Download Microscopic Anatomy of Skeletal Muscle

The Muscular System
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Muscles are responsible for all types of body movement
Three basic muscle types are found in the body
1. Skeletal muscle
2. Cardiac muscle
3. Smooth muscle
Characteristics of Muscles
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Skeletal and smooth muscle cells are elongated (muscle cell  muscle fiber)
Contraction and shortening of muscles is due to the movement of microfilaments
All muscles share some terminology
o Prefixes myo and mys refer to “muscle”
o Prefix sarco refers to “flesh”
Skeletal Muscle Characteristics
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Most are attached by tendons to bones
Cells are multinucleate
Striated—have visible banding
Voluntary—subject to conscious control
Connective Tissue Wrappings of Skeletal Muscle
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Cells are surrounded and bundled by connective tissue
o Endomysium—encloses a single muscle fiber
o Perimysium—wraps around a fascicle (bundle) of muscle fibers
o Epimysium—covers the entire skeletal muscle
o Fascia—on the outside of the epimysium
Skeletal Muscle Attachments
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Sites of muscle attachment
o Bones
o Cartilages
o Connective tissue coverings
Smooth Muscle Characteristics
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Lacks striations
Spindle-shaped cells
Single nucleus
Involuntary—no conscious control
Found mainly in the walls of hollow visceral organs (such as stomach, urinary bladder, respiratory
passages)
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Cardiac Muscle Characteristics
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Striations
Usually has a single nucleus
Involuntary
Found only in the walls of the heart
Skeletal Muscle Functions
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Produce movement
Maintain posture
Stabilize joints
Generate heat
Microscopic Anatomy of Skeletal Muscle
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Sarcolemma—specialized plasma membrane
Myofibrils—long organelles inside muscle cell
o Light (I) bands and dark (A) bands give the muscle its striped appearance
Microscopic Anatomy of Skeletal Muscle
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Banding pattern
o I band  light band
 Contains only thin filaments
 Z disc is a midline interruption
o A band  dark band
 Contains the entire length of the thick filaments
 H zone is a lighter central area
 M line is in center of H zone
Microscopic Anatomy of Skeletal Muscle
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Sarcomere—contractile unit of a muscle fiber
Organization of the sarcomere
o Myofilaments produce banding (striped) pattern
 Thick filaments  myosin filaments
 Thin filaments  actin filament
Stimulation and Contraction of Single Skeletal Muscle Cells
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Irritability (also called responsiveness)—ability to receive and respond to a stimulus
Contractility—ability to shorten when an adequate stimulus is received
Extensibility—ability of muscle cells to be stretched
Elasticity—ability to recoil and resume resting length after stretching
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Mechanism of Muscle Contraction: The Sliding Filament Theory
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Activation by nerve causes myosin heads (cross bridges) to attach to binding sites on the thin
filament
Myosin heads then bind to the next site of the thin filament
This continued action causes a sliding of the myosin along the actin
The result is that the muscle is shortened (contracted)
Energy for Muscle Contraction
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ATP
o Immediate source of energy for muscle contraction
o Stored in muscle fibers in small amounts that are quickly used up
o After this initial time, other pathways must be utilized to produce ATP
Energy for Muscle Contraction
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Three ways to generate ATP
1. Direct phosphorylation of ADP by creatine phosphate
2. Aerobic respiration
3. Anaerobic glycolysis and lactic acid formation
Energy for Muscle Contraction
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Direct phosphorylation of ADP by creatine phosphate (CP)—fastest
o Muscle cells store CP, a high-energy molecule
o After ATP is depleted, ADP remains
o CP transfers a phosphate group to ADP to regenerate ATP
o CP supplies are exhausted in less than 15 seconds
o About 1 ATP is created per CP molecule
Energy for Muscle Contraction
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Aerobic respiration
o Glucose is broken down to carbon dioxide and water, releasing energy (about 32 ATP)
o A series of metabolic pathways occurs in the mitochondria
o This is a slower reaction that requires continuous oxygen
o Carbon dioxide and water are produced
Energy for Muscle Contraction
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Anaerobic glycolysis and lactic acid formation
o Reaction that breaks down glucose without oxygen
o Glucose is broken down to pyruvic acid to produce about 2 ATP
o Pyruvic acid is converted to lactic acid
This reaction is not as efficient, but is fast
o Huge amounts of glucose are needed
o Lactic acid produces muscle fatigue
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Muscle Fatigue and Oxygen Deficit
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If muscle activity is strenuous and prolonged, muscle fatigue occurs because:
o Ionic imbalances occur
o Lactic acid accumulates in the muscle
o Energy (ATP) supply decreases
After exercise, the oxygen deficit is repaid by rapid, deep breathing
Types of Muscle Contractions
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Isotonic contractions
o Myofilaments are able to slide past each other during contractions
o The muscle shortens, and movement occurs
o Example: bending the knee; rotating the arm
Isometric contractions
o Tension in the muscles increases
o The muscle is unable to shorten or produce movement
o Example: pushing against a wall with bent elbows
Muscle Tone
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Muscle tone keeps muscles healthy and ready to react
o Result of a staggered series of nerve impulses delivered to different cells within the muscle
o If the nerve supply is destroyed, the muscle loses tone, becomes paralyzed, and atrophies
Effect of Exercise on Muscles
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Exercise increases muscle size, strength, and endurance
o Aerobic (endurance) exercise (biking, jogging) results in stronger, more flexible muscles with
greater resistance to fatigue
 Makes body metabolism more efficient
 Improves digestion, coordination
o Resistance (isometric) exercise (weight lifting) increases muscle size and strength
Muscles and Body Movements
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Movement is attained as a result of a muscle moving an attached bone
Muscles are attached to at least two points
1. Origin: attachment to a moveable bone
2. Insertion: attachment to an immovable bone
Types of Body Movements
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Flexion
o Decreases the angle of the joint
o Brings two bones closer together
o Typical of bending hinge joints (e.g., knee and elbow) or ball-and-socket joints (e.g., the hip)
Extension
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Opposite of flexion
Increases angle between two bones
Typical of straightening the elbow or knee
Extension beyond 180° is hyperextension
Types of Body Movements
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Rotation
o Movement of a bone around its longitudinal axis
o Common in ball-and-socket joints
o Example: moving the atlas around the dens of axis (i.e., shaking your head “no”)
Types of Body Movements
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Abduction
o Movement of a limb away from the midline
Adduction
o Opposite of abduction
o Movement of a limb toward the midline
Types of Body Movements
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Circumduction
o Combination of flexion, extension, abduction, and adduction
o Common in ball-and-socket joints
o Proximal end of bone is stationary, and distal end moves in a circle
Special Movements
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Dorsiflexion
o Lifting the foot so that the superior surface approaches the shin (toward the dorsum)
Plantar flexion
o Depressing the foot (pointing the toes)
o “Planting” the foot toward the sole
Special Movements
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Inversion
o Turning sole of foot medially
Eversion
o Turning sole of foot laterally
Special Movements
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Supination
o Forearm rotates laterally so palm faces anteriorly
o Radius and ulna are parallel
Pronation
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o Forearm rotates medially so palm faces posteriorly
o Radius and ulna cross each other like an X
Special Movements
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Opposition
o Moving the thumb to touch the tips of other fingers on the same hand
Interactions of Skeletal Muscles in the Body
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In general, groups of muscles that produce opposite actions lie on opposite sides of a joint
We will explore examples in Figure 6.14 next
Types of Muscles
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Prime mover—muscle with the major responsibility for a certain movement
Antagonist—muscle that opposes or reverses a prime mover
Synergist—muscle that aids a prime mover in a movement and helps prevent rotation
Fixator—stabilizes the origin of a prime mover
Naming Skeletal Muscles
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By direction of muscle fibers
o Example: rectus (straight)
By relative size of the muscle
o Example: maximus (largest)
Naming Skeletal Muscles
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By location of the muscle
o Example: temporalis (temporal bone)
By number of origins
o Example: triceps (three heads)
Naming Skeletal Muscles
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By location of the muscle’s origin and insertion
o Example: sterno (on the sternum)
By shape of the muscle
o Example: deltoid (triangular)
By action of the muscle
o Example: flexor and extensor (flexes or extends a bone)
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