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 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

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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 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