Skeletal muscle cells (myocytes) form long, cylindrical, multinucleated fibers that contract to generate movement. Each fiber contains bundled myofibrils composed of actin and myosin filaments. Nerve impulses trigger calcium release from the sarcoplasmic reticulum, allowing myosin heads to attach to actin and generate force as they detach due to ATP hydrolysis, shortening the sarcomere. Muscle relaxation occurs when calcium is resequestered in the sarcoplasmic reticulum, detaching actin and myosin.

1. Ms. O.HARA M.S.
Muscular TissuesMuscular Tissues

2. Contractile cells
generate motile forces through contraction.
Contractile proteins are MYOSIN and ACTIN
1.Myoepithelial cells* - in secretory glands
function to expel secretions from glandular acini.
1.Pericytes* - surround blood vessels.
2.Myofibroblasts* - have a contractile function as
well as a role in secretion of extracellular matrix.
 inconspicuous in normal tissues
becomes dominant cell during scar formation in
tissue repair.
Specialized Contractile cells - single cell contractile units

3. Sweat
gland
Sweat gland
Blood vessel
Myofibroblast
Specialized
Contractile cells

4. Muscle cells  myofibers /
myocytes
contractile cells that function by forming
multicellular contractile units.
3 types of muscles cells – based on structure,
contractile properties, and control mechanisms
1.Skeletal muscle – for movement of skeleton, eye
globe, and tongue
2.Smooth muscle - visceral muscle (blood
vessels, GIT, uterus and urinary bladder)
3.Cardiac muscle - rhythmic contractions of heart.

6. Skeletal Muscle
Referred as voluntary muscle.
The primary function - to move the skeleton.
The muscle contractions required for movement
also produce heat, which contributes to the
maintenance of a constant body temperature.
Special terminology:
Plasma membrane – sarcolemma
Cytoplasm – sarcoplasm
Endoplasmic reticulum – sarcoplasmic
reticulum

7. Epimysium
Perimysium
Endomysium
• A muscle is composed of
FASCICLES of fibers,
wrapped by CT-
EPIMYSIUM.
• Each fascicle is composed of
MYOFIBERS, the cells of
skeletal muscle.
• Each fascicle is
wrapped by CT –
PERIMYSIUM
• Each myofiber is
wrapped by CT –
ENDOMYSIUM
SKELETAL MUSCLES are subdivided by CT

8. Connective Tissue wrappings
Epimysium - dense, irreg. CT; collagen;
binds the fascicles into a single muscle.
Perimysium – less dense, irreg. CT; collagen;
surrounding individual fasciculi through which larger
vessels and nerves run.
Endomysium - barely visible; delicate support;
mainly of reticulin fibres and a small amount of
collagen
surrounding each muscle fiber.
conveys numerous small blood vessels, lymphatics
and nerves throughout the muscle.

10. Skeletal muscle fibers (muscle
cells/myofiber)
Long
Cylindrical
multinucleated cells
with peripheral nuclei
Multiple nuclei – due to the fusion of muscle cell
precursor myoblasts during the embryonic
development.

11. Each myofiber contains MYOFIBRILS that extend the length of the fiber
• Orderly arrays of filamentous subunits

12. Myofibrils are made of MYOFILAMENTS formed by the contractile proteins
• Actin (thin) and Myosin (thick)

14. IMPORTANT role of CT in muscle
- to mechanically transmit the forces generated by
contracting muscle cells, because in most instances,
individual muscle cells do not extend from one end of a
muscle to the other.
Two orientations when cut:
- transverse and longitudinal muscle fibers

18. I A Z
N
Mf
S

19. Skeletal muscles are “striated” muscles due to
distinct cross-striation patterns.
In LM, light I bands and dark A bands are in each
muscle fiber.
Light bands are I bands ; formed by thin actin
filaments
Dark bands are A bands ; located in the middle of
sarcomere.
A bands are formed by overlapping actin and
myosin filaments.

20. I Band is bisected by Z-line that divide each myofibril into numerous
contractile units called SARCOMERE (2 Z-lines).
Each A band is bisected by the lighter H band and the darker M line
Actin
THIN
Myosin
THICK

21. • As the muscle contracts  the sarcomere shortens  the Z
lines are drawn closer together -- the thick and thin filaments
slide past each other.
• I and H bands shorten, while A bands stay the same.
• SLIDING FILAMENT THEORY

22. Muscles are arranged around the skeleton so
as to bring about a variety of movements.
The two general types of arrangements
-opposing antagonists
• muscles that have opposing or opposite functions.
- cooperative synergists
• muscles with the same function or those that work together to
perform a particular function.
Antagonists

23. The role
of the brain
The contraction of
skeletal muscles
depends on the brain.
(voluntary or
conscious control)
The nerve impulses
for movement come
from the frontal lobes
of the cerebrum.
Motor areas of the frontal lobes generate electrochemical
impulses that travel along motor nerves to muscle fibers,
causing the muscle fibers to contract.
Frontal
lobe

24. Motor Unit - Muscles are innervated by large motor
nerves; supplying electrical impulse into the muscle and
groups of muscle fibers (neuromuscular junctions).
Motor unit
- muscle cells controlled
by one nerve cell.

25. Each muscle fiber has its own motor nerve
ending; the neuromuscular junction is where
the motor neuron terminates on the muscle fiber

27. Neuromuscular Junction

28. Surrounding each
sarcomere in a repeating
fashion are the tubules of
sacroplasmic reticulum
and mitochondria T tubule is
surrounded on
each side by the
expanded
terminal
cisternae of the
SR and form
triads
Calcium ions
are stored in the
sarcoplasmic
reticulum (SR).

30. Myofilament components
1. ACTIN – Thin filament
- composed of two chains of G-actin (globular) that
forms F-actin (filamentous) arranged in helix.

31. • Tropomyosin - stabilizes the troponin and
attaches to actin filament.
• Troponin complex (TnT, TnC, TnI) –
initially covers the F-actin during muscle
rest
• TnT – attaches to tropomyosin
• TnC – (+) receptor for calcium ions
• TnI – inhibits contraction

32. a. The heavy chains of myosin molecules form the core of a thick filament.
b. Structure of a myosin
molecule.
2. MYOSIN – Thick filament

33. H Band

34. Skeletal Muscle
Contraction
Before the arrival of the nerve impulse to the muscle, the
muscle is relaxed and the calcium ions are stored in the
cisternae of the sarcoplasmic reticulum.
In summary, a nerve impulse causes depolarization of a
muscle fiber, and this electrical change enables the
myosin filaments to pull the actin filaments toward the
center of the sarcomere, making the sarcomere shorter.

35. Nerve impulse reaches neuromuscular
junction.

36. Acetylcholine / Ach (neurotransmitter) is released from
motor neuron. Ach binds with receptors in the muscle
membrane opens Na+
Channel to allow sodium to enter.

37. Sodium influx causes the sarcolemma to depolarize,
becoming negative outside and positive inside.
This will generate an action potential in the
sarcolemma.

38. Action potential is propagated along the entire
length of the sarcolemma and transmitted
deep to every myofiber by the network of the
T tubules.
The T tubules bring the action potential to the
interior of the muscle cell.

39. Cisternae of the SR release calcium ions into the individual
sarcomeres and the overlapping thick and thin myofilaments of
the myofiber.
At each triad, the action potential is transmitted from the T tubules
to the sarcoplasmic reticulum (SR) membrane and stimulates the
release of calcium ions.

40. Calcium binds with troponin to move the troponin,
tropomyosin complex
Binding sites in the actin filament are exposed
Myosin head attach to binding sites and create a
power stroke
ATP detaches myosin heads and energizes them
for another contaction
When action potentials cease the muscle stop
contracting

43. SLIDING FILAMENT THEORY
Initiated once calcium ions is released from the
sarcoplasmic reticulum.
Calcium ions activate binding between actin and
myosin that results in their sliding past each other
and muscle contraction.
When the stimulus subsides and the membrane
is no longer stimulated, calcium ions are actively
transported back into and stored in the cisternae
of the sarcoplasmic reticulum, causing muscle
relaxation.

44. Muscle sense
(proprioception)
is the brain’s ability to
know where our
muscles are and what
they are doing,
without our having to
consciously look at
them.
Stretch receptors
(proprioceptors or
muscle spindles)

45. Stretch receptors / muscle
spindles
Detect stretching of muscles and generate
impulses, which enable the brain to create a
mental picture to know where the muscles are
and how they are positioned.
Conscious muscle sense is felt by the parietal
lobes.
Unconscious muscle sense is used by the
cerebellum to coordinate voluntary movements.

46. References
Junquiera LC, Carneiro J. 2005. BASIC
HISTOLOGY : TEXT AND ATLAS 11th
Edition.
McGraw-Hill’s ACCESS MEDICINE.
Young B. 2009. WHEATER’S FUNCTIONAL
HISTOLOGY. 5TH
Edition. UK: Churchill
Livingstone. Distributor: Phils: C & E Publishing,
Inc.

47. Assignment
1. Explain the consequence of TETANUS AND
BOTULISM in skeletal muscle contraction.
 What is BOTOX and how does it work?
1. Describe MYASTHENIA GRAVIS and give its
consequence to the process of muscular
contraction.