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Lecture

BIOB32H3 Lecture Notes - Somatic Nervous System, Sarcolemma, Growth Factor


Department
Biological Sciences
Course Code
BIOB32H3
Professor
Kenneth Welch

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Muscles and Muscle Tissue
Muscle Overview
The three types of muscle tissue are skeletal, cardiac, and smooth
These types differ in structure, location, function, and means of activation
Muscle Similarities
Skeletal and smooth muscle cells are elongated and are called muscle fibers
Muscle contraction depends on two kinds of myofilaments actin and myosin
Muscle terminology is similar
Sarcolemma muscle plasma membrane
Sarcoplasm cytoplasm of a muscle cell
Prefixes myo, mys, and sarco all refer to muscle
Skeletal Muscle Tissues
Packaged in skeletal muscles that attach to and cover the bony skeleton
Has obvious stripes called striations
Is controlled voluntarily (i.e., by conscious control)
Contracts rapidly but tires easily
Is responsible for overall body motility
Is extremely adaptable and can exert forces over a range from a fraction of an ounce to over
70 pounds
Cardiac Muscle Tissue
Occurs only in the heart
Is striated like skeletal muscle but is not voluntary
Contracts at a fairly steady rate set by the heart’s pacemaker
Neural controls allow the heart to respond to changes in bodily needs
Smooth Muscle Tissue
Found in the walls of hollow visceral organs, such as the stomach, urinary bladder, and
respiratory passages
Forces food and other substances through internal body channels
It is not striated and is involuntary
Muscle Function
Skeletal muscles are responsible for all locomotion
Cardiac muscle is responsible for coursing the blood through the body
Smooth muscle helps maintain blood pressure, and squeezes or propels substances (i.e.,
food, feces) through organs
Muscles also maintain posture, stabilize joints, and generate heat
Functional Characteristics of Muscles
Excitability, or irritability the ability to receive and respond to stimuli
Contractility the ability to shorten forcibly
Extensibility the ability to be stretched or extended
Elasticity the ability to recoil and resume the original resting length
Skeletal Muscle
Each muscle is a discrete organ composed of muscle tissue, blood vessels, nerve fibers, and
connective tissue
The three connective tissue wrappings are:
Epimysium an overcoat of dense regular CT that surrounds the entire muscle

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Perimysium fibrous CT that surrounds groups of muscle fibers called fascicles
Endomysium fine sheath of CT composed of reticular fibers surrounding
each muscle fiber
Skeletal Muscle: Nerve and Blood Supply
Each muscle is served by one nerve, an artery, and one or more veins
Each skeletal muscle fiber is supplied with a nerve ending that controls contraction
Contracting fibers require continuous delivery of oxygen and nutrients via arteries
Wastes must be removed via veins
Skeletal Muscle: Attachments
Muscles span joints and are attached to bone in at least two places
When muscles contract the movable bone, the muscle’s insertion moves toward the
immovable bone the muscle’s origin
Muscles attach:
Directly epimysium of the muscle is fused to the periosteum of a bone
Indirectly CT wrappings extend beyond the muscle as a tendon or aponeurosis
Microscopic Anatomy of a Skeletal Muscle Fiber
Each fiber is a long, cylindrical cell with multiple nuclei just beneath the sarcolemma
Fibers are 10 to 100 m in diameter, and up to hundreds of centimeters long
Each cell is a syncytium produced by fusion of embryonic cells
Sarcoplasm has numerous glycosomes and a unique oxygen-binding protein
called myoglobin
Fibers contain the usual organelles, myofibrils, sarcoplasmic reticulum, and T tubules
Myofibrils
Myofibrils are densely packed, rodlike contractile elements
They make up most of the muscle volume
The arrangement of myofibrils within a fiber is such that a perfectly aligned repeating series
of dark A bands and light I bands is evident
Sarcomeres
The smallest contractile unit of a muscle
The region of a myofibril between two successive Z discs
Composed of myofilaments made up of contractile proteins
Myofilaments are of two types thick and thin
Myofilaments: Banding Pattern
Thick filaments extend the entire length of an A band
Thin filaments extend across the I band and partway into the A band
Z-disc coin-shaped sheet of proteins (connectins) that anchors the thin filaments and
connects myofibrils to one another
Thin filaments do not overlap thick filaments in the lighter H zone
M lines appear darker due to the presence of the protein desmin
Ultrastructure of Myofilaments: Thick Filaments
Thick filaments are composed of the protein myosin
Each myosin molecule has a rodlike tail and two globular heads
Tails two interwoven, heavy polypeptide chains
Heads two smaller, light polypeptide chains called cross bridges
Ultrastructure of Myofilaments: Thin Filaments
Thin filaments are chiefly composed of the protein actin

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Each actin molecule is a helical polymer of globular subunits called G actin
The subunits contain the active sites to which myosin heads attach during contraction
Tropomyosin and troponin are regulatory subunits bound to actin
Arrangement of the Filaments in a Sarcomere
Longitudinal section within one sarcomere
Sarcoplasmic Reticulum (SR)
SR is an elaborate smooth endoplasmic reticulum that mostly runs longitudinally and
surrounds each myofibril
Paired terminal cisternae form perpendicular cross channels
Functions in the regulation of intracellular calcium levels
Elongated tubes called T tubules penetrate into the cell’s interior at each A band–I band
junction
T tubules associate with the paired terminal cisternae to form triads
T Tubules
T tubules are continuous with the sarcolemma
They conduct impulses to the deepest regions of the muscle
These impulses signal for the release of Ca2+ from adjacent terminal cisternae
Contraction of Skeletal Muscle Fibers
Contraction refers to the activation of myosin’s cross bridges (force generating sites)
Shortening occurs when the tension generated by the cross bridge exceeds forces opposing
shortening
Contraction ends when cross bridges become inactive, the tension generated declines, and
relaxation is induced
Sliding Filament Mechanism of Contraction
Thin filaments slide past the thick ones so that the actin and myosin filaments overlap to a
greater degree
In the relaxed state, thin and thick filaments overlap only slightly
Upon stimulation, myosin heads bind to actin and sliding begins
Each myosin head binds and detaches several times during contraction, acting like a ratchet
to generate tension and propel the thin filaments to the center of the sarcomere
As this event occurs throughout the sarcomeres, the muscle shortens
Role of Ionic Calcium (Ca2+) in the Contraction Mechanism
At low intracellular Ca2+ concentration:
Tropomyosin blocks the binding sites on actin
Myosin cross bridges cannot attach to binding sites on actin
The relaxed state of the muscle is enforced
At higher intracellular Ca2+ concentrations:
Additional calcium binds to troponin (inactive troponin binds two Ca2+)
Calcium-activated troponin binds an additional two Ca2+ at a separate regulatory site
Calcium-activated troponin undergoes a conformational change
This change moves tropomyosin away from actin’s binding sites
Myosin head can now bind and cycle
This permits contraction (sliding of the thin filaments by the myosin cross bridges) to begin
Sequential Events of Contraction
Cross bridge attachment myosin cross bridge attaches to actin filament
Working (power) stroke myosin head pivots and pulls actin filament toward M line
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