Myology: the study of skeletal muscles MUSCLES – 1A Myology: the study of skeletal muscles

Ability to generate electrical impulses (action potentials) Basic Features Excitability Ability to generate electrical impulses (action potentials) Contractility Ability to shorten (contract)

Functions of Skeletal Muscle Heat production Voluntary movement Including speech and breathing Maintenance of body posture Guard body openings Store nutrient reserves

Relationship of Skeletal Muscle to Bone Muscles pull on bones Provide a “force” (effort) Bones serve as levers Lever – a rigid bar (makes it easier to do work. To carry a “load” (resistance; weight) Joints act as fulcrums for the levers Fulcrum = fixed point Force (muscle) Resistance (weight) Fulcrum (joint)

Examples—seesaws, scissors, and lifting your head off your chest First-class lever E - F - R Examples—seesaws, scissors, and lifting your head off your chest Load Effort L Fulcrum Load 1st class lever = E-F-R If the fulcrum near the load, you can lift a lot of weight a short distance! (mechanical advantage) L Effort Fulcrum Fulcrum Load Effort

Second-class lever (F-R-E) Fulcrum-Load - Effort Examples—wheelbarrow or standing on tiptoe An uncommon type of lever in the body Works at a mechanical advantage effort load fulcrum

Arrangement of the elements is Third-class lever Effort is applied between the load and the fulcrum Works speedily Always at a mechanical disadvantage Third-class lever Arrangement of the elements is load-effort-fulcrum. Load Effort 3rd class lever = F-E-R L Fulcrum Load If the fulcrum is far from the load, it’s hard to lift a lot of weight BUT you can lift the load farther! L Fulcrum Effort Most skeletal muscles are 3rd-class levers Example—biceps brachii Fulcrum—the elbow joint Force—exerted on the proximal region of the radius Load—the distal part of the forearm Example: tweezers or forceps Effort Load Fulcrum In the body: Flexing the forearm by the biceps brachii muscle exemplifies third-class leverage. The effort is exerted on the proximal radius of the forearm; the fulcrum is the elbow joint; and the load is the hand and distal end of the forearm.

Tendons: Muscle Attachment to Bone Tendons attach muscle to the periosteum of bone. One bone moves; one bone remains fixed Origin—(head; ceps) - less movable attachment ”ceps” = cephalic, which means “head” Biceps, triceps, quadriceps Insertion— (tail) attachment to more movable attachment Muscle contracting Origin by direct attachment Brachialis (belly of muscle) Tendon Insertion by indirect attachment

Types of Contractions isotonic – muscle contracts and changes length. 2 types: eccentric – lengthening contraction concentric – shortening contraction isometric – muscle contracts but does not change length

Group Actions of Muscles Agonist (prime mover) – muscle that causes the action Synergist (helper) – muscle that acts with an agonist Antagonist – muscle that causes the opposite action of the agonist Fixator (stabilizer) – muscle that stabilizes body position In a flexion, the biceps is the prime mover; the brachialis (not shown)is a synergist; the triceps is an antagonist

SKELETAL MUSCLE Skeletal muscle is voluntary. It is under conscious control. “Conscious” means that the nervous system is involved in getting your muscles to move. Neurons are the cells of the nervous system that send messages. “Motor Neurons” send messages to skeletal muscle, telling them to move.

A single neuron can send messages to many muscle cells (fibers). As more muscle fibers are recruited, the strength of the contraction will increase. Let’s look at a single motor neuron contacting many muscle cells at once.

Motor Unit a single motor neuron innervates (supplies) several hundred skeletal muscle fibers a voluntary motor neuron is called a somatic motor neuron

Structure of a Skeletal Muscle A Skeletal Muscle is an organ Contains different tissues: skeletal muscle nervous blood vessels connective fascia – surrounds each muscle tendon – holds to bone aponeuroses – broad, fibrous sheets, attach muscle to muscle or other structures.

Terminology Specific to Muscle Tissue Myo and mys—prefixes meaning “muscle” Sarco—prefix meaning “flesh” Sarcolemma—plasma membrane of muscle cells Sarcoplasm—cytoplasm of muscle cells Sarcasm – flesh-eating words!

Structure of a Skeletal Muscle 9-4

Skeletal Muscle Fiber

Sarcomere 9-6

Myofilaments are thick and thin filaments composed of actin associated with troponin and tropomyosin Thick Filaments composed of myosin cross-bridges

Neuromuscular Junction Neuron transmits an action potential via a neurotransmitter (acetylcholine)

Calcium binds to troponin, allows actin to bind to myosin, sliding occurs

Sliding Filament Theory When sarcromeres shorten, myosin and actin filaments slide past one another Z lines move closer together

RELAXATION acetylcholinesterase – breaks down acetylcholine muscle impulse stops Ca+2 moves back into SR myosin and actin binding prevented

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