Download Ch. 6 The Muscular System

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Muscle Functions
1. Movement of bones or fluids (e.g., blood)
2. Maintaining posture and body position
3. Stabilizing joints
4. Heat generation (esp. skeletal muscle)
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Skeletal Muscle
• Connective tissue sheaths of skeletal muscle:
• Epimysium: fibrous CT surrounding entire muscle
• Perimysium: fibrous CT surrounding fascicles (groups
of muscle fibers)
• Endomysium: delicate CT surrounding each muscle
fiber
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Epimysium
Epimysium
Tendon
Fascicle
Perimysium
Endomysium
Muscle fiber
in middle of
a fascicle
(wrapped by perimysium)
Endomysium
Perimysium Fascicle
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Muscle fiber
Skeletal Muscle: Attachments
• Muscles attach to bone by an origin and
insertion
• Origin —is fixed and on the immovable bone
• Insertion—is on the movable bone.
• As a contraction occurs the insertion moves
towards the origin
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Microscopic Anatomy of a Skeletal Muscle Fiber
• Multiple peripheral nuclei, many mitochondria
• Also contain sarcolemma, myofibrils, sarcoplasmic reticulum, T
tubules
Sarcolemma
Mitochondrion
Myofibril
Dark A band
Light I band
Nucleus
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I band A band I band
Z disc H zone Z disc
Myofibril
M line
Sarcolemma
Triad:
tubule
•T
• Terminal
cisternae
of the SR (2)
Tubules of the SR
Myofibrils
Mitochondria
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Myofibrils
• Densely packed, rodlike organelles
• ~80% of cell volume
• Composed of sarcomeres
• Exhibit striations:perfectly aligned repeating
series of dark A bands and light I bands
Myofibril
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Sarcomere
• Smallest contractile unit (functional unit) of a
muscle fiber
• The region of a myofibril between two
successive Z discs
• Composed of myofilaments: Thick (myosin)
and thin (actin)
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Regions of a Sarcomere
• A band (Dark Band)- Contains thin & thick filaments
• H zone: lighter midregion where filaments do not
overlap
• M line: proteins that hold adjacent thick filaments
together; center of sarcomere
• I band (Light Band)- Contains only thin filaments
• Z disc: proteins that anchor thin filaments; mark start
and end of one sarcomere
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Thin (actin)
filament
Thick (myosin)
filament
(c)Part
Z disc
H zone
I band
A band
Sarcomere
Z disc
I band
M line
of one myofibril
Sarcomere
Z disc
M line
Z disc
Thin (actin)
filament
Elastic (titin)
filaments
Thick
(myosin)
filament
(d)
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Myofilaments of a Sarcomere
• Thick Filament
• Thin Filament
• Composed of many
myosin proteins
• Twisted double strand
of actin protein
• A single myosin protein
has a “tail” and a
“head” that can:
• Has active sites for the
myosin head
• Bind actin and pull it
during a contraction
• Hydrolyze ATP to
release energy
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Longitudinal section of filaments
within one sarcomere of a myofibril
Thick filament
Thin filament
Thick filament
Thin filament
Portion of a thick filament
Myosin head
Portion of a thin filament
Tropomyosin
Troponin
Actin
Actin-binding sites
ATPbinding
site
Heads
Tail
Flexible hinge region
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Myosin molecule
Active sites
for myosin
attachment
Actin
subunits
Actin subunits
•In the relaxed state, thin
and thick filaments
slightly overlap
•During contraction,
myosin heads bind to
actin, detach, and bind
again, propelling thin
filaments toward M line
•As H zones shorten and
disappear, sarcomeres
shorten, muscle cells
shorten, and whole muscle
shortens
Z
I
1
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I
Fully relaxed
Z
2
Z
H
A
I
Z
A
Fully contracted
I
The Neuromuscular Junction
• Defined as:
• Axons of motor neurons: travel from the
brain/spinal cord via nerves to skeletal
muscles
• Each axon: branches into a number of axon
terminals as it enters a muscle
• Each axon ending forms: a neuromuscular
junction with a single muscle fiber
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Action
potential (AP)
Myelinated axon
of motor neuron
Axon terminal of
neuromuscular
junction
Nucleus
Sarcolemma of
the muscle fiber
1 Action potential arrives at
axon terminal of motor neuron.
2 Voltage-gated Ca2+ channels
open and Ca2+ enters the axon
terminal.
Ca2+
Ca2+
Axon terminal
of motor neuron
Fusing synaptic
vesicles
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Synaptic vesicle
containing ACh
Mitochondrion
Synaptic
cleft
The Neuromuscular Junction
• Axon terminal and muscle fiber are: separated
by space called the synaptic cleft
• Synaptic vesicles within axon terminal
contain: the neurotransmitter acetylcholine
(ACh)
• Junctional folds of the sarcolemma contain:
ACh receptors (chemically-gated channels)
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Events at the Neuromuscular Junction
1) A nerve impulse: arrives at the axon terminal
2) Ca2+ floods into axon terminal
3) Ca2+ entry causes synaptic vesicles to release
Ach
4) ACh diffuses across the synaptic cleft and
binds to receptors on the sarcolemma
5) Ach binding opens channels
6) Na+ floods into muscle fiber and K+ floods out
making the interior of cell less negative
7) Once threshold is reached an AP is generated
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The Action Potential
• The AP is an unstoppable, electrical event
that travels along the entire sarcolemma
conducting the electrical impulse from one
end of cell to the other
• Repolarization :The muscle cell returns to its
resting state mainly by the exit of K+
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Myelinated axon
of motor neuron
Axon terminal of
neuromuscular
junction
Sarcolemma of
the muscle fiber
Action
potential (AP)
Nucleus
1 Action potential arrives at
axon terminal of motor neuron.
2 Voltage-gated
Ca2+
channels
open and Ca2+ enters the axon
terminal.
Ca2+
Ca2+
Axon terminal
of motor neuron
3 Ca2+ entry causes some
Fusing synaptic
vesicles
synaptic vesicles to release
their contents (acetylcholine)
by exocytosis.
ACh
4 Acetylcholine, a
neurotransmitter, diffuses across
the synaptic cleft and binds to
receptors in the sarcolemma.
Na+
K+
channels that allow simultaneous
passage of Na+ into the muscle
fiber and K+ out of the muscle
fiber.
by its enzymatic breakdown in
the synaptic cleft by
acetylcholinesterase.
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Junctional
folds of
sarcolemma
Sarcoplasm of
muscle fiber
5 ACh binding opens ion
6 ACh effects are terminated
Synaptic vesicle
containing ACh
Mitochondrion
Synaptic
cleft
Ach–
Degraded ACh
Na+
Acetylcholinesterase
Postsynaptic membrane
ion channel opens;
ions pass.
Postsynaptic membrane
ion channel closed;
ions cannot pass.
K+
Figure 9.8
Destruction of Acetylcholine
• ACh effects are quickly terminated by the
enzyme acetylcholinesterase
• Prevents continued muscle fiber contraction in
the absence of additional stimulation
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Axon terminal
Open Na+
Channel
Na+
Synaptic
cleft
Closed K+
Channel
ACh
ACh
Na+ K+
Na+ K+
++
++ +
+
K+
Action potential
+
+ +++
+
2 Generation and propagation of
the action potential (AP)
1 Local depolarization:
Sarcoplasm of muscle fiber
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Closed Na+ Open K+
Channel
Channel
Na+
K+
3 Repolarization
Setting the stage
Axon terminal
of motor neuron
Action potential
Synaptic cleft
is generated
ACh
Sarcolemma
Terminal cisterna of SR
Muscle fiber Ca2+
Triad
One sarcomere
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1 Action potential is
Steps in
E-C Coupling:
propagated along the
sarcolemma and down
the T tubules.
Voltage-sensitive
tubule protein
Sarcolemma
T tubule
Ca2+
release
channel
Terminal
cisterna
of SR
Ca2+
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1 Action potential is
Steps in
E-C Coupling:
propagated along the
sarcolemma and down
the T tubules.
Voltage-sensitive
tubule protein
Sarcolemma
T tubule
Ca2+
release
channel
Terminal
cisterna
of SR
Ca2+
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2 Calcium
ions are
released.
Role of Calcium (Ca2+) in Contraction
• At low intracellular Ca2+ concentration:
• Active sites on actin are blocked
• Myosin heads cannot attach to actin
• Muscle fiber relaxes
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Actin
Ca2+
Troponin
Tropomyosin
blocking active sites
Myosin
3 Calcium binds to
troponin and removes
the blocking action of
tropomyosin.
Active sites exposed and
ready for myosin binding
The aftermath
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Role of Calcium (Ca2+) in Contraction
• At higher intracellular Ca2+ concentrations:
• Ca2+ causes binding sites on actin to be
exposed
• Events of the cross bridge cycle occur
• When nervous stimulation ceases, Ca2+ is
pumped back into the SR and contraction ends
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Actin
Ca2+
Troponin
Tropomyosin
blocking active sites
Myosin
3 Calcium binds to
troponin and removes
the blocking action of
tropomyosin.
Active sites exposed and
ready for myosin binding
4 Contraction begins
Myosin
cross
bridge
The aftermath
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Cross Bridge Cycle
• Continues as long as the Ca2+ signal and
adequate ATP are present
• Cross bridge formation: high-energy myosin
head attaches to thin filament
• Power stroke: myosin head pivots and pulls
thin filament toward M line
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Cross Bridge Cycle
• Cross bridge detachment: ATP attaches to
myosin head and the cross bridge detaches
• “Cocking” of the myosin head: energy from
hydrolysis of ATP cocks the myosin head into
the high-energy state
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Thin filament
Actin
Ca2+
Myosin
cross bridge
ADP
Pi
Thick
filament
Myosin
Cross
bridge
formation.
1
ADP
ADP
Pi
Pi
ATP
hydrolysis
2 The power (working)
stroke.
4 Cocking of myosin head.
ATP
ATP
3 Cross bridge
detachment.
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Spinal cord
Motor Motor
unit 1 unit 2
Axon terminals at
neuromuscular junctions
Nerve
Motor neuron
cell body
Motor
Muscle
neuron
axon
Muscle
fibers
• Motor unit = a motor neuron and all (four to
several hundred) muscle fibers it supplies
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Graded Muscle Responses
• Defined: Variations in the degree of muscle
contraction
Responses are graded by:
1. Changing the frequency of stimulation
2. Changing the number of muscle cells being
stimulated at one time (by changing strength
of stimulus)
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Response to Change in Stimulus Frequency
Single stimulus
single twitch
Contraction
Relaxation
Stimulus
A single stimulus results in a single contractile response called
a muscle twitch
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Figure 9.15a
Response to Change in Stimulus Frequency
• Increase frequency of stimulus  muscle
doesn’t have time to completely relax (btwn.
stimuli)
• Ca2+ release stimulates further contraction 
temporal (wave) summation
• Further increase in stimulus frequency 
unfused (incomplete) tetanus
• If stimuli are given quickly enough, fused
(complete) tetanus results
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Low stimulation frequency
unfused (incomplete) tetanus
Partial relaxation
Stimuli
(b) If another stimulus is applied before the muscle
relaxes completely, then more tension results.
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Figure 9.15b
High stimulation frequency
fused (complete) tetanus
Stimuli
(c) At higher stimulus frequencies, there is no relaxation
at all between stimuli. This is fused (complete) tetanus.
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Figure 9.15c
Muscle Metabolism: Energy for Contraction
• ATP is the only source used directly for
contractile activities
• Available stores of ATP are depleted in 4–6
seconds
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Muscle Metabolism: Energy for Contraction
• ATP is regenerated by:
• Direct phosphorylation of ADP by creatine
phosphate (CP)
• Anaerobic pathway
• Aerobic pathway
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Muscle Metabolism: Energy for Contraction
• Direct phosphorylation of ADP by creatine
phosphate (CP)
• CP is more concentrated in muscle fibers than
ATP (~4 X more)
• When ATP stores are depleted: muscle fibers
use CP to regenerate ATP
• Products are: 1 ATP/ CP
• Provides energy for: ~ 15 seconds of activity
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(a)
Direct phosphorylation
Coupled reaction of creatine
phosphate (CP) and ADP
Energy source: CP
CP
ADP
Creatine
kinase
Creatine
ATP
Oxygen use: None
Products: 1 ATP per CP, creatine
Duration of energy provision:
15 seconds
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Figure 9.19a
Anaerobic Pathway
• Under intense muscle activity or when oxygen
delivery is impaired: the body switches to the
anaerobic pathway
• Begins just like aerobic pathway (Glucose
breakdown) but pyruvic acid is converted into
lactic acid
• Products are: 2ATP/glucose
• Provides energy for : 60 seconds of activity
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(b)
Anaerobic pathway
Glycolysis and lactic acid formation
Energy source: glucose
Glucose (from
glycogen breakdown or
delivered from blood)
Glycolysis
in cytosol
2
O2
ATP
Pyruvic acid
net gain
O2
Released
to blood
Lactic acid
Oxygen use: None
Products: 2 ATP per glucose, lactic acid
Duration of energy provision:
60 seconds, or slightly more
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Figure 9.19b
Aerobic Pathway
• Produces 95% of ATP during rest and light to
moderate exercise
• Fuels: stored glycogen, then bloodborne
glucose, pyruvic acid from glycolysis, and free
fatty acids
• Products are: 32 ATP/glucose, CO2 and H2O
• Provides energy for: hours (endurance
activities)
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(c)
Aerobic pathway
Aerobic cellular respiration
Energy source: glucose; pyruvic acid;
free fatty acids from adipose tissue;
amino acids from protein catabolism
Glucose (from
glycogen breakdown or
delivered from blood)
O2
Pyruvic acid
Fatty
acids
O2
Aerobic
respiration
Aerobic respiration
in mitochondria
mitochondria
Amino
acids
32
CO2
H2O
ATP
net gain per
glucose
Oxygen use: Required
Products: 32 ATP per glucose, CO2, H2O
Duration of energy provision: Hours
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Figure 9.19c
Short-duration exercise
ATP stored in
muscles is
used first.
ATP is formed
from creatine
Phosphate
and ADP.
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Glycogen stored in muscles is broken
down to glucose, which is oxidized to
generate ATP.
Prolonged-duration
exercise
ATP is generated by
breakdown of several
nutrient energy fuels by
aerobic pathway. This
pathway uses oxygen
released from myoglobin
or delivered in the blood
by hemoglobin. When it
ends, the oxygen deficit is
paid back.
MUSCLE IDENTIFICATION AND
NAMING
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Naming Skeletal Muscles
• Location—bone or body region associated
with the muscle
• Shape—e.g., deltoid muscle (deltoid =
triangle)
• Relative size—e.g., maximus (largest),
minimus (smallest), longus (long)
• Direction of fibers or fascicles—e.g., rectus
(fibers run straight), transversus, and oblique
(fibers run at angles to an imaginary defined
axis)
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Naming Skeletal Muscles
• Number of origins—e.g., biceps (2 origins)
and triceps (3 origins)
• Location of attachments—named according to
point of origin or insertion
• Action—e.g., flexor or extensor, muscles that
flex or extend, respectively
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Head
Temporalis
Masseter
Shoulder
Trapezius
Deltoid
Arm
Triceps brachii
Biceps brachii
Brachialis
Forearm
Hand, wrist and finger flexors
Pelvis/thigh
Iliopsoas
Pectineus
Thigh
Rectus femoris
Vastus lateralis
Vastus medialis
Leg
Fibularis longus
Extensor digitorum longus
Tibialis anterior
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Facial
Epicranius, frontal belly
Orbicularis oculi
Zygomaticus
Orbicularis oris
Neck
Platysma
Sternocleidomastoid
Thorax
Pectoralis major
Abdomen
Rectus abdominis
Internal oblique
Transversus abdominis
External oblique
Thigh
Sartorius
Adductors
Gracilis
Leg
Gastrocnemius
Soleus
Muscles of Facial Expression
• Epicranius (Frontal belly and Occipital belly)
• Raises eyebrows, wrinkles forehead (frontal belly)
• Pulls scalp posteriorly (occipital belly)
• Orbicularis Oculi
• Closes eyes, squinting, blinking
• Orbicularis Oris
• Closes mouth, protrudes lips
• Buccinator
• Flattens cheek (as in whistling)
• Zygomaticus
• Pulls corners of mouth superiorly (as in smiling)
• Platysma
• Pulls corners of mouth inferiorly
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Orbicularis oculi
Zygomaticus
Buccinator
Orbicularis oris
Platysma
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Temporalis
Masseter
Sternocleidomastoid
Trapezius
Muscles of Mastication
• Temporalis and Masseter
• Elevate the mandible (closing jaw)
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Temporalis
Orbicularis
oris
Buccinator
(a)
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Masseter
Muscles of the Neck
• Sternocleidomastoid—major head flexor (also
rotates the head)
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1st cervical
vertebra
Sternocleidomastoid
(a) Anterior
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Base of
occipital bone
Mastoid
process
AnteriorTrunk Muscles
• Pectoralis Major
• Adducts and flexes the arm
• Rectus Abdominis
• Flexes vertebral column
• External and Internal obliques
• Flex vertebral column
• Transversus Abdominis
• Compresses abdomen
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Subscapularis
Pectoralis minor
Sternum
(a)
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Serratus anterior
Pectoralis major
Linea alba
Tendinous
intersection
External oblique
(a)
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Rectus
abdominis
Posterior Trunk Muscles
• Trapezius
• Extends head; elevates,
depresses scapula
• Latissimus Dorsi
• Adducts and extends
humerus
• Erector Spinae
• Extends vertebral column
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Levator
scapulae
Trapezius
Rhomboid
minor
Rhomboid
major
(c)
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Arm Muscles
• Anterior Flexor Muscles
• Brachialis, Biceps brachii, Brachioradialis
• Forearm flexors
• Posterior extensor muscles
• Triceps brachii
• Extend forearm
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Biceps brachii
Brachialis
Brachioradialis
(a) Anterior view
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Shoulder Muscles
• Deltoid
• Abducts arm, but can do all angular
movements
• Rotator Cuff Muscles
• Supraspinatus
• Infraspinatus
• Subscapularis
• Teres Minor
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Supraspinatus
Infraspinatus
Teres minor
Triceps brachii:
Lateral head
Long head
(b) Posterior view
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Muscles of the Forearm
• Actions: movements of the wrist, hand, and
fingers
• Most anterior muscles are flexors and insert
via the flexor retinaculum
• Most posterior muscles are extensors and
insert via the extensor retinaculum
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Flexor retinaculum
Brachioradialis
Medial head of
triceps brachii
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(a)
Hip/Thigh Muscles
• Iliopsoas and Sartorius
• Hip/thigh flexion
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Iliopsoas
Psoas major
Iliacus
Sartorius
(a)
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5th lumbar vertebra
Hip/Thigh Muscles
• Gluteus Maximus
• Lateral thigh
• Extends leg at hip
• Gluteus Medius
• Abducts thigh
• Adductor Muscles
• Medial thigh
• Adductor thigh
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Gluteus medius (cut)
Gluteus minimus
Gluteus
maximus
(cut)
(c)
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Pectineus
(cut)
Adductor
brevis
Adductor
longus
Adductor
magnus
Femur
(b)
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O = origin
I = insertion
Thigh Muscles
• Hamstring muscles (Biceps femoris,
Semitendinosus, Semimembranosus )
• Posterior thigh
• All three flex leg at knee and extend hip
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Long head Biceps
Short head femoris
Semitendinosus
Semimembranosus
(a)
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Hamstrings
Muscles of the Thigh that Move the Knee
Joint
• Quadriceps femoris (Vastus Medialis,
Lateralis, intermedius and rectus femoris)
• Anterior Thigh
• All extend the knee
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Quadriceps femoris
• Rectus femoris (superficial
to vastus intermedius)
• Vastus lateralis
• Vastus medialis
Tendon of quadriceps femoris
Patella
(a)
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Arm
Triceps brachii
Brachialis
Forearm
Brachioradialis
Extensors of forearm
Extensor carpi
ulnaris
Extensor digitorum
Iliotibial tract
Leg
Gastrocnemius
Soleus
Fibularis longus
Calcaneal
(Achilles) tendon
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Neck
Epicranius, occipital belly
Sternocleidomastoid
Trapezius
Shoulder
Deltoid
Infraspinatus
Teres major
Rhomboid major
Latissimus dorsi
Hip
Gluteus medius
Gluteus maximus
Thigh
Adductor magnus
Hamstrings:
Biceps femoris
Semitendinosus
Semimembranosus
Figure 10.5
Gluteus medius
Gluteus maximus
Adductor magnus
Gracilis
Iliotibial tract
Biceps
femoris
Semitendinosus
Semimembranosus
(a)
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Hamstrings
Muscles of the Anterior Compartment of
the Leg
• Tibialis anterior & Extensor digitorum longus
• Primary toe extensors and ankle dorsiflexors
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Tibialis anterior
Extensor digitorum longus
(a)
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Muscles of the Lateral Compartment of
the Leg
• Fibularis longus
• Plantar flexion and eversion of the foot
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Head of fibula
Fibularis longus
Lateral malleolus
(a)
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Muscles of the Posterior Compartment of
the Leg
• Gastrocnemius and Soleus
• Both do plantar flexion of foot
• Gastrocnemius also does flexion at knee
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Gastrocnemius Medial head
Lateral head
Tendon of
gastrocnemius
Calcaneal tendon
Medial malleolus
Lateral malleolus
Calcaneus
(a) Superficial view of the posterior leg.
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