Histology SSN October 25, 2004
Muscle
Nadia Goodwin – ng2021
Rebecca Nardi – ran2101
SKELETAL MUSCLE
I. Organization
A. General
Muscle fibers run parallel and straight
Muscle tissue has many fasciculi
A fasciculus has many myofibers
A myofiber has many myofibrils
Myofiber = muscle cell
“Muscle fiber” is really a syncytium of small myoblasts
that fused to form a large multinucleated cell
Polygon-shaped in cross section
nuclei pushed to outer edge of cell by the myofibrils
A myofibril is composed of many myofilaments (thick and thin)
B. Surrounding connective tissue
1) Endomysium
Loose connective tissue that surrounds each
muscle fiber
Reticular fibers
Contains the smallest capillaries and neuronal
branches
2) Perimysium
Connective tissue + fat + blood vessels and nerves
Group fibers into fascicles
3) Epimysium
Dense connective tissue that surrounds muscle groups. Large vessels and
nerves penetrate this to reach muscle fibers
II. Myofilaments & Striations
A. Types:
1) Thick: myosin
2) Thin: actin
B. Sarcomere = Basic Contractile Unit
C. Striations: Diagramed in sarcomere. Seen in entire muscle b/c myofibrils and
myofibers are in register.
H-band = myosin not overlapping actin
A-band = all myosin (both overlapping and non-)
I-band = actin not overlapping myosin
Z disc = disc in center of I band from which actin extends
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Histology SSN October 25, 2004
III. Function Tidbits
A. Musculotendinous junction = to dense regularly-arranged connective tissue
(tendon) i.e. collagen type I.
B. Triad = functions to deliver intracellular Ca++ to initiate contraction.
(2 triads/sarcomere)
1. (1) Transverse (T) Tubule = in-pocketing of plasma membrane
2. (2) cisternae of the Sarcoplasmic reticulum
C. Motor unit = neuron + muscle cells it innervates.
CARDIAC MUSCLE
I. Organization
Fiber = one, mononuclear cell
Central nucleus (can be seen in cross section too)
Columns of cells are branched
II. Intercalated discs
A. Gap junctions
Lateral portion
Important for ionic continuity, to
coordinate contraction
B. Fasciae adherens
Transverse portion
Holds cells together at their ends
C. Desmosomes
Lateral and transverse portions
Keeps cells together
** Tight Junction = Zona occludans is not found in cardiac muscle
III. Functional
A. intrinsic rhythm - does not need ACh or norepinephrine for contraction
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Histology SSN October 25, 2004
SMOOTH MUSCLE
I. Characteristics
Surrounds the walls of ducts, blood vessels, GI tract, etc.
Function to give an imprecise squeeze (staggered sheets of cells)
Nuclei
cigar-shaped
lie in the middle of the fiber
appear wrinkled or “puckered”
Plane of section determines visibility of nuclei, cell width
No striations because of diffuse contractile elements
Gap junctions allow for united contractions
Basal Lamina (Collagen IV) and Reticular Fibers (Collagen III) surround
individual fibers
II. Smooth muscle vs. Dense irregularly-arranged connective tissue
A. Color
Smooth muscle is more purple because of combination of actin/myosin and
RNA (red + blue = purple)
Connection tissue is more red because of purity of collagen
B. Nuclei
Smooth muscle nuclei are inside cell
Fibroblast nuclei are in between collagen fibers
C. Homogeneity
Smooth muscle is composed only of muscle
Connective tissue contains fibroblasts, macrophages, lymphocytes, etc., and
will therefore have a ragged appearance
Skeletal Muscle Cardiac Muscle Smooth Muscle
Striated Yes Yes No
T Tubule Location A-I junction Z disk
Sarcoplasmic Reticulum: 2 cisternae in Triad Anastamosing
w/ T tubule network
Electrically coupled via No Yes Yes
Gap Junctions?
Can regenerate? Yes (limited) No Yes
Level of Ca2+ regulation thin filaments thin filaments thick filaments
Location of Nucleus Peripheral Central Central
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Histology SSN October 25, 2004
MUSCLE QUESTIONS
1) Which of the following statements is FALSE?
a. The band marked “A” does not include actin filaments.
b. The M line is found in the band marked “A”.
c. During contraction, the length of the band marked “A” remains the same.
d. During contraction, the length of the band marked “B” is shortened.
2) Which of these statements is FALSE about the muscle type at the pointer?
a. The cells in this muscle type are electrically coupled.
b. This muscle type utilizes a T-tubule system for calcium delivery.
c. This muscle type is not capable of regeneration.
d. Calcium regulation occurs at the level of thick filaments.
3) Which figure(s) demonstrates smooth muscle?
a. Figure A&C
b. Figure A&D
c. Figure B&C
d. Figure B&D
4) The structure at the pointer is composed of all of the following except:
a. desmosomes
b. tight junctions
c. gap junctions
d. fascia adherens
5) The structure at the pointer is a:
a. site of calcium-binding protein (calsequestrin) and a Ca2+-activated-ATPase.
b. site of glycogen storage and degradation to glucose
c. site from while Ca2+ is passively transported during muscle relaxation
d. site into which calcium is released during muscle relaxation
e. form of rough endoplasmic reticulum
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Histology SSN October 25, 2004
MUSCLE ANSWERS
1) a. This is skeletal muscle. A marks the A band and B marks the I band. The A
band consists of myosin and some actin that overlaps the myosin. Thick
filaments of myosin anchor at the M line, which is found in the middle of the
H zone. The figure below demonstrates how the length of the A band stays the
same during contraction, while the I band gets shorter.
2) d. This is cardiac muscle. The cells are electronically coupled via gap junctions,
thus allowing contractile signals to pass from cell to cell and allowing cardiac
muscle to behave as a syncytium. T tubules are used for calcium delivery in
both skeletal and cardiac muscle. Smooth muscle cells have no T tubule
system. Cardiac muscle is not capable of regeneration (mature cardiac muscle
cells do not divide). If injury to cardiac muscle tissue leads to death of cells,
fibrous connective tissue forms (scarring) with consequent loss of cardiac
function. Since tropomyosin and troponin wind around the actin filaments,
calcium regulation does not occur at the level of the thick filament.
3) b. A is smooth muscle and B is skeletal muscle (from the esophagus). C is an EM
of skeletal muscle and D is an EM of smooth muscle. Skeletal muscle has
striations whereas smooth muscle does not. The EM in Figure C shows a
higher magnification of the H band, A band, I band, and Z line in a striated
muscle fiber. Figure C also shows a triad of T tubules and flanking terminal
cisternae, which is only found in skeletal muscle. Whereas skeletal muscle has
multiple, peripherally-located nuclei, smooth muscle has a single, centrally
located nucleus, as seen in Figure D.
4) b. Pointer is at an intercalated disc. An intercalated disc does not have tight
junctions. The desmosomes are on both the transverse and lateral components
of the intercalated disc, while the fasciae adherens are on the transverse. Note
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Histology SSN October 25, 2004
from Dr. Rothman: Gap junctions can also be a component of the transverse
portion of the disc but they are not as prevalent as on the lateral.
5) a. This is an EM of skeletal muscle. The pointer is at the sarcoplasmic reticulum.
The sarcoplasmic reticulum (SR) provides a mechanism for the muscle cell to
regulate the concentration of cytosolic calcium. It is a modified smooth
endoplasmic reticulum that serves alternatively as a storage site and a source
of cellular calcium. Calcium is actively transported from the cytosol to the SR
through the activity of a Ca2+-dependent ATPase. Calsequestrin is a calcium-
binding protein found in the SR that, as it’s name implies, functions in the
sequestion of calcium. Calcium is released from the SR during muscle
contraction and stored during relaxation. Glycogen is stored as particles or
droplets in the cytoplasm, which contains the enzymes required for the
synthesis and breakdown of glycogen.
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