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Home , Cardiac muscle, Muscle cell, Muscle fascicle, Muscle tissue, Sarcoplasm, Sarcoplasmic reticulum

Department of and Embryology, P. J. Šafárik University, Medical Faculty, Košice

MUSCLE : Sylabus for foreign students

Author: doc. MVDr. Iveta Domoráková, PhD.

Revised by: prof. MUDr. Eva Mechírová, CSc.

MUSCLE TISSUE

Function: is specialized for contraction

Types of muscle tissue: 1. Striated 2. Striated 3.

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 .

Fig. 1 – longitudinal and transverse section of skeletal muscle fiber

2. CARDIAC MUSCLE Function: contraction - involuntary; rapid and rhythmic - in the (myocardium) BASIC MORPHOLOGIC UNIT in LM: cardiac muscle (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 (, ) BASIC MORPHOLOGIC UNIT in LM: spindle shaped smooth . 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 that gives rise to mesenchymal cells. • Skeletal muscle develops from paraaxial mesoderm, organized into myotomes in . Muscles of the head develop from of branchial arches. • Cardiac muscle develops from cardiogenic mesoderm. • Smooth muscle develops from splanchnic mesoderm - except of 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 - network of reticular fibers, also contains and lymphatic and nerves. Groups of muscle fibers (anywhere between 10 to 100 or more) form fascicles (bundles). is surrounded by - sheath of . Entire muscle is surrounded by - composed of dense irregular connective tissue. It is continuous with and other connective tissue wrappings of muscle including the endomysium and perimysium. It is also continuous with .

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 (visible in EM). Myofibrils create the biggest part of the . They are oriented longitudinally with long axis of muscle fiber. Myofilbrils are composed of thin filaments () and thick filaments (myofilaments). Actin filament on molecular level is composed of: F-actin, complex (Tn subunits: TnC, TnI, TnT) and . Actin in the is bound to the Z-line by an α-. 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 (sER). Between myofibrils are mitochondria – sarcosomes - energy for contraction. Oval nuclei are situated beneath the sarcolemma. Sarcoplasm contains granules and pigment .

Fig.8 – Structure of muscle fiber in EM

TRIAD 3) 2 of sarcoplasmic reticulum +1 T- (invagination of sarcolemma; encircle each at the border of A-band and I band). Function of T-: Fast transmission of nerve impulses by depolarization of membranes (sarcolemma, membranes of sarcoplasmic reticulum Function of sarcoplasmic reticulum: storrage and release of 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: A- bands (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 α- actinin, 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 - subunits):

- (TnC) – binds calcium ions → contraction

- (TnT) – attachment of troponin to tropomyosin

- (TnI) – inhibits actin-myosin interaction

MYOSIN FILAMENT - composed of hundreds of myosin molecules

- golf stick shape - rod-like straight part (heavy chain; double helix) - (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 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 (ACh). Here the axolemma forms presynaptic membrane.

 Between the 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 , 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 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

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: and

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

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,

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 -screw Contraction of smooth muscle cells is involuntary

1. inervation by autonomic nerve system

2. hormonal stimulation (e.g. smooth muscle in the during ): , estrogen.