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The Muscular System
I. Types of muscles
A. Skeletal
1. Voluntary…under conscious
control
2. Striated
3. Result in the movement of
bones at articulations
4. Contracts at the fastest
rate
5. Has the greatest strength
of contraction
6. Can remain contracted for
the shortest length of time
B. Visceral
1. Involuntary…not able to
be consciously controlled
2. Nonstriated (smooth)
3. Located within the walls of
some organs
4. Contracts at the slowest
rate
5. Has the smallest strength
of contraction
6. Can remain contracted for
the longest amount of
time
C. Cardiac
1. Involuntary
2. Has striations similar to
skeletal muscles
3. Has an intermediate speed
and strength of contraction
4. Can remain contracted for
an intermediate amount of
time
II. Functional Characteristics
1. Excitability:
(responsiveness/irritability)
the ability to receive and
respond to a stimulus
2. Contractility: the ability to
shorten forcibly when
adequately stimulated
3. Extensibility: the ability to
be stretched or extended
4. Elasticity: the ability of a
muscle fiber to recoil and
resume its resting length
after being stretched
III. Muscle Functions
1. Producing movement: all
movements of the body
and its parts are due to
muscular contractions
2. Maintaining posture
3. Stabilizing and
strengthening joints
4. Generating heat: heat is
produced when muscles
contract…maintaining a
normal body temperature…a
vital homeostatic process
5. Muscles also protect fragile
internal organs and regulate
the movement of
substances through the
various body openings by
forming valves.
IV. Skeletal Muscles
• Each skeletal muscle is an
individual organ consisting
of muscle fibers, blood
vessels, nerve fibers and
connective tissue
• Skeletal muscles are the
body’s primary type of
muscles
• They provide motion
through the process of
contracting
• They attach to bones either
by a direct connection with
the bone and the outer layer
of the muscle
or by a connection through
a tendon
A. Connective Tissue
• The most abundant
connective tissue associated
with muscles is
fascia…
• Fascia is: “a sheet or broad
band of dense connective
tissue that may occupy the
narrow space between skin
and muscle or may surround
muscles or other organs.”
• There are two types of
fascia: superficial and deep.
• Deep fascia is the type
associated most commonly
with muscles.
• Deep fascia surrounds a
muscle, supporting it and
holding it together as a single
unit.
• It also provides a passageway
for nerves and blood vessels.
1. Sheaths
a. Epimysium: the outer
covering of dense, irregular
connective tissue that
surrounds the entire muscle.
It is continuous with
adjacent muscles or tendons
b. Perimysium: fibrous
connective tissue that
surrounds each bundle of
muscle fibers (fascicle)
c. Endomysium: fine sheath
of areolar and reticular
connective tissue
surrounding each individual
muscle fiber
2. Tendons
• Thick bands of dense
connective tissue
that forms most of the
connections between bones
and muscles
Tendons form when the
three layers of connective
tissue converge and extend
away from the fleshy part of
the muscle to attach to the
periosteum of the bone
• a. A broad sheet-like tendon
extending away from the
fleshy part of some muscles is
called an aponeurosis
• This can attach muscles to
bones or muscles to muscles.
B. Microscopic Anatomy
• The sarcolemma is the cell
membrane of a muscle
fiber.
• Each muscle fiber is a long,
cylindrical cell with multiple
nuclei
• Skeletal muscle fibers are
extremely large ranging in
diameters of up to 10 times
that of an average cell and
up to 30 cm in length.
• The cytoplasm of a muscle
cell is called sarcoplasm
• Myoglobin is a red pigment
in the sarcoplasm that
transports oxygen
• Myofibrils and sarcoplasmic
reticulum are highly
specialized organelles in the
sarcoplasm.
• T tubules are modifications
to the sarcolemma.
1. Myofibrils
• Rod-like organelles that run
parallel to the length of the
muscle fiber
• They contain the contractile
elements of skeletal muscle
cells
• Skeletal muscles are
characterized by striations…
• A repeating series of dark A
bands and light I bands
• The light stripe in the center
of an A band is the H zone
which is bisected by a dark
M line
• The dark area in the I band
is the Z disk.
• A sarcomere is the region of
a myofibril between 2
successive Z disks…and is
the smallest contractile unit
of a muscle fiber
• There are two smaller
structures in myofibrils
called myofilaments. There
are two types: thick and thin
filaments
• Thick filaments are made
mostly of myosin which has
specific features that form
the cross bridges that
generate the tension
developed by a contracting
muscle cell.
• Thin filaments are made
mostly of actin to which the
myosin cross bridges attach
during contraction
• Elastic fibers are a third
(relatively newly discovered
filament)
• It is made mostly of the
protein titin
• Elastic fibers perform 2
functions:
–Holding thick filaments in
place
–Assisting the muscle cell in
springing back to shape
after stretching or
shortening
C. Regulation of contraction
• Two sets of tubules regulate
muscle contraction:
1. Sarcoplasmic reticulum:
SER of muscle cells – it
regulates intracellular levels
of calcium
• Storing and releasing Ca on
demand when the muscle
fiber is stimulated to
contract.
2. T Tubules:
conduct nervous impulses to
the deepest regions of the
muscle cells and to every
sarcomere signaling the
release of Ca…ensuring
coordinated muscular
contraction
D. Muscles and Motion
1. Sliding Filament Model
Contraction occurs when
the cross bridges create
enough tension to exceed
the forces opposing
contraction
• This occurs in a ratcheting
type of motion…with the
attachments formed and
broken multiple times during
a single contraction
• Contraction ends when the
cross bridges become
inactive and the tension
decreases inducing
relaxation
• In order to cause
movement, the muscle must
have two points of
attachment:
Origin:
The proximal attachment
onto the bone that does
NOT move
Insertion
The distal attachment
onto the bone that
moves
F. Muscle Response
1. Action Potential
The result of a series of
electrical changes that
occurs along the entire
length of the sarcolemma
• The resting state of the
sarcolemma is polarized…
the outside face is positive
the inside face is negative
• The action potential initiates
and propagates
depolarization which leads
to the sliding of
myofilaments and the
contraction of the muscle
• Depolarization waves are
quickly followed by
repolarization waves…
during which the initial,
resting, electrical conditions
of the polarized state is
restored
• The muscle cell cannot be
stimulated again until
repolarization is complete.
• During this time, the muscle
is said to be in a refractory
period
2. The role of ionic calcium
• Because calcium plays
numerous, vital,
physiological roles, the
cytoplasmic concentrations
of calcium is finely regulated
by intracellular proteins
• When intracellular calcium
levels are low, the muscle
cell is relaxed. As calcium
ion levels rise, the ions bind
to sites causing cross bridge
formation, attachments and
detachments which result in
muscular contractions
• As calcium ions are
reclaimed by the
sarcoplasmic reticulum, the
contraction ends and the
muscle fiber relaxes
3. All or none response
The threshold stimulus is the
weakest stimulus capable of
initiating muscular
contractions
Either the threshold stimulus
is reached and the muscle
contracts
or it is not and the muscle
does not contract.
There is no partial
contraction!
The strength of the
contraction results from the
number of and specific
muscles stimulated.
G. Additional Terms
1. Rigor mortis:
most muscles begin to
stiffen 3 – 4 hours after
death…and relax over the
following 48 – 60 hours
• This occurs because dying
cells are unable to keep
calcium ions out of the
muscle cell…resulting in the
formation of cross bridges
and muscular contraction
• Relaxation cannot occur
until the muscle proteins
break down and the cross
bridges detach
2. Muscle tension is the
forces exerted by a
contracting muscle
3. Load is the opposing force
exerted on the muscle by
the weight of the object
being moved
4. Muscle twitch: the
response of a motor unit to
a single action
potential…the muscle fibers
contract quickly and then
relax
5. Aerobic respiration:
provides most of the ATP
used for muscle activity
this takes place in the
mitochondria…glucose
combines with oxygen and
breaks down to form water,
carbon dioxide and ATP
• The carbon dioxide released
diffuses out of the muscle
tissue into the blood and is
removed through exhalation
• The process of aerobic
respiration provides high
levels of ATP but requires a
continuous source of oxygen
and nutrients
6. Oxygen debt:
occurs when the
respiratory/cardiovascular
system cannot bring in
enough oxygen to meet
the demand of the
activity
This leads to an accumulation
of lactic acid in the muscle
fibers
Prolonged build up of lactic
acid causes muscle fatigue
which may make the muscle
unresponsive to stimuli
7. Muscle cramps
occur when a muscle
contracts spasmodically
without relaxing
8. Isotonic contractions
produce movement as the
muscle pulls the bone
towards a stationary
structure
9. Isometric contractions
produces muscle tension,
but does not shorten the
muscle – therefore, does
not produce movement.
This can occur when you
push against an immovable
object such as a wall.
V. Skeletal Muscle
Nomenclature
1. Location of the muscle:
some muscles are named
for the bone or region with
which the muscle is
associated
2. Shape of the muscle:
some muscles are named
for their distinctive shapes
(deltoid – triangular shape;
trapezius – both right and
left form a trapezoid)
3. Relative size:
maximus – largest
minimus – smallest
longus – long
brevis - short
4. Direction of muscle fibers:
rectus – straight…fibers run
parallel to the axis
transversus – fibers run at
right angles to the axis
oblique – fibers run
obliquely to the axis
5. Number of origins:
biceps – two origins (heads)
triceps – three origins
quadriceps – four origins
6. Location of the attachments:
some muscles are named for
both their origins and
insertions…the origin is
named first
sternocleidomastoid: two
origins: sternum and clavicle
and an insertion on the
mastoid process
7. Action:
flexor
extensor
adductor
suppinator
etc…
VI. Some “fun” muscles
A. Muscles of the face:
1. Frontalis:
raises the eyebrows
2. Masseter:
elevates the mandible
3. Orbicularis oculi:
controls the eyelids
4. Orbicularis oris:
controls the lips –
facilitates speech
5. Zygomaticus:
raises the corners of the
lips…smiling
6. Platysma:
depress mandible…lower
lip back and down
7. Buccinator:
whistling, sucking, holding
food between teeth
8. Mentalis:
protrudes lower lip…pout
B. Muscles of the back
1. Rhomboids:
adducts and rotates the
scapula
2. Deltoids:
abducts the arm, aids in
extending and flexing the
humerus
3. Trapezius
elevates and rotates the
scapula; adducts the
scapula; depresses the
shoulder; extends the
head
4. Latissimus dorsi
extends the arm; adducts
and medially rotates the
arm; pulls the shoulder
downward and back
C. Muscles of the torso
1. Rectus abdominus
compresses the abdomen
by flexing the vertebral
column
2. Pectoralis major
flexes, adducts and
medially rotates the arm
3. Internal and External
obliques
together they flex the
vertebral column and
compress the abdominal
wall. Individually they aid
back muscles in trunk
rotation and lateral flexion
Linea Alba
• The tendinous seam that
runs from the sternum to
the pubic symphysis.
4. Diaphragm
• Inspiration…flattens on
contraction increasing the
vertical dimensions of the
thorax
• Center is a tendon…allowing
for the passage of the vena
cava
D. Muscles of the arm
1. Biceps brachii:
two heads of origin
flexes the forearm at the
elbow; supinates the hand
2. Triceps brachii:
three heads of origin
extends the forearm
E. Muscles of the leg
1. Adductor longus:
adducts, flexes and
laterally rotates the thigh
2. Quadraceps femoris:
a group of muscles that
extend the leg
3. Gastrocnemius:
the major calf muscle;
facilitates plantar flexion,
and flexion of the leg at
the knee
the distal tendon is the
Achilles tendon
4. Sartorius:
flexes, abducts and laterally
rotates the thigh
weakly flexes the knee
5. Soleus:
Plantar flexes foot…
important locomotor and
postural muscle during
walking, running and
dancing
F. Muscles of the lower torso

1. Gluteus maximus
Extends the thigh
2. Gluteus medius
abducts and medially
rotates the thigh