Chapter 12

12.1 Skeletal Muscle

Types of Muscles

Muscle tissue in the human body is classified into three main types, each with distinct structure and function.

• Skeletal muscle: Voluntary, striated muscle attached to bones for movement. Multinucleated

• Cardiac muscle: Involuntary, striated muscle found only in the heart. Uninucleate, Intercalated disks, branches

• Smooth muscle: Involuntary, non-striated muscle found in the walls of hollow organs.

Key Anatomical Terms

Understanding muscle anatomy requires familiarity with several key terms:

• Tendon: Connective tissue attaching muscle to bone.

• Origin: The fixed attachment point of a muscle.

• Insertion: The movable attachment point of a muscle.

• Joint: The location where two bones meet, allowing movement.

• Flexor: Muscle that decreases the angle at a joint.

• Extensor: Muscle that increases the angle at a joint.

• Antagonistic muscle groups: Muscles that produce opposite movements at a joint.

Skeletal Muscle Structure

Skeletal muscles are composed of bundles of muscle fibers, which are further organized into fascicles.

• Muscle fiber: A single muscle cell, multinucleated and elongated.

• Fascicle: A bundle of muscle fibers surrounded by connective tissue.

• Besides muscle fibers, skeletal muscle contains blood vessels, nerves, and connective tissue.

Muscle Fiber Anatomy

Muscle fibers have specialized structures for contraction and energy storage.

• Sarcoplasm: The cytoplasm of a muscle fiber, containing organelles and glycogen granules., like cytoplasm

• Myofibril: Rod-like units within muscle fibers, composed of contractile proteins.

• Sarcoplasmic reticulum (SR): Specialized endoplasmic reticulum storing calcium ions.

• Transverse tubules (T-tubules): Invaginations of the sarcolemma that transmit action potentials.

• Triad: A structure formed by a T-tubule flanked by two SR cisternae.

• Sarcolemma: cell MeMbrane

Glycogen granules are abundant in muscle fibers to provide a rapid source of glucose for ATP production during contraction.

Myofibrils: Contractile Structures

Myofibrils contain the proteins responsible for muscle contraction.

• Contractile proteins: Actin (thin filament) and myosin (thick filament).

• Regulatory proteins: Troponin and tropomyosin, which control access to actin's binding sites.

• Accessory proteins: Titin and nebulin, which stabilize and organize the filaments.

Myosin is a motor protein with a head that binds actin and hydrolyzes ATP, enabling movement. Actin exists as G-actin (globular) and F-actin (filamentous).

Sarcomere Structure

The sarcomere is the functional unit of muscle contraction, defined by specific bands and lines.

• Z disk: Boundary of the sarcomere, anchoring actin filaments.

• I band: Region containing only thin filaments (actin).

• A band: Region containing the entire length of thick filaments (myosin).

• M line: Center of the sarcomere, anchoring myosin filaments.

Titin provides elasticity and stabilizes myosin; nebulin helps align actin filaments.

Muscle Contraction Physiology

Key Terms

• Muscle tension: Force generated by muscle contraction.

• Load: The weight or resistance a muscle works against.

• Contraction: The process of generating tension within muscle fibers.

• Relaxation: The return of muscle fibers to their resting state.

Steps of Muscle Contraction

Muscle contraction involves a sequence of events:

1. Action potential travels along the sarcolemma and T-tubules.

2. Calcium is released from the SR.

3. Calcium binds to troponin(ENCOURAGES CONTRACTION), moving tropomyosin(PREVENTS CONTRACTION) and exposing actin binding sites.

4. Myosin heads bind to actin, forming crossbridges.

5. Power stroke occurs as myosin heads pivot, pulling actin filaments.

6. ATP binds to myosin, causing detachment from actin.

7. Calcium is pumped back into the SR, leading to relaxation.

Sliding Filament Theory

Muscle contraction is explained by the sliding filament theory, where actin and myosin filaments slide past each other to shorten the sarcomere.

• Tension generated in a muscle fiber is directly proportional to the number of crossbridges formed.

Sarcomere Band Changes During Contraction

• Sarcomere: Z disk to Z disk, they move during contraction

• A bands do not shorten during contraction. represents length of the myosin

  • I band (Light band, actin, shortens)and H zone decrease in width as filaments slide.

• M line- think middle of A band

• Crossbridgers where actin and myosin overlap

Crossbridge Cycle and Myosin ATPase

The crossbridge cycle describes the interaction between actin and myosin during contraction.

• Power stroke: Myosin head pivots, pulling actin filament.

• Myosin ATPase: Enzyme activity that hydrolyzes ATP, providing energy for contraction.

Calcium Signals and Excitation-Contraction Coupling

Calcium ions play a central role in initiating muscle contraction.

• Troponin and tropomyosin regulate actin's binding sites in response to calcium.

• Calcium is released from the SR via signal transduction pathways.

Relaxation

• Muscle fiber ends contraction by pumping calcium back into the SR via Ca2+-ATPase.

Timing of E-C Coupling

• A single contraction-relaxation cycle is called a twitch.

• Latent period: The delay between stimulus and contraction, due to E-C coupling events.

ATP Supply and Muscle Fatigue

• Phosphocreatine: Provides rapid ATP regeneration in muscle.

  • Creatine kinase is an enzyme that converts and breaks down atp

  • ATP +creatine uses creatine kinase to make ADP + phosphocreatine

  • Phosphocreatine+ ADP uses creatine kinase to creat creatine and ATP

• Aerobic metabolism yields more ATP per glucose than anaerobic metabolism(produces lactic acid).

• Muscle fatigue: Decline in ability to generate force, influenced by factors such as energy supply, ion imbalances, and central nervous system.

• Central fatigue: Originates in the CNS; peripheral fatigue: Originates in the muscle.

Muscle Fiber Types

Muscle fibers are classified by speed and resistance to fatigue.

• Type I (slow-twitch): Slow contraction, high fatigue resistance, high myoglobin.

• Type IIa (fast-twitch oxidative): Fast contraction, moderate fatigue resistance.

• Type IIb/x (fast-twitch glycolytic): Fast contraction, low fatigue resistance.

Duration of contraction is determined by fiber type and myosin ATPase activity.

Resting Fiber Length and Tension

• Tension generated is optimal at intermediate sarcomere length; too short or too long reduces tension.

Summation and Tetanus

• Summation: Increased force due to rapid, repeated stimulation.

• Unfused tetanus: Partial relaxation between stimuli.

• Fused (complete) tetanus: No relaxation; maximal force.

Motor Units

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

• Each muscle fiber is innervated by only one motor neuron.

• Movements requiring fine control use motor units with few muscle fibers.

Plasticity and Recruitment

• Muscle fibers can adapt their metabolic properties with use (plasticity).

• Force of contraction varies by recruiting more or fewer motor units.

• Asynchronous recruitment: Alternating active motor units to prevent fatigue during sustained contraction.

12.2 Mechanics of Body Movement

Muscle Mechanics

• Mechanics: The study of forces and movement in muscle physiology.

Types of Contractions

• Isotonic contraction: Muscle changes length, moves a load.

• Isometric contraction: Muscle generates force without changing length.

Series Elastic Elements

• Structures such as tendons and connective tissue that stretch during contraction, affecting force transmission.

Levers and Fulcrums

• Bones act as levers; joints serve as fulcrums for movement.

• Increasing load size decreases contraction speed.

12.3 Smooth Muscle

Groups and Types

• Six major groups of smooth muscles are found in humans, each with specific functions.

• Contraction patterns include phasic (intermittent) and tonic (sustained).

• Single-unit smooth muscle: Cells contract together via gap junctions.

• Multi-unit smooth muscle: Cells contract independently.

Functional Principles

• Some principles of skeletal muscle function apply to smooth muscle, such as force generation and response to stimuli.

• Smooth muscle differs in organization, control, and contractile proteins.

• Do not have sarcomers

• Actin is more plentiful, less myosin

• Myosin filaments are longer, surface covered with myosin heads.

Smooth Muscle Structure

• Smooth muscle lacks sarcomeres; contractile fibers are arranged in a network, under autonomic nervous system control, involuntary

• This arrangement allows for greater flexibility and varied contraction patterns.

Latch State and Relaxation

• Latch state: Sustained tension with low energy use.

• Relaxation involves molecular events that reduce crossbridge cycling.

MLCK/MLCP Ratio and Ca2+ Sensitivity

• MLCK: Myosin light chain kinase, promotes contraction. Activated by calcium binding to calmodulin

  • increases myosin ATPase activity.

• MLCP: Myosin light chain phosphatase, promotes relaxation, antagonistic to kinase.

• Force of contraction depends on the balance between MLCK and MLCP activity.

Calcium Initiates Contraction

• Contraction can be triggered by calcium from the SR or extracellular fluid (ECF).

• Electromechanical and pharmacomechanical coupling describe different pathways for calcium entry and contraction.

Relexation

• Ca pumped back into SR or out of the cell

• Ca unbinds from CaM decreased MLCK activity

• MLCP removes phospotase decreasing myosin ATPase activituy

Membrane Potentials

• Some smooth muscle cells have unstable membrane potentials, such as slow wave and pacemaker potentials.

Chemical Signals

• Autonomic neurotransmitters and hormones regulate smooth muscle activity.

• Antagonistic and tonic control depend on receptor types and signaling pathways.

• Increased IP3 promotes contraction; increased cAMP promotes relaxation.

• Paracrine signals (e.g., histamine, NO) can affect smooth muscle function.

12.4 Cardiac Muscle

Comparison with Other Muscle Types

• Cardiac muscle shares features with both skeletal and smooth muscle, such as striations and involuntary control.

• Unique features include intercalated discs, automaticity, and rhythmic contractions.

Summary Table: Muscle Types Comparison

Feature	Skeletal Muscle	Cardiac Muscle	Smooth Muscle
Striations	Present	Present	Absent
Control	Voluntary	Involuntary	Involuntary
Location	Bones	Heart	Hollow organs
Cell Shape	Long, cylindrical	Branched	Spindle-shaped
Unique Features	Multinucleated, fast contraction	Intercalated discs, rhythmic	Lack sarcomeres, slow contraction

Additional info: Where questions referenced diagrams or figures, academic context and definitions were expanded to ensure completeness. For tables, a summary comparison was inferred based on standard textbook content.