MUSCULAR SYSTEM
CHAPTER 6
Muscles

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Are muscles organs?
Yes, each individual muscle is an organ
– Why?

Muscles are composed of more than just muscle
tissue.
– Example: Skeletal muscles are composed of skeletal
muscle tissue, nervous tissue, blood, and connective
tissue.

3 types:
– Skeletal (We will concentrate mostly on this one.)
– Smooth
– Cardiac
Skeletal muscle
Cells - long, cylindrical, multinucleated,
striated
 Voluntary - controlled by the nervous
system

– When are they not voluntary?
Speed of contraction varies (fast to slow)
 Location?

Cardiac muscle
Cells - branching, intercalated discs, striated
 Involuntary - controlled by its own
pacemaker and hormones
 Slow rhythmic contractions

Smooth muscle
Cells - no striations, appear smooth and
“stringy” in microscope
 Involuntary
 Very slow contraction

– Can be rhythmic

Location
Functions of Skeletal Muscles

Produce Movement
– Muscles cross joints and work in opposite forces
to move the bones

Maintain Posture
– Always working against gravity

Stabilize joints
– Many joints don’t fit together well, muscles help
reinforce those (ex. shoulder)

Generate Heat
– Heat is a by product of muscle activity
Rules of Skeletal Muscles

1. All muscles cross at least one joint

A few minor exceptions
2. Typically, the bulk of the muscle is going
to be proximal to the joint crossed
 3. All muscles have at least two attachment
points: the origin and insertion
 4. Muscles can only pull, they cannot push
 5. During contraction, the insertion point
moves toward the origin.

Structure of Skeletal Muscles

Muscles have multiple layers
– Cylinders within cylinders
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Levels of muscles, largest to smallest:
Muscle  fascicle  muscle fiber  myofibril 
filaments
Each level of a muscle is enclosed in a layer of
connective tissue
Fascia - connective tissue that covers and is
intertwined with the muscles.
Has three purposes
– DQ – What do you think those might be?
Structure of Skeletal Muscles
1. Separate Muscle levels from one another
Endomysium – separates each muscle fiber
Perimysium – separates each fascicles
2. Separate muscles from one another
Epimysium – holds the group of fascicles together
3. Hold muscles in place
–
Tendons - connective tissue that connects muscles
to the bone.

–
It is an area beyond the end of the muscle where the
fascia gets very dense and compact.
Aponeuroses - associated with sheet-like muscles,
connects muscle to muscle.
Skeletal Muscle Fibers


DQ - What is the purpose of all the levels?
Help with contraction, all of them can work
together
– Muscle contracts at the smallest level, many small
contractions add up to one big one


Muscle fiber – individual muscle cell that
contracts in response to stimulation.
These have different names for the cell
membrane and cytoplasm
– Cell membrane = sarcolemma
– Cytoplasm = sarcoplasm
Skeletal Muscle Fibers


Each muscle fiber contains numerous threadlike
myofibrils that lie parallel to one another.
Myofibrils contain two kinds of protein filaments.
– Myosin – “Thick” filaments

Cross bridges for contraction
– Actin - “Thin” filaments


Active sites for contraction
These filaments overlap, what type of appearance
does this create in skeletal muscle?
– Striated
Skeletal Muscle Fibers

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Striation pattern parts:
I band - (light bands) where only actin
filaments are.
Z – line – where actin filaments meet
A bands - (dark bands) where both myosin
and actin are overlapping
H zone – where only myosin is
From Z line to Z line is one sarcomere.
Structure of Skeletal Muscle

Within the muscle fiber there are networks of
tubes:
– Sarcoplasmic reticulum - run parallel to each
myofibril
– Transverse tubules – perpendicular to SR and run in
between myofibrils (a.k.a. T tubules)


DQ - What would these structures be
used for?
Activate entire fiber when contraction occurs
Muscle activation


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DQ - How are muscles activated to
contract?
The neuromuscular junction (pg 185)
Each skeletal muscle fiber is connected to
an extension of a motor neuron.
The nerve fiber and the muscle meet at the
neuromuscular junction.
– The two do not actually touch – Synaptic cleft
– Then how does the nerve stimulate the fiber?

Neurotransmitters (Acetylcholine)
Motor Units


DQ - Does each nerve only control one
muscle fiber?
NO, we have motor units. (pg 184)
– consists of a motor neuron and all of the muscle
fibers it controls.
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The number of muscle fibers in a motor unit
varies considerably.
DQ - Why would that be?
The fewer muscle fibers in the motor units,
the finer the movement.
– Examples – hands vs legs
Muscle Response

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When the nerve receives an impulse, the neuron
releases the neurotransmitter into the synaptic
cleft.
DQ - What will this cause?
All muscle fibers in the motor unit contract
– All or none response


DQ - Does this mean entire muscle
contracts?
NO, just the motor unit
– DQ - How do we control force of muscles?

More motor units = More force
Skeletal Muscle Contraction


The sliding filament theory is the most widely
accepted theory on how muscle contraction works.
It says that the head of a myosin cross-bridge can
attach to an actin binding site and bend slightly
– What will this cause?


The actin to move (or slide) with it
The head can then release, straighten itself, and
combine with another binding site farther down the
actin filament.
– Example, arms with a meter stick

This can happen several times causing the muscle
fiber to shorten dramatically
- Shortening of
a sarcomere
Skeletal Muscle Contraction
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All of this binding and changing of shape will take
energy, where will it come from?
ATP
DQ – How is ATP made? (3 ways) (pg 190)
Direct Phosphorylation
– Fastest, Lasts ~15 sec

No O2 needed
Anaerobic glycolysis
– Lasts ~ 30 – 60 sec Causes lactic acid, No O2

Aerobic respiration
– Slowest, Lasts hours, Requires O2
Skeletal Muscle Contraction
Two types of contractions
 Isotonic contractions

– Contractions that shorten the muscle.
– This is the more common one

Isometric contractions
– Tension in the muscle but no change in
length.
Steps to muscle contraction
1. Nerve impulse arrives at the
neuromuscular junction
2. Acetylcholine diffuses across the gap at the
neuromuscular junction
3. T Tubules carry acetlycholine to
sarcoplasmic reticulum, which causes cell
membrane to be permeable to Ca2+
4. Ca2+ binds to troponin
Steps to muscle contraction
5. Conformation shift - troponin pulls
tropomyosin off active binding sites
6. One ATP per myosin Head releases and
activates myosin
7. Actin and myosin filaments form linkages
8. Myosin cross-bridges pull actin filaments
inward
9. Muscle fiber shortens and contracts
Muscle Relaxing

DQ - What happens when a muscle does
not need to contract anymore?
– Think about how it all started


The acetylcholine needs to be removed
the Ach is rapidly decomposed by action of an
enzyme called acetylcholinesterase. (Ach-ase)
– This enzyme is present at the neuromuscular junction

The Ach-ase stops a single nerve impulse from
continuously stimulating the muscle fiber.
Muscle Relaxing
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As Ach is broken down, the calcium pump
quickly moves Ca2+ back into the sarcoplasmic
reticulum.
DQ - What does this do?
The linkages will break
– the troponin and tropomyosin return to the normal
conformation, blocking the binding sites.

The muscle fiber relaxes.
Muscle Fatigue

Muscle fatigue most often occurs because of
oxygen debt.
– Causing?
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Anaerobic respiration – accumulation of lactic acid.
Oxygen debt is “paid back” after activity
– Heavy breathing


DQ - How are people able to “train” to
prolong activity before muscle fatigue?
Exercise stimulates new capillaries to grow within
the muscles and it also causes an increase in the
number of mitochondria
Rigor Mortis
What is it?
 Stiffening of joints and locking into place
after death

– Why does it happen?

As the cells begin to shut down, the
sarcoplasmic reticulum becomes permeable
to Ca2+
– Causing what?

Sets in place within a few hours of death
– Lasts ~72 hrs
Fast and Slow Muscles

There are two types of muscle fibers
relating to the speed of contraction.
– Fast and Slow

The speed of contraction is related to
the specialized function of a muscle.
– Example: Eye muscles that blink contract
ten times faster than the muscles involved
in posture.
Fast and Slow Muscles
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Slow-contracting (slow twitch) fibers
Often called red muscles
– Why?
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The fibers contain red, oxygen-storing myoglobin.
Well supplied with blood.
What would these be useful for?
Fast and Slow Muscles
Fast-contracting (fast twitch) muscle
fibers
 Also called white muscles.

– Why?
They contain less myoglobin and have a
poorer blood supply than red muscles.
 What would these be used for?
 Shown to have a very well developed
sarcoplasmic reticulum?

– Why is that important?
Fast and Slow Muscles
Research has discovered an
intermediate fiber.
 How would you classify these?

Fast and Slow twitch

How would you describe the ratio of fast
twitch to slow twitch for Usain Bolt? Ryan
Hall?
Muscle Hypertrophy
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Hypertrophy – increasing in size
Occurs when more proteins are produced, not
more muscle fibers/muscle cells.
Occurs from working muscles hard or
isometric exercises
When worked hard the muscle becomes
damaged, which causes an increased number
of nuclei.
– DQ - Why would increased nuclei cause
increase in muscle mass (think transcription and
translation)?
Muscle Atrophy
Atrophy – decrease in size
 Caused by lack of use

– Proteins broken down
Muscle interactions
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Prime movers – responsible for major
movements
Antagonists – muscles that oppose or reverse a
movement (examples?)
Synergists - help prime movers by helping
with movement or stabilizing unwanted
movement. (Examples?)
Fixators – specialized synergists, stabilize
bones (Ex. rhomboideus major)
DIDN’T USE

ANY SLIDES AFTER THIS POINT, I DID
NOT USE, BUT KEPT IN CASE I
WANTED TO.
Skeletal Muscle Contraction
In the presence of calcium ions, the
myosin cross-bridges react with actin
filaments and form linkages with them.
 This reaction between the myosin and
actin filaments provides the force that
shortens myofibrils during muscle
contraction.

Skeletal Muscle Contraction
Actin accounts for about 1/4 of the total
protein in skeletal muscle.
 Actin molecules, arranged together in a
double twisted strand, form an actin
filament.
 Tropomyosin & troponin are two
proteins associated with actin filaments.

Skeletal Muscle Contraction
The tropomyosin-troponin complex
blocks the binding sites on the actin
molecules when the muscle is at rest.
 If a high concentration of calcium ions is
present, the calcium ions bind to the
troponin, and this modifies the position
of the tropomyosin.
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Skeletal Muscle Contraction
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The tropomyosin molecules move,
exposing the binding sites on the actin
filaments, and the linkages form
between the actin and myosin filaments.
Skeletal Muscle Contraction
The sliding filament theory of muscle
contraction suggests that the head of a
myosin cross-bridge can attach to an
actin binding site and bend slightly,
pulling the actin filament with it.
 Then the head can release, straighten
itself, and combine with another binding
site farther down the actin filament.

Skeletal Muscle Contraction

The enzyme ATPase causes the
breakdown of ATP to supply energy for
these actions.
Stimulus for Contraction
Acetylcholine (ACh) is a
neurotransmitter that is synthesized in
the cytoplasm of the motor neuron and
is stored in vesicles.
 A nerve impulse reaches the end of the
axon, some of these vesicles release
ACh into the gap between the nerve
and the motor end plate.

Stimulus for Contraction
The ACh diffuses rapidly across the
gap, combines with certain protein
molecules in the sarcolemma, and thus
stimulates the muscle fiber membrane.
 This stimulus causes a muscle
impulse that passes in all directions
over the surface of the sarcolemma.

Stimulus for Contraction
It also travels through the sarcoplasmic
reticulum and the transverse tubules.
 The sarcoplasmic reticulum contains a
high concentration of calcium ions
compared to the sarcoplasm.

Stimulus for Contraction

In response to a muscle impulse, the
membranes of the cisternae become
more permeable to these ions and the
calcium ions diffuse into the sarcoplasm
of the muscle fiber.
Stimulus for Contraction

When a relatively high concentration of
calcium ions is present in the
sarcoplasm, linkages form between the
actin and myosin filaments, and a
muscle contracts.
Energy Sources of
Contraction
The energy for muscle contractions
comes from ATP.
 The muscle has enough ATP to contract
briefly. Therefore, when a fiber is
active, ATP must be regenerated.
 Creatine phosphate supplies the
energy to change ADP back to energy
rich ATP

Oxygen Supply and Cellular
Respiration
Oxygen is transported by the red blood
cells. It is loosely bound to molecules of
hemoglobin.
 The hemoglobin releases the oxygen in
areas of the body that are low in oxygen
content

Oxygen Supply and Cellular
Respiration
Myoglobin, in the muscle cells, can
store oxygen temporarily.
 This reduces a muscle’s need for a
continuous blood supply during a
contraction.
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Oxygen Debt
During strenuous exercise, the available
oxygen supply may be used up. The
body then relies on anaerobic
respiration creating an oxygen debt.
 Anaerobic respiration builds up lactic
acid. The liver converts lactic acid back
to glucose, but it takes several hours to
complete the conversion.
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Muscle Cramps
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Cramps occur when the muscle contracts
spasmodically, but does not relax completely.
The condition is due to a lack of ATP needed
to move calcium or other ions, can also be
caused by a lack of those ions
How would a lack of ATP be a problem?
ATP required to release cross bridges
Exercise stimulates new capillaries to grow
within the muscles and it also causes an
increase in the number of mitochondria.
Heat Production
Since muscle tissue represents a large
proportion of the total body mass, it is a
major source of heat.
 About 25% of the energy released in
cellular respiration is available for use in
metabolic processes.

Heat Production
Active muscles release large amounts
of heat.
 Blood transports this heat to other
tissues to help maintain body
temperature.
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Smooth Muscle
Smooth muscle characteristics:
 Shorter than the fibers of skeletal
muscle.
 Single, central nucleus.
 Cells are elongated with tapering ends.
 Filaments are more randomly arranged
than skeletal muscle.
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Smooth Muscle
The two types of smooth muscle are
multiunit and visceral.
 Multiunit muscles are less organized
and occur as separate fibers.
 Multiunit are found in the irises of the
eyes and walls of blood vessels.
 They contract through motor nerve
impulse or hormone action.

Smooth Muscle
Visceral smooth muscle is composed of
sheets of spindle shaped cells held
together by gap junctions.
 Visceral smooth muscles are found in
walls of hollow organs (ex. Stomach,
intestines)
 There usually will be two muscle layers,
a longitudinal and a circular layer.

Smooth Muscle
When one fiber is stimulated, the
impulse will also excite adjacent cells
causing a rhythmic contraction.
 Peristalsis - wavelike contraction that
occurs in tubular organs.

Smooth Muscle Contraction
Smooth muscle contraction is similar to
skeletal muscle contraction.
 Smooth muscle lacks troponin. Instead
it uses a protein called calmodulin to
bind calcium ions.
 Calcium diffuses into the cell from the
extracellular fluid.

Smooth Muscle Contraction
Smooth muscle reacts to the
neurotransmitter ACh and
norepinephrine.
 Some hormones cause smooth muscle
contraction. (Ex. Childbirth)
 Stretching smooth muscle can cause
contractions. (Ex. Digesting food in the
stomach)

Smooth Muscle Contraction
Smooth muscle is slower to contract
and slower to relax.
 Smooth muscle can contract for a
longer time with the same amount of
ATP.
 Smooth muscles stretch without
changing tautness while a hollow organ
fills.

Cardiac Muscle
Found only in the heart.
 Striated cells joined end to end forming
interconnecting branched threedimensional network.
 Each cell contains a single nucleus.
 Well developed sarcoplasmic reticulum
and transverse tubules.

Cardiac Muscle
Sarcoplasmic reticulum stores less
calcium but the enlarged transverse
tubules store extra calcium.
 The extra calcium allows cardiac
muscle to maintain a contraction longer
than skeletal muscles.

Cardiac Muscle
Intercalated disks separate opposing
ends of cardiac cells.
 The disks help hold adjacent cells
together and transmit the force of
contraction from cell to cell.

Cardiac Muscle

When one portion of the cardiac
network is stimulated, the impulse
passes throughout the network causing
the whole structure to contract as a unit.
Muscle Action
The main muscle is the prime mover.
 Muscles that contract along with the
prime mover are called synergists.
 Muscles opposing the prime mover are
called antagonists.
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