Muscle System
Overview of Muscle Tissues
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Skeletal muscle is attached to the skeleton, is striated and can be controlled voluntarily
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Cardiac muscle forms the heart, is striated and is controlled involuntarily
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Smooth muscle, located chiefly in the walls of hollow organs, is controlled involuntarily. Its fibers are not striated
Functional Characteristics of muscles fibers
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Excitability
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Contractility
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Extensibility
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Elasticity
Muscle Functions:
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Move internal and external body parts
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Maintain posture
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Stabilize joints
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Generate heat
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Protect some visceral organs
Gross Anatomy of Skeletal Muscle:
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Protected and strengthened by connective tissue coverings o Deep to superficial
 Endomysium
 Perimysium

Muscle System
 Epimysium
Muscle attachments (origins/insertions) may by direct or indirect via tendons or aponeuroses. Indirect withstand friction better
Microscopic Anatomy of Muscle Fibers
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Skeletal muscle fibers are long, striated and multinucleated
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Myofibrils are the contractile elements that occupy most of the cell volume o Banded appearance results from the regular alternation of the dark (A) and light (I) bands o Myofibrils are chains of sarcomeres: each sarcomere contains thick (myosin) and thin (actin) myofilaments. The heads of the myosin form cross bridges that interact with the thin filaments.
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The sarcoplasmic reticulum (SR) is a system of membranous tubules surrounding each myofibril. Its function is to release and sequester calcium ions
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T-tubules are invaginations of the sarcolemma that run between the terminal cisternae of the SR. They allow electrical stimuli to be delivered deep into the cell regions
Sliding Filament Model of Contraction:
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The thin filaments are pulled toward the sarcomere centers by cross bridge activity of the thick filaments

Muscle System
Physiology of Skeletal Muscle Fiber:
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Regulation of skeletal muscle cell contraction involves o Generation and transmission of an action potential along the sarcolemma
 These involve the flow of Na + and K + ions o Excitation and contraction coupling o The action potential is propagated down the T-tubules, causing the release of Ca + from the SR into the cells interior. o Sliding filaments is triggered by the rise of intracellular Ca + levels.
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Contraction of Skeletal Muscle: o A motor unit is one motor neuron and all the cells it innervates
 A motor units response to a single brief threshold stimulus is a twitch
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A twitch has 3 phases; o The latent period (preparatory events occurring) o The period of contraction (the muscle may tense or shorten) o The period of relaxation (muscle tension declines and resumes its resting length)
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Depending on the strength and duration of stimuli, muscles have a graded response from twitch to tetanus.
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Isotonic contractions occur when the muscle shortens (concentric contraction) or lengths
(eccentric contraction) as the load is moved.

Muscle System
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Isometric contraction occur when muscle tension produces neither shortening nor lengthening
Muscle Metabolism:
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Energy source for muscle contraction is ATP obtained from a coupled reaction of creatine phosphate with ADP and from the aerobic and anaerobic metabolism of glucose
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When ATP is produced by nonaerobic pathways, lactic acid accumulates, membrane potential is disturbed, oxygen debt occurs.
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Only about 40% of energy released during ATP hydrolysis powers contractile activity, the rest is lost to heat.
Force of muscle contraction:
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Affected by number and size of contracting muscle cells. (the more and larger the cells, the greater the force)
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When the thick and thin filaments are optimally overlapping the muscle can generate maximum force
Velocity and Duration of Contraction:
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Load (the greater the load, the slower the contraction)and fiber type o 3 types of muscle fiber
 Fast glycolytic (fatigable fibers)
 Slow oxidative (fatigue resistant)
 Intermediate fast oxidative (fatigue resistant) o Most muscles contain a mixture of fiber types

Muscle System
Developmental Aspects of Muscles:
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For the most part, specialized skeletal and cardiac muscle lose their ability to divide but retain their ability to hypertrophy
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Women’s muscles account for about 36% of their total body weight
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Men’s muscles account for about 42% of their total body weight.
The difference between men and women is the effects of male hormone on skeletal muscle growth
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Skeletal muscles are richly vascularized and quite resistant to infection
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Old age: skeletal muscle becomes fibrous, decline in length, and atrophy unless appropriately exercised.

Muscle System
Interactions of Skeletal Muscles in the Body:
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Skeletal muscles are arranged in opposing groups across joints so that one group can reverse or modify the action of the other
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Muscles are classified as o prime movers, (bear the chief responsibility for producing movement) o antagonists (reverse or oppose the action of another muscle), o syngerists (aid a prime mover by affecting the same action, stabilizing joints, or preventing undesirable movements) o fixators, function to immobilize a bone or a muscle’s origin
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A muscle may act as a prime mover in one movement, an antagonist for another movement, a synergist for a third movement and so on.
Naming Skeletal Muscles:
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Criteria used to name muscles include: o Location: some muscle names indicate the bone or body region the muscle is associated. Ex. Temporalis, intercostal o Shape: some have distinctive shapes. Ex. Deltoid are roughly triangular. Trapezius, shaped like a trapazoid o Relative size: maximus (largest), minimus smallest), brevis,
(short) longus, (long). Ex. gluteus maximus (he said butt  ) o Fiber direction: reveal the direction the fibers run in reference to (usually) the midline of the body. Ex. transversus, oblique o Number of origins: Biceps, triceps or quadriceps 2,3 or 4 origins

Muscle System o Attachment sites: some muscles are named for their points of origin or insertion. The origin is always first. Ex. sternocleidomastoid. o Action: When muscles are named for their action, action words such as flexor, extensor, or adductor appear in the name. Ex. adductor longus, supinator
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Several criteria are combined to name some muscles
Muscle Mechanics: The Importance of Fascicle Arrangement and
Leverage
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Common patterns of fascicle arrangement are (see page 327) o Parallel, the long axis of the fascicles run parallel to the long axis of the muscles. The muscles are strap-like. o Fusiform, strap-like - spindle shaped with an expanded mid section (belly) Ex. biceps brachii o Pennate, the fascicles are short and run obliquely to a central tendon that runs the length of the muscle. If the fascicles insert to only one side of the tendon, it is
unipennate (extensor digitorum of the leg). If the fascicles insert into the tendon from the opposite side so the muscle’s grain looks like a feather, it is bipennate (rectus femoris – thigh). Multipennate, if the arrangement looks like many feathers situated side by side with all their quills inserted into a large tendon (deltoid) o Convergent, has a broad origin, and its fascicles converge toward a single tendon of insertion. (triangular shaped pectoralis major)

Muscle System o Circular: arranged in concentric circles. These muscles surround the external body openings –sphincters
(orbicularis muscles of the eyes and mouth)
Lever Systems:
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Lever is a ridged bar that moves on a fixed point.
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Applied force is used to move the resistance (load)
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Joints are the fulcrums and bones are the levers
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Muscle system provides the effort
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The load is the bone itself and the overlying tissue and anything else you are trying to move
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Mechanical Advantage: o A lever allows a given effort to move a heavier load, (or move a load farther or faster) than it otherwise could.
If the load is close to the fulcrum, and the effort is applied far from the fulcrum, a small effort exerted over a large distance can be used to move a large load over a small distance. Ex. jack lifting a car
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Mechanical Disadvantage: (speed lever) o The load is far from the fulcrum and the effort is applied near the fulcrum, the force exerted by the muscle must be greater than the load moved or supported. Ex. wielding a shovel
All levers follow the same basic principles:
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Effort farther than load from the fulcrum = mechanical advantage
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Effort nearer than load to fulcrum = mechanical disadvantage

Muscle System
Types of levers:
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First-class levers o The effort is applied at one end of the lever and the load is at the other end, the fulcrum somewhere in between. Ex.
Seesaw and scissors.
 Some first class levers in your body operate at a mechanical advantage but others (triceps) operate at a mechanical disadvantage (extending the forearm against resistance. o Second class levers
 the effort is applied at one end of the lever and the fulcrum is located at the other end, with the load between them. Ex. Wheelbarrow
 these are uncommon in the body. Ex. Standing on your toes
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These are levers for strength, speed and range of motion are sacrificed for strength
Third Class levers
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The effort is applied between the load and the fulcrum o These levers operate with great speed and always at a mechanical disadvantage. Ex. Tweezers and forceps o Most skeletal muscles of the body act in third class levers systems; the biceps muscle of the arm, lifting the distal forearm and anything carried in the hand. o Permits a muscle to be inserted very close to the joint across which movement occurs. This allows rapid extensive movements with relatively little shortening of the muscle.

Muscle System o Muscles involved in third class levers tend to be short, thick and powerful
Differences in the positioning of the three elements modify muscle activity with respect to : 1. Speed of contraction, 2. Range of movement, 3. The weight of the load that can be lifted.
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The lever systems that operate at a mechanical disadvantage
(speed levers), force is lost but speed and range of movement are gained and this can be a benefit. Systems that operate at a mechanical advantage (power levers) are slower, more stable and used where strength is a priority.