Transcript Life: The Science of Biology, 8e

David Sadava H. Craig Heller Gordon H. Orians
William K. Purves David M. Hillis
Biologia.blu
C – Il corpo umano
Musculoskeletal System
Musculoskeletal System
• How do muscles contract?
• What roles do skeletal systems play in
movement?
Musculoskeletal System - How do muscles contract?
Muscles and skeletons are the effectors that
produce movement.
Three types of vertebrate muscle:
• skeletal - voluntary movement, breathing;
• cardiac - beating of heart;
• smooth - involuntary, movement of internal
organs.
Musculoskeletal System - How do muscles contract?
Skeletal muscle (striated):
• multinucleate cells are called muscle
fibers;
• form from fusion of embryonic
myoblasts;
• one muscle consists of many muscle
fibers bundled together by connective
tissue.
Musculoskeletal System - How do muscles contract?
The structure of skeletal muscle (part 1)
Musculoskeletal System - How do muscles contract?
Contractile proteins:
• actin - thin filaments;
• myosin - thick filaments.
Each muscle fiber has several
myofibrils: bundles of actin and
myosin filament.
Musculoskeletal System - How do muscles contract?
Each myofibril consists of repeating
units: sarcomeres.
Sarcomere: overlapping actin and
myosin filaments.
Bundles of myosin filaments are held in
place by the protein titin, the largest
protein in the body.
Musculoskeletal System - How do muscles contract?
The structure of skeletal muscle (part 2)
Musculoskeletal System - How do muscles contract?
The structure of skeletal muscle (part 3)
Musculoskeletal System - How do muscles contract?
The sliding filament theory of muscle
contraction:
• depends on structure of actin and
myosin molecules;
• myosin heads can bind specific sites
on actin molecules to form cross
bridges, myosin changes conformation
causing actin filament to slide 5–10
nm.
Musculoskeletal System - How do muscles contract?
Sliding filaments
Musculoskeletal System - How do muscles contract?
Actin and myosin filaments overlap to form myofibrils
Musculoskeletal System - How do muscles contract?
Action potentials also travel deep within
muscle fiber via T tubules.
T tubules (transverse tubules) descend
into the sarcoplasm (muscle fiber
cytoplasm).
T tubules run close to the sarcoplasmic
reticulum (ER): a closed compartment
that surrounds every myofibril.
Musculoskeletal System - How do muscles contract?
T tubules in action
Musculoskeletal System - How do muscles contract?
Sarcoplasmic reticulum has Ca2+ pumps.
At rest there is high concentration of
Ca2+ in the sarcoplasmic reticulum.
Action potential reaches receptor proteins
and opens the Ca2+ channels, Ca2+ flows
out of sarcoplasmic reticulum and
triggers interaction of actin and myosin.
Musculoskeletal System - How do muscles contract?
Actin filaments also include tropomyosin
and troponin.
Troponin has three subunits: one binds
actin, one binds myosin, and one binds
Ca2+.
At rest, tropomyosin blocks the binding
sites on actin.
Musculoskeletal System - How do muscles contract?
When Ca2+ is released, it binds to
troponin, which changes conformation.
Troponin is bound to tropomyosin—
twisting of tropomyosin exposes
binding sites on actin.
When Ca2+ pumps remove Ca2+ from
sarcoplasm, contraction stops.
Musculoskeletal System - How do muscles contract?
The release of Ca2+ from the sarcoplasmic reticulum
triggers muscle contraction
Musculoskeletal System - How do muscles contract?
Cardiac muscle is also striated; cells
are smaller than skeletal and have one
nucleus.
Cardiac muscle cells also branch and
interdigitate: can withstand high
pressures.
Intercalated discs provide mechanical
adhesions between cells.
Musculoskeletal System - How do muscles contract?
There are three kinds of muscle
Musculoskeletal System - What roles do skeletal systems play in movement?
The human skeleton
Human skeleton: 206
bones, 2 connective
tissue types (cartilage
and bone).
Musculoskeletal System - What roles do skeletal systems play in movement?
Cartilage cells produce a tough, rubbery
extracellular matrix of polysaccharides
and protein, mostly collagen.
Cartilage is found on bone surfaces in
joints, also ears, nose, larynx.
Musculoskeletal System - What roles do skeletal systems play in movement?
Bone: extracellular matrix of calcium
phosphate.
Bone cells:
• osteoblasts make new bone matrix, when
they become enclosed in bone they are
called osteocytes;
• osteoclasts reabsorb bone.
Bone is constantly being replaced and
remodeled.
Musculoskeletal System - What roles do skeletal systems play in movement?
Types of joints
Musculoskeletal System - What roles do skeletal systems play in movement?
Joints, ligaments, and tendons
Musculoskeletal System - What roles do skeletal systems play in movement?
Bones are a system of levers moved by
muscles.
Levers have a power arm and a load
arm that work around a fulcrum (or
pivot point).
Musculoskeletal System - What roles do skeletal systems play in movement?
Bones and joints work like systems of levers