Muscular System Functions
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Body movement (Locomotion)
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Maintenance of posture
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Respiration
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Diaphragm and intercostals contractions
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Communication (Verbal and Facial)
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Constriction of organs and vessels
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Heart beat
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Production of body heat (Thermogenesis)

Muscle cell structures not found in other cells
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Myofibrils : bundles of very fine fibers
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Thick and thin myofilaments : very fine fibers that make up myofibrils
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Sarcomere : segment of myofibril between two Z lines; contractile unit
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T tubules : transmit electrical impulses through cell

Myofilaments
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Properties of Muscle
Excitability : Capacity of muscle to respond to a stimulus
Contractility : Ability of a muscle to shorten and generate pulling force
Extensibility : Ability stretches when pulled
Elasticity : Ability to return to original shape and length after contraction or extension

Muscle structure
Connective Tissue Sheaths
Connective Tissue (CT) of a Muscle
Epimysium : Dense regular CT surrounding entire muscle
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Separates muscle from surrounding tissues and organs
Perimysium : Collagen and elastic fibers surrounding a group of muscle fibers called a fascicle
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Contains blood vessels and nerves
Endomysium : Loose CT that surrounds individual muscle fibers
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Also contains blood vessels and nerves
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Collagen fibers of all 3 layers come together at each end of muscle to form a tendon or aponeurosis.

Motor neurons
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Stimulate muscle fibers to contract
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Neuron axons branch so that each muscle fiber
(muscle cell) is innervated
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Form a neuromuscular junction
Capillary beds surround muscle fibers
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Muscles require large amount of energy
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Extensive vascular network delivers necessary oxygen and nutrients and carries away metabolic waste produced by muscle fibers

Energy for Muscle Contractions
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ATP : adenosine triphosphate
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CP : creatine phosphate
Glucose & Oxygen
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Glucose stored in form of glycogen in muscle
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Excess oxygen molecules in sarcoplasm bound to myoglobin

Anaerobic respiration
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Types of Muscle
Skeletal
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Attached to bones
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Makes up 40% of body weight (Women’s skeletal muscle makes up
36% of their body mass, Men’s skeletal muscle makes up 42% of their body mass)
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Responsible for locomotion, facial expressions, posture, respiratory movements, other types of body movement
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Voluntary in action ; controlled by somatic motor neurons
Smooth
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In the walls of hollow organs, blood vessels, eye, glands, uterus, skin
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Functions : propel urine, mix food in digestive tract, regulating blood flow
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Controlled involuntarily by endocrine and autonomic nervous systems
Cardiac
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Heart : major source of movement of blood

Basic Features of a Skeletal Muscle
Muscle attachments
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Most skeletal muscles run from one bone to another
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One bone will move – other bone remains fixed
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Origin – less movable attachment
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Insertion – more movable attachment
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Muscles attach to origins and insertions by connective tissue
Fleshy attachments – connective tissue fibers are short
Indirect attachments – connective tissue forms a tendon

Skeletal Muscle Structure
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Composed of muscle cells
(fibers), connective tissue, blood vessels, nerves
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Fibers are long, cylindrical, and multinucleated
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Tend to be smaller diameter in small muscles and larger in large muscles. 1 mm - 4 cm in length
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Striated appearance
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Nuclei are peripherally located

Muscle Fiber Anatomy
Sarcolemma - cell membrane
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Surrounds the Sarcoplasm (cytoplasm of fiber)
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Punctuated by openings called the transverse tubules (T-tubules)
Myofibrils -cylindrical structures within muscle fiber
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Are bundles of protein filaments (= myofilaments )
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Two types of myofilaments
1.
Actin filaments (thin filaments)
2.
Myosin filaments (thick filaments)
When myofibril shortens, muscle shortens
(contracts)

Sarcoplasmic Reticulum (SR)
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SR is an elaborate, smooth endoplasmic reticulum
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Runs longitudinally and surrounds each myofibril
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SR stores Ca ++ when muscle not contracting
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When stimulated, calcium released into sarcoplasm
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SR membrane has Ca ++ pumps that function to pump Ca ++ out of the sarcoplasm back into the
SR after contraction

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Cells are not striated
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Fibers smaller than those in skeletal muscle
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Spindle-shaped; single, central nucleus
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More actin than myosin
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No sarcomeres
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Not arranged as symmetrically as in skeletal muscle, thus no striations.

Smooth Muscle
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Grouped into sheets in walls of hollow organs
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Longitudinal layer: muscle fibers run parallel to organ’s long axis
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Circular layer: muscle fibers run around circumference of the organ
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Both layers participate in peristalsis

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Found only in heart where it forms a thick layer called the myocardium
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Striated fibers that branch
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Each cell usually has one centrally-located nucleus
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Fibers joined by intercalated disks

Disorders of Muscle Tissue
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Muscle Fatigue
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Lack of oxygen causes ATP deficit
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Lactic acid builds up from anaerobic respiration
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Muscle Atrophy
* a decrease in the mass of the muscle
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Weakening and shrinking of a muscle
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May be caused
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Immobilization
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Loss of neural stimulation

Disorders of Muscle Tissue
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Muscle Hypertrophy
* Enlargement of a muscle
* More capillaries
* More mitochondria
* Caused by
* Strenuous exercise
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Steroid hormones
* Muscle Tonus
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Tightness of a muscle
* Some fibers always contracted