PowerLecture:
Chapter 6
The Muscular System
Learning Objectives




Explain the structure of muscles, from the
molecular level to the organ systems level.
Explain how biochemical events occur in
muscle contractions and how antagonistic
muscle action refines movements.
Explain the differences in stimulation
required for each type of muscle and how
each responds.
Demonstrate how muscle disorders impact
the function of both the muscular and
skeletal systems.
Impacts/Issues
Pumping Up Muscles
Pumping Up Muscles
Can you bulk up your muscles by using a
pill and do so safely?



Androstenedione and THG are available as
supplements but are really steroid-like drugs
unapproved by the FDA.
Another unapproved performance enhancer is
creatine, a substance normally produced by the
body during muscle activity.
Pumping Up Muscles
 The
intricate movements of
the human body are the
result of interactions between
the skeleton and muscle.
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 Dietary
supplements are largely unregulated.
Should they be subject to more stringent
testing for effectiveness and safety?


a. Yes, or companies could make any claim
about their product.
b. No, so long as the extract does no harm, let
the buyer decide what to buy.
Section 1
The Body’s Three Kinds
of Muscle
The Body’s Three Kinds of Muscle
The three kinds of muscle are built and
function in different ways.




Skeletal muscle, composed of long thin cells
called muscle “fibers,” allows the body to move.
Smooth muscle is found in the walls of hollow
organs and tubes; the cells are smaller than
those of skeletal muscle and are not striated.
The heart is the only place where cardiac
muscle is found.
One skeletal muscle fiber
Skeletal muscle
Fig. 6.1, p.104
Smooth muscle fibers
Smooth muscle
Fig. 6.1, p.104
Cardiac muscle fibers
Cardiac muscle
Fig. 6.1, p.104
The Body’s Three Kinds of Muscle


Cardiac muscle and smooth muscle are
considered involuntary muscles because we
cannot consciously control their contraction;
skeletal muscles are voluntary muscles.
Skeletal muscle comprises the body’s
muscular system.
TRICEPS BRACHII
BICEPS BRACHII
PECTORALIS MAJOR
DELTOID
SERRATUS ANTERIOR
TRAPIZIUS
EXTERNAL OBLIQUE
LATISSIMUS
DORSI
RECTUS ABDOMINUS
GLUTEUS MAXIMUS
ADDUCTOR LONGUS
BICEPS FEMORIS
SARTORIUS
QUADRICEPS FEMORIS
GASTROCNEMIUS
TIBIALIS ANTERIOR
2007Brooks/Cole
- Thomson Higher
Education
© ©2001
©2005
Brooks/Cole
..Thomson
Thomson
Fig. 6.2, p.105
flexor digitorum
superficialis
Fig. 6.20a, p.118
zygomaticus major
Fig. 6.20b, p.118
Section 2
The Structure and
Function of Skeletal
Muscles
The Structure and Function
of Skeletal Muscles
A skeletal muscle is built of bundled muscle
cells.



Inside each cell are threadlike myofibrils,
which are critical to muscle contraction.
The cells are bundled together with connective
tissue that extends past the muscle to form
tendons, which attach the muscle to bones.
muscle
tendon
(attached
to bone)
fluid
tendon sheath
bone
Fig. 6.4, p.106
muscle’s outer sheath
(connective tissue)
two bundles of muscle
cells (each has its own
connective tissue sheath)
one muscle cell
one myofibril
Fig. 6.3, p.106
The Structure and Function
of Skeletal Muscles
Bones and skeletal muscles work like a
system of levers.




The human body’s skeletal muscles number
more than 600.
The origin end of each muscle is designated
as being attached to the bone that moves
relatively little; whereas the insertion is
attached to the bone that moves the most.
Because most muscle attachments are located
close to joints, only a small contraction is
needed to produce considerable movement of
some body parts (leverage advantage).
The Structure and Function
of Skeletal Muscles
Many muscles are arranged as pairs or in
groups.


Many muscles are arranged as pairs or
grouped for related function.
•
•

Some work antagonistically (in opposition) so that
one reverses the action of the other.
Others work synergistically, the contraction of one
stabilizes the contraction of another.
Reciprocal innervation dictates that only one
muscle of an antagonistic pair (e.g. biceps and
triceps) can be stimulated at a time.
triceps relaxes
origin
biceps contracts at
the same time, and
pulls forelimb up
triceps contracts,
pulls the forelimb
down
at the same time,
biceps relaxes
insertion
Fig. 6.5, p.107
The Structure and Function
of Skeletal Muscles
“Fast” and “slow” muscle.


Humans have two general types of skeletal
muscles:
•
•
“Slow” muscle is red in color due to myoglobin and
blood capillaries; its contractions are slower but
more sustained.
“Fast” or “white” muscle cells contain fewer
mitochondria and less myoglobin but can contract
rapidly and powerfully for short periods.
Fig. 6.6, p.107
The Structure and Function
of Skeletal Muscles

When athletes train, one goal is to increase the
relative size and contractile strength of fast
(sprinters) and slow (distance swimmers) muscle
fibers.
Figure 6.6b
Section 3
How Muscles Contract
How Muscles Contract
A muscle contracts when its cells shorten.



Muscles are divided into contractile units called
sarcomeres.
Each muscle cell contains myofibrils composed
of thin (actin) and thick (myosin) filaments.
•
•

Each actin filament is actually two beaded strands of
protein twisted together.
Each myosin filament is a protein with a double head
(projecting outward) and a long tail (which is bound
together with others).
The arrangement of actin and myosin filaments
gives skeletal muscles their characteristic
striped appearance.
one actin
molecule
part of
a thin
filament
a Arrangement of actin molecules in the thin filaments
part of a
myosin
molecule
part of
a thick
filament
b Arrangement of myosin molecules in the thick filaments
Fig. 6.8, p.108
One myofibril
inside cell:
a. Skeletal muscle cell, longitudinal section.
All bands of its myofibrils are in register and
give the cell a striped appearance.
Fig. 6.7a, p.108
sarcomere
Z band
sarcomere
Z band
H zone
Z band
b. Sarcomeres. Many
thick and thin filaments
overlap in an A band.
Only thick filaments
extend across the H
zone. Only thin filaments
extend across I bands
to the Z bands.
I band
A band
I band
Fig. 6.7b, p.108
How Muscles Contract
 Muscle
cells shorten when actin filaments
slide over myosin.

Within each sarcomere there are two sets of
actin filaments, which are attached on opposite
sides of the sarcomere; myosin filaments lie
suspended between the actin filaments.
actin
myosin
actin
a Sarcomere when muscle cell is
relaxed
b Same sarcomere, contracted
Fig. 6.9, p.109
myosin
actin
actin
p.116
How Muscles Contract

During contraction, the myosin filaments
physically slide along and pull the two sets of
actin filaments toward each other at the center
of the sarcomere; this is called the slidingfilament model of contraction.
•
•
•
When a myosin head is energized, it forms crossbridges with an adjacent actin filament and tilts in a
power stroke toward the sarcomere’s center.
Energy from ATP drives the power stroke as the
heads pull the actin filaments along.
After the power stroke the myosin heads detach and
prepare for another attachment (power stroke).
myosin head
one of many myosin binding sites on actin
cross bridge
cross bridge
Fig. 6.9cd, p. 109
Fig. 6.9ef, p. 109
ATP
ATP
Fig. 6.9g, p. 109
How Muscles Contract

When a person dies, ATP production stops,
myosin heads become stuck to actin, and rigor
mortis sets in, making the body stiff.
Section 4
How the Nervous
System Controls Muscle
Contraction
How the Nervous System
Controls Muscle Contraction
Calcium ions are the key to contraction.


Skeletal muscles contract in response to
signals from motor neurons of the nervous
system.
•
•
•
Signals arrive at the T tubules of the sarcoplasmic
reticulum (SR), which wraps around the myofibrils.
The SR responds by releasing stored calcium ions;
calcium binds to the protein troponin, changing the
conformation of actin and allowing myosin crossbridges to form.
Another protein, tropomyosin, is also associated
with actin filaments.
How the Nervous System
Controls Muscle Contraction

When nervous stimulation stops, calcium ions
are actively taken up by the sarcoplasmic
reticulum and the changes in filament
conformation are reversed; the muscle relaxes.
section from spinal cord
motor neuron
a Signals from the
nervous system
b Endings of
motor neuron
T tubule
section from a
skeletal muscle
part of one muscle cell
c Signals travel along muscle cell’s plasma
membrane to sarcoplasmic reticulum around
myofibrils.
© 2007 Thomson Higher Education
sarcoplasmic reticulum
(calcium in storage)
plasma
membrane
of skeletal
muscle
fiber
one of the
myofibrils
inside the
muscle fiber
Z line
Z line
d Signals trigger the release of calcium ions
from sarcoplasmic reticulum threading
among the myofibrils.
Fig. 6.10, p. 110
troponin
a Actin molecule
myosin binding site blocked
b Cross-section of (a). Red dots are calcium ions bound to
troponin (green).
c Calcium ions flow in; troponin binds additional calcium.
d Troponin changes shape,moving away from the myosin
binding site.
myosin head
e The binding site is now exposed; actin can bind the myosin head.
myosin head
cross-bridge
f Cross-bridge forms between
actin and myosin.
Fig. 6.11, p. 111
troponin
a Actin molecule
myosin
binding site
blocked
b Cross-section of (a). Red dots
are calcium ions bound to
troponin (green).
c Calcium ions flow in; troponin
binds additional calcium.
Fig. 6.11a.c, p.111
myosin head
d Troponin changes shape,moving
away from the myosin binding site.
e The binding site is now exposed;
actin can bind the myosin head.
myosin head
f Cross-bridge forms between
actin and myosin.
Fig. 6.11d.f, p.111
How the Nervous System
Controls Muscle Contraction
Neurons act on muscle cells at
neuromuscular junctions.


At neuromuscular junctions, impulses from
the branched endings (axons) of motor neurons
pass to the muscle cell membranes.
•
•

Between the axons and the muscle cell is a gap
called a synapse.
Signals are transmitted across the gap by a
neurotransmitter called acetylcholine (ACh).
When the neuron is stimulated, calcium
channels open to allow calcium ions to flow
inward, causing a release of acetylcholine into
the synapse.
Vesicles containing ACh molecules
Axon ending of
motor neuron
Synapse
Muscle cell
Muscle cell receptor for ACh
Fig. 6.12, p.111
Section 5
How Muscle Cells Get
Energy
How Muscle Cells Get Energy
ATP supplies the energy for muscle
contraction.



Initiation of muscle contraction requires much
ATP; this will initially be provided by creatine
phosphate, which gives up a phosphate to
ADP to make ATP.
Cellular respiration provides most of the ATP
needed for muscle contraction after this, even
during the first 5-10 minutes of moderate
exercise.
How Muscle Cells Get Energy


During prolonged muscle action, glycolysis
alone produces low amounts of ATP; lactic acid
is also produced, which hinders further
contraction.
Muscle fatigue is due to the oxygen debt
that results when muscles use more ATP
than cellular respiration can deliver.
ADP + Pi
Pathway 1
Phosphate Transferred from
Creatine Phosphate
Relaxation
Contraction
creatine
ATP
Pathway 2
Aerobic
Respiration
oxygen
Pathway 3
Glycolysis
Alone
glucose from bloodstream and from
glycogen breakdown in cells
Fig. 6.13, p. 112
Section 6
Properties of Whole
Muscles
Properties of Whole Muscles
Several factors determine the
characteristics of a muscle contraction.


A motor neuron and the muscle cells under its
control are a motor unit; the number of cells in
a motor unit depends on the precision of the
muscle control needed.
•
•
A single, brief stimulus to a motor unit causes a brief
contraction called a muscle twitch.
Repeated stimulation makes the twitches run
together in a sustained contraction called tetanus
(tetany).
motor unit
slice from spinal cord
motor neuron leading
from spinal cord to
muscle fibers
neuromuscular
junction
Fig. 6.14a, p. 112
relaxation starts
stimulus
contraction
six stimulations per second
tetanic contraction
twitch
repeated stimulation
Time
Fig. 6.15ac, p.113
Properties of Whole Muscles

Not all muscle cells in a muscle contract at the
same time.
•
•
The number of motor units that are activated
determines the strength of the contraction: Small
number of units = weak contraction; large number of
units at greater frequency = stronger contraction.
Muscle tone is the continued steady, low level of
contraction that stabilizes joints and maintains
general muscle health.
Properties of Whole Muscles

Muscle tension is the force a contracting
muscle exerts on an object; to contract, a
muscle’s tension must exceed the load
opposing it.
•
•
An isotonically contracting muscle shortens and
moves a load
An isometrically contracting muscle develops
tension but does not shorten.
contracted
muscle can
shorten
contracted
muscle can’t
shorten
Fig. 6.16, p. 113
Properties of Whole Muscles
Tired muscles can’t generate much force.



Muscles fatigue when strong stimulation keeps
a muscle in a state of tetanus too long.
After resting, muscles will be able to contract
again; muscles may need to rest for minutes up
to a day to fully recover.
Section 7
Muscle Disorders
Muscle Disorders
Strains and tears are muscle injuries.



Muscle strains come from movement that
stretches or tears muscle fibers; ice, rest and
anti-inflammatory drugs (ibuprofen) allow
damage to repair.
If the whole muscle is torn,
scar tissue may develop,
shortening the muscle and
making it function less
effectively.
Figure 6.17
Muscle Disorders
Sometimes a skeletal muscle will contract
abnormally.



A muscle spasm is a sudden, involuntary
contraction that rapidly releases, while cramps
are spasms that don’t immediately release;
cramps usually occur in calf and thigh muscles.
Tics are minor, involuntary twitches of muscles
in the face and eyelids.
Muscle Disorders
Muscular dystrophies destroy muscle fibers.


Muscular dystrophies are genetic diseases
leading to breakdown of muscle fibers over
time.
•
Duchenne muscular
dystrophy (DMD) is
common in children; a
single mutant gene
interferes with
sarcomere contraction.
Figure 6.18
Muscle Disorders
• Myotonic muscular dystrophy is usually found in
adults; muscles of the hands and feet contract strongly
but fail to relax normally.

In these diseases, muscles progressively weaken
and shrivel.
 Exercise

makes the most of muscles.
Muscles that are damaged or which go unused
for prolonged periods of time will atrophy (waste
away).
Muscle Disorders

Aerobic exercise improves the capacity of
muscles to do work.
•
•
Walking, biking, and jogging are examples of
exercises that increase endurance.
Regular aerobic exercise increases the number and
size of mitochondria, the number of blood capillaries,
and the amount of myoglobin in the muscle tissue.
Figure 6.19a
Muscle Disorders


Strength training improves function of fast
muscle but does not increase endurance.
Even modest activity slows the loss of muscle
strength that comes with aging.
Figure 6.19b