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Muscles and Skeleton
Muscles work by contracting (active movement) and
lengthening (passive). Three types in mammals:
1. Skeletal muscle – used to move the skeleton, most
abundant form of muscle in body
• also called striated muscle (has “striped”
appearance)
• most is under voluntary control
• contracts as a simple twitch (single, quick
contraction)
• most movement occurs as a result of skeletal
muscle receiving a series of separate stimuli timed
very close together – these produce a single
sustained contraction called tetanus
Two types of skeletal muscle fibers (muscle
cells):
•
white muscle fibers – specialized for rapid
response, generate a lot of power but fatigue
quickly – obtain most of their energy from
glycolysis - have few mitochondria
• red (dark) muscle fibers – specialized for slower,
endurance activities
– require a steady flow of oxygen, have many
mitochondria, rich blood supply
– rich in myoglobin (red pigment similar to
hemoglobin) which stores oxygen and
enhances rapid diffusion of oxygen during
strenuous exercise
Skeletal Muscle
2. Cardiac muscle – located only in the
heart
– spontaneously active – initiates its own
contractions
– influenced by nerves and hormones
– striated in appearance
3. Smooth muscle – lacks striations
– lines the walls of digestive tract and large blood
vessels
– produces slow, sustained contractions
– involuntary
Muscles are made up of muscle cells called
muscle fibers
–
each muscle fiber runs the entire length of the
muscle
–
each muscle fiber is made up of myofibrils –
individual contractile subunits that extend from one
end of the fiber to the other
•
each myofibril is surrounded by the sarcoplasmic
reticulum
– series membranes forming hollow tubes that
store high concentrations of calcium
•plasma membrane
(sarcolemma) has multiple
inward extensions that form
transverse tubules (T
tubules)
•T tubules pass very
close to sarcoplasmic
reticulum and are crucial
to muscle control
•
each myofibril is made up of two types of
myofilaments:
– myosin filaments – thick filaments
– actin filaments – thin filaments, also
contain tropomyosin and the troponin
complex (regulate actin’s interactions
with myosin)
– myosin and actin filaments slide past
each other during contraction
• myofilaments are arranged in repeating
subunits called sarcomeres – basic unit of
contraction – aligned end to end the
length of the myofibril
Muscle contraction – Sliding Filament Model
• muscle contraction occurs when the
sarcomeres contract
• myosin “heads” bond to exposed receptor
sites on actin forming cross bridges
• at rest, receptor sites are blocked by
tropomyosin and troponin complex
• when stimulated, complex pulls away from
actin exposing receptor sites to form cross
bridges
•
•
•
•
myosin heads bend in toward the center
of the sarcomere then cross bridges
break
cross bridges reform at the next actin
receptor site and bend some more
process repeats causing thin filaments to
slide over thick pulling the sarcomere
inward (shortens between Z lines)
ATP is used to break cross bridges and
allow myosin heads to bond to the next
receptor site after powerstroke
•
•
•
•
A band length stays the
same
I band length shortens
H zone length shortens
(may disappear)
Z lines are drawn
closer to each other
Role of calcium in muscle contraction
• muscle fiber membrane is polarized – action
potential is generated in similar manner as neurons
– action potential activates contraction indirectly by
causing the release of calcium
• action potential is generated by the release of
acetylcholine (a neurotransmitter) at the
neuromuscular junction – acetylcholine binds to the
sarcolemma and causes the membrane to
depolarize
• action potential depolarizes membrane of T tubules
which are closely associated with sarcoplasmic
reticulum – reticular membranes become
permeable to and release large amounts of
calcium
•troponin complex and tropomyosin block
the receptor sites on actin when muscle
is at rest
•calcium binds to troponin complex and
tropomyosin and pulls them out of the
way to expose receptor sites to myosin
heads
Skeletal Structure
•
•
•
•
Arthropod and vertebrate skeletal and
muscular systems have many functional
similarities
each have hard jointed skeleton
most skeletal muscle is arranged w/one
end attached to one section of skeleton
and the other attached to a different
section
Arthropods – exoskeleton – hard body
covering w/muscles and organs located
inside – requires molting for growth
Vertebrates – endoskeleton – framework w/muscles attached
embedded within the organism – composed of two types of
bone:
1. Cancellous – “spongy”
– very porous and light weight
– filled w/red bone marrow (produce RBCs)
– found in flat bones of ribs, skull, and ends of long bonds
2. Compact Bone – composed of structural units
called Haversian Systems
–
–
concentric layers of hard inorganic matrix
surrounding a central Haversian Canal
has radiating canaliculi that allow for exchange of
mat’l b/w bone cells and blood vessels
Cartilage – firm, flexible tissue
• primary component of embryonic
skeletons
• replaced w/bone as fetus develops –
retained in some areas where firmness
and flexibility is needed: ends of ribs,
external ear, tip of nose, where some
bones meet
• Chondricthyes and Agnathans – retain
cartilagenous skeleton throughout life
•
Vertebrate skeleton divided into 2 major parts:
– Axial skeleton – skull, vertebral column and
rib cage
– Appendicular skeleton – pectoral and pelvic
girdles w/paired appendages
•
•
•
•
•
•
Ligaments – tough “strings” of tissue holding
bone to bone
Tendons – attach muscle to bones
Origin – end of musc. attached to an essentially
stationary bone
Insertion – end of musc. attached to bone that
moves
a single muscle may have multiple
origins/insertions
Movement resulting from a musc. contraction
depends on position of origin/insertion and type
of joint b/w bones (read about diff’t types of joints
in text)