Skeletal Muscle histology

1. Maha Hammady,MBBCh,MMSc

2. SKELETAL MUSCLE
By/ Maha Hammady Hemdan
MBBCh- MMSc - Faculty of Medicine, Alexandria University. 2015

3. THE MYOTENDINOUS
JUNCTION (MTJ)

4. THE MYOTENDINOUS JUNCTION (MTJ)
4
The Myotendinous Junction (MTJ)
It is the region where muscle fibers interface with tendons.
The transmission of force from muscle contraction to the
skeleton through this junction is essential for movement.
Histologically, the myotendinous junction exhibits
distinctive features:
• At the cellular level, skeletal muscle fibers become
tapered.
• presence of finger-like projections known as junctional
folds or digitations.
• Collagen fibers of the tendon penetrate deep into these
infoldings and become continuous with the reticular fibers
of the endomysium. forming a continuous structural link.
These structures increase the surface area of the junction,
facilitating a more efficient transmission of force. The
folds also play a role in distributing stress evenly,
preventing excessive concentration of force at specific
points.
The size and number of folds are increased as a response
to heavy training and reduced during inactivity.

5. THE MYOTENDINOUS JUNCTION (MTJ)
5
Dissection of the gastrocnemius–soleus MTJ
https://www.physio-
pedia.com/Myotendinous_Junction

6. THE MYOTENDINOUS JUNCTION (MTJ)
6
actF: Actin filament from last Z-band;
Bm: Basement membrane;
C: Collagen fibers;
Flp: Finger-like process;
M: Muscle;
Sm: Sar-comer;
Diagram of adult MTJ.
1-The collagen fibers, produced by tenocytes, are anchored
perpendicularly to the sarcolemma of the finger-like processes.
2-The sub-sarcolemmaa densities present at the tips of finger-like
processes correspond to the muscle side of MTJ. These densities
result from the massive recruitment of protein linkage-complexes
that connect actin filaments from the last Z-band to the tendinous
extracellular matrix.
https://www.ncbi.nlm.nih.gov/pmc/articles/PMC3666507/
Ssd: Sub-sarolemmal density;
StC: Satellite cell;
T: Tendon;
Tc: Tenocyte;
Zb: Z-band.

7. THE MYOTENDINOUS JUNCTION (MTJ)
7

8. THE MYOTENDINOUS JUNCTION (MTJ)
8
(c,d) Sirius red staining of the myotendinous junction of the
palmaris brevis muscle. (c) Note the clear endings of the individual
muscle fibers (green) at collagen fiber bundles (red), marked by
arrowheads. (d) Higher magnification shows the parallel finger-like
protrusions of the muscle fibers (asterisks) toward the tendon-like
dense collagen fibers
https://onlinelibrary.wiley.com/doi/10.1111/joa.13419

9. THE MYOTENDINOUS JUNCTION (MTJ)
9
FIGURE 1. The MTJ of a human
semitendinosus muscle fiber
viewed with EM. The protrusions
(arrows) from the tendon (T) into
the muscle fiber (M) increase the
contact area between the muscle
and tendon. Scale bar is 10 μm.
https://www.frontiersin.org/article
s/10.3389/fphys.2021.635561/full
#:~:text=should%20be%20studie
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10. INNERVATION OF
SKELETAL MUSCLE

11. INNERVATION OF SKELETAL MUSCLE
11
Innervation of Skeletal Muscle
Each skeletal muscle :
1-motor nerve functions in eliciting contraction
2-the sensory fibers pass to muscle spindles and Golgi tendon organs
Additionally:
autonomic fibers supply the vascular elements of skeletal muscle.

12. INNERVATION OF SKELETAL MUSCLE
12
Innervation of Skeletal Muscle
Motor unit:
• A motor unit is a functional unit of the neuromuscular system, consisting of a
motor neuron and the muscle fibers it innervates.
• The muscle fibers of a single motor unit contract in unison and follow the all-or-
none law of muscle contraction.

13. INNERVATION OF
SKELETAL MUSCLE
1-IMPULSE TRANSMISSION AT
THE NEUROMUSCULAR
JUNCTION
The neuromuscular junction (NMJ) is a highly specialized synapse between
a motor neuron nerve terminal and its muscle fiber that are responsible for
converting electrical impulses generated by the motor neuron into electrical
activity in the muscle fibers.
The NMJs are very small structures (∼30 μm long) compared to the length
of the muscle fibers they innervate which can be anything from less than a
cm (e.g., intercostal muscle) to more than 20 cm (e.g., sartorius, the long
muscle of the thigh). Typically, each skeletal muscle fibers has a single NMJ
where the motor axon joins the muscle fiber.

14. NEUROMUSCULAR JUNCTION
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Neuromuscular Junction

15. NEUROMUSCULAR JUNCTION
15
Motor fibers are
myelinated axons of α-
motor neurons that pass in
the connective tissue of the
muscle. The axon arborizes,
eventually losing its myelin
sheath (but not its Schwann
cells). The terminal of each
arborized twig becomes
dilated and overlies the cell
membrane of individual
muscle fibers. Each of these
muscle–nerve junctions,
known as a neuromuscular
junction , it is composed of
1-an axon terminal,
2-a synaptic cleft
3- a modified muscle cell
membrane
Neuromuscular Junction

16. NEUROMUSCULAR JUNCTION
The axon terminal is covered by Schwann cells on its entire surface except on
its presynaptic membrane( the surface facing the postsynaptic membrane)
The axon terminal houses mitochondria, SER, and as many as 300,000 synaptic
vesicles (each 40 to 50 nm in diameter) containing the neurotransmitter
acetylcholine. The nerve terminal has complex machinery in place to allow the
synthesis, exocytosis and recycling of these synaptic vesicles
16
Neuromuscular Junction
1- Axon Terminal (Synaptic Bouton)

17. NEUROMUSCULAR JUNCTION
The sarcolemma at the postsynaptic membrane is modified, forming a depression, known
as the primary synaptic cleft, occupied by the axon terminal. The synaptic cleft is the gap
between the presynaptic terminal and the postsynaptic muscle membrane, which is filled with
a specialized form of extracellular matrix called synaptic basal lamina. This matrix is crucial
for the alignment, organization and structural integrity of the NMJ. In particular, it is of
relevance that the enzyme acetylcholinesterase (AChE), which terminates synaptic
transmission by breaking down acetylcholine, is attached to the basal lamina
.
17
Neuromuscular Junction
2- Synaptic Cleft

18. NEUROMUSCULAR JUNCTION
• Opening into the primary synaptic clefts are numerous tubular invaginations known as
junctional folds (secondary synaptic clefts), a further modification of the sarcolemma.
They increase the overall surface of the postsynaptic membrane
• The sarcoplasm in the vicinity of the secondary synaptic cleft is rich in glycogen, nuclei,
ribosomes, and mitochondria.
18
Neuromuscular Junction
3- Postsynaptic Membrane
Electron microscopy image of
the NMJ. The presynaptic
nerve terminal is filled with
synaptic vesicles containing
acetylcholine (*). The
postsynaptic muscle
membrane exhibits a high
degree of folding which
extends into the muscle
sarcoplasm (arrows) in order
to increase the total endplate
surface. The NMJ is covered
by terminal Schwann cells.
https://www.frontiersin.org/arti
cles/10.3389/fnmol.2020.6109
64/full

19. NEUROMUSCULAR JUNCTION
19
Neuromuscular Junction
Scanning electron micrograph of a
neuromuscular junction from the tongue of
a cat(×2315). Arrows indicate striations.
MJ, Neuromuscular junction; N, nerve fiber.

20. NEUROMUSCULAR JUNCTION
20
Stimulus transmission across a
synaptic cleft
involves the following sequence of
events:
1.A stimulus, traveling along the axon,
depolarizes the membrane of the axon
terminal, thus opening the voltage-
gated calcium channels
2. The influx of calcium ions into the
axon terminal results in the fusion of
about 120 synaptic vesicles per nerve
impulse with the axon terminal’s
membrane (presynaptic membrane)
and subsequent release of
acetylcholine (along with
proteoglycans and ATP) into the
primary synaptic cleft.
3. The neurotransmitter acetylcholine
(ligand) is liberated in large quantities
from the nerve terminal.
Neuromuscular Transmission

21. NEUROMUSCULAR JUNCTION
21
4. Acetylcholine then diffuses
across the synaptic cleft and binds
to postsynaptic acetylcholine
receptors in the muscle cell
membrane. These receptors,
located in the vicinity of the
presynaptic active sites, are
transmitter-gated sodium ion
channels, which open in response
to the binding of acetylcholine.
The resulting ion influx leads to
depolarization of the muscle cell
membrane and creation of an
action potential .
5. The impulse generated spreads
quickly throughout the muscle
fiber via the system of T tubules
(see previous section on muscle
contraction and relaxation),
initiating muscle contraction.
Neuromuscular Transmission

22. NEUROMUSCULAR JUNCTION
22
To prevent a single stimulus from
eliciting multiple responses,
acetylcholinesterase, an enzyme
located in the external lamina lining
the primary and secondary synaptic
clefts, degrades acetylcholine into
acetate and choline, thus permitting
the reestablishment of the resting
potential.
Neuromuscular Transmission

23. NEUROMUSCULAR JUNCTION
23
Botulism is usually caused by ingestion of improperly
preserved canned foods. The toxin, produced by the
microbe Clostridium botulinum, interferes with the
release of acetylcholine, with resultant muscle paralysis
and,without treatment, death.
Myasthenia gravis is an autoimmune disease in
which autoantibodies attach to acetylcholine receptors,
blocking their availability to acetylcholine. Receptors
thus inactivated are endocytosed and replaced by new
receptors, which are also inactivated by the
autoantibodies. Thus, the number of locations for the
initiation of muscle depolarization is reduced, and the
skeletal muscles (including the diaphragm) weaken
gradually.
Clinical Correlations

24. NEUROMUSCULAR JUNCTION
24
Certain neurotoxins, such as the bungarotoxin of some
poisonous snakes, also bind to acetylcholine receptors,
causing paralysis and eventual death due to respiratory
compromise.
Botulinum Toxin Type A (Botox Cosmetic) is an
inhibitor of acetylcholine release by motor fibers that
cause skeletal muscle contraction. This toxin, produced
by Clostridium botulinum, when injected into particular
muscles, specifically inhibits the contraction of that
muscle. For cosmetic purposes, the procerus and
corrugator muscles are usually injected with Botox, thus
diminishing the frown lines that the contraction of those
facial muscles otherwise produces and, by eradicating
the “wrinkles,” making the face appear smoother and
younger.
Clinical Correlations

25. INNERVATION OF
SKELETAL MUSCLE
2- SENSORY IMPULSES
THROUGH MUSCLE SPINDLES
AND GOLGI TENDON ORGANS
The neural control of muscle function requires not only the capability of
inducing or inhibiting muscle contraction but also the ability to monitor the
status of the muscle and its tendon during muscle activity. This monitoring is
performed by two types of sensory receptors:
• Muscle spindles, which provide feedback about the changes in muscle length
as well as the rate of alteration in muscle length
• Golgi tendon organs, which monitor the tension as well as the rate at which
the tension is being produced during movement.

26. MUSCLE SPINDLE

27. 27
Muscle Spindle
MUSCLE SPINDLE
When muscle is stretched, it normally
undergoes reflex
contraction, or stretch reflex. This
proprioceptive
response is initiated by the muscle
spindle, an encapsulated sensory
receptor located among, and in parallel
with, the muscle cells Each muscle
spindle
is composed of 8 to 10 elongated,
narrow, very small,
modified muscle cells called intrafusal
fibers, surrounded by the fluid-
containing periaxial space, which, in
turn, is enclosed by the capsule. The
connective tissue elements of the
capsule are continuous with the collagen
fibers of the perimysium and
endomysium. The skeletal muscle fibers
surrounding the muscle spindle are
unremarkable and are called extrafusal
fibers. Intrafusal fibers are of two types:
nuclear bag fibers and the more
numerous, thinner nuclear chain fibers.

28. 28
Muscle Spindle
MUSCLE SPINDLE
Nuclear bag Nuclear chain
Number 2-4/spindle 6-8/spindle
Length Longer Shorter
Width Wider thinner
Nuclei many aggregated nuclei are present
in the central non-contractile
region
Chain of nuclei are present in the
central non-contractile region
Sensory :
1ry afferent
(annulospiral)
-Myelinated
-Thick
-Encircle the central noncontractile part
-Present in both types
Sensory :
2ry afferent (flower
spray ending)
Not present Synapse at the peripheral contractile
part of the fibers
Motor/efferent from
anterior horn cell
-γ-Myelinated
-Synapse at the peripheral contractile part of the fibers
-Present in both types
-regulates the sensitivity of muscle spindles

29. 29
Muscle Spindle
MUSCLE SPINDLE
Muscle spindle in longitudinal section, located between normal or
extrafusal muscle fibers. Two types of fiber are distinguished in its
interior: nuclear chain fibers and nuclear bag fibers.

30. 30
Muscle Spindle
MUSCLE SPINDLE
Muscle Spindle with Intrafusal Fibers-Gomori trichrome stain
Examples of semi-thin (1-μm-thick) transverse sections through the
central part of a cat muscle spindle.
https://onlinelibrary.wiley.com/doi/full/10.1111/joa.12297

31. MUSCLE SPINDLE
31
Muscle Spindle
Light microscopic views of muscle spindles. Transverse section showing 2
spindles (arrow) arranged side-by side and forming a paired complex. Their
outer capsules are fused but their inner contents remain separate and distinct.
Each spindle contains several intrafusal fibers (arrowheads).
The scale bar represents 100 mm.
doi:10.1371/journal.pone.0051538.g008

32. GOLGI TENDON
ORGAN

33. GOLGI TENDON ORGAN
33
Golgi Tendon Organ
• Golgi tendon organs, also called neurotendinous spindles, are cylindrical structures about 1 mm in length and 0.1 mm in
diameter.
• They are located at the juncture of a muscle with its tendon and are positioned in series with the muscle fibers.
• Golgi tendon organs are composed of wavy collagen fibers and the nonmyelinated free nerve endings in the interstices between the
collagen fibers.
• When the muscle contracts, it places tensile forces on the collagen fibers, straightening them, with a consequent compression and
firing of the Golgi tendon organs monitor the force of muscle contraction, whereas muscle spindles monitor the stretching of the
muscle in which they are located. These two sensory organs act in concert to integrate spinal reflex systems. entwined nerve
endings. The rate of firing is directly related to the amount of tension placed on the tendon. When a muscle undergoes strenuous
contraction, it may generate a great amount of force. To protect the muscle, bone, and tendon, Golgi tendon organs provide an
inhibitory feedback to the α-efferent neurons (motoneurons) of the muscle, resulting in relaxation of the contracting tendon’s
muscle.
• Thus, the Golgi tendon organs monitor the force of muscle contraction, whereas muscle spindles monitor the stretching of the
muscle in which they are located. These two sensory organs act in concert to integrate spinal reflex systems.

34. GOLGI TENDON ORGAN
34
Rabbit, formic acid-gold chloride
https://www.anatomyatlases.org/MicroscopicAnatomy/Section06/Plate
06122.shtml
Golgi Tendon Organ

35. GOLGI TENDON ORGAN
35
Camillo Golgi
Winner of Nobel Prize for Physiology
or Medicine, Camillo Golgi,
discovered (one of his discoveries) the
Golgi Tendon Organ. The
contributions of Camillo Golgi (1843–
1926) to the study of the nervous
system are a pillar of modern
neuroscience. The Golgi impregnation
first offered to microscopic studies
individual neurons and glial cells in
their entirety, and has therefore laid
the foundation of neurohistology and
neuroanatomy, opening a new era in
neuroscience
https://synapse.koreamed.org/articles/
1145503?viewtype=pubreader
https://www.frontiersin.org/articles/
10.3389/fnana.2019.00003/full

36. GOLGI TENDON ORGAN
36
Camillo Golgi
Camillo Golgi at his
laboratory bench in the
Institute of General
Pathology of the
University of Pavia
around 1920. Reproduced
with permission of the
University Museum
System of Pavia.
https://www.frontiersin.
org/articles/10.3389/fn
ana.2019.00003/full

37. GOLGI TENDON ORGAN
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Camillo Golgi
Photographs of some of the slides.
The labels in (A,B) are signed by
Golgi with the indication of the year
1899; (C) shows an example of a
wooden slide; the label in (D) has
the indication “Cajal”; the label
in (E) has a comment signed by
Dominick Purpura in 1973.
https://www.frontiersin.org/article
s/10.3389/fnana.2019.00003/full

38. GOLGI TENDON ORGAN
38
Camillo Golgi
Equipment of the laboratory of Camillo Golgi in the years that followed his appointment as Professor of
Histology at the University of Pavia in 1876, when Golgi's studies focused on the nervous system. The equipment
is on display at the Golgi museum (Berzero et al., 2018). (A) Microtome by the German anatomist and
physiologist Gustav Fritsch (1838–1927), bought in 1878. (B) Microtome by the French histologist and anatomist
Louis Ranvier (1835–1922) to cut by hand, with the razor shown in the figure, sections sufficiently thin for
microscopic examination from tissue blocks fixed to the cylinder; this microtome was bought in
1879. (C) Hartnack-Prazmowski microscope, bought in 1877 from the firm Hartnack had established in Paris in
partnership with the Polish mathematician and astronomer Adam Prazmowski (1821–1885). (D) Microscope by
Edmund Hartnack (1826–1891), renowned German microscope maker, bought in 1876.
https://www.frontiersin.org/articles/10.3389/fnana.2019.00003/full

39. THANK YOU
39