Connective tissue
MUSCLE TISSUE
Part 1
General features of MT
• Develop from mesoderm
• Many cells, less intercellular matrix
• Function – contraction (shortening)
Types of MT
• Skeletal
(striated, voluntary)
• Cardiac
(striated, involuntary)
• Smooth
(nonstriated,
involuntary)
Structure of skeletal muscle
• Unit of SM – muscle cell (fiber)
• Muscle fibers form muscle fascicle
• Muscle fascicles form muscle
Connective tissue of skeletal muscle
• Epimysium (covers muscle from the outside)
• Perimysium (surrounds muscle fascicles)
• Endomysium (surrounds muscle fibers)
Connective tissue of skeletal muscle
Black arrows - perimysium; green arrows - endomysium
Terminology
Sarx (greek) - meat
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Sarcolemma – plasma membrane of muscle fiber
Sarcoplasm – cytoplasm of muscle fiber
Sarcoplasmic reticulum - SER of muscle fiber
Sarcomere – structural unit of myofibril
Muscle fiber (Cell)
 Consist of
myofibrils
 Myofibrils consist
of myofilaments:
actin and myosin
 Many nuclei on
the periphery
(syncytium)
Skeletal muscle fiber
structure
Muscle fiber
Myofibrils
Myofilaments
Myofilaments
Actin
(thin filament)
• Responsible for band like
structure of skeletal muscle
• Interaction between actin
and myosin results in
contraction
Myosin
(thick filament)
Thin actin filaments
• Composed of 3 major proteins:
- F (fibrous) actin
- Tropomyosin
- Troponin
Two strands of F actin make a double helix
F actin consists of G (globular) actin monomers
• Tropomyosin – winds along the groove of F actin double helix
• Troponin composed of 3 subunits:
-Tn-I: binds to actin
-Tn-T: binds troponins to tropomyosin
-Tn-C: binds Calcium ions
Actin filament
Thick myosin filaments
• Consist of myosin protein
• Has a golf club shape
• Two heavy myosin chains
make the tail
• Two heads that extend
laterally
• Hinge region in between
Skeletal muscle fiber
structure
Muscle fiber
Myofibrils
Myofilaments
Sarcomere
• Part of myofibril between two Z-lines
• Z-line – connection of actin filaments in the
middle of I-band
• M-line – connection of myosin filaments in the
middle of the sarcomere
• A-band (anisotropic, dark) - contains both myosin and
actin filaments
• I-band (isotropic, light) – contains only thin actin
filaments
• H-band – contains only thick myosin filaments
Sarcoplasmic reticulum
• Network of tubules and cylinders surrounding each myofibril
• Resembles smooth endoplasmic reticulum
• Stores Calcium ions
• Transverse tubule (T-tubule) – invagination of the sarcolemma at the level of I- and
A-band junction, surrounding the myofibril
• Terminal cisterns - extensions of sarcoplasmic reticulum on the sides of T-tubules
• Triade of skeletal muscle = 1 Т-tubule + 2 terminal cisterns of sarcoplasmic
reticulum
Motor unit = efferent (motor)
neuron + all muscle fibers
innervated by it
The more precise the work of
the muscle, the more motor
units in it (and less muscle
fibers per one motoneuron)
Neuromuscular junction (synaps)
• Terminal bud of
motor neuron
(presynaptic
membrane)
• Sarcolemma of
muscle fiber
(postsynaptic
membrane)
• Synaptic cleft
between them
Function:
Nervous impulse
transmitting from
neuron to muscle
fiber
Impulse conduction
• Release of
neurotransmitter
(acetylcholine) into the
synaptic cleft
• Binding of acetylcholine
with receptors on the
postsynaptic membrane
leads to changing of the
membrane charge –
firing of the Action
potential
Skeletal muscle contraction
1. Action potential is spread along the sarcolemma onto the T-tubules and than
terminal cisterns
2.Са++ ions are realized into the sarcoplasm
Skeletal muscle contraction
3. Са++ binds to troponin С
4. Movement of TnI away from the myosin-binding sites on the actin filaments
Skeletal muscle contraction
5. Binding of the myosin heads to the actin filaments
6. Activation of myosin ATPasa producing energy and
ADP
7. Power Stroke: conformational change in myosin at
hinge region pulling attached actin filaments toward
the center of the sarcomere
8. Shortening of the sarcomeres (I bands narrow, A
bands do not)
9. Shortening of the entire muscle fiber
10. New ATP molecule binds to myosin and it gets
ready again to bind to actin (step 5)
11. End of neural stimulation – Са++ sequestration
back to the sarcoplasmic reticulum, TnI returns to its
position covering the tropomyosin binding sites
Skeletal muscle contraction
Types of skeletal muscle fibers
Cardiac muscle
• Consist of cardiomyocytes
• Striated
• Involuntary
• Diads instead of triades
(1 T-tubule + 1 terminal cistern)
• Conducting system
Cardiomyocytes
• Branched cells
• One central nucleus
• Intercalated disc –
connections of
cardiomyocytes allowing
rapid transmission of
impulses
Three types of cardiomyocytes
• Typical cardiomyocytes – predominant type,
function - contraction
• Atypical cardiomyocytes – less myofibrils,
function – impulse generation and conduction
• Secretory cardiomyocytes – located in right
atrium, secrete hormone atrial natriuretic factor,
which causes sodium and water loss, reducing
blood pressure
Conducting system
of the heart
•impulse generation
and transmission
•Autonomous nervous
fibers only slow down
or speed up the
intrinsic beat
Purkinje cells
Intercalated discs
• Specialized junctional
complexes
•Consist of:
- desmosomes
- gap junctions
Smooth muscle
• Spindle shaped cells with gap junctions
• Involuntary
• One central nucleus
• No sarcomeres
• Dense bodies on the
membrane –
analogous to the Z
lines
• Thin filaments (actin)
are attached to
dense bodies
• Thick filaments
(myosin) are
unstable and
dispersed in the
cytoplasm
• Intermediate
filaments (desmin,
vimentin)
Smooth muscle contraction
1. Influx of Ca into the
cytoplasm
2. Binding of Ca with
Calmodulin (instead of
troponin)
3. Ca-Calmodulin complex
activates myosin light
chain kinase
4. Phosphorilation and
activation of myosin
5. Myosin binds to actin =
initiation of contraction
• Sliding actin filaments pull dense bodies close together, contracting the cell
• Contraction of smooth muscle cells is slow and wavelike
Smooth muscle
Transverse section
Longitudinal section
Regeneration
• Skeletal muscle fibers – regeneration is
possible due to satellite cells
• Smooth muscle cells – regeneration is
possible due to the division of mature
myocytes
• Cardiomyocytes – do not regenerate, only
intracellular renewal is possible
Satellite cells
• Stem cells of skeletal muscle
• Lay between the sarcolemma and
the basal lamina
• Activated in case of injury of
muscle or excessive exercising
Skeletal muscle regeneration
Smooth muscle regeneration
Cardiomyocytes regeneration???
Thank you for attention