Connective tissue

Part 1

MUSCLE TISSUE

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

• 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 Myosin

(thin filament) (thick filament)

• Responsible for band like

structure of skeletal muscle

• Interaction between actin

and myosin results in

contraction

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