Skeletal Muscle Overview Quiz

Skeletal Muscle Overview

• Skeletal muscles are attached to bones by tendons
• They produce body movements relative to each other
• They are voluntary, meaning you control when and how they work
• The majority of muscles in the body are skeletal
• They make up 30-40% of total body mass
• Composed of integrated tissues: skeletal muscle fibers, blood vessels, nerve fibers, and connective tissue

Behavioral Goals

• Define skeletal muscle
• Illustrate skeletal muscle function
• Classify skeletal muscles by shape
• Relate skeletal muscle facts to pathological conditions
• Describe skeletal muscle structure
• Explain skeletal muscle contraction mechanism
• Explain energy source consumption
• Discuss muscle conditions (contracture, fatigue, and tone)

Muscle Functions

• Body movement (e.g., limb movement)
• Maintaining body position (e.g., erector spinae)
• Supporting internal organs (e.g., pelvic diaphragm)
• Regulating materials (e.g., sphincters of the digestive tract)
• Temperature regulation

Muscle Origins and Insertions

• Origin: bony attachment at the stationary end of the muscle
• Belly: the thicker, middle region of the muscle between origin and insertion
• Insertion: bony attachment to the mobile end of the muscle

Skeletal Muscle Shapes

• Circular: surrounding openings (e.g., orbicularis oris)
• Convergent: wider origin, narrower insertion (e.g., pectoralis major)
• Parallel: fibers parallel to each other (e.g., sartorius)
• Fusiform: spindle-shaped, wider belly (e.g., biceps brachii)
• Unipennate: fibers arranged diagonally to a tendon (e.g., extensor digitorum longus)
• Bipennate: two rows of fibers facing opposite directions (e.g., rectus femoris)
• Multipennate: multiple rows of fibers converging on multiple tendons (e.g., deltoid)

Skeletal Muscle Structure

• Epimysium: outer connective tissue layer surrounding the entire muscle
• Perimysium: surrounds bundles of muscle fibers (fascicles)
• Endomysium: surrounds individual muscle fibers
• Fascicle: bundle of muscle fibers
• Sarcolemma: muscle fiber membrane
• Sarcoplasm: cytoplasm of the muscle fiber
• Sarcomere: smallest contractile unit of a muscle fiber
• Myofibrils: thread-like protein filaments within the muscle fiber
• Myofilaments: actin and myosin filaments within myofibrils

Skeletal Muscle Movement

• Prime mover: muscle primarily responsible for a movement
• Antagonist: muscle that opposes the prime mover
• Synergist: muscle that assists the prime mover

Ordinary Body Movements

• Flexion: bending a joint
• Extension: straightening a joint
• Rotation: turning a joint
• Abduction: moving a limb away from the midline
• Adduction: moving a limb toward the midline
• Circumduction: circular movement of a limb

How Skeletal Muscles Move

• Sensory neurons: detect stimuli and transmit signals to the central nervous system
• Motor neurons: carry signals from the central nervous system to muscles to cause movement

Neuromuscular Junction

• Synaptic connection between a motor neuron and a muscle fiber
• Site for action potential transmission from nerve to muscle

Skeletal Muscle Excitation

• Triggered by signaling from the nervous system at the neuromuscular junction
• Acetylcholine (ACh) diffuses across the synaptic cleft
• ACh binds to ACh receptors within the sarcolemma
• Sodium (Na+) ions enter the muscle fiber, depolarizing the membrane
• Calcium (Ca++) is released from the sarcoplasmic reticulum

Skeletal Muscle Contraction (Cross-Bridge)

• Myosin heads split ATP and become reoriented
• Myosin heads bind to actin, forming cross-bridges
• Myosin heads rotate, pulling actin filaments
• ATP binds to myosin, detaching it from actin

Sliding Filament Model of Contraction

• Thin filaments (actin) are pulled past thick filaments (myosin)
• Sarcomeres shorten, leading to muscle contraction

Providing Energy for Contraction

• Direct phosphorylation: creatine phosphate (CP) to ADP
• Anaerobic mechanism: glycolysis and lactic acid formation
• Aerobic mechanism: oxidative phosphorylation

Types of Skeletal Muscle Fibers

• Type I (slow oxidative): slow contraction, high resistance to fatigue, aerobic metabolism, red color
• Type IIA (fast oxidative-glycolytic): fast contraction, moderate resistance to fatigue, aerobic and anaerobic metabolism, red color
• Type IIB (fast glycolytic): fast contraction, low resistance to fatigue, anaerobic metabolism, white color

Muscle Relaxation

• Motor neuron stops releasing ACh
• Muscle fiber repolarizes
• Calcium (Ca++) is actively transported back into the sarcoplasmic reticulum
• Actin-binding sites are shielded
• Loss of ability to form cross-bridges

Muscle Tone

• Slight, ongoing muscle contraction even when not actively moving
• Maintains posture and stabilizes joints

Muscle Contracture

• Continuous muscle contraction due to ATP depletion
• Actin filaments fail to detach from myosin heads

Muscle Fatigue

• Depletion of ATP or glycogen
• Blood interruption
• Reduced oxygen levels
• Impaired signaling from nerves to muscles