Cell and tissue organization

1. 1 /
MUSCLE TISSUE
Skeletal, Cardiac and Smooth Muscle

2. Muscle Tissue: General Protperties
 One of 4 basic tissue types
along with epithelial,
connective, and nerve tissue
 3 types of muscle:
 Skeletal: mostly attached to
skeleton; vouluntary
 Cardiac: in heart; involuntary
 Smooth: in visceral organs;
involuntary
 Made up specialised
elongated cells called
myofibres or muscle
fibres; capable of
contraction
 Contraction results from
conversion of chemical
energy in ATP into
mechanical energy
2 /

3. Skeletal Muscle: Striated and Voluntary
 Myofibres form long
multinucleated syncytium,
grouped in bundles
ensheathed in connective
tissue membranes
 Extend from site of origin to
their insertion
 Connective tissue sheaths
continuous with tendons;at
myotendinous junction
 Connective tissue sheaths:
 Epimysium: dense
connective tissue; entire
muscle
 Perimysium: muscle
bundles (fascicles)
 Endomysium: delicate
reticular fibres and matrix;
individual fibres

4. Connective
Tissue
Coverings
 Endomysium:
surrounds individual
fibres
 Perimysium:
Surrounds fascicles
 Epimysium: surrounds
entire muscle
 Coverings continuous
with tendons

5. Skeletal Myocytes

6. Skeletal Myofibres:
Characteristics
 Formed from fusion of
embryonic myoblasts into
multinucleated myotube
 Myotube matures into a
long muscle fibre (diameter,
10-100 µm; length, several
cm.)
 Cell membrane called
sarcolemma surrounded by
basement membrane and
satellite cells
 Sarcolemma forms tubular
invaginations into cell
cytoplasm (sarcoplasm);
transverse (T) tubules
 T tubules associated with
sarcoplasmic reticulm
(SR); an intracellular Ca2+
store
 Muscle action potential
spreads deep into cell along
T tubules, and cause
release of Ca2+ from SR

7. Skeletal Myofibres:
Characteristics(2)
 80% sarcoplasm occupied
by myofibrils surrounded by
mitochondria; sarcosomes
 Myofibrils contain thick and
thin myofilaments;
contractile proteins
 Thick filaments (15 nm
width) contain myosin
 Thin filaments (7 nm width)
contain actin
 Thin filaments insert into Z-
disk and alternate with
thick filaments
 Sarcomere: alternating
thick and thin filaments is
basic contractile unit of
striated muscle
 Sarcomere extends
between 2 successive Z
disks

8. Sarcosomes,
T tubules, SR
Sarcosome:
Myofibril bundle
surrounded by
mitochondria and
SR network
T tubule:
invagination of
sarcolemma
SR: Network of
Ca2+ containing
tubules

10. Skeletal Myofibres:
Characteristics(3)
 Banding pattern observed
under microscope in cardiac
and skeletal muscle due to
alternating arrangement of
thin and thick filaments
(striated)
 A (dark) band: region of
sarcomere with thick
filaments
 I (light) band: region with
thin filaments
 H band (zone): region of A
band with thick filaments
only; contains enzyme
creatine kinase
 M line: line in H zone due to
bridges and filaments linking
thick filaments
 Z disk: borders sarcomeres
and provide attachment for
filaments

12. Banding in
Striated
Muscle
 Z disc interval is
sarcomere
 I band: thin
filaments
 A band: thin and
thick filaments
 H zone: area of
thick filaments
only

13. Composition of Thin and Thick
Myofilaments
 Thin filaments:
 (1) F-actin: double-
stranded actin
twisted in a spiral;
inserts into Z disk
 (2) Tropomyosin:
lies in a groove
formed by actin helix
 (3) Troponin:
complex of 3
proteins; T
(tropomyosin), I
(actin) and C
(Ca2+).

15. Composition of Thin and Thick
Myofilaments(2)
 Thick filaments:
 Main component is
myosin: 4 light chains
and 2 heavy chains
 Myosin is a motor
protein; uses ATPase
activity to generate
movement
 Binds to F-actin in a
reversible manner
 Attached to Z disks by
protein titin

16. Composition of Thin and Thick
Myofilaments(3)
 Other molecules
present in
muscle:
 Nebulin:
associated with
actin; stabilises
 Desmin: links
and stabilises
myofibrils and
links them to
sarcolema via
interaction with
dystrophin-
associated
complex

17. Neuromuscular Junction (NMJ)
 Junction b/n muscle
fibre and motor nerve
 Made up of membrane
of muscle, membrane
of synaptic bouton
and nanometer cleft
 Chemical released at
NMJ: acetylcholine
ACh
 Motor unit: a motor
nerve and all muscle
fibre innervated

18. Motor Unit

19. Mechanism of Muscle
Contraction
 3 important facts about contraction:
1. Length of myofilaments does not change (e.g. A band length
does not change)
2. Muscle shortening occurs due to sliding of thin filaments
over thick filaments (I & H band width narrow): sliding
filament theory
3. Force of contraction is generated by ATP-powered
conformational changes of myosin proteins
 Maintenance of steady ATP levels provided by creatine
phosphate

20. Creatine Phosphate Replenishes ATP
Levels during Muscle Contraction

21. Contraction Mechanism
 Excitation of muscle fibre at NMJ releases acetylcholine
(ACh)
 ACh induces muscle action potential, which spreads along
sarcolemma and T tubules
 Muscle action potential causes release of Ca2+ from SR
 Ca2+ binds troponin, enabling displacement of tropomyosin
and exposure of myosin binding sites on actin
 Myosin binds and undergoes changes that generates the
force for filament sliding

22. Muscle Disorders: Synaptic
Transmission

23. Muscle Disorders

24. Cardiac Muscle (Cardiocytes)
 Branched; 85-100 µm long, 15 µm wide
 Single, centrally-located nucleus
 Larger T tubules than in skeletal myocytes
 SR not as extensive as skeletal myocytes
 More abundant mitochondria
 Cardiocytes joined end-to-end by junctional complexes;
intercalated disks
 Intercalated disks contain gap junctions: enable signals to
spread through out large regions to contract simultaneously

25. Skeletal and Cardiac Muscle Compared

26. The Heart as a Pump

27. Smooth Muscle
 Found in viscera: wall of gut, bile ducts, ureters, bladder,
respiratory tract, uterus, blood vessels
 Cells are spindle-shaped, tapering with central nucleus
 Myofilaments not organised into sarcomeres; no striations
 Have caveolae in place of T tubules, transmit signals to a
small SR
 Linked to one another by gap junctions, enabling
synchronous contraction
 Lack troponin, dependent on another Ca2+ sensor,
calmodulin

28. Smooth Muscle in Long and
Tranverse Sections

29. Smooth Muscle: Caveolae and
Arrangement of Myofibrils