Skeletal and Muscular System
By Vignesh Vudatha
and Oliver Wiegel
• Motion is caused by the various forms of
sensory inputs to the nervous system.
• These inputs cause muscles to work against
the skeleton to facilitate movement of the
body.
• What are the functions of a skeleton?
Support
Protection
Movement
Types of Skeletons
Hydrostatic skeletons: fluid
is held together under
pressure in compartments,
whose form can be altered
for movement
Examples include
flatworms, nematodes,
and cnidarians such as
this hydra
Movement terminology
• Muscles: provide the force needed for muscle contraction
• Tendons: attach muscles to bones
• Bones: provide a firm anchorage for muscles. They also
act as levers, changing the size or direction of forces
generated by muscles
• Ligaments: connect bone to bone, restricting movement at
joints and helping to prevent dislocation
• Nerves: stimulate muscles to contract at a precise time and
extent, so that movement is coordinated
IB standard
Hydrostatic Skeletons con.
Organisms such as
annelids, that have
hydrostatic skeletons,
display peristaltic
behavior.
Through peristalsis,
muscles contract
rhythmically, allowing
for forward movement.
Types of Skeleton: Exoskeleton
•A hard layer surrounding the
epidermis of animals
•A key characteristic of
arthropods
•Expands as animal grows
•For arthropods, their jointed
exoskeleton is called a
cuticle
•A major component is
chitin, a polysaccharide that
hardens the cuticle
Cuticle is hardened with
organic compounds
when protection is
necessary
Type of Skeleton: Endoskeleton
•Tough elements
within soft tissues
•Sponges,
echinoderms,
humans
•Mammals have
skeletons of more
than 200 bones,
some of which are
fused and others
connected by
ligaments. (How
many do we have?)
•The human skeleton is
designed (evolution…)
such that the legs are
positioned under the body,
bearing most of the body
weight.
•Muscles and tendons
keep legs relatively
straight and provide
movement in the skeletal
system.
•Muscles only contract.
After contraction, muscles
relax passively
Ex. Elbow joint
Joints
• Cartilage: reduces friction between bones
where they meet
• Synovial fluid: lubricates the joint to reduce
friction
• Joint capsule: seals the joint and holds in
the synovial fluid.
IB standard
Biceps bend the arm at the
elbow and triceps straighten
the arm
These muscles are antagonistic
because they oppose each
other’s motion
radius
Radius transmits forces from
biceps, and ulna transmits
forces from triceps
Humerus:
anchorage for
muscles
IB wants you to know how
to label diagram
ulna
Hip vs. Knee Joints: IB wants you to know this
Move your knee, it’s like a hinge. It only moves along
one plane. (You can try the other planes of motion if you
want) The knee exhibits a bending (flexion) or
straightening (extension) effect.
Stand up and move your hip in any way you want. This
is possible because the hip can move in all three planes
(the technical terms are protraction/retraction,
abduction/adduction, and rotation
The elbow joint in the previous example is like the knee,
it’s a hinge with only one plane of motion
Skeletal Muscle terminology
•Skeletal Muscle: attached to
bones and is responsible for
movement. They consist of
bundles of long fibers, which are
cells with multiple nuclei.
•Fibers themselves are made up
of myofibrils, which are
composed of myofilaments (thin
and thick). Thin ones consist of
two strands of actin and one
strand of protein, while thick
ones are arrays of myosin
molecules
IB standard
• Muscle cells
have multiple
nuclei and
thousands of
mitochondria
• Sarcoplasmic
reticulum
encompasses
individual fibers
http://fitnessratchet.com/images/musclefiber.jpeg
Muscle Structure and Basic Characteristics of Contraction
Skeletal Muscle is
striated, meaning it
is arranged to create
a pattern of light and
dark bands.
Each unit is called a
sarcomere.
Sarcomeres contain
Z lines, I bands, A
bands, H bands,
and thick (myosin)
and thin (actin)
filaments
IB standard
The top one is a relaxed muscle, while the bottom one is a
contracted muscle cell. The easy way to distinguish is to
notice the width of the sarcomere. In the relaxed muscle,
the sarcomere is wide, while in the contracted muscle, the
sarcomere is contracted (easy to remember)
IB standard
Muscle Motion
• Sliding-filament model: neither thin filaments
nor thick filaments change in length; thin
filaments only slide past each other further to
contract the muscle
• Contraction is when these filaments are moved
together. Hence in the micrograph, the
sacromeres look to be fatter, since the fibers
are overlapping each other
• ATP is bound
and hydrolyzed
to ADP by
individual
myosin molecule
in a thick
filament
• This triggers the
binding of a
myosin (thick
filament) to an
actin (thin
filament)
http://physioweb.med.uvm.edu/muscle_physio/muscle_contraction
• The thick filament makes several of these
connections at once to a thin filament.
• Each individual actin-myosin bond will last
until a new ATP molecule severs it and binds
to a myosin head. The ATP molecule will be
broken, and the myosin will connect to another
actin molecule.
• Because a single myosin head forms about five
of these bonds per second, and there are ~350
heads on a thick filament, the thin filaments
“slide” to contract rather rapidly
• To keep them passive, thick muscle filaments’
myosin heads are usually blocked by an
allosteric regulator protein called tropomyosin.
• Tropomyosin is rearranged in position by a
rising concentration of Calcium ions from the
sarcoplasmic reticulum, which bind to the
troponin complex, which regulates where
tropomyosin is on the thick filament.
• This allows the heads of myosin to be exposed
and contraction can take place.
• Motor neurons control the contraction of
muscles.
• All contractions begin with a twitch lasting
less than 1/10 of a second, with more twitches
added on to increase tension
• Eventually, these twitches add up in the
tension they produce and then are expanded
into a smooth contraction (the kind you’re
used to feeling) called a tetanus
Muscle
Contraction
Listen
carefully
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