Download Muscular System 1

Copyright © 2006 Pearson Education, Inc., publishing as Benjamin Cummings
Muscle Overview

The three types of muscle tissue are skeletal,
cardiac, and smooth

These types differ in structure, location, function,
and means of activation
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Muscle Overview

Structure:
1. Skeletal muscle tissue
2. Connective tissues
3. Nervous tissue
4. Blood
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Connective Tissue Coverings

Layers of dense connective tissue, called fascia,
surround and separate each muscle. It extends
beyond the ends of the muscle and gives rise to
tendons that are fused to the periosteum of bones.
Fascia from the underside of the sternum (top image).
Note how the fibers look like crystals. Fascia extends
between individual muscle fibers which have been
teased out from the gluteal muscle (bottom image).
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Fascia is essentially all of the
connective tissue in the body.
It is a tough covering, much
like a sausage casing, that
surrounds every muscle. It
forms a vast supporting
network found throughout the
body and is continuous from
head to toe It is the white in
the picture shown here. The
tendons that join the muscle to
the bone, the joint capsules and
the ligaments are all fascia.
Scar tissue and adhesions
occur within the fascia; these
areas are typically more
restricted and disorganized.
Microscopic Anatomy of a Skeletal Muscle
Fiber

Each fiber is a long, cylindrical cell with multiple nuclei
just beneath the sarcolemma

Beneath each sarcolemma (cell membrane) lies sarcoplasm
(cytoplasm) with many mitochondria and nuclei; the
sarcoplasm contains myofibrils.

Thick filaments of myofibrils are made up of the protein
myosin.

Thin filaments of myofibrils are made up of the protein
actin.

The organization of theses filaments produces striations or
dark bands.
Copyright © 2006 Pearson Education, Inc., publishing as Benjamin Cummings
Copyright © 2006 Pearson Education, Inc., publishing as Benjamin Cummings
Copyright © 2006 Pearson Education, Inc., publishing as Benjamin Cummings
Microscopic Anatomy of a Skeletal Muscle
Fiber

The area between 2 Z-bands is called a sarcomere.

I bands (light bands) made up of actin are anchored
to Z lines.

A bands (dark bands) are made up of overlapping
thick and thin filaments.
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Myofibrils
Figure 9.3b
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Myofilaments: Banding Pattern
Figure 9.3c,d
Copyright © 2006 Pearson Education, Inc., publishing as Benjamin Cummings
Copyright © 2006 Pearson Education, Inc., publishing as Benjamin Cummings
Neuromuscular Junction

The site where motor neuron and muscle fiber meets.

The neuromuscular junction is formed from:

The motor end plate of a muscle, which is a
specific part of the sarcolemma that is tightly
folded and where nuclei and mitochondria are
abundant.

The cytoplasm of the motor neuron contains
numerous mitochondria and small membranous
sacs (synaptic vesicles) that contain the
neurotransmitter acetylcholine (ACh)
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Neuromuscular Junction

When a nerve impulse reaches the end of an
axon at the neuromuscular junction:


Neurotransmitters are released so impulse
can cross synapse.
A motor unit is made up of:

A motor neuron

Muscle fibers that it controls
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Neuromuscular Junction
Figure 9.7 (a-c)
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Skeletal Muscle Contraction

Muscle contraction involves several
components that result in the shortening of
sarcomeres, and the pulling of the muscle
against its attachements.
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Sarcomeres
Figure 9.3c
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Sliding Filament Model of Contraction

Thin filaments slide past the thick ones so that the
actin and myosin filaments overlap to a greater
degree

In the relaxed state, thin and thick filaments
overlap only slightly

Upon stimulation, myosin heads bind to actin and
sliding begins
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Sliding Filament Model of Contraction

Each myosin head binds and detaches several
times during contraction, acting like a ratchet to
generate tension and propel the thin filaments to
the center of the sarcomere

As this event occurs throughout the sarcomeres,
the muscle shortens

This cycle will last as long as ATP is present and
the muscle fiber is stimulated.

Example
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Sliding Filament Model of Contraction
Copyright © 2006 Pearson Education, Inc., publishing as Benjamin Cummings
Sliding Filament Model of Contraction
Copyright © 2006 Pearson Education, Inc., publishing as Benjamin Cummings
Sliding Filament Model of Contraction
Copyright © 2006 Pearson Education, Inc., publishing as Benjamin Cummings
Skeletal Muscle Contraction

Skeletal muscles are stimulated by motor neurons of the somatic
nervous system which release acetylcholine from its synaptic vesicles
into the synaptic cleft in order to initiate a muscle contraction

In order to contract, a skeletal muscle must:

Be stimulated by a nerve ending

Propagate an electrical current, or action potential, along its
sarcolemma

Have a rise in intracellular Ca2+ levels, the final trigger for
contraction

The high concentration of Ca2+ in the sarcoplasm interacts with
the troponin and tropomyosine molecules, which move aside,
exposing the myosin binding sites on the actin filaments.
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Nerve Stimulus of Skeletal Muscle

Mysoin cross-bridges now bind and pull on the
actin filaments, causing the sarcomere to shorten.

After the nervous impulse has been received ACh
bound to ACh receptors is quickly destroyed by
the enzyme acetylcholinesterase

This destruction prevents continued muscle fiber
contraction in the absence of additional stimuli

Ca2+ concentrations decrease in the sarcoplasm,
and the linkages between the mysoin & actin are
broken.
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Nerve Stimulus of Skeletal Muscle

Animation

Animation #2
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Energy Sources for Contraction

Energy for contraction comes from ATP

Creatine phosphate, which stores excess energy
released by the mitochondria, is present to
regenerate ATP from ADP and phosphate

Whenever the supply of ATP is sufficient, creatine
phosphokinase promotes the synthesis of creatine
phosphate

As ATP decomposes, the energy from creatine
phosphate can be transferred to ADP molecules,
converting them back to ATP
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Oxygen Supply & Cellular Respiration

Muscles require lots of
oxygen to enable the
breakdown of glucose

Hemoglobin in the RBCs
carries oxygen to muscles

The pigment myoglobin
stores oxygen in muscle
tissue
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Oxygen Debt

During rest or moderate activity, there is enough oxygen to
support aerobic respiration.

Oxygen deficiency may develop during strenuous exercise,
and lactic acid accumulates as an end product of anaerobic
respiration.

Oxygen debt refers to the amount of oxygen the liver cells
need in order to convert lactic acid into glucose, plus the
amount needed by muscle cells to resynthesize ATP &
creatine phosphate to their original
concentrations.

This process may take several hours
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Muscle Fatigue

When muscles lose their ability to contract during
strenuous exercise.

Muscle fatigue usually arises from the
accumulation of lactic acid in the muscle. This
cause the pH level to drop, preventing the muscle
from contracting. (Can’t contract)

A muscle cramp occurs due to a lack of ATP
required to return Ca2+ back to the sarcoplasm so
muscle fibers can relax. (Can’t relax)
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Muscle Fatigue

Rigor mortis is the partial contraction of muscles
that fix joints.

It occurs several hours after death

Due to an increase in cell membrane permeability
to Ca2+, but a decrease in ATP

Therefore actin hooks to myosin without ATP to
stop the myosin hooks and the muscle contracts

Eventually the muscle decomposes
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Remember, this is the
process that occurs to make
muscles contract. This
process stops when life
ceases to exist.
Copyright © 2006 Pearson Education, Inc., publishing as Benjamin Cummings
Heat Production

There are 2 sources of heat production for the
body:

Contraction of skeletal muscles

Cellular respiration (increases the rate at which you
burn fat & sugar)
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Smooth Muscle Tissue

Found in the walls of hollow visceral organs, such
as the stomach, urinary bladder, and respiratory
passages

Forces food and other substances through internal
body channels

It lacks striations and is involuntary
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Smooth Muscle Contraction

Similar myosin-binding-to-actin

Both acetylcholine & norepinephrine stimulate and
inhibit smooth muscle contraction, depending on
the target muscle

Hormones can also stimulate or inhibit contraction

Slower to contract & relax than skeletal muscle,
but can contract longer using the same amount
of ATP
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Cross section of intestinal muscle
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Cardiac Muscle Tissue

Occurs only in the heart

Similar mechanism for contraction as skeletal muscle

Is striated like skeletal muscle but is not voluntary

Contracts at a fairly steady rate set by the heart’s
pacemaker

Has transverse tubules that supply extra calcium, and can
contract for longer periods of time

Neural controls allow the heart to respond to changes in
bodily needs
Copyright © 2006 Pearson Education, Inc., publishing as Benjamin Cummings
Copyright © 2006 Pearson Education, Inc., publishing as Benjamin Cummings
Skeletal Muscle Tissue

Packaged in skeletal muscles that attach to and cover the
bony skeleton

Has obvious stripes called striations

Is controlled voluntarily (i.e., by conscious control)

Contracts rapidly but tires easily

Is responsible for overall body motility

Is extremely adaptable and can exert forces ranging from a
fraction of an ounce to over 70 pounds
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Skeletal Muscle Actions

Origin & Insertion

The immovable end of a muscle is the origin, while
the moveable end is the insertion; contraction pulls
the insertion toward the origin

Some muscles have more than one insertion or
origin
Copyright © 2006 Pearson Education, Inc., publishing as Benjamin Cummings
Copyright © 2006 Pearson Education, Inc., publishing as Benjamin Cummings
Skeletal Muscle Actions

Interaction of Skeletal Muscles

Of a group of muscles, the one doing the majority
of the work is the prime mover

Helper muscles are called synergists; opposing
muscles are called antagonists.

Example: Tricep & bicep muscles
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Antagonist. The triceps brachii
extends the forearm at the
elbowwhile the biceps brachii, its
antagonist, flexes the elbow.
Copyright © 2006 Pearson Education, Inc., publishing as Benjamin Cummings
Copyright © 2006 Pearson Education, Inc., publishing as Benjamin Cummings
Muscle Function

Skeletal muscles are responsible for all locomotion

Cardiac muscle is responsible for coursing the
blood through the body

Smooth muscle helps maintain blood pressure, and
squeezes or propels substances (i.e., food, feces)
through organs

Muscles also maintain posture, stabilize joints, and
generate heat

Review
Copyright © 2006 Pearson Education, Inc., publishing as Benjamin Cummings