CHAPTER 7
MUSCULAR SYSTEM
FUNCTIONS OF SKELETAL
MUSCLE
SKELETAL MUSCLE TISSUE IS THE
MOST ABUNDANT IN THE BODY!
- there are approximately 700 skeletal
muscles
Functions:
1. Produce movement- pull on tendons
and move bones
2. Maintain posture- continuous muscle
contractions maintain posture
3. Support soft tissues- abdominal
wall and floor of pelvic cavity
consist of layers of skeletal
muscle
4. Guard entrances and exitsskeletal muscles encircle
openings to digestive and
urinary tracts
5. Maintain body temperaturemuscle contractions require
energy --> converted to HEAT
GROSS ANATOMY
Each muscle is made up of 3
connective tissues:
1. Epimysium- layer of collagen fibers
that surrounds the muscle and
separates it from others
2. Perimysium- connective tissue
fibers that divide the muscle into
compartments, each containing a
bundle of fibers called a FASCICLE
(bundle of muscle cells)
3. Endomysium- within each
fascicle; surrounds each muscle
fiber and ties adjacent muscle
fibers together
- at the ends of muscles, collagen
fibers come together to form a
TENDON
- these connective tissues contain
the blood vessels and nerves that
supply the muscle
- skeletal muscles contract only
under stimulation from the central
nervous system
- AXONS span all 3 layers of
tissue; control individual muscle
fibers
RECALL: Skeletal muscles are
often called VOLUNTARY
MICROANATOMY- a look
inside a muscle fiber
Skeletal muscle fibers are NOT typical
cells:
- they are much larger
- each muscle fiber is
MULTINUCLEATED
Sarcolemma- the cell membrane;
surrounds the cytoplasm
Sarcoplasm- cytoplasm
- in the sarcolemma there are openings
that lead to a network of narrow tubes
called T TUBULES
- these T tubules are like tunnels
through the muscle fiber; also
play a role in coordinating
muscle contraction
MYOFIBRILS
- branches of T tubules encircle
cylindrical structures called
MYOFIBRILS
- these are as long as the entire
muscle cell, and are bundles of
MYOFILAMENTS (contain
proteins actin and myosin)
Actin- found in THIN FILAMENTS
Myosin- found in THICK
FILAMENTS
- myofibrils are responsible for
muscle contraction
- mitochondria are also found
scattered among the myofibrils,
and they provide energy for
muscle contraction
SARCOPLASMIC RETICULUM
- the sarcoplasmic reticulum surrounds
each myofibril
- the SR contains chambers called
CISTERNAE, which contain high
concentrations of calcium ions
- a muscle contraction begins when
these stored calcium ions are
released by the cisternae
SARCOMERES
- myofilaments are organized into
repeating functional units called
SARCOMERES
- each myofibril consists of about
10,000 sarcomeres arranged end
to end
** THE SARCOMERE IS THE
SMALLEST FUNCTIONAL UNIT
OF THE MUSCLE FIBER
- interactions between thin and thick
filaments of sarcomeres are
responsible for contraction
All myofibrils are arranged
parallel to one another, with
their sarcomeres lying side by
side
- so a muscle fiber appears
striated, or banded
MUSCLE FIBER CONTRACTIONA QUICK OVERVIEW
The contraction of a muscle fiber
involves a series of complex steps
- actin and myosin molecules of
thin and thick filaments move
past one another
- This movement cannot occur
without the release of calcium
ions within the muscle fiber
- Each muscle fiber is controlled by
a nerve cell called a motor
neuron
- it is this motor neuron that
controls the initial release of
calcium ions to begin the
muscle contraction
- Muscle cells also require a
constant supply of an energy
molecule called ATP (made
in mitochondria)
- the muscle contraction ends
and the muscle returns to a
resting state when calcium
ions drop back to normal
MUSCLE MECHANICS
- muscle mechanics are
concerned with an entire
population of muscle fibers
Individual muscle cells in muscle
tissue are tied together and
surrounded by connective tissue
- when muscles contract, they pull
on collagen fibers, producing
TENSION- this tends to pull the
object toward the source of the
tension
- before the object can move
though, tension must
overcome the object’s
RESISTANCE- a force that
opposes movement
- COMPRESSION- a push
applied to an object; forces
object away from source of
compression
- muscle cells use energy to
generate tension, but not
compression
An entire muscle contracts
when its component muscle
fibers are stimulated
Tension as a whole is
determined by:
1. frequency of stimulation
2. number of muscle fibers
activated
FREQUENCY
TWITCH- a single stimuluscontraction-relaxation
sequence in a muscle fiber
- can last from 7.5-100 msec
During a Twitch:
LATENT PERIOD- begins at
stimulation and lasts about 2
msec
- calcium ions are released, but no
tension is yet produced by the
muscle fiber
CONTRACTION PERIOD- tension
rises to a peak
RELAXATION PERIOD- muscle
tension falls to resting levels
- a single stimulation produces
a single twitch, but twitches
do not produce anything
useful
- normal activities involve
sustained muscle
contractions, which result
from repeated stimulations
SUMMATION &
INCOMPLETE TETANUS
SUMMATION- addition of one
twitch to another
- when a second stimulus
arrives before the relaxation
period ends
- the muscle doesn’t relax
completely
- a muscle producing peak
tension during rapid cycles of
contraction and relaxation is
said to be in INCOMPLETE
TETANUS
- Most normal muscle
contractions involve
incomplete tetanus of
participating muscle fibers
COMPLETE TETANUS
COMPLETE TETANUS- rate of
stimulation is increased until
relaxation phase is
completely eliminated
- this produces continuous
contraction
NUMBER OF MUSCLE
FIBERS
During normal movement, our
muscles contract smoothly
because fibers are responding
in complete tetanus
- total force exerted by the
muscle depends on how many
muscle fibers are activated
Some motor neurons control a
single muscle fiber, while
some control hundreds or
thousands of fibers
- a MOTOR UNIT consists of all
of the muscle fibers
controlled by a single motor
neuron
The size of a motor unit indicates
how fine the control of
movement can be
Ex: In muscles of eye where
movement must be precise, a
motor neuron may control 2 or 3
muscle fibers
- in the leg, a motor neuron may
control 2000 muscle fibers
Muscle fibers of each motor unit
overlap with those of others
- this ensures that direction of pull
on a tendon does not change
despite variations in the number
of activated motor units
Peak tension production occurs
when all motor units in the
muscle are contracting in
complete tetanus
- these contractions do not last
long because the muscle fibers
use up energy quickly
- during a sustained
contraction, motor units are
activated on a rotating basis
so that some are resting
while others are contracting
- So, when your muscles
contract for long periods,
they produce slightly less
than maximal tension
MUSCLE TONE
Some motor units are always
active, even when the muscle is
not contracting
- these contractions don’t produce
enough tension to cause
movement, but they tense and
firm the muscle
- a muscle with little tone appears
limp and flaccid, but one with
tone is firm and solid
A muscle not stimulated by a
motor neuron on a regular basis
will ATROPHY:
- muscle fibers become smaller
and weaker
- paralysis; fractures
- initially reversible, but dying
muscle fibers cannot be
replaced– why physical therapy
is so important for patients who
are unable to move normally
ISOTONIC & ISOMETRIC
CONTRACTIONS
ISOTONIC CONTRACTIONS- tension
rises to a level that is maintained
until relaxation occurs
- muscle length shortens as the tension
remains constant
Ex: lifting an object off of a desk,
walking and running
ISOMETRIC- tension continues to rise
but muscle as a whole does not
change in length
Ex: pushing against a wall; trying to
pick up a car
- many everyday reflexive muscle
contractions that keep the body
upright involve isometric
contractions that oppose gravity
- normal daily activities involve a
combination of the 2:
as you sit in this class, isometric
contractions of postural muscles
maintain your position, and
movements of your arm, forearm,
hand, and fingers are produced
by isotonic contractions
ANATOMY OF MUSCULAR
SYSTEM
ORIGIN, INSERTION,
ACTION
Each muscle begins at an ORIGIN
Each muscle ends at an INSERTION
Each muscle contracts to produce
a specific ACTION
- Origin end usually remains
stationary while the insertion
moves
- almost all skeletal muscles have
either their origin or insertion on
the skeleton
- when these contract, they
produce flexion, extension,
adduction, etc.
ACTIONS OF MUSCLES
Muscles can be described by
their PRIMARY ACTIONS:
1. Prime Mover/Agonist- muscle
whose contraction is
responsible for producing a
particular movement
Ex: The biceps brachii is a prime
mover that flexes the elbow
2. Antagonists- muscles whose
actions oppose the
movement of another muscle
`
Ex: The triceps brachii extends the
elbow, so it is an antagonist of the
biceps brachii
- agonists and antagonists are
opposites: if one produces flexion,
the other produces extension
3. Synergist- a muscle that helps the
prime mover to work efficiently
Ex: The deltoid muscle acts to abduct
the arm; the supraspinatus muscle
assists the deltoid in beginning this
movement
AXIAL MUSCLES
GROUPS OF AXIAL
MUSCLES
Axial muscles fall into 4 groups:
1. Muscles of head and neck- include
muscles for facial expression,
chewing, and swallowing
2. Muscles of the spine- flexors and
extensors of the head, neck, and
spinal column
3. Muscles of the trunk- walls of
thoracic and abdominopelvic
cavities
4. Muscles of pelvic floorextend between sacrum and
pelvic girdle and form the
muscular perineum which
closes the pelvic outlet
MUSCLES OF HEAD AND
NECK
- muscles of the face originate
on the surface of the skull
and insert into the dermis of
the skin
- when these muscles contract,
the skin moves
1. Orbicularis oris- constricts
opening of mouth; compresses
purses lips
2. Buccinator- compresses cheeks,
as when blowing forcefully
(buccinator translates as
“trumpet player”)
CHEWING:
3. Masseter- elevates mandible
4. Temporalis- also elevates
mandible
5. Pterygoids- elevate, protract,
move mandible to either side
FACIAL EXPRESSIONS:
6. Frontalis- muscle of
forehead; raises eyebrows
and pulls on the skin of the
scalp
7. Occipitalis- tenses and
retracts scalp
- these 2 muscles are
separated by an aponeurosis
or tendinous sheet called the
galea aponeurotica
NECK:
8. Platysma- covers ventral
surface of neck; tenses skin
of neck and depresses
mandible
9. Digastric- opens mouth by
depressing mandible
10. Mylohyoid- elevates floor of
mouth; support of tongue
11. Stylohyoid- elevates larynx
12. Sternocleidomastoidstogether flex neck; alone
bend head to one side and
rotates head
MUSCLES OF SPINE
- muscles of the spine are
covered by more superficial
back muscles (covered later)
13. Splenius capitis- back of
neck; rotating head
14. Semispinalis capitis- also
back of neck, deeper
- both assist each other in
extending the head when left
and right pairs contract
together
- when they contract on one side,
this tilts the head
15. Erector spinae- maintain erect
spinal column and head; can be
subdivided into 3 divisions
- moving towards the spine they
are the iliocostalis, longissimus,
and spinalis
- contracting together extends
spinal column
- contracting on only one side
causes spine to bend to the side
AXIAL MUSCLES OF TRUNK
THORAX:
16. Intercostals- elevate and
depress ribs
17. Diaphragm- expands
thoracic cavity, compresses
abdominopelvic cavity
ABDOMEN:
18. External and Internal
obliques- compressing
abdomen, depressing ribs
19. Transversus abdominiscompressing abdomen
- External obliques (outer),
Internal obliques (middle),
Transversus (inner)
20. Rectus abdominisdepresses ribs; flexes
vertebral column
- “6-pack abs”
21. Muscles of pelvic floor
- the floor of the pelvic cavity is
called the perineum
- formed by a broad sheet of
muscles that connects the
sacrum and coccyx to the
ischium and pubis
- support organs of pelvic cavity,
flex the coccyx, control
movement of materials through
urethra and anus
APPENDICULAR
MUSCLES
APPENDICULAR
MUSCLES:
INCLUDE:
1. Muscles of shoulders and
upper limbs
2. Muscles of pelvic girdle and
lower limbs
SHOULDERS AND UPPER
LIMBS
SHOULDER:
1. Trapezius- large, superficial
muscles that cover back and
parts of neck; form broad
diamond
- varied actions: elevation,
adduction, depression, rotation
of scapula; elevation of clavicle;
extension/hyperextension of
neck
2. Rhomboideus- covered by
trapezius; adduction of
scapula (pulling toward
center of back)
3. Levator scapulae- also
covered by trapezius;
elevation of scapula
(shrugging shoulders)
4. Serratus anterior- on chest;
pulls shoulder anteriorly
5. Pectoralis minor- depression
and protraction of scapula
MOVEMENT OF ARM:
6. Deltoid- major abductor of
arm
7. Supraspinatus- assists deltoid;
part of rotator cuff
8. Rotator cuff muscles (5)- rotate
arm- supraspinatus,
infraspinatus, subscapularis,
teres minor, teres major
9. Pectoralis major- flexion,
adduction, rotation at shoulder
10. Latissimus dorsi- extension,
adduction, rotation of shoulder
MOVEMENT OF FOREARM AND WRIST:
11. Biceps brachii- flexes elbow;
supinates wrist
- when palm is facing back
(pronated) biceps cannot function
properly; you are strongest when
you flex your elbow with a
supinated forearm
12. Triceps brachii- extends elbow
13. Brachialis- flexion of elbow
14. Brachioradialis- flexion of elbow
15. Flexor carpi ulnaris- flexes and
adducts wrist
16. Flexor carpi radialis- flexes and
abducts wrist
17. Extensor carpi radialis- extension
and abduction of wrist
18. Extensor carpi ulnaris- extension
and adduction of wrist
MOVEMENT OF HANDS AND
FINGERS:
19. Extensor digitorumextension at finger joints and
wrist
20. Flexor digitorum- flexion at
finger joints and wrist
MUSCLES OF PELVIS AND
LOWER LIMBS
CAN BE DIVIDED INTO 3
GROUPS
- Muscles that move the thigh,
working across the hip joint
- Muscles that move the leg,
working across the knee joint
- Muscles that move the
ankles, feet, and toes across
various joints of the foot
MOVEMENT OF THIGH:
1. Gluteus maximus- largest
and most posterior; produces
extension, rotation, and
abduction at the hip
2. Gluteus medius- abduction
and medial rotation at hip
3. Gluteus minimus- abduction
and medial rotation
4. Adductor magnus
5. Adductor brevis
6. Adductor longus
7. Pectineus
8. Gracilis
- all adductors of thigh
- a pulled groin results from a
strain (tear or break) in one
of these muscles
9. Iliopsoas- largest flexor of hip
MOVEMENT OF LOWER LEG:
10. Biceps femoris
11. Semimembranosus
12. Semitendinosus
- all three make up the
“hamstrings”
- all flexors of knee
- a pulled hamstring is a strain
in one of these muscles
13. Sartorius- also flexor of knee
14. Vastus intermedius
15. Vastus lateralis
16. Vastus medialis
17. Rectus femoris
- all 4 make up Quadriceps
femoris; all are knee
extensors
18. Popliteus- medially rotates
tibia
- moves tibia back into position
so that knee joint can be
flexed
MOVEMENT OF FOOT AND
TOES:
19. Gastrocnemius- largest
muscle of calf; plantar flexion
20. Soleus- assists
gastrocnemius
- these share a common
tendon, the Calcaneal
tendon (Achilles tendon)
21. Fibularis- pair produces
eversion of foot and plantar
flexion of ankle
22. Tibialis- inversion of foot
23. Flexor and extensor
digitorum- same as in fingers
24. Flexor hallucis- flexion of
hallux
25. Extensor hallucis- extension
of hallux