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Muscles
Smooth muscle
• Found in the walls of hollow organs and the blood vessels
• Lack striations
• Contain less myosin
• Cannot generate as much tension as striated muscle
• Can contract over a great range of lengths
• No T tubule system
• No well developed sarcoplasmic reticulum
• Contractions are relatively slow
Cardiac muscle
• Heart muscle• Striated• Electrical properties• Membranes differ• Intercalated discs- junction between cardiac muscle
cells• These gap junctions provide direct electrical coupling
among cells• Cardiac muscle cells can generate action potentials on
their own w/out any input from NS
Skeletal muscle
• Can only contract (to flex)
• Extend passively ( to extend)
• Attached to bones
• Multi-nucleated muscle fibers (cells)
• Fiber- bundle of myofibrils
myofibrils
• Made up of filaments (myofilaments)
• Thick filaments- myosin
• Thin- 2 strands of actin and strand one of a regulatory protein
• Look like dark and light bands under a microscope
Z lines
• Borders of sarcomere
• Lined up with next myofilaments
• Thin filaments are attached to the Z lines
• Thick filaments are centered in sarcomere
I bands
• Area where only thin filaments are found
Sarcomere
• Unit of thick and thin filaments
• Basic unit of muscle
A band
• Length of thick filaments
H zone
• Center of A band where only thick filaments are found
Contraction
• The length of each sarcomere is reduced
• the distance between one Z line to the next is shorter
• A bands do not change in length, but the
I bands shorten
• H zone disappears
Sliding filament model
• Neither thin nor thick filaments change in length; they slide pass each other longitudinally
• Therefore the degree of overlap increases
• Based on the inter action of myosin and actin
• Myosin has a “head “ and a “tail” region• Like golf clubs lined up• The head region can bind to ATP• When energized- the myosin takes on a
“high energy” configuration• Binds to a site on the actin forming a
CROSS-BRIDGE• Stored energy is released • Myosin changes back to a low energy
configuration
• The relaxation changes the angle of attachment of the head to the tail
• bends inward • pulls thin filament toward the center of the
sarcomere• Bond is broken when a new ATP molecule
binds to the myosin head• Process is repeated with the head forming
cross-bridge to actin farther down the molecule
• @350 heads of the myosin filament form and reform @ 5 cross-bridges/ sec.
• Only enough ATP is stored for a few contractions
• Glycogen is stored in between myofilaments
• Most energy is from creatine phosphate- the phosphagen of vertebrates, which can supply a phosphate group to ADP to make ATP
http://media.pearsoncmg.com/bc/bc_campbell_biology_6/cipl/ins/49/HTML/source/71.html
Control of Muscle Contraction
• Skeletal muscle is stimulated by motor neurons
• At rest the myosin binding site is blocked by tropomyosin (regulatory protein)
• Troponin complex is a set of regulatory proteins that control the position of tropomyosin on the actin
• Ca ++ bind to troponin, changing the shape of the complex, exposing myosin-binding sites on the actin
• Ca++ conc. in cytoplasm is regulated by sarcoplasmic reticulum ( a special type of ER)
• SR actively transports Ca++ from the cytoplasm to the interior of the SR
• An action potential of neuron releases Ca++
• Contraction stops when sarcoplasmic reticulum pumps Ca++ back into storage.
Graded contractions of whole muscles
• Muscles can contract completely or a little• @ the cellular level, any stimulus that depolarizes
the plasma membrane of a single muscle fiber triggers an all-or-none contraction
• Therefore a graded contraction is produced when the frequency of the action potential is varied in the motor neurons controlling the muscle
• Motor unit
• Recruitment
• Fast muscle fibers
• Slow muscle fibers