Muscle
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Muscle is a specialised tissue of mesodermal origin. About 40-50 per cent of the body weight of a human adult is contributed by muscles. They have special properties like excitability, contractility, extensibility and elasticity.
Muscles have been classified using different criteria namely location, appearance and nature of regula Han of their activities. Based on their location, three types of muscles are identified:
Types of Muscles
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Skeletal
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Visceral
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Cardiac
The muscles that act together to produce a movement are called synergists and the muscle that act in opposition to each other are antagonists. The muscles that act most powerfully during any given movements are called prime movers.
Flexor and Extensor: Muscles that bend one part over another joint is called flexor. Extensor muscle is antagonist of flexor muscle. The contraction of an extensor extends a joint by pulling one of the articulating bone apart from another.
(ii) Pronator and Supinator: The contraction of a pronator rotates the forearm to turn the palm downward or backward. Supinator is antagonist of pronator. A supinator contracts to rotate the forearm and thus to make palm face upward or forward.
(iii) Abductor and Adductor: An abductor contracts to draw a bone away from the body midline. Muscle that brings the limb away from midline is called abductor. An adductor draws a bone towards the body midline. Muscles that brings the limb towards midline is called adductor. Abductor muscle is antagonist of adductor muscle. Abduction is elevation and adduction is depression.
(iv) Protractor and Retractor: Protractor muscle pulls the lower jaw, tongue and the head forward. Retraction is opposite to protaction. Retractor muscle draws the lower jaw, tongue and the head backward.
(v) Inversion and Eversion: Turning of feet so that the soles face one another in inversion. Eversion is the opposite of inversion. In this movement, the soles of the feet face laterally.
(vi) Rotation: Rotation is term that indicates the partial revolving of a body .part on the part's long axis.
(vii) Arrector: Raises hairs of skin.
Skeletal Muscles
Skeletal Muscles are closely associated with the skeletal components of the body. They have' a striped appearance under the microscope and hence are caned striated muscles. As their activities are under the voluntary control of the nervous system, they are known as voluntary muscles too. They are primarily involved in locomotory actions and changes of body postures.
Visceral Muscles
Visceral muscles are located in the inner walls of hollow visceral organs of the body like the alimentary canal, reproductive tract, etc. They do not exhibit any striation and are smooth in appearance. Hence, they are called smooth muscles (non striated muscle). Their activities are not under the voluntary control of the nervous system and are therefore known as involuntary muscles. They assist, for example, in the transportation of food through the digestive tract and gametes through the genital tract.
Cardiac Muscles
Cardiac muscles are the muscles of heart. Many cardiac muscle cells assemble in a branching pattern to form a cardiac muscle. Based on appearance, cardiac muscles are striated. They are involuntary in nature as the nervous system does not control their activities directly.
Each organised skeletal muscle in our body is made of a number of muscle bundles or fascicles held together by a common collagenous connective tissue layer called fascia. Each muscle bundle contains a number or muscle fibres, Each muscle fibre is lined by the plasma membrane called sarcolemma enclosing the sarcoplasm. Muscle fibre is a syncitium as the sarcoplasm contains many nuclei. The endoplasmic reticulum, i.e., sarcoplasmic reticulum of the muscle fibres is the store house of calcium ions.
A characteristic feature of the muscle fibre is the presence of a large number of parallelly arranged filaments in the sarcoplasm called myofilaments or myofibrils.
Each myofibril has alternate dark and light bands on it. A detailed study of the myofibril has established that the striated appearance is due to the distribution pattern of two important proteins - Actin and Myosin. The light bands contain actin and is called I-band or Isotropic band, whereas the dark band called 'A' or Anisotropic band and contains myosin. Both the proteins are arranged as rod-like structures, parallel to each other and also to the longitudinal axis of the myofibrils. Actin filaments are thinner as compared to the myosin filaments, hence are commonly called thin and thick filaments respectively. In the centre of each 'I' band is an elastic fibre called' 'Z' line which bisects it. The thin filaments are firmly attached to the 'Z' line. The thick filaments in the ‘A’ band are also held together in the middle of this band by a thin fibrous membrane called ‘M' line. The 'A' and ‘I’. bands are arranged alternately throughout the length of the myofibrils. The portion of the myofibril 'between two successive 'Z' lines is considered as the functional unit of contraction and is called a sarcomere. In a resting state, the edges of thin filaments on either side of the thick filaments partially overlap the free ends of the thick filaments leaving the central part of the thick filaments. This central part of thick filament, not overlapped by thin filaments is called' the 'H' zone.
Structure of Contractile Proteins
Each actin (thin) filament is made of two 'F' (filamentous) actins helically wound to each other. Each 'F' actin is a polymer of monomeric 'G' (Globular) actins. Two filaments of another protein, tropomyosin also run close to the 'F' actins throughout its length. A complex protein Troponin is distributed at regular intervals on the tropomyosin. In the resting state a subunit of troponin masks the active binding sites for myosin on the actin filaments. Each myosin (thick) filament is also a polymerisedprotein,
Many monomeric proteins called Meromyosins (Figure 20.3b) constitnte one thick filament. Each meromyosin has two important parts, a globular head with a short arm and a tail, the former being called the heavy meromyosin (HMM) and the latter, the light meromyosin (LMM). The HMM component, i.e.; the head and short arm projects outwards at regular distance and angle from each other from the surface of a polymerised myosin filament and is known as cross arm. The globular head is an active ATPase enzyme and has binding sites for ATP and active sites for actin.
Mechanism of Muscle Contraction
Mechanism of muscle contraction is best explained by the sliding filament theory which states that contraction of a muscle fibre takes place by the sliding of the thin filaments over the thick filaments.
Muscle contraction is initiated by a signal sent by the central nervous system (CNS) via a motor neuron.
A motor neuron alongwith the muscle fibres connected to it constitute a motor unit-The junction between a motor neuron and the sarcolemma of the muscle fibre is called the neuromuscular junction or motor-end plate. A neural signal reaching this junction releases a neurotransmitter (Acetyl choline) which generates an action potential in the sarcolemma. This spreads through the muscle fibre and causes the release of calcium ions into the sarcoplasm. Increase in Ca++ level leads to the binding of calcium with a subunit of troponin on actin filaments and thereby remove the masking of active sites for myosin. Utilising the energy from ATP hydrolysis, the myosin head now binds to the exposed active sites on actin to form a cross bridge. This pulls the attached actin filaments towards the centre 'of 'A' band. The 'Z' line attached to these actins are also pulled inwards thereby causing a shortening of the sarcomere, i.e., contraction.
During shortening of the muscle, i.e., contraction, the ‘I’ bands get reduced, whereas the' A' bands retain the length. The myosin, releasing the ADP and P goes back to its relaxed state. A new ATP binds and the cross-bridge is broken. The ATP is again hydrolysed by the myosin head and the cycle of cross bridge formation and breakage is repeated causing further sliding. The process continues till the Ca++ ions are pumped back to the sarcoplasmic cisternae resulting in the masking of actin filaments. This causes the return of 'Z' lines back to their original position. i.e., relaxation. Repeated activation of the muscles can lead to the accumulation of lactic acid due to anaerobic breakdown of glycogen in them, causing fatigue.
Muscle contains a red coloured oxygen storing pigment called myoglobin. Myoglobin content is high in some of the muscles which gives a reddish appearance. Such muscles are called the Red fibres. These muscles also contain plenty of mitochondria which can utilise the large amount of oxygen stored in them for ATP production. These muscles, therefore, can also be called aerobic muscles.
Some of the muscles possess very less quantity of myoglobin and therefore, appear pale or whitish. These are the white fibres. Number of mitochondria are also few in them, but the amount of sarcoplasmic reticulum is high. They depend on anaerobic process for energy.
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