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Muscles II: Microscopic Anatomy
and Contraction
October 31 – Nov 3 2014
Muscle
Structure
Muscle
Fascicle
(bundle of
fibers)
Muscle Fiber
(single cell)
Myofibril
Sarcomere
(unit of
contraction)
Microscopic Anatomy of
Skeletal Muscle
• Large, cylindrical, multinucleate cells
• Contain many mitochondria; nearly filled with myofibrils
• Some organelles have unique vocabulary:
– Sarcolemma:
cell membrane
– Sarcoplasm:
cytoplasm
– Sarcoplasmic
reticulum:
modified ER,
surrounds each
myofibril; store
Ca2+
Microscopic Anatomy of
Skeletal Muscle
• Each myofibril can
be divided into
contractile units
called sarcomeres.
• Sarcomeres consist
of overlapping
protein filaments
of actin and
myosin.
• Regular
arrangement of
dark and light
bands. Dark
bands occur where
myosin is present.
Microscopic Anatomy of
Skeletal Muscle
• The M line is
where the myosin
attaches
• Z discs (a
membrane) mark
the edge of each
sarcomere; serve
as attachment site
for actin
Microscopic Anatomy of
Skeletal Muscle
Use the picture to
come up with a
definition of the
following:
I band
A band
H zone
Microscopic Anatomy of
Skeletal Muscle
Use the picture to
come up with a
definition of the
following:
I band – area without
myosin fibers; aka
light band
A band – area with
myosin fibers; aka
dark band
H zone – area
without actin fibers
Turn & Talk
First match the words …
actin
cell
myofibril
group of cells
sarcomere
cell membrane
fascicle
protein
muscle fiber
organelle
sarcolemma
contractile unit
Then, write a paragraph that uses all the words in both
columns above and explains that structure of the muscle.
Contraction Overview
• Globular heads of
myosin filaments
attach to actin
filaments.
• Myosin pulls actin
filaments :
“Sliding filament
theory”
• Causes sarcomere
to shorten,
particularly the
light bands
Contraction Overview
• Globular heads of
myosin filaments
attach to actin
filaments.
• Myosin pulls actin
filaments :
“Sliding filament
theory”
• Causes sarcomere
to shorten,
particularly the
light bands
light
dark
light
dark
light
light
Contraction Overview
Which shows contracted muscle fibers?
How can you tell?
Contraction Overview
Relaxed muscle
(large light bands)
Contracted muscle
(small light bands)
Contraction Overview
What are these?
See animation!
Mitochondria
Contraction Details
1.
A motor neuron stimulates the muscle cell by releasing
the neurotransmitter acetylcholine ACh into the
synaptic cleft between the neuron and muscle cell.
Note:
A motor unit is a single motor neuron and all the muscle
fibers it activates
Contraction Details
1.
A motor neuron stimulates the muscle cell by releasing
the neurotransmitter acetylcholine ACh into the
synaptic cleft between the neuron and muscle cell.
2.
ACh causes an electric current called an action potential
to move through the muscle cell.
Contraction Details
1.
A motor neuron stimulates the muscle cell by releasing
the neurotransmitter acetylcholine ACh into the
synaptic cleft between the neuron and muscle cell.
2.
ACh causes an electric current called an action potential
to move through the muscle cell.
3.
The action potential causes the release of Ca2+ from the
sarcoplasmic reticulum.
Contraction Details
4.
Ca2+ exposes myosin-binding sites on actin filaments.
5.
Myosin heads (& ADP) attach to actin binding sites,
forming cross-bridges.
actin
ADP + P
myosin head
myosin
Muscle relaxed.
No Ca2+ present.
Ca2+ present.
Cross-bridge formed.
Contraction Details
6.
Myosin heads release ADP, move the actin filament in
“power stroke”
actin
myosin
Power stroke,
ADP + P released
Contraction Details
6.
Myosin heads release ADP, move the actin filament in
“power stroke”
7.
ATP binds to myosin head. The crosslink between actin
and myosin breaks.
8.
ATP becomes ADP + P, readying the myosin head to
reattach to actin.
actin
myosin
Power stroke,
ADP + P released
ATP binds,
cross-links break
Contraction Details
•
If Ca2+ is still present, cycle will repeat, with myosin
heads reattaching and contracting the muscle even
more.
•
Once the action
potential is over, the
Ca2+ is reabsorbed into
the sarcoplasmic
reticulum. Without
Ca2+, myosin cannot
attach to actin.
Watch me!
Contraction Details
NOTE:
ATP is required to break
cross-links, not to form
them.
Explains rigor mortis
Why then do muscles need
ATP?
To reset head so it can
contract further -contraction is a series of
sliding motions.
Turn & Talk
Describe the role of each of the following in muscle contraction
Scholar with more siblings….
• ACh
• Ca2+
Scholar with less siblings …
• ATP
• Action potential
Exit Ticket
1. In comparing electron micrographs of a relaxed
skeletal muscle fiber and a fully contracted
muscle fiber, which would be seen only in the
relaxed fiber?
a)
b)
c)
d)
e)
Z discs
Triads
I bands
A bands
H zones
Exit Ticket
2.
Which word describes the unit of
contraction of a muscle?
a)
b)
c)
d)
Myofibril
Sarcomere
A band
H band
Exit Ticket
3.
Which of the following correctly lists the
order of structure of the muscle from
largest to smallest?
a)
b)
c)
d)
fascicle, myofibril, sarcomere, muscle fiber
myofibril, fascicle, sarcomere, muscle fiber
fascicle, muscle fiber, myofibril, sarcomere
muscle fiber, fascicle, myofibril, sarcomere
Exit Ticket
4. Which of these stores calcium ion?
a)
b)
c)
d)
Sarcoplasmic reticulum
Sarcomere
Sarcolemma
mitochondria
Exit Ticket
5. Which of these best describe the process of
muscle contraction?
a)
b)
c)
d)
The actin filaments shorten
The myosin filaments shorten
The light bands shorten
The dark bands shorten
Exit Ticket
6. Which of these best describe the process of
muscle contraction?
a) Myosin heads attach to actin filaments that are
exposed by the presence of ATP
b) Myosin heads attach to actin filaments that are
exposed by the presence of Ca2+
c) Actin heads attach to myosin filaments that are
exposed by the presence of ATP
d) Actin heads attach to myosin filaments that are
exposed by the presence of Ca2+

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