Page 1 : MUSCLE CONTRACTION, INTRODUCTION, ❖ Mobility is one of the most important features of animals. In higher, animals movement of body parts or the whole body is due to, specialised tissue called muscle., ❖ Muscles are the effector organs which respond to various stimuli like, pressure, light, heat etc,., ❖ The muscles are mesodermal in origin, ❖ They constitute 60 to 75% of the total body weight of the animal., ❖ They have great power of contraction and relaxation., ❖ The study of muscles is called “ Myology”., ❖ The muscles exhibit irritability, contractility, conductibility,, extensibility, elasticity, ❖ The structural and functional unit of muscle is “Sarcomere”., ❖ Most sources state that there are over 650 named skeletal muscles in the, human body, although some figures go up to as many as 840., ❖ skeletal muscles are attached to your bones via tendons., , ❖ Muscle contraction theory was explained by “Sliding Filament, Theory” put forward by “Huxley & Hanson” scientists., ❖ The haemoglobin present in muscle is called “Myoglobin”., ❖ Muscle “Contractile proteins” are- “ACTIN” and “MYOSIN”., ❖ Muscle “ Regulatory proteins” are-”Troponin” and “Tropomyosin”, ❖ Muscle “Stabilizing Proteins” are- “Titin”, “Nebulin”, “Myomesin”,, “C-protein”, and “Alpha Actinin”., ❖ The largest protein is- “Nebulin”.

Page 2 : CLASSIFICATION OF MUSCLES, , Based on arrangement the muscles are divided into two types, 1. Phasic muscles, 2. Tonic muscles., 1.Phasic muscles: These muscles have the origin and insertion on Endo or, exoskeleton or on the skin. They move the appendages. They show rapid, contractions. These muscles always occur in opposite pairs of which one, shows contraction and the other shows extension., 2.Tonic muscles:These muscles appear on the heart, urinary bladder,, digestive tract and body wall form tonic muscles. they do not have origin, and insertion. They contract slowly., , Based on morphological features, muscles of vertebrates are classified, into three types. They are, 1. Skeletal / striated/ voluntary/ striped muscles., 2. Smooth/ unstriated/ involuntary/ unstriped muscles ., 3. Cardiac muscles.

Page 3 : 1. Skeletal / striated/ voluntary/ striped muscles:, ❖ They are attached to the skeletal system hence the name skeletal, , muscles, ❖ They have dark ( A) and light bands (I) ( striped muscles), ❖ They are tubular, cylindrical unbranched, and have a, “multinucleate condition”., ❖ The contraction and relaxation of these muscles are under the, control of will and wish of the organism , hence these muscles, are said to be “voluntary muscles”., ❖ They are also called “Myotomal muscles”.Eg. Triceps, Biceps., 2. Smooth/ unstriated/ involuntary/ unstriped muscles ., ❖ These muscles are spindle shaped., ❖ They have a single nucleus., ❖ These muscles are attached to the internal organs( digestive tract,, respiratory and urinary bladder). Hence they are also called “Visceral, muscles”., ❖ They are “unstriated/ unstriped” due to absence of dark( A) and, , light(I) bands.

Page 4 : ❖ The contraction and relaxation of these muscles are under the control, of the autonomic nervous system., ❖ Hence, these muscles are said to be “involuntary muscles”., , 3. Cardiac muscles., ❖ They are found in the “heart muscles” only., ❖ They are short, cylindrical, branched and have a “single nucleus”., ❖ They have “intercalated discs” with “gap junctions”., ❖ They are “striped or striated”, having dark and light bands., ❖ They are “involuntary muscles” controlled by “Autonomic nervous, system”., , STRUCTURE OF SKELETAL MUSCLE, , ❖ They are attached to the skeletal system hence the name skeletal, , muscles. They are“voluntary muscles”.

Page 5 : ❖ The muscle is attached to the bone by tendons., ❖ The muscle is covered by a thin envelope called “Epimysium”., ❖ The muscle is made up of a large number of muscle fibres that are, , arranged in the form of bundles called “fasciculi”., ❖ Each fasciculus is covered by “Perimysium”., ❖ Each muscle fibre is is enveloped by “Endomysium”., , ❖ Each muscle fibre is made up of “myofibrils” or “myofilaments” ., ❖ Each muscle fibre is consists of 1000 “myofibrils” or “myofilaments”, , ULTRASTRUCTURE OF SKELETAL MUSCLE FIBRE, ❖ Each muscle fibres is enclosed by a plasma membrane called, “Sarcolemma”., ❖ The cytoplasm is known as “sarcoplasm” which contains a large, number of mitochondria called “sarcosomes”., ❖ Muscle fibre is composed of a large number of myofibril arranged, parallel to each other. Around each myofibril, a network of, sarcoplasmic reticulum runs parallel forming transverse tubules, (T-tubules).---- triad sytem, ❖ Close examination of myofibril shows it is composed of two types of, longitudinal filament system. They are, 1. Thin filament system, 2. 2.Thick filament system.

Page 7 : ❖ Tropomyosin is a long protein that runs along the actin filament and, blocks the myosin head binding sites., , 2. Thick filament system.(A-Band):Thick filament is made up of myosin, protein., ❖ The myosin monomers that comprise the thick filaments are fibrous, proteins containing a “head” and “tail” portion., ❖ Each myosin molecule is composed of two heavy polypeptide chains, of MW 200,000 and four light chains of MW 20,000., ❖ Myosin can be cleaved into three pieces using proteolytic en­zymes., ❖ Treatment with trypsin releases part of the tail (i.e., light, meromyosin, LMM) from the remainder of the molecule., ❖ Light meromyosin has no ATPase ac­tivity and cannot combine with, actin., ❖ The remaining portion of the molecule, called heavy meromyosin, (HMM), contains ATPase activity and binds actin. Treatment of

Page 8 : HMM with the enzyme papain severs the head (the portion, containing the ATPase activity) from the tail., ❖ In thick filaments, the myosin molecules are ar­ranged with their tails, parallel to each other and their heads projecting away from the long, axis of the fila­ment at intervals ., ❖ No heads are present in the center of the filament, this region, coinciding with the H zone. The heads of the myosin molecules form, cross-bridges with adjacent actin filaments., ❖ Two functional domains can be identified in each head. One domain, contains the two cross-bridges that link the myosin to actin, and the, other domain provides a flexi­ble connection or hinge between the, cross-bridges and the remainder of the myosin molecule., , These myofilaments are arranged in such a way that they form alternate, dark and light bands.

Page 9 : ❖ The dark band is formed where actin and myosin interlocked each, other, called A-band., ❖ The light band between the A-band formed by actin only known as, I-band., ❖ Cutting across the I-band is Z-line., ❖ Within an A-band there is a somewhat lighter H-zone which contains, only myosin., ❖ Extending across the H-zone there is a delicate M-line, which, connects adjacent myosin filaments., , The distance between Two Z-line represents a fundamental unit for muscle, contraction known as Sarcomere., ❖ Z line – where the actin filaments are anchored., ❖ M line – where the myosin filaments are anchored., ❖ I band – contains only actin filaments., ❖ A band – the length of a myosin filament, may contain overlapping actin, filaments., ❖ H zone – contains only myosin filaments., A useful acronym is MHAZI – the M line is inside the H zone which, is inside the A band, whilst the Z line is inside the I band.

Page 11 : MECHANISM OF MUSCLE CONTRACTION, There are two important theories that explain the process of muscle, contraction. They are, 1. Electro kinematics theory, 2. Sliding filament theory, , 1.Electro kinematics theory:This theory explains that the myosin, filaments carry negative charge. The actin filaments carry neutral charge., In the resting condition, there is no electrostatic attraction between the, two types of filaments., During activation,Ca2+ ions are released into the sarcoplasm and, makes the actin filaments positive. Now, because of charge variations, the, positively charged actin filament and the negatively charged myosin, filament attract one another. Hence, the actin filaments slide towards the, myosin filaments. This theory is not accepted., , 2. Sliding Filament theory, Mechanism of muscle contraction is best explained by the “Sliding Filament, Theory”. It states that muscle contraction takes place by the sliding of thin, filaments over the thick filaments. It was proposed by Jean Hanson and, Hugh Huxley scientists. The sequences of muscle contraction explained by, sliding filament theory are as follows under the following 5 headings, 1. Excitation of muscle, 2. Formation of cross bridges, 3. Powerstroke, 4. Recovery stroke, 5. Relaxation of muscle

Page 12 : 1. Excitation of muscle, When the nerve impulse from the CNS is carried along motor neurons to, neuromuscular junctions, it stimulates the release of neurotransmitters, (Acetylcholine) in the synaptic cleft. The neurotransmitter binds to the, receptor on the sarcolemma and generates action potential across muscle, fiber for muscle contraction. The action potential propagates over the entire, muscle fiber and moves to the adjacent fibers along transverse tubules. The, action potential in transverse tubules reaches the triad system through Ttubules, sarcoplasmic reticulum causes the release of calcium ions into the, sarcoplasm., , 2. Formation of cross bridges: Increase in the calcium levels leads to, the binding of Ca2+ions to the Tn-C Subunit of troponin of the thin, filaments. This makes Troponin- Tropomyosin complex to move away from, the active sites of actin molecules. Now, the active sites are exposed to the, heads of the myosin. Utilising the energy released from hydrolysis of ATP,, the myosin head now binds to the exposed “active sites” on the actin, molecules to form a Cross -Bridge and P1 is released., , 3. Powerstroke, The cross bridge pulls the attached actin filaments towards the centre of, “A” band. The “Z” lines attached to these actin filaments are also pulled, inwards from both the sides, thereby causing the shortening of the, sarcomere. i.e., muscle contraction. During the shortening of muscle, the, “I” bands get reduced in size/ length( Z membranes of sarcomere are, brought closer), whereas ‘A’ bands retain their size/ length. It is important, to note that myofilaments do not actually shorten. As the thin filaments are

Page 13 : pulled deep into the ‘A’ bands making the ‘H’ zone narrow, the muscle, shows the effect- muscle contraction., , 4. Recovery stroke, The myosin goes back to its relaxed state and releases ADP, A new ATP, molecule binds to the head of myosin and the cross bridge is broken down., The new ATP is again hydrolysed by ATPase of the myosin head and the, cycle of cross Bridge formation, and breakage is repeated causing further, sliding., , 5. Relaxation of muscle, When motor impulse stops, the calcium ions are pumped back into the, sarcoplasmic reticulum cisternae. It results in the masking of the Active, sites of the actin filaments. The myosin head fails to bind with the active, sites of Actin molecules. These changes cause the return of ‘Z’ lines back to, their original position, i.e., relaxation.

Page 15 : 2. Isometric and isotonic contractions., (i) Isometric contraction: It is a type of muscle contraction in which the, muscle fibre develops tension but does not shorten. In this contraction,, some sarcomere muscles shorten, while some are stretched, without, shortening the muscle. Such a muscle cannot do external mechanical work, and hence it cannot lift a load., (ii) Isotonic contraction: It is a type of contraction of muscle in which the, fibres are shortened but the tension remains constant. All the contractile, elements in a sarcomere maintain the same tension throughout the, contraction. This is seen in a muscle when it is freely lifting a load. It does, external work., , PHYSICO-CHEMICAL CHANGES DURING MUSCULAR, CONTRACTION