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SI Tuesday October 20
Skeletal Muscle Histology
1. Outline some skeletal muscle histology mnemonics:
1)STRIATION: The striated appearance of skeletal muscle is due to alternating A-bands and Ibands. If asked what accounts for the striated appearance, the answer is already in the question!
The I-band has the lIght appearance and the A-band has the dArk appearance.
2) A is for always: During contraction, the A-band does not change in length…it is Always the same
length.
3) HI, then bye. The H zone and I band are flighty. They shorten and ultimately disappear as the
sarcomere shortens, so they say “HI”, then they say bye. The H-zone will disappear first and is
listed first in the word HI.
4) TRIAD. The triads are located at the A-band / I-band junctions.
2. What terminology do we use to describe the muscle fiber plasma membrane, cytoplasm, and smooth ER? What do
we call the functional contractile unit of the muscle fiber? What are T-tubules? What is a triad?
Plasma Membrane = Sarcolemma
Cytoplasm = Sarcoplasm
Modified sER = Sarcoplasmic Reticulum (membranous pouches that sequester Calcium Ions)
Functional Unit = Sarcomere (extends from Z-disk to Z-disk)
T-Tubules = invaginations of the sarcolemma that penetrate to the core of the muscle fiber. They
allow for uniform distribution of the action potential so that all of the myofibrils contract in
unison. Just as the intestinal tract is considered as outside of the body even though it runs
through us, the contents of the T-tubules are essentially extracellular (so they have a much
higher concentration of sodium…essential for generating action potentials).
Triads are the coupling of one T-tubule to two terminal cisternae (outpouchings of sarcoplasmic
reticulum) Later we will learn that an action potential propagated along the T-tubule is
coupled to the release of Calcium Ions from the Terminal Cisternae.
3. Briefly describe the process through which mature skeletal muscle fibers possess multiple nuclei
Multiple myoblasts (stem cells) fuse to form a mature muscle fiber. Each myoblasts contributes one
nucleus. Note that the nuclei are squeezed out into the periphery of the muscle fiber.
Outline the components of thick and thin filaments:
Thick Filaments
Myosin: Protein with a tail (extending from the M-line), a hinge region that enables pivoting, and a
head with a binding affinity for G-Actin and with ATPase activity.
M-Line: Region at the center of the sarcomere where multiple thick filaments are anchored together.
Titin: Huge Protein that runs from the M-line to the Z-disk within the core of the thick filament.
Keeps the A-band in the proper alignment within the sarcomere and elasticity prevents overstretching of the sarcomere.
Thin Filaments
Nebulin: Filamentous protein that establishes length of F-Actin
G-Actin: Globular Actin. Individual polar actin molecules that will link together plus to minus ends.
F-Actin: The filament of actin that results from G-actin polymerization
Tropomyosin: Filamentous proteins, each running approximately 7 g-actin units, and overlapping
with each other…at rest tropomyosin cover G-actin binds sites thus preventing cross-bridge
formation
Troponin: Globular protein with binding sites for 1)Tropomyosin 2)G-actin and 3)Calcium Ions.
Actinin (Z-Disk): Protein that creates a meshwork that anchors thin filaments in transverse plane.
SI Tuesday October 20
Skeletal Muscle Histology
Use arrows to assign each cross section to its corresponding location in the longitudinal image
Z-line
I-Band
M-Line
H-Zone
Zone of Overlap
Multiple Choice Blitz:
1. The presence of a membrane potential is primarily due to:
A. A greater concentration of potassium ions inside of the cell versus outside of the cell
B. The presence of negative charges extending from the plasma membrane into the cytoplasm
C. The presence of positive charges surrounding the outer surface of the membrane.
D. All of the above
2. Which of the following neurotransmitters might you expect to have cytosolic receptors?
A. Neuropeptide Y
B. Glycine
C. Nitric Oxide
D. Acetylcholine
3. Direct activation of calcium channels by serotonin release at an axo-axonic synapse would be an example of:
A. Spatial Summation
B. Second messenger mediated stimulation
C. Presynaptic Faciliation
D. All of the above
4. Often activation of metabotropic receptors (receptors whose activation is coupled to second messenger activation)
by a neurotransmitter leads to the opening or closing of an ion channel. Why not just activate the ion channel
directly?
A. Inhibitory stimuli require second-messengers to mediate their effects
B. Changes in cell metabolism and gene expression can be simultaneously initiated
C. Metabotrobic receptors are required for spatial summation to occur
D. Actually, activation of metabotropic receptors never indirectly leads to ion channel activation
5. Which of the following is not a function of skeletal muscle:
A. Maintenance of posture and body position
B. Red blood cell production
C. Maintenance of body temperature
D. Stabilization of joints
SI Tuesday October 20
Skeletal Muscle Histology
6. Skeletal muscle fibers are said to be excitable cells. What specifically accounts for this?
A. Presence of Voltage Gated Channels
B. Sarcomeres
C. The Sarcoplasmic Reticulum
D. Abundant mitochondria
E. All of the above
7. Which of the following contains the correct order of organization from large to small:
A. Skeletal muscle, muscle fibers, fascicles, myofibrils, sarcomeres
B. Muscle fibers, biceps muscle, fascicles, sarcomeres, myofibrils
C. Biceps muscle, muscle fibers, fascicles, myofilaments, sarcomeres
D. Biceps muscle, fascicles, muscle fibers, myofibrils, sarcomeres
8. The functional contractile unit of muscle fibers is the:
A. A-band
B. Sarcomere
C. T-tubule/sarcoplasmic reticulum triad
D. Z-disk
9. Binding sites for calcium are present on:
A. G-Actin
B. Troponin
C. Tropomyosin
D. Nebulin
10. Where might you find proteins running perpendicular to the longitudinal proteins within a sarcomere?
A. The M-line
B. The zone of overlap
C. The Z-disk
D. A and C
11. How many terminal cisternae exist per sarcomere?
A. 6
B. 4
C. 3
D. 1
12. Which protein is responsible for establishing the correct length of f-actin?
A. Titin
B. Tropomyosin
C. Nebulin
D. Actinin
13. Which protein prevents overstretching of the sarcomere?
A. Troponin
B. Nebulin
C. Titin
D. M-line proteins
14. The striated appearance of skeletal muscle is due to:
A. The transverse tubule system
B. Alternating A-bands and I-bands in a parallel arrangement
C. Alternating M-lines and Z-disks in a parallel arrangement
D. Alternating H-zones and I-bands in a parallel arrangement
15. Muscle Fascicles are encased in:
A. Perimysium
B. Sarcoplasmic Reticulum
C. Endomysium
SI Tuesday October 20
Skeletal Muscle Histology
D. Epimysium
16. Every ____________is completed enveloped in sarcoplasmic reticulum:
A. Muscle fascicle
B. Sarcomere
C. Myofibril
D. Endomysium
17. Satellite cells are found in:
A. The Central Nervous System
B. Epimysium
C. Perimysium
D. Endomysium
18. Actin-Myosin cross-bridge formation is directly prevented at rest due to the position of:
A. Nebulin
B. Titin
C. Tropomyosin
D. Mitochondria
19. At resting length, which zone in the sarcomere would contain only thin filaments and titin?
A. The I-band
B. The M-Line
C. The zone of overlap
D. The H-zone
20. Each thick filament can theoretically form cross bridges with ___different thin filaments:
A. 3
B. 6
C. 1
D. 12
21. Endomysium, Epimysium, and Perimysium can fuse together at the ends of muscle to form:
A. Tendons
B. Ligaments
C. Periosteum
D. Articular Cartilage
(Aponeurosis – flat and broad tendinous tissue – would also be correct if it was listed as an option)
22. The boundaries that define a sarcomere are:
A. From one M-line to the next M-line
B. From one Z-line to the next Z-line
C. Longitudinally from the start to the end of a myofibril
D. From one zone of overlap to the next zone of overlap
23. Which protein is present on both sides of f-actin and extends for approximately 6-7 g-actin molecules and
overlaps with itself to form essentially continuous strands?
A. Troponin
B. Nebulin
C. Titin
D. Tropomyosin
25. Which of the following statements is false:
A. Depending on the extent of activation, a muscle can grade the strength of its contraction by only
activating some of its myofibrils.
B. Satellite cells have some capacity to differentiate and repair damaged muscle
C. G-actin monomers have a plus and a minus end
D. The fluid contained within the t-tubules has essentially the same ionic composition as extracellular fluid.