Department of Histology and Embryology, P. J. Šafárik University, Medical
Faculty, Košice
MUSCLE TISSUE: Sylabus for foreign students
Author: doc. MVDr. Iveta Domoráková, PhD.
Revised by: prof. MUDr. Eva Mechírová, CSc.
MUSCLE TISSUE
Function: muscle tissue is specialized for contraction
Types of muscle tissue:
1. Striated skeletal muscle
2. Striated cardiac muscle
3. Smooth muscle
STRUCTURE OF MUSCLE IN THE LIGHT MICROSCOPE
1. SKELETAL MUSCLE
Function: contraction - voluntary and rapid
- body movement, muscle tissue in the tongue (speech, mixing of food),
breathing, voice
BASIC MORPHOLOGIC UNIT in LM: multinucleated skeletal muscle fiber with
cross striation. Nuclei are situated below the sarcolemma.
Fig. 1 – longitudinal and transverse section of skeletal muscle fiber
2. CARDIAC MUSCLE
Function: contraction - involuntary; rapid and rhythmic
-
in the heart (myocardium)
BASIC MORPHOLOGIC UNIT in LM: cardiac muscle cell (cardiomyocyte) with
cross striation. Nuclei (1-2) are situated in the centre of the cell. Cardiomyocytes
are connected by intercalated discs.
Fig. 2. – longitudinal and transverse section of cardiomyocytes
3. SMOOTH MUSCLE
Function: contraction is involuntary; weak and slow
- in the wall of hollow organs (stomach, small intestine)
BASIC MORPHOLOGIC UNIT in LM: spindle shaped smooth muscle cell.
Cytoplasm has no cross striation. Oval or rod-like nuclei in the centre.
Fig. 3. – longitudinal and transverse section of smooth muscle cells
Cytoplasm of muscle fibers and muscle cells is eosinophilic.
ORIGIN
Most of muscle tissue develops from mesoderm that gives rise to mesenchymal cells.
• Skeletal muscle develops from paraaxial mesoderm, organized into
myotomes in somites. Muscles of the head develop from mesenchyme of
branchial arches.
• Cardiac muscle develops from cardiogenic mesoderm.
• Smooth muscle develops from splanchnic mesoderm
- except of iris where smooth muscle arises from
neuroectoderm.
Fig. 4 Development of multinucleated muscle fiber from myoblasts.
ORGANISATION OF SKELETAL MUSCLE
Each muscle fiber is surrounded by endomysium - network of reticular fibers, also
contains blood and lymphatic capillaries and nerves.
Groups of muscle fibers (anywhere between 10 to 100 or more) form fascicles
(bundles). Muscle fascicle is surrounded by perimysium - sheath of connective tissue.
Entire muscle is surrounded by epimysium - composed of dense irregular
connective tissue. It is continuous with fascia and other connective tissue wrappings
of muscle including the endomysium and perimysium. It is also continuous with
tendons.
Fig. 5. Organisation of skeletal muscle
SEQUENCE OF ORGANISATION from muscle to molecular structure (Fig.6.)
Muscles are composed of muscle fascicles. They are composed of skeletal
muscle fibers (visible in LM). Muscle fiber contains myofibrils (visible in EM).
Myofibrils create the biggest part of the sarcoplasm. They are oriented longitudinally
with long axis of muscle fiber. Myofilbrils are composed of thin actin filaments
(myofilaments) and thick myosin filaments (myofilaments).
Actin filament on molecular level is composed of: F-actin, troponin complex
(Tn subunits: TnC, TnI, TnT) and tropomyosin. Actin in the sarcomere is bound to
the Z-line by an α-actinin.
Myosin filament is composed of hundreds of myosin molecules (golf stick
shape).
Fig.6. Organisation of skeletal muscle from muscle to molecular structure
Fig. 7. Sarcomere is a functional unit of striated muscles.
Sarcomeres are repeated functional unites visible on the myofibrils in EM.
Sarcomere is present between two Z-lines. Z-line serves for attachment of actin
filaments. I band is formed only by actin filaments. A band is composed of myosin
filaments and partly of actin filaments. H band is present in the centre of A band and
is composed of myosin filaments. Width of H band changes during contraction of
sarcomere. H band is shorter after contraction and longer after relaxation in
dependence how much actin filaments penetrate between myosin filaments.
STRUCTURE OF SKELETAL MUSCLE FIBER in ELECTRON MICROSCOPE
Muscle fiber (Fig. 8) is covered by sarcolemma. Sarcoplasm is filled predominantly
with myofibrils surrounded by sarcoplasmic reticulum (sER). Between myofibrils
are mitochondria – sarcosomes - energy for contraction. Oval nuclei are situated
beneath the sarcolemma. Sarcoplasm contains glycogen granules and pigment
myoglobin.
Fig.8 – Structure of muscle fiber in EM
TRIAD
3)
2 terminal cisternae of sarcoplasmic reticulum +1 T-tubule (invagination of
sarcolemma; encircle each myofibril at the border of A-band and I band).
Function of T-tubules:
Fast transmission of nerve impulses by depolarization of membranes (sarcolemma,
membranes of sarcoplasmic reticulum
Function of sarcoplasmic reticulum: storrage and release of calcium ions necessary for
contraction. Calcium ions are bound to the troponin complex of TnC subunit.
MYOFIBRILS – structural and functional units of muscle fiber
- sarcoplasm contains 80% of myofibrils
-
oriented longitudinally
-
surrounded by SER, mitochondria (2 %)
-
composed of myofilaments (actin, myosin)
In LM and EM are visible alternating isotropic: I-bands (light) and anisotropic: Abands (dark)
SARCOMERES – basic functional units of striated muscle (skeletal, cardiac).
Sarcomere is limited by two Z- lines. Z-line is in the centre of isotropic band. Contains αactinin, protein that binds actin filaments to Z-line.
ACTIN and MYOSIN filaments form myofibril.
ACTIN FILAMENT
1. F-actin- double helix filament composed of G-actin monomeres
2. Tropomyosin – double helix peptide chain; runs in the groove of F-actin chains
3. Troponin complex (3 globular proteins - subunits):
- Troponin C (TnC) – binds calcium ions → contraction
- Troponin T (TnT) – attachment of troponin to tropomyosin
- Troponin I (TnI) – inhibits actin-myosin interaction
MYOSIN FILAMENT - composed of hundreds of myosin molecules
- golf stick shape
- rod-like straight part (heavy chain; double helix)
- myosin head (flexible; binds to actin filament)
Myosin head has:
- actin binding site,
- ATP binding site,
- ATP-ase activity
Mechamisms of contraction: Adamkov – Functional Histology, lecture + included
figures.
During contraction occures shortening of:
1. sarcomere
2. I-band
3. H-band (in maximal contraction could disapear)
!!! No changes in lenght of :
A-band
actin and myosin filaments
INNERVATION OF SKELETAL MUSCLE - efferent motor nerve endings
Motor end-plate or myoneural junction
 Myelinated motor nerves form terminal arborisation on the surface of skeletal muscle fibers.
At their terminal parts they loose myeline sheath and form dilated axonal bouttons covered
only by a thin layer of Schwann cell cytoplasm. Bouttons contain axoplasm rich in
mitochondria and synaptic vesicles with acetylcholine (ACh). Here the axolemma forms
presynaptic membrane.

Between the axon and muscle fiber is a synaptic cleft. Sarcolemma at the site of junction
forms many invaginations that increase the postsynaptic surface. Sarcoplasm directly below
the folds does not contain myofibrils.

Postsynaptic membrane - sarcolemma contains acetylcholine receptors. When action
potencial reach the motor end plate, acetylcholine is released from the axon terminal to the
synaptic cleft and is bound to acetylcholine receptors of the sarcolemma. Sarcolemma
becomes more permeable to Na+ and this resultes in membrane depolarization.

At each triad , the depolarization signal is passed to the sarcoplasmic reticulum and results
in Ca2+ release (calcium ions are bound to troponinC).

When the depolarization stops, the Ca2+ is transported back to the sarcoplasmic reticulum
and muscle relaxes. In the synaptic cleft is an active acetylcholinesterase that brake down
excess of Ach.
CARDIAC MUSCLE CELLS – 3 types
1. Contractile cells
2. Impuls generating and conducting cells (initiate heart beat)
3. Myoendocrine cells (production of hormone for regulation of : Na+ , K+ balance and
water in the body)
Intercalated discs (ID)
Intercalated discs (ID) are connections between cardiac muscle cells.
On the transverse site are:
 fasciae adherentes (α – actinin) – serves for actin filament attachment
 desmosomes that serve for strong connection between cardiac muscle cells
On the lateral site are:
 gap junctions (nexus) – serve for the transport of ions, spreading of impulses
and metabolism
Cardiac muscle cells in LM: ID - intercalated discs, nuclei (1-2) in the centre of cell
Transverse and longitudinal
section of smooth muscle
EM: differences between smooth muscle cell and skeletal muscle fiber
1. actin and myosin filaments oriented „criss-cross“ (in striated muscles
longitudinally with long axis of the cell)
2. No myofibrils !!!
3. Thin actin filaments have no troponin complex
4. Intermediate filaments: desmin and vimentin
5. Dense bodies ( function like „Z-line“, contain α-actinin)
a) cytoplasmic
b) below sarcolemma
6. Sarcolemma – invaginations involved in transport of Ca2+ from extracellular
space into smooth muscle cell cytoplasm – pinocytotic vesicles
7. Mitochondria, rER and Golgi apparatus
8. Basal lamina + reticular fibers on the cell surface (produced by cell)
9. Nexus – smooth muscle cells spread ions through gap junctions
Function:
1. Contraction (smooth muscle in the hollow organs forms compact layers that
regulate diameter of the lumens)
2. Secretion (rER, GA):
synthesis of colagen type III, elastic fibers, proteoglycans
CONTRACTION OF SMOOTH MUSCLE CELL
CONTRACTION
• criss cross orientation of myofilaments give special shape to the cell after contraction
• after contraction rod-like nucleus is changed to cork-screw
Contraction of smooth muscle cells is involuntary
1. inervation by autonomic nerve system
2. hormonal stimulation (e.g. smooth muscle in the uterus during pregnancy): oxytocin,
estrogen.