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

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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 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