electrical impulse travels down the T tubule, these proteins change shape, resulting in the release of Ca²⁺ into the cytoplasm.

Sliding Filament Model of Contraction

• Contraction involves the activation of cross bridges to generate force.
• Muscle shortening occurs when the tension generated by these cross bridges exceeds the opposing forces.
• Contraction ceases when the cross bridges become inactive.

Mechanism of Sliding Filaments

• In a relaxed state, thin and thick filaments overlap minimally at the ends of the A band.
• The sliding filament model posits that during contraction, the thin filaments slide past the thick filaments, increasing their overlap.
• Importantly, neither thick nor thin filaments change in length; they merely overlap more during contraction.
• Upon stimulation by the nervous system, the myosin heads bind to actin molecules, initiating the crossbridge cycle, leading to muscle contraction.

Crossbridge Cycling

• Cross bridges form and break multiple times, with each action pulling thin filaments closer to the center of the sarcomere.
• This process leads to:
  • Shortening of the muscle fiber.
  • Z discs being drawn closer together.
  • Reduction in I band length.
  • Disappearance of H zones.
  • Movement of A bands closer together.

Excitation-Contraction Coupling

• Excitation-contraction coupling refers to the mechanisms linking action potentials along the sarcolemma to the contraction of muscle myofilaments.
• Events leading to muscle fiber contraction include:
  1. Action potential is generated at the neuromuscular junction.
  2. Action potential travels along the sarcolemma and down T tubules.
  3. Voltage-sensitive proteins undergo a conformational change, stimulating Ca²⁺ release from the sarcoplasmic reticulum.
  4. Ca²⁺ binds to troponin, leading to exposure of myosin binding sites on actin and subsequent muscle contraction via crossbridge cycling.

Events at the Neuromuscular Junction

1. Action potential (AP) arrives at the axon terminal.
2. Voltage-gated calcium channels open, and calcium enters the motor neuron.
3. Calcium entry triggers the release of acetylcholine (ACh) neurotransmitter into the synaptic cleft.
4. ACh diffuses across the cleft and binds to ACh receptors on the sarcolemma.
5. Binding of ACh opens ion channels, allowing Na⁺ to enter the muscle fiber, resulting in an end plate potential.
6. Acetylcholinesterase breaks down ACh in the synaptic cleft, terminating the signal.

Generation of an Action Potential Across the Sarcolemma

• The resting sarcolemma is polarized, possessing a negative inside relative to the outside.
• Action Potential Generation Steps:
  1. End Plate Potential: ACh binds to receptors, causing ion channels to open; Na⁺ flows in causing depolarization.
  2. Depolarization: If EPP reaches threshold, voltage-gated Na⁺ channels open, allowing a large influx of Na⁺ and resulting in action potential.
  3. Repolarization: Na⁺ channels close while K⁺ channels open, restoring resting conditions.
  4. Refractory Period: The muscle fiber cannot be stimulated until it repolarizes.

Muscle Fiber Contraction Background

• For muscle contraction to occur, the decision to move is initiated by the brain, with signals transmitted down the spinal cord to motor neurons, activating muscle fibers.
• Neurons and muscle fibers are excitable cells, capable of changing their resting membrane potential through action potentials.
• Action potentials cross from neuron to muscle via the neurotransmitter acetylcholine (ACh).
• Two classes of ion channels play crucial roles in changing membrane potentials:
  • Chemically gated ion channels: Opened by chemical messengers like neurotransmitters.
  • Voltage-gated ion channels: Open or close in response to changes in membrane potential.

Muscle Twitch and Graded Muscle Responses

Muscle Twitch

• A muscle twitch is the simplest contraction, resulting from a muscle fiber’s response to a single action potential from a motor neuron.
• It consists of three phases:
  1. Latent Period: Excitation-contraction coupling events occur; no tension is generated.
  2. Period of Contraction: Cross bridge formation occurs, and tension increases.
  3. Period of Relaxation: Ca²⁺ returns to the SR; tension declines to zero.

Graded Muscle Responses

• Muscle responses vary in strength and are termed graded muscle responses, which allow for smooth control of muscle contractions depending on the demands placed on the muscle.
• Responses can be modified by:
  • Changing frequency of stimulation: Rapid successive stimuli result in added tension (summation).
  • Changing strength of stimulation: Includes subthreshold (too weak to induce contraction), threshold (first observable contraction), minimal, and maximal stimuli.

Adaptation to Exercise

Aerobic (Endurance) Exercise

• Leads to increased muscle capillaries, more mitochondria, and higher myoglobin levels, enhancing endurance and strength while resisting fatigue.

Resistance Exercise

• Often anaerobic (weight lifting), leads to:
  • Muscle hypertrophy: Increase in fiber size, enhanced strength.
  • More mitochondria, extensive myofilaments, increased glycogen stores, and more connective tissue.

Smooth Muscle

Characteristics

• Found in the walls of most hollow organs except the heart.
• Organized into sheets of tightly packed fibers, including:
  • Longitudinal layer: Fibers run parallel, and contraction shortens the organ.
  • Circular layer: Fibers encircle the organ, constricting the lumen upon contraction.

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Differences Between Smooth and Skeletal Muscle

• Smooth muscle fibers are shorter and narrower than skeletal muscle fibers and only have one nucleus.
• They lack striations and have less elaborate sarcoplasmic reticulum, using extracellular calcium primarily for contraction.
• Smooth muscle fibers are connected via gap junctions unlike electrically isolated skeletal muscle fibers.

Mechanism of Smooth Muscle Contraction

• Smooth muscle contractions occur in response to neural, hormonal, or local chemical signals, with the contraction mechanism involving the interaction of actin and myosin.
• Ca²⁺ binds to calmodulin, activating myosin kinase, leading to phosphorylation and crossbridge formation requiring more steps to relax compared to skeletal muscle.

Special Features of Smooth Muscle

• Displays a stress-relaxation response, adapting to stretch but retaining contractile ability.
• Capable of contraction at varying lengths, allowing container organs (e.g., stomach and bladder) to store contents without flaccid states.