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Muscles
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Makes up almost half the body’s mass
Essential function- contraction (shortening)
o Unique to muscle cells
o Responsible for essentially all body movement (so called the “machines” of the body)
Two special characteristics
o Excitability (responsiveness)- ability to receive and respond to stimulus
o Contractility-ability to shorten when adequately stimulated
 Extensibility- ability of muscle cells to be stretched
 Elasticity- their ability to recoil and resume their resting length after being
stretched
Muscle fibers- smooth and skeletal because they are elongated
Contraction ability depends on two typed of myofilaments
Myo, mys, sarco means flesh
Muscle functions
o Producing movement- most important; result from muscle contraction; responsible for
whole body movement such as locomotion and manipulation; also allows us to respond
quickly to stimuli; also allow us to make facial expressions; cardiac and smooth propel
blood throughout the body and maintain blood pressure; smooth also forces fluids
throughout the body
o Maintaining posture and body position- function almost continuously to do this; act
against gravity when not lying down
o Stabilizing joints- tendons really do this; especially important for joints with poor
articular surfaces, such as the shoulder joint
o Additional functions
 Protection
 Valves (formed by smooth muscles) regulate passage of materials
 Dilate and constrict out eyes (smooth)
 Arrector pili (smooth)
Muscle types

Type
Body location

Skeletal
Attached to skeleton
and some in the face
attached to skin

# of nuclei
Cell shape

multinucleated
Long (some can be
almost a foot),
cylindrical, striated
 Largest of
muscle fibers
 Some visible
(such as the

Smooth
Found mainly in the
hollow visceral organs
such as stomach,
urinary bladder, and
respiratory passages
Uninucleate
No striations; fusiform,
spindle shaped

Cardiac
Walls of heart

Uninucleate
Branching cells joined
by intercalated discs
 This along with
spiral
arrangement
of heart
muscles allows

Tissue shape

Connective tissue

big, coarse
antigravity
muscles in the
hips)
Arranged
longitudinally; may
have enlarged belly, be
spindle-shaped, fanshape, or circle-shaped
 Fascicles- a
bundle of
fibers
Deep to superficial
wrappings
 Endomysiumencloses each
muscle fiber
 Perimysiumcoarser, fibrous
membrane
that wraps
fascicles
 Epimysiumcovers the
entire muscle
 Fascia
Aponeuroses- sheetlike; attach muscles
indirectly to bones,
cartilages, or
connective tissue
covering
Tendons- the epimysia
blend into this, which
anchors the muscles to
the bone; mostly made
of touch, collagen
fibers; more tendons
than fleshy muscles can
pass over a joint
 Functions
 Anchor muscle
to bone
 Provide
durability
 Conserve space

it to be a
coordinated
pump
Arranged in layers
(usually two), which
alternately relax and
contract (allows
substances to be
propelled)

Surrounded by small
endomysium

Spiral

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How they contract

Speed of contractions
Rhythm of contractions
Other info

Function

Can cross bony
projections due
collagen fibers
Voluntary (although
can be controlled
without willed
command as well)
Slow to fast (tires easily
so must rest)
No
Fibers surprisingly
fragile, making
connective tissue
important
Help form smoother
contours of the body
Movement like walking

Involuntary

Involuntary

Very slow

Slow (can speed up)

Yes

Yes, in some

Propelling substances
along a tract in the
body, such as moving
food trough the
digestive tract or
emptying the bowels
and bladder

Allows the heart to
serve a pump,
propelling blood
vessels into large
arteries

Microscopic anatomy of skeletal muscle
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Oval nuclei can be seen just below sarcolemma; nuclei pushed aside by myofibrils
Myofibrils- ribbon like organelles that nearly fill the organelles
o Sarcomere- tiny contractile units; make up myofibrils
 Myofilaments- threadlike proteins; arrangement of these causes badning
patterns or striations in skeletal muscle cells
 Thick (myosin) filaments- mostly made up of bundled myosin; also have
ATPase enzymes, which split ATP to generate power of muscle
contraction; extend entire A band
o Midline smooth but ends covered in myosin heads
 Cross bridges- myosin heads when they link together
thin and thick filaments during contraction
 Thin (actin) filaments- composed of contractile protein actin plus some
regulatory proteins (trophin and trophinmysine) in allowing or
preventing cross bridges; anchored to Z disc; thin filaments not in H
zone, which is why it’s called the bare zone
 I (light) bands- start from end of one thick filament in one sarcomere to
beginning of one in another; just thin filaments
 A (dark band)-from beginning to end of thick filament in one sarcomere
 I and A band give myofibrils their stripped appearance

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Z disc- darker area with just thin filaments; border of sarcomeres
H zone- lighter area with just thick filaments; in middle of sarcomere; between
end of thin filament and beginning of another; disappears during contraction
because actin and myosin completely overlap
 M line- in center of H zone; has protein rods that hold thick filaments together
Sarcoplasmic Reticulum (SR)- a specialized smooth endoplasmic reticulum; interconnecting
tubules and sacs of the SR surround each and every myofibril; stores calcium and release it on
demand when muscle cell is stimulated; calcium provides “go” signal for contraction

How they contract
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1
...

3
...


5
...


Cells must be stimulated by nerve impulses
...
More Na come in than K go out, which causes reverse the electrical conditions of the
sarcolemma, making depolarization
This causes an electrical current called an action potential, which is unstoppable
...

While action potential is going on, ACh breaks down acectic acid and choline by enzymes
(acetylcholinesterase, AChE) on sarcolemma and in synaptic cleft
a
...
So relaxes until stimulated by another release of ACh

Returning cells to resting state includes:
1
...
Operation of sodium-potassium pump (the active transport mechanism that moves sodium and
potassium ions back into their initial position)
Sliding filament theory:

1
...

a
...

2
...
They bind and unbind multiple time, a
...
a cross bridges attach and detach, generating tension
that helps to pull thin filaments toward center of sarcomere
a
...
Some myosin heads always in contact with thin filament so thin filament doesn’t go
backwards
Graded Responses
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Muscle cell will contract to its fullest extent when stimulated enough (never partial)
WHOLE MUSCLE can react to stimuli with graded responses
o Graded responses- different degrees of shortening of muscles
 Graded muscle contractions changed in two ways
 Frequency of stimulation
 Number of muscles being stimulated at one time
Muscle twitches- single brief jerky contractions that can result from nervous system problems;
not the way we normally operate
Usually nerve impulses are delivered to the muscle at a very fast rate so muscles do not get a
chance to relax completely between stimuli
o Causes successive contractions are added together, causing contractions to get stronger
and smoother
Fused (Complete) tetanus or Tetanic contraction- all contractions smooth and sustained and no
evidence of contractions
Unfused (incomplete) tetanus- muscle until at state of complete tetanus
Tetanus’s primary role is to produce smooth and prolonged muscled contractions
# of cells stimulated= force of contraction
o Strongest muscle contraction can be is when all motor units are active and all cells are
stimulated

Providing Energy for Muscle Contraction
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To contract, ATP must be hydrolyzed to release needed energy; muscles only store seconds wort
o ATP only power muscles can use directly
Working muscles replenish ATP supply in three ways
o Direct phosphorylation of ADP by creatinine phosphate- High energy creatinine
phosphate (only found in muscle fibers); as ATP is depleted, the high energy transfer of
phosphate group from CP to ADP results in ATP; muscle cells store 5x as much CP as ATP
but this supply quickly depleted (in 15 seconds)
o Aerobic respiration- during moderate exercise, 95% of ATP comes from this; metabolic
pathways(oxidative phosphorylation) that use oxygen and glucose, which is broken
down in carbon dioxide and water and energy; generates about 32 ATP per 1 glucose;
slow

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Anaerobic glycolysis and lactic acid formation- initial step of glucose breakdown and
does not require oxygen; in cytosol; glucose broken down to pyruvates and 2 ATP per 1
glucose molecule generated; if no oxygen, pyruvates converted to lactic acid; it is 2
...

2
...

4
...

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Most skeletal muscles cross at least one joint
Bulk of skeletal muscle lies proximal to the joint crossed
All skeletal muscles have at least two points of attachments: the origin and insertion
Skeletal muscles can only pull
During contraction skeletal muscle insertion moves toward origin
Origin- point at muscle attached to immovable and less movable bone
Insertion- attached to movable bone and when muscle contracts, the insertion moves toward
the origin
Body movement occurs when muscle contracts across joints; depends on
o Mobility of the joint
o Where the muscle is located in relation to the joint
Flexion- decreases angle of joint and brings two bones closer together; typical in hinge and ball
and socket joint
Extension- opposite of flexion; increases angle and distance between two bones; extension
greater than 180 called hyperextension (like moving your arms way behind your body)
Rotation- around longitudinal axis; common for ball and socket (axis and atlas)
Abduction- moving limb away (usually on frontal plane) from midline
Adduction- opposite of abduction; limb toward midline of body
Circumduction- combination of flexion, extension, abduction, and adduction; seen in ball and
socket (shoulder); proximal end stationary and its distal end moves in a circle (outlines a cone)

Special Movements
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Dorsiflexion- lifting the foot up so it approaches the shin (standing on your heels); like
extension of the hand to wrist
Plantar flexion- depressing the foot; tippy toes; corresponds to flexion of the hand
Inversion- sole of foot medially
Eversion- sole of foot laterally
Supination- forearm rotates laterally so palm is up (radius and ulna parallel)
Pronation- palm down; ulna and radius make an X

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Opposition- thumb can touch other fingers on hand; saddle joint between metacarpal 1 and
carpals

Interactions of muscles
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Groups of muscle that produce opposite movements lie on opposite side of joint
Prime mover- muscle that has major responsibility for some movement
Antagonists- muscles that oppose or reverse a movement
o Prime mover- active (contracted) and Antagonist- relaxed and stretched
Synergists- helps prime movers by producing the same movement or reducing undesirable
movement
o Muscle that crosses two or more joints will cause movement in all joints unless
synergists (finger fleer muscle take both wrist and finger joints but can make fists
without bending wrist due to synergist muscles)
o Fixators- specialized synergists; hold bone still or stabilize origin of prime mover so
tension used to move insertion (ex, scapulae anchored to thorax and postural muscles
that stabilize vertebral column)

How Muscles are Named
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Direction of muscle fibers- in reference to imaginary line; ex
...
Frontalis and temporalis named after bones they lie over
Number of Orgins- ex
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Sternocleidomastoid- origins and sternum and clavicle and
insertion in mastoid
Shape of muscle- ex
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Flexor extensor, adductor

Arrangement of Fascicles- determines range of motion; more parallel to long axis the more the muscle
can shorten (but not very powerful); muscle power= # of muscle cells; stocky bipennate and
multipennate in most fibers shorten very little but very strong
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Circular- fascicles arranged in concentric rings; usually found surrounding external body
openings which they close by contracting; muscles with circular fascicles are sphincters; ex
...
Pectoralis major; triangular
and fan shaped
Parallel- length fascicles run parallel to the long axis of muscle; strap-like
Fusiform- modification of parallel; spindle-shaped muscle with expanded belly such as biceps
brachii
Pennate- short fascicles attach obliquely to central tendon
o Unipennate- fascicles insert only one side of tendon and muscle; ex extensor digitorum
o Bipennate- fascicles insert on opposite sides of the tendon
o Multipennate-on several different sides

Head and Neck Muscles
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Facial Muscles- inserted in soft tissue such as other muscles and skin; allow us to make facial
expressions (smilem kiss, pout etc)
o Frontalis- covers frontal bones (from the cranial aponeurosis to skin of eyebrow ridges);
allows you to raise skin of eyebrows and wrinkle forehead
 Occipitalis- small muscle at cranial aponeurosis (covers posterior part of skull);
pulls scalp posteriorly
o Orbicularis Oculi- fibers that run in circles around the eyes; allows you to close your
eyes, blink, and squint
o Orbicularis Oris- circular muscle of the lips; closes the mouth and protrudes the lips
(kissing muscles)
o Buccinator- runs horizontally across cheek and inserts into the orbicularis oris; flattens
cheek (for blowing); it is also chewing muscle to compress cheek to hold food between
teeth during chewing
o Zygomaticus- extends from corner of mouth to cheekbone
Chewing Muscles- begin to break down food for the body
o Masseter- from zygomatic process of temporal bone to mandible; covers angle of lower
jaw; closes jaw by elevating mandible
o Temporalis- fan-shaped muscle overlying temporal bone; inserts into mandible; acts as
synergist for masseter
Neck Muscles- move the head and shoulder girdle (small and strap-like)
o Platysma- single sheetlike muscle; covers the anterolateral neck; originates from
connective tissue covering of the chest muscles an inserts into area around the mouth
o Sternocleidomastoid- pair of muscles; two-headed muscles, one head of each muscle
comes from clavicle, other sternum; the heads fuse and insert into mastoid process of
temporal bone; flex neck; if one muscle contracts, head rotated to opposite side (lateral
flexion)

Trunk Muscles- include those that move vertebral column (most of these are posterior anti-gravity
muscles) anterior thorax muscles, which move ribs, head, arms, and muscles of the abdominal wall,
which helped to move the vertebral column and most importantly form muscular natural girdle of the
abdominal wall
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Anterior muscles
o Pectoralis Major- large fan-shaped muscle covering upper part of chest; origin in
sternum, shoulder girdle, and the 1st six ribs; inserts on proximal end of humerus and
forms anterior wall of axilla and acts to adduct and flex the arm
o Intercostal muscles- deep muscles found between the rib; external intercostal helps
raise ribcage when you inhale; internal intercostal depress ribcage which helps move air
out of lungs when you exhale forcibly
Abdominal girdle muscles- anterior abdominal muscles form natural girdle that reinforces trunk;
resemble plywood cause the fibers of muscle (o pair) run in different directions; contains and
protects the abdominal contents

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Rectus Abdominis- paired straplike; most superficial muscles of abdomen; pubis to rib
cage; enclosed by aponeurosis; compress abdominal contents during defeaction and
childbirth and help in forced breathing
o External oblique- paired superficial muscles; make up lateral walls of abdomen; fibers
run downward and medially from the last eight ribs and insert into ilium; flex vertebral
column and rotate trunk and bend it laterally
o Internal oblique- paired muscles under external oblique; fibers run at right angles; origin
iliac crest and insert into last three ribs; do same thing as external
o Tranvsversus abdominis- deepest muscle of abdominal wall; fibers run horizontally
Posterior muscles
o Trapezius- most superficial muscles of posterior neck and upper trunk; form a diamond
or kite-shaped mass; origin very broad; runs from occipital bone of the skull down the
vertebral column to end of thoracic vertebrae; flare laterally and insertion of scapular
spine and clavicle; antagonists of sternocleidomastoids; elevate, depress, and adduct
the scapula
o Latissimus Dorsi- two large, flat muscles that cover lower back; originate on lower back
and ilium, sweep superiorly to insert into proximal end of humerus; extends and
adducts humerus; help in power stroke
o Erector Spinae- prime mover of back extension; paired muscles; deep; consists of three
muscle columns that spine entire vertebral column: longissimus, iliocostalis, and
spinalis; back extensors and provide resistance to control bending of waist; injury can
cause painful lower back spasms
o Quadratus Lumborum- form part of posterior abdominal wall; when each muscle acts
separately and flex spine laterally; when they act together, they extend lumbar spine;
from iliac crest and insert into upper lumbar vertebrae
o Deltoid- triangle shaped muscles that form the rounded shape of shoulders; bulky,
injection site for medication; origin of deltoid winds across shoulder girdle from spine of
scapular to clavicle; inserts into proximal humerus; prime movers of arm abduction

Muscles of Upper Limb- divided into three groups: ones that arise from shoulder girdle and insert into
humerus (pectoralis major, latissimus dorsi, and deltoid), second group that cause movement at elbow
joint (enclose humerus and insert on forearm bones), third group includes muscle of forearm, which
insert on hand bones and cause their movement (they are thin, spindle-shaped, and numerous) forearm
muscles reflect function
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Muscles of the Humerus that act of Forearm- all anterior arm muscles cause elbow flexion
o Biceps brachii- second most strong of bicep; bulges when elbow is flexed; originates by
two heads from the shoulder girdle and inserts into radial tuberosity; prime mover for
flexion of the forearm and acts to supinate forearm
o Brachialis- most strong- lie deep to bicep muscles and is important as the biceps in elbow
flexion; liftsthe ulna as the biceps lifts radius
o Brachioradialis- weakest muscle; arises on the humerus and insert into distal forearm
(mainly in forearm)

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Trcieps Brachii- only muscle at back of humerus; three heads arise the shoulder girdle and
proximal humerus and inserts into olecranon process of ulna; prime mover of elbow
extension and antagonist of biceps brachii; called boxer’s muscle

Muscles of Lower Limb- cause movement at hip, knew, and foot joints; largest, strongest muscles in
body; specialized for walking and balancing the body; pelvic girdle composed of heavy, fused bones that
allow little movement, no special muscle groups is necessary to stabilize it (versus shoulder girdle, which
requires lots of fixators); many muscles span two joints and can cause movement at both them (making
origin and insertion interchangeable); (technical term for leg is from ankle to knee); thigh muscles help
hold body upright against gravity and cause various movements at hip joint (attachment at pelvis girdle);
leg muscles cause movements of ankle, foot, and toe joint
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Muscles that Cause Movement at the Hip Joint
o Gluteus Maximus- superficial muscle of hip (makes up most of butt); hip extensor that
acts to bring thigh in a straight line to pelvis; most important for extending hip when
power needed (like jumping); originates from sacrum and iliac bones and inserts on
gluteal tuberosity of the femur and into large tendinous iliotibial tract
o Gluteus Medius- runs from ilium to the femur beneath the gluteus maximus for most of
its length; hip abductor and important in steadying pelvis during walking; important site
for intramuscular injections; overlies the large sciatic nerve; contains superolateral
quadrant of buttocks
o Iliopsoas- fused muscle composed of two muscles(iliacus and psoas major); from iliac
bone to lower vertebrae deep inside pelvis to insert on lesser trochanter of the femur;
prime mover of hip flexion; also keeps upper body from falling backward when standing
erect
o Adductor muscles- form the muscle mass at the medial side of each thigh; they adduct
or press thighs together; become flabby very easily (gravity doesn’t really affect them);
origin on pelvis and insert on the proximal aspect of the femur
Muscles Causing Movement at Knee Joint
o Hamstring Group- muscle mass of posterior thigh; originate on ischial tuberosity and run
down thigh to instert on both sides of tibia; prime movers of thigh extension and knee
flexion; tendons can be felt at back of knee; muscles consists of
 Biceps femoris
 Semimembraneous
 Semitendinous
o Sartorius- thin, strap-like; most superficial muscle of the thigh; run obliquely across the
thigh from the anterior iliac crest to the medial side of tibia; weak thigh flexor; synergist
to bring about cross legged position
o Quadriceps- anterior thigh; vastus muscles originate from femur; rectus femoris
originate from pelvis; all muscles insert into tibial tuberosity via patellar ligament; group
acts as a whole to extend knee power and rectus femoris helps flex hips; sometimes
used as intramuscular injection sites
 Rectus femoris
 Vastus muscles
 Vastus lateralis

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 Vastus medialis
 Vastus intermedius- under rectus femoris
Muscles that cause movement of at the Ankle at Foot
o Tibialis Anterior- superficial muscle on anterior of leg; arises from upper tibia and then
parallels the anterior crest as it runs to tarsal bones where in inserts via long tendon;
acts to dorsiflex and invert foot
o Extensor digitorum lohus- lateral to tibilias anterior, from lateral tibial condyle and
proximal 75% of the fibula and inserts into phalanges 2 to 5; prime mover of toe
extension
o Fibularis muscles- found on lateral part of legs; from fibula and insert into metatarsal
bones of foot; causes plantar flexes and everts the foot
 Longus
 Brevis
 Tertius
o Gastrocnemius- two- bellied muscle that forms curved calf of posterior leg; two heads,
one on each side of distal femur and inserts though large calcaneal (Achilles) tendon;
prime mover for plantar flexion of foot and flexes knee
o Soleus- deep to gastrocnemius; arises on tibia and fibula; inserts in calcaneal tendon and
is strong plantar flexor of foot

Developmental Aspects of Muscular System

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Embryo- muscular system is laid down in segments; each segment invaded by nerves; muscles
of thoracic and lumbar regions become very extensive because cover and move bones of limbs;
first movement called quickening (16th week of pregnancy); control of muscles by nervous
system develops quickly
Baby- all baby’s movements gross reflex movements; development proceeds in cephalic/caudal
direction and gross muscular movements precede fine ones; babies can do gross movements
before fine ones and can use more proximal muscles than distal muscles
Childhood- nervous system becomes more precise
Midadolescence- peeked development
o Throughout life, muscle resistant to infection due to rich blood supply but most be
worked out to prevent atrophy
Elderly- muscle tissue decreases, connective tissue decreases (causes muscles to become
stringy or sinewy); body weight declines in older person as loss of muscle mass occurs (50% by
80); regular exercise helps decrease these effects

Homeostatic Imbalances
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Muscular dystrophy- group of inherited muscle destroying diseases (affect specific groups);
enlarge due to fat and connective tissue, but fibers degenerate and atrophy
o Duchenne’s muscular dystrophy- only in boys; diagnosed between 2-7; starts
extremities upward; seemingly healthy children become clumsy and begin to fall; fibers
lack dystrophin that helps maintain sarcolemma

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Myasthenia gravis- autoimmune disease; drooping eyelids difficulty swallowing and talking, and
generalized muscle weakness and fatigability; involves a shortage of ACh receptors at
neuromuscular junctions (antibodies destroy ACh receptors); death occurs as a result of
respiratory failure
Torticollis (wryneck)- sternocleidomastoid muscles may be injured and develop spams
Flaccid- soft and flabby muscles
Atrophy- decrease in size, waste away

Closer Looks
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Anabolic steroids- originally used to treat disease and prevent muscle atrophy but used by
athletes later to get stronger; banned by many international competitions; cause bloated faces,
shriveled testes, and infertility; liver damage and psychiatric problems
Massage Therapy- muscle may contract and pinch nerve so massage to release it



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