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

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)

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.

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, 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

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)

Transverse and longitudinal

section of smooth muscle

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.