Anatomy and Physiology 2A Chapter 9

Main Functions of Muscle

Primary Function: Movement, stability, heat production.

Muscle Terminology

Prefixes and Terms: Myo and Mys: Refer to muscle (e.g., myocardium).

Sarco: Refers to flesh (e.g., sarcomere).

Muscle Fiber vs Muscle Cell: Both terms can be used interchangeably to denote muscle cells; however, 'muscle fiber' is often used specifically for skeletal muscle cells.

Types of Muscle

Skeletal Muscle

• Function: Facilitates movement of skeleton during voluntary actions such as walking, running, and lifting.

• Description: Striated, multi-nucleated fibers, allowing rapid contraction and high force generation.

• Control: Voluntary, controlled consciously by the somatic nervous system.

Cardiac Muscle

• Function: Pumps blood throughout the body, maintaining circulation and oxygen delivery.

• Description: Striated, branched fibers with intercalated discs that allow for synchronized contraction.

• Control: Involuntary, regulated by the autonomic nervous system and pacemaker cells.

Smooth Muscle

• Function: Moves substances through hollow organs such as intestines, blood vessels, and the bladder.

• Description: Non-striated, spindle-shaped fibers that allow for slow, sustained contractions.

• Control: Involuntary, controlled autonomously or by hormones, responding to various stimuli.

Characteristics of Muscle Tissue

Excitability: Ability to respond to stimuli such as nerve impulses or chemical signals, initiating a contraction.

Contractility: Ability to shorten forcibly when stimulated, leading to movement.

Extensibility: Ability to be stretched without damage, enabling muscles to operate across various ranges of motion.

Elasticity: Ability to recoil to original resting length, ensuring muscles return to their starting position after contraction.

Important Muscle Functions

2. Movement: Facilitates locomotion and mobility, enabling activities such as walking, running, and manipulation of objects.

3. Posture/Body Position: Maintains body position against gravity, supporting balance during static and dynamic activities.

4. Joint Stability: Stabilizes joints during movement to prevent injuries and ensure proper movement mechanics.

5. Heat Production: Generates heat as a byproduct of muscle contraction, playing a vital role in thermoregulation.

Skeletal Muscle Gross Anatomy

Nerve and Blood Supply: Richly supplied by nerves and blood vessels for function and metabolism; adequate blood supply is essential for energy delivery and waste removal.

Connective Tissue Sheaths:

• Endomysium: Surrounds individual muscle fibers, providing nutrients and support. areolar C.T

• Perimysium: Binds groups of muscle fibers into fascicles, containing nerves and blood vessels. dense irregular C.T.

• Epimysium: Encases the entire muscle, providing structural integrity and facilitating muscle function. dense irregular C.T.

Muscle Attachments:

• Insertion: The movable attachment, typically located at the distal end of a muscle.

• Origin: The stationary attachment, usually located at the proximal end of a muscle.

• Direct Attachment: Connects directly to bone, providing a strong union.

• Indirect Attachment: Involves tendons or aponeurosis, allowing for greater range of movement and less bony exposure.

Muscle Fiber Structure

Sarcolemma: Cell membrane of a muscle fiber, responsible for electrical excitability and maintaining the structural integrity of the fiber.

Sarcoplasm: The cytoplasm of a muscle fiber containing myofibrils, organelles, and stored glycogen and myoglobin.

Myofibrils: Long, thread-like structures made up of repeating units called sarcomeres, essential for muscle contraction.

Sarcomere Structure

Parts of a Sarcomere: Composed of thick (myosin) and thin (actin) filaments that interact to facilitate contraction.

Myosin Myofilament: Contains myosin heads that interact with actin, initiating contraction through cross-bridge cycling.

Actin Myofilament: Main component of the thin filament containing binding sites for myosin, playing a key role in muscle contraction.

Other Components

Sarcoplasmic Reticulum (SR): Stores calcium ions (Ca2+); essential for muscle contraction, releasing ions in response to stimuli.

T-tubule: Invagination of the sarcolemma that helps conduct electrical impulses, facilitating synchronized contraction across the muscle fiber.

Triad: Composed of a T-tubule flanked by two terminal cisternae of the SR; involved in excitation-contraction coupling, which translates electrical signals to mechanical responses.

Sliding Filament Model

Overview: Describes how muscle fibers contract through the sliding of actin and myosin filaments past each other, resulting in sarcomere shortening and leading to muscle contraction. This model explains the mechanism underlying muscle movement at the microscopic level.

Muscle Contraction Steps

• Activation: Nerve impulse stimulates muscle fiber, leading to an action potential.

• Calcium Release: SR releases Ca2+ ions into the cytosol, initiating contraction.

• Cross-Bridge Formation: Myosin heads attach to actin filaments, forming cross-bridges essential for contraction.

• Power Stroke: Myosin heads pivot and pull actin filaments, generating force and movement.

• Cross-Bridge Detachment: Myosin heads detach from actin, allowing relaxation and recharging for the next cycle.

• Calcium Reuptake: Ca2+ ions are pumped back into the SR, stopping contraction and allowing the muscle to relax.

Governing Muscle Contraction

Motor Unit: A motor neuron and all the muscle fibers it innervates; the basic functional unit of muscle activity.

Muscle Twitch: Response of a muscle fiber to a single action potential; consists of:

• Latent Period: Time between stimulation and contraction.

• Period of Contraction: Muscle fibers are actively shortening, generating force.

• Period of Relaxation: Muscle fibers return to resting state, restoring readiness for subsequent contractions.

Graded Muscle Contraction: Varies in strength based on frequency of stimulation (wave summation, recruitment), allowing for precise control of muscle force according to the demands of the activity.

Types of Muscle Contractions

Isotonic Contractions: Muscle changes length while maintaining tension, facilitating movement.

• Concentric: Muscle shortens, generating force while overcoming resistance (e.g., lifting).

• Eccentric: Muscle lengthens under tension, controlling the speed of movement (e.g., lowering).

Isometric Contraction: Muscle exerts force but does not change length, stabilizing posture or holding an object in place.

Muscle Tone: The constant low-level tension in muscles when at rest, contributing to posture and readiness for action.

Muscle Metabolism

ATP Storage: Limited ATP is stored; ongoing metabolism required during exercise for energy production.

Direct Phosphorylation: Fast ATP regeneration via creatine phosphate, providing immediate energy for short bursts of activity.

Anaerobic Pathway: Produces ATP without oxygen; leads to lactic acid accumulation during intense exercise.

Aerobic Pathway: Produces ATP using oxygen; efficient for long-duration activities and relies on steady-state energy production.

Aerobic Endurance: Influenced by aerobic metabolism efficiency, allowing sustained exercise capabilities over time.

Anaerobic Threshold: Point at which anaerobic metabolism increases during exercise, marking a transition in energy production pathways.

Muscle Fatigue: Inability to contract due to prolonged exertion and depletion of energy reserves.

Excess Postexercise Oxygen Consumption (EPOC): Oxygen required post-exercise to restore muscle to resting state, facilitating recovery and restoring oxygen debt.

Factors Affecting Contraction Force

Muscle Fiber Type: Different types (slow oxidative, fast oxidative, fast glycolytic) affecting contraction speed and ATP source, influencing performance.

Load: Heavier load results in slower contraction, requiring more force generation.

Recruitment: Activating more motor units for greater force, enhancing muscle strength during activity.

Adaptations to Exercise

Endurance Training: Increases oxidative capacity and endurance, enhancing performance in prolonged activities.

Resistance Training: Increases muscle fiber size and strength, promoting hypertrophy and overall muscle function.

Smooth Muscle Characteristics

Differences with Skeletal Muscle: Involuntary, non-striated, operates autonomously without conscious control.

Peristalsis: Rhythmic contractions to move substances through the digestive tract, essential for digestion and absorption.

Structural Composition: Includes varicosities, diffuse junctions, and caveolae for communication and calcium release, facilitating coordinated contractions.

Calcium Source: Primarily from extracellular fluid, distinguishing smooth muscle contraction from skeletal muscle.

Smooth Muscle Contraction: Involves slow, sustained contractions compared to skeletal muscle, allowing for prolonged and controlled movements.

Skeletal Muscle Development

Development involves myogenesis and differentiation of muscle precursor cells into mature muscle fibers, vital for muscle growth and