1. Muscles II: Microscopic Anatomy and Contraction
October

2. Fascicle (bundle of fibers)
Muscle Structure
Muscle
Fascicle (bundle of fibers)
Muscle Fiber (single cell)
Myofibril (organelle)
Sarcomere (unit of contraction)
Single muscle cell up to 30 cm -- relate back to cell theory
Light / dark regions are ??
Small gray things between myofibrils are … ?

3. 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+
Sarco = greek root for “flesh” or “muscle”
T-tubules spread action potential to sarcoplasmic reticulum -- sarcoplasmic reticulum open to allow flood of Ca2+ which initiates contraction

4. 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.
Sarco = greek root for “flesh” or “muscle”
T-tubules spread action potential to sarcoplasmic reticulum -- sarcoplasmic reticulum open to allow flood of Ca2+ which initiates contraction

5. 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
Sarco = greek root for “flesh” or “muscle”
T-tubules spread action potential to sarcoplasmic reticulum -- sarcoplasmic reticulum open to allow flood of Ca2+ which initiates contraction

6. Microscopic Anatomy of Skeletal Muscle
Use the picture to come up with a definition of the following:
I band
A band
H zone
Sarco = greek root for “flesh” or “muscle”
T-tubules spread action potential to sarcoplasmic reticulum -- sarcoplasmic reticulum open to allow flood of Ca2+ which initiates contraction

7. 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
Sarco = greek root for “flesh” or “muscle”
T-tubules spread action potential to sarcoplasmic reticulum -- sarcoplasmic reticulum open to allow flood of Ca2+ which initiates contraction

8. 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.
Sarco = greek root for “flesh” or “muscle”
T-tubules spread action potential to sarcoplasmic reticulum -- sarcoplasmic reticulum open to allow flood of Ca2+ which initiates contraction

9. 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
Sarco = greek root for “flesh” or “muscle”
T-tubules spread action potential to sarcoplasmic reticulum -- sarcoplasmic reticulum open to allow flood of Ca2+ which initiates contraction

10. 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
On the HW Question 7 turned today, I will be looking for diagrams that clearly show appropriate lengths … change if its not there. If you have already done it correctly then quiz a partner on vocabulary – 2 min.
light dark light

11. Which shows contracted muscle fibers?
Contraction Overview
Can have degrees of contraction, even within sarcomere
Which shows contracted muscle fibers?
How can you tell?

12. Contraction Overview Relaxed muscle Contracted muscle
Can have degrees of contraction, even within sarcomere
Relaxed muscle Contracted muscle
(large light bands) (small light bands)

13. Contraction Overview See animation! What are these?
Where are the z disks? The m line? A single sarcomere
What are these?
Are the myofibrils organized left to right or top to bottom?
Mitochondria
Left to right
See animation!

14. Contraction Details
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
Motor neurons can activate a few cells or 100s of cells

15. Contraction Details
A motor neuron stimulates the muscle cell by releasing the neurotransmitter acetylcholine ACh into the synaptic cleft between the neuron and muscle cell.
ACh causes an electric current called an action potential to move through the muscle cell.
Motor neurons can activate a few cells or 100s of cells

16. Contraction Details
A motor neuron stimulates the muscle cell by releasing the neurotransmitter acetylcholine ACh into the synaptic cleft between the neuron and muscle cell.
ACh causes an electric current called an action potential to move through the muscle cell.
The action potential causes the release of Ca2+ from the sarcoplasmic reticulum.
Can have degrees of contraction, even within sarcomere

17. Contraction Details
Ca2+ exposes myosin-binding sites on actin filaments.
Myosin heads (& ADP) attach to actin binding sites, forming cross-bridges.
actin
ADP + P
myosin head
NOT necessary to know that tropomysin blocks binding sites, and troponin exposes them (by changing position on tropomysin) when Ca2+ is bound to them.
myosin
Muscle relaxed Ca2+ present.
No Ca2+ present Cross-bridge formed.

18. Contraction Details
Myosin heads release ADP, move the actin filament in “power stroke”
actin
Can have degrees of contraction, even within sarcomere
myosin
Power stroke,
ADP + P released

19. Contraction Details
Myosin heads release ADP, move the actin filament in “power stroke”
ATP binds to myosin head. The crosslink between actin and myosin breaks.
ATP becomes ADP + P, readying the myosin head to reattach to actin.
actin
Can have degrees of contraction, even within sarcomere
myosin
Power stroke, ATP binds,
ADP + P released cross-links break

20. Contraction Details Once the action potential is over, the
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.
Can have degrees of contraction, even within sarcomere
Watch me!

21. 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.
Can have degrees of contraction, even within sarcomere

22. 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
Sequence these!
Stimulation of contraction is different – have self-excitory cells. Cardiac fairly similar.
Also, in smooth muscle Ca2+ comes from outside cell, process slightly different but still involves sliding of actin past myosin.
Less organized – no striations – means shortening occurs in all directions, not just length-wise (like tightening a drawstring bag).
Challenge question:
We discussed the process of contraction in skeletal muscles. How do you expect it differs for cardiac and smooth muscles?

23. Exit Ticket
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?
Z discs
Triads
I bands
A bands
H zones
Can have degrees of contraction, even within sarcomere
MORE TIME? Have them draw contracted and relaxed sarcomeres, labeling zones.

24. Exit Ticket
2. Which word describes the unit of contraction of a muscle?
Myofibril
Sarcomere
A band
H band
Can have degrees of contraction, even within sarcomere

25. Exit Ticket
3. Which of the following correctly lists the order of structure of the muscle from largest to smallest?
fascicle, myofibril, sarcomere, muscle fiber
myofibril, fascicle, sarcomere, muscle fiber
fascicle, muscle fiber, myofibril, sarcomere
muscle fiber, fascicle, myofibril, sarcomere
Can have degrees of contraction, even within sarcomere

26. Exit Ticket Which of these stores calcium ion? Sarcoplasmic reticulum
Sarcomere
Sarcolemma
mitochondria
Can have degrees of contraction, even within sarcomere

27. Exit Ticket
Which of these best describe the process of muscle contraction?
The actin filaments shorten
The myosin filaments shorten
The light bands shorten
The dark bands shorten
Can have degrees of contraction, even within sarcomere

28. Exit Ticket 6. Which of these best describe the process of
muscle contraction?
Myosin heads attach to actin filaments that are exposed by the presence of ATP
Myosin heads attach to actin filaments that are exposed by the presence of Ca2+
Actin heads attach to myosin filaments that are exposed by the presence of ATP
Actin heads attach to myosin filaments that are exposed by the presence of Ca2+
Can have degrees of contraction, even within sarcomere