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Skeletal & Muscular Systems
Learning Objectives (3/11/09)
Describe the types of skeletons that support and
enable movement in animals, with examples.
2. Describe how muscles exert force against
skeletal elements to maintain posture and
produce movement.
3. Compare the structure and function of the three
types of muscle tissue.
4. Differentiate between whole muscle contraction
and contraction of a single muscle cell,
including the sliding filament theory of muscle
Musculoskeletal Machines
 The skeleton and muscles work together in lever systems
 Muscles can only shorten by contraction, they cannot actively
 An external force is needed to stretch a muscle back to its resting
 Opposing muscle sets provide this external force.
Hydrostatic Skeletons
How do soft-bodied animals like worms and other forms that
lack rigid skeletons operate opposing muscles?
p. 1046
 Fluid held in internal
compartments as a
hydraulic fluid transfers
force between opposing
muscle sets.
 As muscles contract,
internal volume remains
the same, so the
opposing muscle set
must stretch.
 This stretch creates the
potential to do work
Hydrostatic Skeleton
Sea anemones (Phylum ?) have cylindrical fluid-filled
bodies that function as a hydrostatic skeleton.
They have both
circular and
longitudinal muscle
that contract against
the fluid in their
Nematode Worms
See p. 694
 Roundworms have only longitudinal muscles, innervated
by two nerve cords, and use a hydrostatic skeleton.
 Their body can assume curved and S-shaped configurations
to help them move through soil and other media.
Name the closed, waterfilled body cavity that acts
as the hydroskeleton.
nematode locomotion
Annelid Worms
Each segment in the worm body
can act as an independent
hydrostatic skeleton.
This permits much more complex
changes in body shape.
 The head is extended forward
by contraction of circular
 A wave of contraction of
longitudinal muscles then
anchors the segments near the
earthworm locomotion
See p. 724
Polychaete Worms
Contraction of longitudinal muscles on one side of a segment
stretches the longitudinal muscles on the other side.
Parapodia act like
paddles to push
each segment
toward the rear
of the animal.
See p. 724
polychaete worm swimming
Exoskeletons are hardened
outer surfaces to which internal
muscles are attached.
Increased leg length allows greater
speed and power in locomotion
(simple lever systems).
Multiple, long legs create a potential
problem of tripping over one’s legs.
p. 1046
Centipedes and crustaceans have
staggered activity in their legs to
prevent tripping.
More advanced forms (e.g.
crustaceans and insects) fuse
segments and reduce the number of
Endoskeletons are internal,
articulated systems of rigid
supports consisting of bone
and cartilage to which
muscles are attached.
What are some of the advantages
associated with endoskeletons, over
How could you improve the efficiency
of the lever system for arm flexion? 
p. 1046
Lever Systems
 Muscles and bones work
together around joints as
systems of levers.
 Lever systems of muscles and
skeletons can be designed either
for power or speed.
 The ratio of load arm
(resistance) to power arm
(effort) determines the power.
 A low load arm to power arm
ratio provides high power but
low speed
 A high load arm to power arm
L:P = 2
ratio provides high speed but
lower power.
L:P = 5
What is stored within cisternae of
muscle cells?
What is a myofibril?
p. 1072
Internal organization of a muscle cell
What is the functional unit of contraction in a
muscle fiber? Circle and/or label one in this
Each t-tubule is an
extension of the
A sarcomere within a myofibril
p. 1070
= actin
= myosin
Organization of Myofilaments in a Sarcomere
Events at the
p. 1072
Is the release of
neurotransmitter active
transport or passive
Is the influx of Na+ ions by
active transport or passive
How does the influx of Na+
ions change the
electrochemical potential?
Are the calcium channels in cisternae voltage-gated or chemically-gated?
Besides Ca+2, what must also be present in order for myosin to bind to