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Muscular System
I. Introduction
A. What are they?
Organs
B. Why do we need them?
to move
C. How do they work?
use chemical energy to contract
D. Three kinds:
smooth (visceral), skeletal, cardiac
II. Structure of a Skeletal Muscle
A. Each muscle is an organ, composed of
1. skeletal muscle tissue
2. connective tissue
3. nervous tissue
4. blood
II. Structure of a Skeletal Muscle
(cont.)
B. Connective Tissue Coverings
1. Fascia
a. layers of dense connective tissue
b. surround & separate each muscle
2. Fascia extend beyond ends of muscle;
give rise to: tendons, which are fused to
periosteum of bone
II. Structure of a Skeletal Muscle
(cont.)
3. Aponeuroses: broad sheets of
connective tissue that connects muscles
to each other
4. Epimysium: layer of connective tissue
around each whole muscle
5. Perimysium: surrounds individual
bundles (fascicles) within each muscle
6. Endomysium: covers each individual
muscle fiber (cell)
II. Structure of a Skeletal Muscle
(cont.)
C. Skeletal Muscle Fibers
1. description: single long cylinder with rounded ends
2. Sarcolemma: cell membrane
3. Sarcoplasm: (cytoplasm of muscle fiber) contains
myofibrils made up of:
a. myosin: thick filaments
b. actin: thin filaments
c. striations: due to the organization of actin & myosin
(sarcoplasm has many mitochondria and nuclei)
II. Structure of a Skeletal Muscle
(cont.)
4. Sarcoplasmic reticulum: network of
membranous channels around each
myofibril (the sarcolemma’s endoplasmic
reticulum)
a. T tubule: invaginations of the
sarcolemma open to outside of the
muscle fiber (allows extracellular fluid in)
b. Cisternae: thickened areas where actin
& myosin filaments meet
II. Structure of a Skeletal Muscle
(cont.)
c. Arrangement allows extracellular
fluid in
d. Sarcoplasmic reticulum and
transverse tubules activate muscle
contraction
e. When fiber is stimulated: length of
sarcomere shortens, causing
contraction.
f. *Sarcomere extends from Z line to
Z line*
Sarcomeres
PLAY
InterActive Physiology ®:
Anatomy Review: Skeletal Muscle Tissue, page 9
Figure 9.3c
Copyright © 2006 Pearson Education, Inc., publishing as Benjamin Cummings
Myofilaments: Banding Pattern
Figure 9.3c,d
Copyright © 2006 Pearson Education, Inc., publishing as Benjamin Cummings
II. Structure of a Skeletal Muscle
(cont.)
D. Neuromuscular Junction
1. What is it? site where motor neuron &
muscle fiber meet
a. Motor end plate
(1) formed by: muscle fiber
membrane
(2) sarcolemma: tightly folded
(3) many nuclei and mitochondria
II. Structure of a Skeletal Muscle
(cont.)
b. Synaptic clefts: recesses or gaps of
motor end plate; branched motor neuron
fibers project into them
c. Cytoplasm of motor neuron contains:
numerous mitochondria & synaptic
vesicles storing neurotransmitters
Day 2
II. Structure of a Skeletal Muscle
(cont.)
E. Motor Units
1. made up of: motor neuron & many
muscle fibers it controls
2. when stimulated: muscle fibers of
motor unit contract all at once
Motor Unit: The Nerve-Muscle Functional Unit
PLAY
InterActive Physiology ®:
Contraction of Motor Units, pages 3-9
Copyright © 2006 Pearson Education, Inc., publishing as Benjamin Cummings
Figure 9.13a
III. Skeletal Muscle Contraction
(cont.)
A. Result of Muscle contraction:
1. shortening of sarcomeres
2. pulling of muscle against its
attachments
III. Skeletal Muscle Contraction
(cont.)
B. Role of Myosin and Actin
1. Myosin: 2/3 protein within skeletal
muscle. 2 twisted protein strands with
globular protein parts called myosin heads
projecting outward along the strands
2. Actin: globular protein with myosin
binding sites
*2 other important proteins: tropomyosin &
troponin associated with surface of actin
filaments
Ultrastructure of Myofilaments: Thin
Filaments
Figure 9.4c
Copyright © 2006 Pearson Education, Inc., publishing as Benjamin Cummings
III. Skeletal Muscle Contraction
(cont.)
3. Sliding filament theory of muscle
contraction:
a. needs: calcium
b. theory: when calcium ions are
present, binding sites on the actin
filament are exposed. Cross bridges on a
myosin filament form linkages by
attaching to the binding sites on the actin,
and bend, pulling on the actin filament
using energy from ATP. The linkage
breaks, then the myosin cross bridge
forms a linkage with the next binding site
III. Skeletal Muscle Contraction
(cont.)
c. Active sites for cross bridges: ADP
molecules attached to surface of actin
4. Where does the energy come from?
conversion of ATP to ADP + P is catalyzed
by enzyme ATPase; energy is
provided
to crossbridges & causes them to be in
‘cocked’ position
III. Skeletal Muscle Contraction
(cont.)
C. Stimulus for Contraction:
1. to initiate a muscle contraction, motor
neuron must: release neurotransmitter
acetylcholine from its synaptic vesicles
into the synaptic cleft
2. protein receptors: in motor end plate
detect the neurotransmitters; muscle
impulse spreads over the surface of the
sarcolemma & into the T tubules, where it
reaches the SR
III. Skeletal Muscle Contraction
(cont.)
3. cisternae of SR: release stored calcium
to sarcoplasm of muscle fiber (SR has
high concentration of calcium due to
active transport)
4. High concentration of calcium ions in
sarcoplasm interacts with troponin &
tropomyosin; they move aside & expose
the myosin binding sites on the actin
filaments
III. Skeletal Muscle Contraction
(cont.)
5. sarcomeres shorten because: myosin
cross bridges bind & pull on actin
filaments
6. Acetylcholinesterase: enzyme that
decomposes acetylcholine rapidly after
nervous impulse is received
7. Calcium returns to SR; linkages
between myosin & actin are broken.
Cross Bridge Animation
Myosin head
(high-energy
configuration)
ADP
Pi
1 Myosin head attaches to the actin
myofilament, forming a cross bridge.
Thin filament
ATP
hydrolysis
ADP
ADP
Thick filament
Pi
Pi
2 Inorganic phosphate (Pi) generated in the
previous contraction cycle is released, initiating
the power (working) stroke. The myosin head
pivots and bends as it pulls on the actin filament,
sliding it toward the M line. Then ADP is released.
4 As ATP is split into ADP and Pi, the myosin
head is energized (cocked into the high-energy
conformation).
ATP
ATP
Myosin head
(low-energy
configuration)
3 As new ATP attaches to the myosin head,
the link between myosin and actin weakens,
and the cross bridge detaches.
Day 3
III. Skeletal Muscle Contraction
(cont.)
D. Energy Sources for Contraction:
1. comes from: ATP. Limited supply—
must be regenerated constantly
2. creatine phosphate: 4-6X more
abundant than ATP: stores excess energy
released by mitochondria; regenerates
ATP from ADP & P.
III. Skeletal Muscle Contraction
(cont.)
3. When there’s enough
ATP, creatine
phosphokinase: promotes
synthesis of creatine
phosphate
4. As ATP breaks down:
energy from creatine
phosphate can be
transferred to ADP
molecules, converting
them back to ATP.
*Supply of creatine
phosphate quickly
exhausted in active
muscles*
After Glycerinated Muscle Lab
Cellular Respiration
Why is oxygen important in C.R.?
III. Skeletal Muscle Contraction
(cont.)
E. Oxygen Supply and Cellular Respiration
1. review of CR: oxygen enables the
complete breakdown of glucose in
mitochondria to release energy to form
ATP
2. early phase of CR: little ATP made
(glycolysis review)
III. Skeletal Muscle Contraction
(cont.)
3. muscle has high requirement for
oxygen
4. Hemoglobin: pigment in RBC that
carries oxygen
5. Myoglobin pigment produced in
muscles—stores oxygen temporarily
III. Skeletal Muscle Contraction
(cont.)
F. Oxygen Debt
1. may develop when: there’s strenuous
exercise
2. Lactic acid accumulation:
a. lactic acid: accumulates as an end
product of anaerobic respiration
b. carried in blood to liver
III. Skeletal Muscle Contraction
(cont.)
3. Oxygen Debt: = the amount of oxygen
that the liver needs to change the
accumulated lactic acid to glucose + the
amount of oxygen muscle cells need to
resynthesize ATP & Creatine Phosphate to
the levels they were before
4. Repaying debt: takes several hours
III. Skeletal Muscle Contraction
(cont.)
G. Muscle Fatigue
1. definition when a muscle loses its ability
to contract during strenuous exercise
2. usually arises from: accumulation of lactic
acid in the muscle decreases pH and
prevents muscle from contracting
3. Lack of ATP leads to: muscle cramp (due
to inability to return calcium ions back to the
sarcoplasmic reticulum so muscle fiber can
relax)
III. Skeletal Muscle Contraction
(cont.)
H. Heat Production
1. Skeletal muscle contraction: important
heat source for body; heat is transported
by blood to maintain body temperature
2. CR: also a source of heat. Only about
25% of energy released by CR is
available
IV. Muscular Responses
A. How can we study muscle function? Remove a
single fiber, connect it to a device that records its
responsiveness to electrical stimulation (myogram)
B. Threshold Stimulus—definition: minimal strength
of a stimulus to cause a fiber to contract
(muscle fiber remains unresponsive until the
stimulus reaches a certain strength)
C. All-or-None Response: when a muscle fiber
contracts, it contracts to its full extent (or not at all)
IV. Muscular Responses (cont.)
D. Recording a muscular contraction:
1. myogram recording of an electrically
stimulated muscle contraction
2. twitch—single, short contraction that only
involves a few motor units
3 phases: latent, contraction, relaxation
3. latent- excitation contraction coupling
occurring, no response yet seen; <0.01sec
4. latent period followed by: period of
contraction, and a period of relaxation
IV. Muscular Responses (cont.)
E. Summation
1. process in which: force of individual
twitches combine
muscle fiber receives a series of stimuli of
increasing frequency until reaches a point
where it can’t completely relax
2. Tetanic contraction if sustained
contraction has NO relaxation, its called
tetany or complete tetanus
(sustained & forceful contraction)
Muscle Response to Varying Stimuli
 A single stimulus results in a single contractile
response – a muscle twitch

Frequently delivered stimuli (muscle does not have
time to completely relax) increases contractile
force – wave summation
Copyright © 2006 Pearson Education, Inc., publishing as Benjamin Cummings
Figure 9.15
Summation, Tetanic Contraction
Copyright © 2006 Pearson Education, Inc., publishing as Benjamin Cummings
IV. Muscular Responses (cont.)
F.
Recruitment of Motor Units
1. What is recruitment? increase in the number of
activated motor units within a muscle at higher
intensities of stimulation
2. Whole muscle has many motor units
a. each responds to:different threshold
b. lower intensities: stimulate fewer motor
units to contract
c. higher intensities stimulate more motor
units to contract
IV. Muscular Responses (cont.)
G. Sustained Contractions—occur when:
1. muscle tone: achieved by continuous
state of partially sustained contraction of
motor units in a muscle
2. features of muscle tone
a. important in posture
b. totally lost with loss of
consciousness
IV. Muscular Responses (cont.)
H. Use and Disuse
1. hypertrophy size of fibers increases;
not the number
2. atrophy decrease in size
if immobilized, 5% per day decrease in
muscle strength & may decrease up to ¼
normal size
V. Smooth Muscles
A. Smooth Muscle Fibers
1. What does a smooth muscle cell look
like? elongated tapered ends, no
striations, underdeveloped sarcoplasmic
reticulum
V. Smooth Muscles (cont.)
2. 2 types:
a. multiunit fibers occur separately
not in sheets (not well organized)
blood vessels and iris of eye
b. visceral sheets
walls of hollow organs (stomach, intestine,
bladder, uterus)
Responsible for peristalsis
Fibers can stimulate each other & display
rhythmicity/”pacemakers”
V. Smooth Muscles (cont.)
B. Smooth Muscle Contraction
1. myosin binding to actin mechanism:
calcium interacts with calmodulin, which
interacts with kinase enzyme to activate myosin
heads
2. 2 neurotransmitters stimulate & inhibit
contraction, depending on target muscle
a. acetylcholine
b. norepinephrine
(bronchiole example)
V. Smooth Muscles (cont.)
3. Hormones stimulate or inhibit
contraction in muscle layers that have no
nerve supply
4. Comparison to skeletal muscle:
smooth muscle is slower to contract &
relax; but it can contract longer using the
same amount of ATP
VI. Cardiac Muscle
A. Mechanism of contraction: similar to that
of skeletal & smooth muscle BUT selfexcitable, contracts as a unit, long
refractory period
B. Can contract for longer periods than
skeletal or smooth muscle because:
transverse tubules supply extra calcium;
leads to longer lasting contraction
VI. Cardiac Muscle (cont.)
C. Intercalated disks
1. join cells
2. transmit force of contraction from one
cell to the next. Also, help in rapid
transmission of impulses throughout the
heart
VI. Cardiac Muscle (cont.)
D. Features of Cardiac Muscle:
1. self exciting
2. rhythmic
3. whole structure contracts as a unit
VII. Skeletal Muscle Actions
A. Origin & insertion some muscles have
more than one insertion or origin
1. immovable end = origin. moveable
end is the insertion
2. contraction pulls insertion TOWARDS
origin
VII. Skeletal Muscle Actions
(cont.)
B. Interaction of Skeletal Muscles
1. Prime mover of a group of muscles,
the one doing the majority of the work
2. synergists helper muscles
3. antagonists opposing muscles
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