1.Striated/striped/skeletal/voluntary/hard muscle. 2.Unstriated/unstriped/nonskeletal/involuntary/smooth muscle 3. Cardiac muscle. # Actin filaments #Myosin filaments

1. DEPARTMENT OF ZOOLOGY
MUSCLE CONTRACTION
PRESENTED BY :-SONALI GHOSAL
+3 1st YEAR ZOOLOGY (HONS).

2. INTRODUCTION
I. The muscular tissue are also known as contractile tissue.
II. Muscles are considered as fleshy part of the body.
III. The ability to contraction,excitation and conductivity of impulse are the characteristics properties of
muscular tissue or muscle.
IV. 40% to 50%of the body weight is considered by muscles.
V. It consist of elongated muscle cells or muscle fibres.
VI. Muscular tissue originate from embryonic mesoderm.
VII. It have no matrix,no power of division and regeneration.
VIII.Study of muscle is known as myology or sarcology.
IX. Muscular tissue is formed of greatly elongated,and highly contractile muscle cells called muscle fibre.
X. Muscle fibres contain contractile units called myofilaments.
XI. Due to modification in general property of protoplasm the cytoplasm in muscle is called sarcoplasm and its
limiting membrane is called sarcolemma.

3. Functions
• It brings about movement of body, parts of the body organs.
• By muscular action it produces body heat.
• It contracts heart and blood vessel.
• Muscles are the agents of brain.
Types of muscle:-
1. Muscles are classified on the basis of their structure and function into three type
2. Striated/striped/skeletal/voluntary/hard muscle.
3. Unstriated/unstriped/nonskeletal/involuntary/smooth muscle
Cardiac muscle.
Striated muscle/striped/skeletal/voluntary:-
 These are called striated or striped muscle due to these are thread like structure.
 These are mostly attached to the skeleton and hence known as skeletal muscle .
 As these muscles are attached to bone hence called as skeletal muscle.
 On the basis of contraction and relaxation these are controlled by will so, called voluntary muscle .
 These are also hard muscle .

4.  The striated muscles are formed of large number of long,unbranched muscles cells or muscle fibres
 The length of fibre vary from 1.0 to 40mm and the breadth from 0.01mm to 0.1mm.
 A single muscle fibre is surrounded by a sheath of connective tissue known as endomysium.
 Bundle of musclefibre form fasciculi which are covered by a sheath of connective tissue known as
perimysium.
 Several fasciculi in a muscle is coverd by a sheath of connective tissue called epimysium.

5.  Each muscle fibre is cylinderical in shape but may also be spindle,flat irregular shape.
 Each myofibril contains nearly 300 light and dark band in one millimeter.
 When studied under polarised light the dark band is known as anisotropic or A-band and the light band is
known isotropic or i-band.

6.  The I-band is bisected by a thin line, called as z-line or krause’s membrane.
 The portion enclosed by two adjacent z- line of a myofibril is considered as contractile unit or sarcomere.
 The central portion of A-band has a H-band or Hensen’s disc which is lighter in colour.
 In the centre of H-band ,there is a narrow dark line called M-line or M-band(mesophragm) ,where the
myosin filaments are thickened.
 The two darker bands of A-band are named as O-band on both the side of Z-line of I-band ,there are thin
darker lines called N-line.
 Darker bands contains myosin filaments are 100A0 in diameter .light band contains actin filaments of 50A0 in
diameter .
 Myosin filaments are also called thick filament and actin filaments are called thin filament.

7. Actin Filament:-
At regular intervals ,these two filaments are linked together forming an acto-myosin
complex.
MYOSIN + ACTIN → Acto-myosin. ATP
ca++

8. •Actin filament is formed by the actin protein about 25% ,tropomyosin protein about 2 to 10% and small
amount of troponin proteins.
•It is represented by 2 helical strands of globular monomers of G-actin.
•In helical strand G-actins are polymerised to form F-actin filament (fibrous actin filament)
•Each troponin protein is made up of 3 subunit to form troponin complex.
•The troponin complex consists of troponin-C ,a small protein that specifically binds with calcium ion at the
time of muscle contraction ,troponin-I a protin that, binds to both G actin and troponin-C and troponin-I the
largest subunit that binds tropomyosin with other troponin.

9. Mysoin Filament
• Myosin filament are formed of myosin protein molecule(About 50% of myofibril proteins).
• Each myosin protein molecule contains 2 long polypeptide chain with 4 short polypeptide chain.
• Many monomeric proteins called meromyosins.
• Each meromyosin has 2 important parts a globular head with a short arm and a tail former being HMM
(heavy meromyosin) and LMM(light meromyosin).

10. • HMM catalyzes the hydrolysis of ATP and binds to actin.
• LMM lacks ATPase activity and does not combine with actin.

11. MUSCLE CONTRACTION :-
 Muscles are contractile in nature. The process by which muscle contract is known as muscle contraction.
 Theory regarding muscle contraction is sliding filament theory explained by H.E. Huxley and Hanson.
 According to this theory the length of actin and myosin filaments remain constant.
 During contraction the length of the A-band remains constant while the length of I-band and H-zone
gradually decreases and finally disappear.
 The Z-line touch the end of myosin filament.
 The sliding of filaments takes place by means quick formation of crossbridge and their break down . So
that, sarcomere becomes shorter in length as a result of which muscle contract and shorten.
The following condition prevails in the sarcoplasm prior to muscle contraction.
 Cross bridges of the myosin heads are oriented at 900 to the actin filaments long axis.
 Strong interaction is not possible between these two filaments due to the presence of regulatory proteins
such as troponin.
 Troponin –I inhibits the activity of ATPase of myosin heads, while tropomyosin present in the grooves of F-
actin covers the actin binding sites of the myosin heads.
 The myosin head contain tightly bound ADP and phosphate molecules.
 The concentration of ATP and Mg++ ions in the sarcoplasm is more than the level of calcium ion.
 Sarcoplasmic reticulum has higher concentration of calcium ions.

12. There is a cyclic mechanism of contraction and relaxation of a muscle cell. It involves four steps such as ,
A.INITIATION
B.POWER STROKE
C.RELAXATION
D.ENERGY GENERATION
A.INITIATION
The muscle cell contraction is initiated by depolarization of its sarcolemma because of stimulation by nerve
impulse generated by any physical and chemical agents .
The depolarization causes change in the permeability of the sarcoplasmic reticulum to ions .
Rapid diffusion of calcium ions to the sarcoplasm.The level of calcium ions now increases in sarcoplasm to
10-5 M.
Calcium ion binds to troponin-C causing a shift in the position of tropomyosin .
The binding sites on the G-actin are exposed to the actin binding site of myosin heads .so that, the
interaction become stronger .

13. B.POWER STROKE
The myosin head undergoes a conformational change from 900 orientation to 450 causing the actin
filaments to slide about 12nM . This movement of the filament is called power stroke .
This phenomenon is accompanied by release of the bound phosphate and ADP .
Detachment of myosin head causing the restoration of its previous 900 orientation .
Binding of ATP to the myosin head and subsequently the hydrolysis of ATP .
C.RELAXATION
When the nervous stimulation stops ,the sarcolemma is repolarised and the normal permeability of
sacoplasmic reticulum is restored .
This result in the active transport of calcium ions form sarcoplasm to sarcoplasmic reticulum.
So,that reduces the concentration of calcium ions in the sarcoplasm to 10-7M .
This is accomplished by the membrane bound calcium ion pump protein of the reticulum (By the activty of
enzyme ATPase).
The withdrawal of calcium ions make free troponin-C which changes the position of tropomyosin .

14. The tropomyosin comes back to the groove of F-actin which shields the binding sites of G-actin .As a result it
prevents the interaction between the cross bridge of myosin head and actin filament.
D.ENERGY GENERATION
A considerable amount of ATP molecules are consumed during molecular activity .The ultimate source of
ATP is due to the phosphorylation of ADP during glycolysis.
The source of phosphate is phosphor-creatin that is present in the sarcoplasm.
As we know that the globular double head of myosin molecule has two active sites ,one for actin and other
for ATP .The active site for ATP has ATPase activity which splits ATP into ADP+iP
myosin ATPase. Contracting muscle
ATP +H2O→ADP+iP+Energy+Phosphocreatin+ADP -----→ATP+Creatin.
Two molecule of ATP are consumed for each interaction between the cross bridge of myosin head and G-
actin.
One molecule of ATP is required for active transport of each calcium ion from sarcoplasm to sarcoplasmic
reticulum.

15. Muscle Contraction diagram

16. In summary the mechanism of contraction expressed in the following scheme CONTRACTION-Membrane
depolarization→ca++ released from sarcoplasmic reticulum→myosin ATP are activated→cross bridge
formed→myosin slides along actin
RELAXATION
 ca++pumped back into sarcoplasmic reticulum→myosin ATPase is depressed→cross bridge
broken→myosin is pulled back to its resting site
 It takes 0.1 sec /contraction .
Muscle fatigue –
By repeated stimulation ,the muscle loose irritability and fails to contract .It is known as muscle fatigue.It
happens due to exahustion of energy and accumulation of lactic acid.
Single twitch-
The single nerve impulse or an electrical shock of sufficient amount brings about contraction in a single
muscle fibre. It is known as muscle twitch.
Rigor mortis-Due to non-availability of ATP ,enzymes,oxygen and food,the muscle remain contracted after
death .It is called rigor mortis
Summation:-When a second stimulus is given to muscle fibre after the 1st one brings about contration.Thus
it adds contraction strength of first stimulus. It is known as summation.
Tetanus:-when a series of stimuli are given to the muscle fibre in a quick succession, the contraction of
muscle fibre continues.The continued state of contraction is known as tetanus.

17. Thresold Stimulus:-
 Each Muscle contracts due to some stimulus. The muscle fibre remain relaxed when it is not stimulated.
 The specific minimum strength requird for muscle contraction is called as Threshold stimulus.
 The stimulus which is unable to bring contraction is called as subliminal stimulus and the stronger stimulus is known as
supraliminal stimulus.
Unstriated/unstriped/Non-Skeletal/Smooth/Non-voluntary muscle:-
i. These muscles are not directly attached to skeleton hence known as non skeletal muscle.
ii. These are connected with visceral organ hence called as visceral muscle.
iii. These are also called smooth, involuntary work will (of ANS) and unstriped.
iv. Each muscle fibre is elongated, spindle shaped tappering at both the ends, length 20 ꜡ to 500 ꜡, breadth 6꜡.
v. Central part is wide.
vi. Uninucleate and nucleus is centric.
vii. Such muscle fibres are less vascular with less mitochondria.
viii. Each muscle fibre contains contractile unit called myofilaments having actin and myosin.
ix. Such type of muscle never fatigue.
x. It takes 0.3sec /contraction.
xi. Absence of dark and light band.

18. Location:-Alimentary canal,blood vessel,lungs,urinary bladder etc.
Function:-Helps in peristalsis, causing slow and prolonged contraction.
Cardiac Muscles:-These are structurally striated and functionally unstriated.
Resembalance with striated muscle:-Cylindrical,vascularised,more mitochondria,Glylogen
granules in sarcoplasm,having light and dark bands.
Resembalance with unstraited muscle:-Uninucleate,involuntary.

19.  Own Character:-
i. Branched cardiac muscle fibres are joined with each other by thick junctions called intercalated disc.
ii. Such muscles never fatigue.
iii. Cells attached with each other forming continuous network.
iv. Wave of excitation self generated.
v. Contract quickly powerfully and rhythmically.
vi. It takes 0.8sec/contraction.

20. CONCLUSION
Efficent blood circulation is essentials. Constant supply of food and oxygen . Removal of
waste products. Muscle provides support to bone and other organs. Muscles bring
movement of the body parts and helps in locomotion of an individual. Muscles contracts
heart blood vessels and alimentary canal. Muscle contraction produces heat. Maintains
equilibrium of body. Helps in child birth.