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Muscular System


Nervous input
Muscle cell function relies on microfilaments
(actin components of the cytoskeleton)

Made up of:
Cytoskeleton

Microfilament
movement and
chemical energy brings
about contraction and
allows for passive
muscle extension


Concept 49.6: Muscles move skeletal parts by
contracting (they are directly attached)
The action of a muscle
◦ Is always to contract

Skeletal muscles are attached to the skeleton in
antagonistic pairs
◦ With each member of the pair working against each
other
Human
Grasshopper
Extensor
muscle
relaxes
Biceps
contracts
Triceps
relaxes
Extensor
muscle
contracts
Forearm
extends
Triceps
contracts
Flexor
muscle
contracts
Forearm
flexes
Biceps
relaxes
Figure 49.27
Tibia
flexes
Tibia
extends
Flexor
muscle
relaxes

Vertebrate skeletal muscle
◦ Is characterized by a hierarchy of smaller and smaller
units
Muscle
Bundle of
muscle fibers
Nuclei
Single muscle fiber
(cell)
Plasma membrane
Myofibril
Z line
Light
band
Dark band
Sarcomere
0.5 µm
TEM
I band
A band
I band
M line
Thick
filaments
(myosin)
Figure 49.28
Thin
filaments
(actin)
Z line
H zone
Sarcomere
Z line

A skeletal muscle consists of a bundle of long
fibers
◦ Running parallel to the length of the muscle
◦ Each fiber is a single cell with many nuclei =
formation by fusion of embryonic cells

A muscle fiber
◦ Is itself a bundle of smaller myofibrils arranged
longitudinally (made of thin and thick filaments)

The myofibrils are
composed to two kinds
of myofilaments
◦ Thin filaments, consisting
of two strands of actin
and one strand of
regulatory protein (that
coil)
◦ Thick filaments,
staggered arrays of
myosin molecules


Edges are thin
Center is thick
The areas that contain
the myofilments
Are the I band, A
band, and H zone
Bordered by Z lines

Skeletal muscle is also called striated muscle
◦ Because the regular arrangement of the myofilaments
creates a pattern of light and dark bands

Each repeating unit is a sarcomere (basic
contractile unit of muscle)

According to the sliding-filament model of
muscle contraction
◦ The filaments slide past each other longitudinally,
producing more overlap between the thin and thick
filaments

As a result of this sliding
◦ The I band and the H zone shrink
0.5 µm
(a) Relaxed muscle fiber. In a relaxed muscle fiber, the I bands
and H zone are relatively wide.
(b) Contracting muscle fiber. During contraction, the thick and
thin filaments slide past each other, reducing the width of the
I bands and H zone and shortening the sarcomere.
Figure 49.29a–c
(c) Fully contracted muscle fiber. In a fully contracted muscle
fiber, the sarcomere is shorter still. The thin filaments overlap,
eliminating the H zone. The I bands disappear as the ends of
the thick filaments contact the Z lines.
Z
H
A
Sarcomere

The sliding of filaments is based on
◦ The interaction between the actin and myosin
molecules of the thick and thin filaments

The “head” of a myosin molecule binds to an
actin filament
◦ Forming a cross-bridge and pulling the thin filament
toward the center of the sarcomere

Myosin-actin interactions underlying muscle
fiber contraction
Thick filament
1 Starting here, the myosin head is
bound to ATP and is in its lowenergy confinguration.
Thin filaments
5 Binding of a new molecule of ATP releases the
myosin head from actin,
and a new cycle begins.
Thin filament
Myosin head (lowenergy configuration)
ATP
ATP
Thick
filament
Thin filament moves
toward center of sarcomere.
Figure 49.30
+
4 Releasing ADP and ( P i), myosin
relaxes to its low-energy configuration,
sliding the thin filament.
Actin
Pi
ADP
Pi
Cross-bridge
binding site
ADP
Myosin head (lowenergy configuration)
ADP
2 The myosin head hydrolyzes
ATP to ADP and inorganic
phosphate ( P I ) and is in its
high-energy configuration.
Pi
Cross-bridge
Myosin head (highenergy configuration)
1
3 The myosin head binds to
actin, forming a crossbridge.


Contain very little ATP (and thus can only
produce a few contractions)
Most energy comes from creatine phosphate
or glycogen

A skeletal muscle fiber contracts
◦ Only when stimulated by a motor neuron

When a muscle is at rest
◦ The myosin-binding sites on the thin filament are
blocked by the regulatory protein tropomyosin
Tropomyosin
Actin
Figure 49.31a
Ca2+-binding sites
(a) Myosin-binding sites blocked
Troponin complex

For a muscle fiber to contract
◦ The myosin-binding sites must be uncovered

This occurs when calcium ions (Ca2+)
◦ Bind to another set of regulatory proteins, the
troponin complex
◦ Which then bind actin and expose myosin
Ca2+
Myosinbinding site
Figure 49.31b
(b) Myosin-binding sites exposed

Increase calcium ions
(Ca2+), increase muscle
contraction

Decrease calcium ions
(Ca2+), muscle relaxes
Ca2+
Myosinbinding site
(b) Myosin-binding sites exposed

The stimulus leading to the contraction of a
skeletal muscle fiber
◦ Is an action potential in a motor neuron that makes a
synapse with the muscle fiber
Motor
neuron axon
Mitochondrion
Synaptic
terminal
T tubule
Sarcoplasmic
reticulum
Ca2+ released
from sarcoplasmic
reticulum
Myofibril
Figure 49.32
Plasma membrane
of muscle fiber
Sarcomere

The synaptic terminal of the motor neuron
◦ Releases the neurotransmitter acetylcholine,
depolarizing the muscle and causing it to produce an
action potential
Motor neurons can cause muscle contractions by
the release of calcium ions (Ca2+)
Synaptic
1 Acetylcholine (ACh) released by synaptic terminal diffuses across synaptic
cleft and binds to receptor proteins on muscle fiber’s plasma membrane,
terminal
triggering an action potential in muscle fiber.
of motor
neuron
Synaptic cleft
ACh
2 Action potential is propagated along plasma
membrane and down
T tubules.
SR
Ca2+
7 Tropomyosin blockage of myosinbinding sites is restored; contraction
ends, and muscle fiber relaxes.
ADP
P2
Figure 49.33
PLASMA MEMBRANE
T TUBULE
3 Action potential
triggers Ca2+
release from sarcoplasmic reticulum
(SR).
4 Calcium ions bind to troponin;
troponin changes shape,
removing blocking action
of tropomyosin;
myosin-binding
Ca2+
sites exposed.
CYTOSOL
2+
6 Cytosolic Ca is
removed by active
transport into
SR after action
potential ends.
5 Myosin cross-bridges alternately attach
to actin and detach, pulling actin
filaments toward center of sarcomere;
ATP powers sliding of filaments.

Action potentials travel to the interior of the
muscle fiber
◦ Along infoldings of the plasma membrane called
transverse (T) tubules


The action potential along the T tubules
◦ Causes the sarcoplasmic reticulum to release Ca2+
The Ca2+ binds to the troponin-tropomyosin
complex on the thin filaments
◦ Exposing the myosin-binding sites and allowing the
cross-bridge cycle to proceed


Skeletal muscle fibers contractions are all or
none twitches
Contraction of a whole muscle is graded
◦ Which means that we can voluntarily alter the extent
and strength of its contraction

There are two basic mechanisms by which the
nervous system produces graded contractions
of whole muscles
◦ By varying the number of fibers that contract
◦ By varying the rate at which muscle fibers are
stimulated

In a vertebrate skeletal muscle
◦ Each branched muscle fiber is innervated (controlled)
by only one motor neuron

Each motor neuron
◦ May synapse with multiple muscle fibers
Motor
unit 1
Spinal cord
Motor
unit 2
Synaptic terminals
Nerve
Motor neuron
cell body
Motor neuron
axon
Muscle
Muscle fibers
Figure 49.34
Tendon

A motor unit
◦ Consists of a single motor neuron and all the muscle
fibers it controls
◦ When motor neuron = action potential, all muscle
fibers in unit contract as a group
◦ Strength of contraction dept on # fibers motor neuron
controls

Force of muscle increases as more motor
neurons are activated
◦ Depending on the # of motor neurons the brain
recruits and size of motor units = what you can lift

A twitch
◦ Results from a single action potential in a motor
neuron
More rapidly delivered action potentials
◦ Produce a graded contraction by summation
Tetanus
Tension

Summation of
two twitches
Single
twitch
Action
potential
Figure 49.35
Time
Pair of
action
potentials
Series of action
potentials at
high frequency

Tetanus is a state of smooth and sustained
contraction
◦ Produced when motor neurons deliver a volley of
action potentials

Skeletal muscle fibers are classified as slow
oxidative, fast oxidative, and fast glycolytic
◦ Based on their contraction speed and major pathway
for producing ATP

Oxidative
◦ Relies on aerobic
respiration
◦ Uses a steady energy
supply
◦ Rich in mitochondria,
blood and myoglobin
Dark meat

Glycolytic
◦ Uses glycolysis as ATP
source
◦ Large in diameter
◦ Low levels of myoglobin
◦ Fatigue quickly
Light meat


Used in brief, rapid ,
powerful contractions
Heart and eye
exclusively this form




Fast twitch
Used in posture;
sustain long
contraction
Less sarcoplasmic
reticulum
Pumps calcium ions
slowly
Twitches last longer
Slow twitch

Cardiac muscle, found only in the heart
◦ Consists of striated cells that are electrically
connected by intercalated discs (gap junctions)
◦ Can generate action potentials without neural input

In smooth muscle, found mainly in the walls of
hollow organs
◦ The contractions are relatively slow and may be
initiated by the muscles themselves
◦ Not striated
◦ Decreased myosin

The three main functions of a skeleton are
◦ Support, protection, and movement

The three main types of skeletons are
◦ Hydrostatic skeletons, exoskeletons, and
endoskeletons

A hydrostatic skeleton
◦ Consists of fluid held under pressure in a closed body
compartment
◦ Muscles change the shape of fluid filled spaces =
movement

This is the main type of skeleton
◦ In most cnidarians, flatworms, nematodes, and
annelids

Annelids use their hydrostatic skeleton for
peristalsis
◦ A type of movement on land produced by rhythmic
waves of muscle contractions
(a) Body segments at the head and just in front
of the rear are short and thick (longitudinal
muscles contracted; circular muscles
relaxed) and anchored to the ground by
bristles. The other segments are thin and
elongated (circular muscles contracted;
longitudinal muscles relaxed.)
Longitudinal
muscle relaxed
(extended)
Bristles
(b) The head has moved forward because
circular muscles in the head segments have
contracted. Segments behind the head and
at the rear are now thick and anchored, thus
preventing the worm from slipping backward.
Figure 49.25a–c
(c) The head segments are thick again and
anchored in their new positions. The rear
segments have released their hold on the
ground and have been pulled forward.
Circular
muscle
contracted
Circular
muscle
relaxed
Longitudinal
muscle
contracted
Head

An exoskeleton is a hard encasement
◦ Deposited on the surface of an animal (chitin)
◦ Muscles attach to knobs and plates of body’s interior

Exoskeletons
◦ Are found in most mollusks and arthropods

An endoskeleton consists of hard supporting
elements
◦ Such as bones, buried within the soft tissue of an
animal

Endoskeletons
◦ Are found in sponges, echinoderms, and chordates

The mammalian skeleton is built from more
than 200 bones
◦ Some fused together and others connected at joints
by ligaments that allow freedom of movement

The human skeleton
key
Axial skeleton
Appendicular
skeleton
Skull
Examples
of joints
Head of
humerus
Scapula
1
Shoulder
girdle
Clavicle
Scapula
Sternum
Rib
Humerus
2
Vertebra
3
Radius
Ulna
Pelvic
girdle
1 Ball-and-socket joints, where the humerus contacts
the shoulder girdle and where the femur contacts the
pelvic girdle, enable us to rotate our arms and
legs and move them in several planes.
Humerus
Carpals
Phalanges
Ulna
Metacarpals
Femur
Patella
2 Hinge joints, such as between the humerus and
the head of the ulna, restrict movement to a single
plane.
Tibia
Fibula
Ulna
Figure 49.26
Tarsals
Metatarsals
Phalanges
Radius
3 Pivot joints allow us to rotate our forearm at the
elbow and to move our head from side to side.

In addition to the skeleton
◦ Muscles and tendons help support large land
vertebrates


Concept 49.7: Locomotion requires energy to
overcome friction and gravity
Movement is a hallmark of all animals
◦ And usually necessary for finding food or evading
predators

Locomotion
◦ Is active travel from place to place

Overcoming friction
◦ Is a major problem for swimmers
◦ Torpedo shaped bodies

Overcoming gravity is less of a problem for
swimmers
◦ Than for animals that move on land or fly

Walking, running, hopping, or crawling on land
◦ Requires an animal to support itself and move against
gravity
◦ Leg muscles propel and keep an organism from
collapsing due to gravity

Diverse adaptations for traveling on land
◦ Have evolved in various vertebrates
◦ Tails for balance
Figure 49.36

Flight requires that wings develop enough lift
◦ To overcome the downward force of gravity
◦ Wing shape is key
 Airfoils
Comparing Costs of Locomotion
•The energy cost of locomotion
–Depends on the mode of locomotion and the
environment
EXPERIMENT
Physiologists typically determine an animal’s rate of energy use during locomotion by measuring
its oxygen consumption or carbon dioxide production while it swims in a water flume, runs on a treadmill, or flies in a
wind tunnel. For example, the trained parakeet shown below is wearing a plastic face mask connected to a tube that
collects the air the bird exhales as it flies.
RESULTS
This graph compares the energy cost, in joules per kilogram of
body mass per meter traveled, for animals specialized for running, flying, and
swimming (1 J = 0.24 cal). Notice that both axes are plotted on logarithmic scales.
CONCLUSION
Flying
Energy cost (J/Kg/m)
For animals of a given
body
mass, swimming is the most energyCONCLUSION
efficient and running the least energyefficient mode of locomotion. In any mode,
a small animal expends more energy per
kilogram of body mass than a large animal.
102
10
1
Swimming
10–1
10–3
Figure 49.37
Running
1
103
Body mass(g)
106

Animals that are specialized for swimming
◦ Expend less energy per meter traveled than
equivalently sized animals specialized for flying or
running