Chapter 10 Muscle Tissue Lecture Presentation Notes

Introduction to Muscle Tissue

• Muscle tissue is a primary tissue type.
  • Includes three types: skeletal, cardiac, and smooth muscle.

10.1 Functions of Muscles

• Muscle tissue cells are specialized for contraction.
  • Skeletal muscles facilitate body movement by pulling on bones.
  • Cardiac and smooth muscles control internal movements.
• Common properties of muscle tissue:
  • Excitability (responsiveness): Ability to respond to stimuli.
  • Contractility: Ability of cells to shorten.
  • Extensibility: Ability to stretch.
  • Elasticity: Ability to recoil to the original length.
• Functions of skeletal muscle:
  • Producing movement.
  • Maintaining posture and body position.
  • Supporting soft tissues.
  • Guarding body entrances and exits.
  • Maintaining body temperature.
  • Storing nutrients.

10.2 Organization of Skeletal Muscle

• Skeletal muscles consists of:
  • Primarily skeletal muscle tissue.
  • Connective tissues.
  • Blood vessels.
  • Nerves.
• Skeletal muscles have three layers of connective tissue:
  • Epimysium.
  • Perimysium.
  • Endomysium.

Epimysium

• Layer of collagen fibers surrounding the entire muscle.
• Connected to deep fascia.
• Separates muscle from surrounding tissues.

Perimysium

• Surrounds muscle fiber bundles called fascicles.
• Contains:
  • Collagen fibers.
  • Elastic fibers.
  • Blood vessels.
  • Nerves.

Endomysium

• Surrounds individual muscle cells (muscle fibers).
• Contains:
  • Capillary networks.
  • Myosatellite cells (stem cells) for repair.
  • Nerve fibers.

Tendons and Aponeuroses

• Collagen fibers of epimysium, perimysium, and endomysium converge to form:
  • Tendon (bundle-like).
  • Aponeurosis (sheet-like).
• These structures attach skeletal muscles to bones.

Vascular Networks and Innervation

• Skeletal muscles have extensive vascular networks that:
  • Deliver oxygen and nutrients.
  • Remove metabolic wastes.
• Muscles contract when stimulated by the central nervous system.
  • Often referred to as voluntary muscles.
  • Note: the diaphragm typically operates subconsciously.

10.3 Skeletal Muscle Fibers

• Skeletal muscle fibers are large in size compared to other cells.
  • Multinucleate, containing hundreds of nuclei.
  • Develop through the fusion of embryonic cells called myoblasts.
  • Known as striated muscle cells due to their striped appearance.

Sarcolemma

• Plasma membrane of a muscle fiber.
• Surrounds the sarcoplasm (cytoplasm of a muscle fiber).
• A sudden change in membrane potential triggers a contraction.

Transverse Tubules (T-tubules)

• Tubes extending from the sarcolemma deep into the sarcoplasm.
• Transmit action potentials from the sarcolemma into the cell interior.
  • These action potentials trigger muscle contraction.

Sarcoplasmic Reticulum (SR)

• Tubular network surrounding each myofibril.
• Similar to smooth endoplasmic reticulum.
• Forms chambers called terminal cisternae that attach to T-tubules.
  • Two terminal cisternae plus a T-tubule form a triad.
• Specialized for storage and release of calcium ions (Ca^{2+}).
  • These ions are actively transported from the cytosol into the terminal cisternae.

Myofibrils

• Lengthwise subdivisions within a muscle fiber.
• Responsible for muscle contraction.
• Made of bundles of protein filaments (myofilaments).
  • Thin filaments: Composed primarily of actin.
  • Thick filaments: Composed primarily of myosin.

Sarcomeres

• Smallest functional units of a muscle fiber.
• Interactions between filaments produce contraction.
• Arrangement of filaments creates a striated pattern in myofibrils.
  • Dark bands (A bands).
  • Light bands (I bands).

A Band

• M line:
  • Located in the center of the A band.
  • Proteins stabilize the positions of thick filaments.
• H band:
  • Located on either side of the M line.
  • Contains thick filaments but no thin filaments.
• Zone of overlap:
  • Dark region where thick and thin filaments overlap.

I Band

• Contains thin filaments but no thick filaments.
• Z lines:
  • Bisect I bands.
  • Mark boundaries between adjacent sarcomeres.
• Titin:
  • Elastic protein.
  • Extends from tips of thick filaments to the Z line.
  • Maintains filament alignment and aids in restoring resting sarcomere length.

Thin Filaments

• Composed of F-actin, nebulin, tropomyosin, and troponin proteins.
• Filamentous actin (F-actin):
  • Twisted strand composed of two rows of globular G-actin molecules.
  • Active sites on G-actin bind to myosin.
• Nebulin:
  • Holds F-actin strands together.

Tropomyosin

• Covers active sites on G-actin.
• Prevents actin-myosin interaction in a resting muscle.

Troponin

• Globular protein.
• Binds to tropomyosin, G-actin, and calcium ions (Ca^{2+}).
  • Crucial for initiating muscle contraction.

Thick Filaments

• Each thick filament contains about 300 myosin molecules.
• Myosin molecule structure:
  • Tail: Binds to other myosin molecules.
  • Head: Consists of two globular protein subunits; projects toward the nearest thin filament.
• Core of titin recoils after stretching, aiding in restoring the sarcomere's resting length.

Sliding-Filament Theory

• During muscle contraction:
  • H bands and I bands narrow.
  • Zones of overlap widen.
  • Z lines move closer together.
  • Width of A band remains constant.
• This occurs because thin filaments slide toward the center of the sarcomere.

10.4 The Neuromuscular Junction (NMJ)

• Excitable membranes:
  • Present in skeletal muscle fibers and neurons.
  • Depolarization and repolarization events produce action potentials (electrical impulses).
• Skeletal muscle fibers contract due to stimulation by motor neurons.
  • This stimulation occurs at the neuromuscular junction (NMJ).

Neuromuscular Junction (NMJ)

• Synapse between a neuron and a skeletal muscle fiber.
• Events at the NMJ:
  • Axon terminal of the motor neuron releases the neurotransmitter acetylcholine (ACh) into the synaptic cleft.
  • ACh binds to chemically gated Na^{+} channels on the muscle fiber, opening them.
  • Na^{+} enters the cell, depolarizing the motor end plate.
  • An action potential is generated.

Excitation-Contraction Coupling

• Process linking the action potential to muscle contraction:
  • Action potential travels down T-tubules to triads.
  • Calcium ions (Ca^{2+}) are released from terminal cisternae of the sarcoplasmic reticulum (SR).
  • Ca^{2+} binds to troponin, changing its shape.
  • The troponin-tropomyosin complex shifts position, exposing active sites on thin filaments.
  • The contraction cycle is initiated.

Contraction Cycle

• Begins with the arrival of calcium ions (Ca^{2+}) within the zone of overlap in a sarcomere.
• Active-site exposure: Calcium ions (Ca2+) bind to troponin, weakening the bond between actin and the troponin-tropomyosin complex, which rolls tropomyosin away from active sites on actin.
• Cross-bridge formation: Energized myosin heads bind to exposed active sites on actin, forming cross-bridges.
• Myosin head pivoting (power stroke): Energy stored in the myosin head is released as the myosin head pivots toward the M line, pulling the actin filament along with it which also releases ADP and a phosphate group.
• Cross-bridge detachment: ATP binds to the myosin head, breaking the link between the myosin head and the active site on the actin molecule.
• Myosin reactivation: Myosin hydrolyzes ATP into ADP and P, recocking the myosin head.
• The cycle repeats as long as Ca^{2+} concentrations remain elevated and ATP is available.

Generation of Muscle Tension

• Muscle cells generate tension (pull) upon contraction.
• Movement occurs when tension overcomes the load (resistance).
• The entire muscle shortens at the same rate, because all sarcomeres contract together.
• Speed of shortening depends on the cycling rate (number of power strokes per second).

Duration of Contraction

• Depends on:
  • Duration of the neural stimulus.
  • Availability of ATP.

Relaxation

• Relaxation occurs when:
  • Calcium ions (Ca^{2+}) are pumped back into the sarcoplasmic reticulum (SR).
  • Calcium ion (Ca2+) detaches from troponin.
  • Troponin returns to its original position.
  • Active sites are re-covered by tropomyosin, ending the contraction.

Rigor Mortis

• Fixed muscular contraction after death.
• Results from:
  • ATP depletion, causing ion pumps to cease function.
  • Build-up of calcium ions (Ca^{2+}) in the cytosol.

10.5 Tension Production

• Muscle fiber tension is determined by:
  • Number of contracting sarcomeres (fixed).
  • Number of power strokes performed.
  • Fiber’s resting length at the time of stimulation.
  • Frequency of stimulation.

Length-Tension Relationship

• Tension produced is related to the length of the sarcomeres.
• Dependent on:
  • Number of power strokes performed by cross-bridges.
  • Amount of overlap between thick and thin filaments.
• Maximum tension occurs when the zone of overlap is large, allowing for the maximum number of cross-bridges to form.

Frequency of Stimulation

• A single neural stimulation produces a single contraction or twitch (lasting 7–100 msec).
• Sustained muscular contractions require many repeated stimuli.
• A myogram illustrates tension development in muscle fibers.

Phases of a Single Twitch

• Latent period: Action potential travels across the sarcolemma, calcium ion (Ca^{2+}) is released from the sarcoplasmic reticulum (SR).
• Contraction phase: Calcium ions (Ca^{2+}) bind to troponin, cross-bridges form, and tension builds to a peak.
• Relaxation phase: Calcium ion (Ca^{2+}) levels in cytosol fall, cross-bridges detach, and tension decreases.

Treppe

• A stair-step increase in tension.
• Caused by repeated stimulations immediately after the relaxation phase.
  • Stimulus frequency < 50/second.
• Generates a series of contractions with increasing tension.
• Typically observed in cardiac muscle, not skeletal muscle.

Wave Summation

• Increasing tension due to the summation of twitches.
• Caused by repeated stimulations before the end of the relaxation phase.
  • Stimulus frequency > 50/second.

Tetanus

• Maximum tension.
• Incomplete tetanus: Muscle produces near-maximum tension due to rapid cycles of contraction and relaxation.
• Complete tetanus: Higher stimulation frequency eliminates the relaxation phase; muscle is in continuous contraction and all potential cross-bridges form.

10.6 Muscle Contractions

• Tension production depends on the number of stimulated muscle fibers.

Motor Unit

• A motor neuron and all the muscle fibers it controls.
  • May contain a few or thousands of muscle fibers.
• All fibers in a motor unit contract simultaneously.
• Fasciculation is an involuntary “muscle twitch” involving more than one muscle fiber.

Recruitment

• Increase in the number of active motor units.
• Produces a smooth, steady increase in tension.
• Maximum tension is achieved when all motor units reach complete tetanus (sustained for only a short time).
• Sustained contractions produce less than maximum tension; motor units are allowed to rest in rotation.

Muscle Tone

• Normal tension and firmness of a muscle at rest.
  • Motor units actively stabilize the positions of bones and joints, maintaining balance and posture.
• Elevated muscle tone increases resting energy consumption.

Types of Muscle Contractions

• Classified based on the pattern of tension production:
  • Isotonic.
  • Isometric.

Isotonic Contractions

• Skeletal muscle changes length, resulting in motion.
  • Isotonic concentric contraction: Muscle tension exceeds the load (resistance), and the muscle shortens.
  • Isotonic eccentric contraction: Muscle tension is less than the load, and the muscle elongates.

Isometric Contractions

• Skeletal muscle develops tension but never exceeds the load.
• Muscle does not change length.

Load and Speed of Contraction

• Inversely related; the heavier the load, the longer it takes for movement to begin.
• Tension must exceed the load before shortening can occur.

Muscle Relaxation and Return to Resting Length

• Elastic forces:
  • Tendons recoil after a contraction, helping to return muscle fibers to their resting length.
• Opposing muscle contractions:
  • Opposing muscles return a muscle to its resting length quickly.
• Gravity:
  • Assists opposing muscles.

10.7 Energy to Power Contractions

• Adenosine triphosphate (ATP) is the direct energy source for muscle contraction.
  • Contracting muscles consume significant amounts of ATP.
  • Muscles store enough ATP to initiate a contraction.
  • More ATP must be generated to sustain a contraction.

ATP Generation

• At rest, skeletal muscle fibers produce more ATP than needed; this energy is transferred to creatine, creating creatine phosphate (CP).
  • CP stores energy and converts ADP back to ATP.
• The enzyme creatine kinase (CK) catalyzes the conversion of ADP to ATP using the energy stored in CP.
• When CP is depleted, other mechanisms are used to generate ATP.

Mechanisms for ATP Generation

• Direct phosphorylation of ADP by creatine phosphate (CP).
• Anaerobic metabolism (glycolysis).
• Aerobic metabolism (citric acid cycle and electron transport chain).

Glycolysis

• Anaerobic process that breaks down glucose from glycogen stored in skeletal muscles.
• An important energy source for peak muscular activity.
• Produces two ATP molecules per molecule of glucose.

Aerobic Metabolism

• Primary energy source of resting muscles, breaking down fatty acids, it requires oxygen.

Muscle Metabolism

• At rest, skeletal muscles metabolize fatty acids and store glycogen and CP.
• During moderate activity, muscles generate ATP through aerobic breakdown of glucose.
• At peak activity, pyruvate produced via glycolysis is converted to lactate.

Recovery Period

• Time required after exertion for muscles to return to normal.

Lactate Removal and Recycling (Cori Cycle)

• Lactate is transferred from muscles to the liver.
• The liver converts lactate to pyruvate, then to glucose.
• Glucose is used to rebuild glycogen reserves in muscle cells.

Oxygen Debt

• The body needs more oxygen than usual to normalize metabolic activities after exercise, increasing breathing rate and depth; excess postexercise oxygen consumption (EPOC).

Heat Production and Loss

• Active skeletal muscles produce heat, releasing up to 85 percent of the heat needed to maintain normal body temperature.

Hormones and Muscle Metabolism

• Several hormones increase metabolic activities in skeletal muscles:
  • Growth hormone
  • Testosterone
  • Thyroid hormones
  • Epinephrine

10.8 Muscle Performance

• Muscle performance is influenced by force and endurance.
  • Force: The maximum amount of tension produced.
  • Endurance: The amount of time an activity can be sustained.
• Force and endurance depend on:
  • The types of muscle fibers.
  • Physical conditioning.

Types of Skeletal Muscle Fibers

• Fast fibers
• Slow fibers
• Intermediate fibers

Fast Fibers

• Majority of skeletal muscle fibers.
• Contract very quickly.
• Large diameter.
• Large glycogen reserves.
• Few mitochondria.
• Produce strong contractions, but fatigue quickly.

Slow Fibers

• Slow to contract and slow to fatigue.
• Small diameter.
• Numerous mitochondria.
• High oxygen supply from extensive capillary network.
• Contain myoglobin (red pigment that binds oxygen).

Intermediate Fibers

• Mid-sized.
• Little myoglobin.
• Slower to fatigue than fast fibers.

Muscle Performance and Fiber Distribution

• White muscles: mostly fast fibers (e.g., chicken breast).
• Red muscles: mostly slow fibers (e.g., chicken legs).
• Most human muscles contain a mixture of fiber types and are pink.

Muscle Hypertrophy

• Muscle growth from heavy training that causes increases in:
  • Diameter of muscle fibers.
  • Number of myofibrils.
  • Number of mitochondria.
  • Glycogen reserves.

Muscle Atrophy

• Reduction of muscle size, tone, and power due to lack of activity.

Changes in Muscle Tissue with Aging

• Skeletal muscle fibers become smaller in diameter.
• Skeletal muscles become less elastic (fibrosis).
• Tolerance for exercise decreases.
• Ability to recover from muscular injuries decreases.

Muscle Fatigue

• Muscles can no longer perform at a required level; symptoms:
  • Depletion of metabolic reserves.
  • Damage to sarcolemma and sarcoplasmic reticulum.
  • Decline in pH, which affects calcium ion binding and alters enzyme activities.
  • Weariness due to low blood pH and pain.

Physical Conditioning

• Improves power and endurance.
  • Anaerobic endurance (e.g., 50-meter dash, weight lifting):
  • Uses fast fibers and stimulates hypertrophy.
  • Improved by frequent, brief, intensive workouts.
  • Aerobic endurance (prolonged activities):
  • Supported by mitochondria.
  • Does not stimulate muscle hypertrophy.
  • Training involves sustained, low levels of activity.

Effects of Training

• Improvements in aerobic endurance result from:
  • Alterations in the characteristics of muscle fibers.
  • Improvements in cardiovascular performance.

10.9 Cardiac Muscle Tissue

• Cardiac muscle found only in the heart has excitable membranes and the cells are striated like skeletal muscle cells.

Structural Characteristics

• Cardiac Muscle cells are small, branched with a single nucleus, have short wide T tubules(no triads), Sarcoplasmic Reticulum with no terminal cisternae, almost totally dependent on aerobic metabolism, contain lots of myoglobin and many mitochondria.
  -Contact each other via intercalated discs.

Intercalated Discs

• Specialized connections joining sarcolemmas of adjacent cardiac muscle cells via gap junctions and desmosomes.
• Functions include stabilizing positions of adjacent cells, maintaining three-dimensional structure of tissue, allowing ions to move from one cell to another, so cardiac muscle cells beat in rhythm.

Functional Characteristics

• Automaticity: Contraction without neural stimulation is controlled by the pacemaker cells, the nervous system can alter pace and tension of contractions, contractions lasts ten times longer than those in skeletal muscles and refractory periods are longer, and wave summation and tetanic contractions are prevented due to special properties of sarcolemma.

10.10 Smooth Muscle Tissue

• Functions include integumentary system (arrector pili muscles erect hairs), cardiovascular and respiratory system (regulates blood pressure and air flow), digestive and urinary systems(forms sphincters moves materials along and out of the body), and the reproductive system (transports gametes and expels fetus).

Structural Characteristics

• Long, slender, spindle-shaped cells.
• Single, central nucleus.
• no T tubules, myofibrils, or sarcomeres(nonstriated muscle).
• Scattered thick filaments with many myosin heads.
• Thin filaments attached to dense bodies that connect adjacent cells, transmitting contractions.
• No tendons or aponeuroses.

Functional Characteristics

• Excitation-contraction coupling- free Calcium ions (Ca^{2+}) in cytoplasm triggers contraction, Calcium ions (Ca^{2+}) bind with calmodulin, activates myosin light chain kinase, and allows myosin heads to attach to actin.

Length-tension relationships

• Lack of sarcomeres tension and resting length are not directly related and a stretched smooth muscle can also contract which means the smooth muscle cells have plasticity(ability to function over a wide range of lengths).

Control of Contractions

• Multiunit smooth muscle cells- Innervated in motor units and each cell be connected to more than one motor neuron.
• Visceral smooth muscle cells- Not connected to motor neurons, arranged in sheets or layers, and the rhythmic cycles of activity are controlled by pacesetter cells.

Smooth Muscle Tone

• Normal background level of activity.
• Can be decreased by neural, hormonal, or chemical factors.