Notesale: Turn your study into money

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Skeletal muscle cells are muscle fibers
o Large with many nuclei
o Form through the fusion of individual embryonic muscle cells called myoblasts
o A single muscle is composed of hundreds of thousands of muscle fibers bundled
together by connective tissues
Each muscle fiber is packed with myofibrils - bundles of thin actin filaments and thick
myosin filaments
o Myofibril is made up of repeating units called sarcomeres
Each sarcomere is bounded by Z lines which anchor the actin filaments
The A band contains all the myosin filaments in the center of the sarcomere
The H zone is the region of myosin that does not overlap actin
The I zone is the region of actin that does not overlap myosin
M band contains proteins that hold the myosin filaments in their place at the center of the
sarcomere
o Titin hold the myosin in place and runs from Z line to Z line through the myosin
bundles
Myosin heads stick out on either end of the filaments
Tropomyosin is a protein that wraps around the actin filaments and is attached to troponin
at certain intervals
When actin and myosin bind, they hydrolyze ATP and the energy released by it changes
the conformation of the myosin head, causing it to relax
Action potential arrives at neuromuscular junction → presynaptic cell releases ACh →
ACh diffuses across synaptic cleft and binds to receptors → ion channels in motor end
plate open → Na+ flows in and cell is depolarized → action potential is fired → action
potential spreads across membrane and down T-tubules into the cell → DHP receptor
changes shape in response to the voltage change caused by the action potential →
Ryanodine receptor uncorks from the membrane of the sarcoplasmic reticulum → Ca2+
is released from sarcoplasmic reticulum into the sarcoplasm (cytoplasm) → Ca2+ binds
to troponin on actin, changing its conformation → troponin twists tropomyosin,
uncovering myosin binding sites on actin
Cardiac muscle fibers are smaller and only have one nucleus
o They are interwoven and connected by adhesive intercalated discs
§ The intercalated discs have gap junctions that allow for cytoplasmic
continuity
• Causes cells to be electrically coupled and action potentials spread
rapidly
Smooth muscle provides the contractile force of most of our intestines and lines organs
o Each cell has a single nucleus and is small
§ Actin and myosin filaments are not regularly arranged, so there is no
striation
o Have gap junctions
o Stretching causes depolarization and action potential which has a positive
feedback cycle
The strength of a muscle contraction depends on how many fibers are contracting and at
what rate
One action potential causes the minimum unit of contraction = a twitch

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The ultimate force generated by an action potential varies enormously depending
on how many muscle fibers it reaches
§ Tension the muscle generates depends on the number of motor units
activated and at what frequency the motor units fire

Slow twitch fibers:
o Lots of mitochondria and hemoglobin
o Oxidative metabolism
o Resistant to fatigue and have substantial energy reserves
o Maintain steady, prolonged production of ATP
o Long term aerobic work
Fast twitch fibers:
o White
o Few mitochondria and little myoglobin
o Develop maximum tension quickly
o Fatigue rapidly
o Cannot reproduce ATP fast enough to sustain contraction for a long time
A muscle has an optimal length for generating maximum tension
o When a muscle is stretched (hanging from a pullup bar), there is less overlap of
actin and myosin and therefore, fewer cross-bridges are formed and less force is
produced
o When muscle is fully contracted the action and myosin are fully overlapped and
press up against the Z lines with no where else to go making further contraction
difficult
Exercise increases muscle strength and endurance
Anaerobic increases strength (weight lifting)
Aerobic increases endurance - changes oxidative capacity; more mitochondria; more and
bigger capillaries
More actin and myosin in a muscle fiber means more tension can be produced = strength
A hydrostatic skeleton consists of fluid enclosed in a body cavity surrounded by muscle
o When muscle contracts in one direction, the fluid is pushed in the perpendicular
direction
An exoskeleton is a hardened outer shell to which muscles can be attached
o Contractions of muscles can cause segments of exoskeleton to move
Arthropods have the most complex exoskeleton
o Cuticle is a type of exoskeleton that covers the outer surfaces and all its
appendages
§ Made up of chitin
Endoskeletons consist of cartilage and bone
Cartilage produces an extracellular matrix that is tough and rubbery
o Contains collagen fibers
Bone also contains collagen fibers but gets its hardness and rigidity from a Calcium
extracellular matrix
o Serve as a reservoir of calcium to the rest of the body
Osteoblast cells lay down the new matrix material on bone surfaces

Gradually get surrounded by matrix and become enclosed within the bone
§ They cease to lay down matrix and are then called osteocytes
• Remain in contact with each other through tiny channels
Osteoclasts reabsorb bone
o Erode bone to form cavities and tunnels
Osteoblasts follow osteoclasts and deposit new bone
Bones develop from connective tissues
o Divided into two types depending on how they develop
§ Membranous bone forms on a scaffold of connective tissue membrane
§ Cartilage bone forms first as cartilaginous structure, resembling the future
mature bone, then gradually hardens (ossifies) to become real bone
• Grow throughout the ossification process
• Growth can continue until areas of ossification join
Bone structure may be compact (solid and hard) or cancellous (spongy)
o Most bones have both
§ Solid on the ends and spongy in the middle
§ All solid would be too heavy
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