Mechanism Of Muscular Movement :
Muscular tissue is characterized by its property of shortening called contractility. It also has the properties of extensibility, elasticity, flexibility, conductivity, etc. The muscular tissue consists of highly, elongated modified cells called muscle fibres. There are three kinds of muscular tissue in the body of vertebrates; striated, unstriated and cardiac muscles. The striated muscles perform voluntary movements while unstriated and cardiac muscles perform involuntary movements.
1. Voluntary movements :
The movements of body parts carried under the control of our will are called voluntary movements e.g. movements of limbs, movements of head, etc. All types of voluntary movements are performed by striated muscles.
Striated muscle cells are elongated, cylindrical unbranched, multinucleated and with cross striations. The striated muscle cells are covered by modified cell membrane called sarcolemma, basement membrane and reticular connective tissue. Sarcolemma is electrically charged membrane of the body.
Contractile proteins :
The muscle fibres show presence of many myofibrils. Each myofibril has many myo-filaments made up of Proteins, actin and myosin. Actin filaments is thin and made-up of pair of filamentous ('F') proteins which is a polymer of globular ('G') actins. Myosin filaments are thick and they are made-up of many meromyosins. Each meromyosin has globular head with short arm called Heavy meromyosin (HMM) and a tail called Light meromyosin (LMM). Head shows an active ATP ase enzyme and has binding sites for ATP and active sites for actin.
Mechanism of muscle contraction :
Most accepted theory of muscle contraction is sliding filament theory. It states that during muscle contractions, sliding of thin filaments over the thick filaments takes place.
Nerve endings of motor neuron innervates each muscle. The junction between motor neuron and sarcolemma of muscle fibre is called motor end plate.
At the axonic ends, neurotransmitter called acetylcholine is released on getting signals from CNS. It generates an action potential in the sacrolemma. It causes release of Ca++ with troponin on actin filaments and removes masking of active site of actin filament between myosin filaments. It results in shortening of sarcomere by reducing I band. Myosin releases ADP and becomes relaxed. Again, while process is repeated causing further sliding. This continues till Ca++ ions are pumped. When Ca++ ions are pumped back the masking of actin filaments takes place. Repeated activation of muscles leads to accumulation of lactic acid due to anaerobic breakdown of glycogen causing muscle fatigue.
The contractile units of muscle are sacromeres formed by thin actin filaments and thick myosin filaments.
Unlike the order two muscular tissues, the striated muscle fibres are packed together in connective tissue into distinct bundles called muscle bundles. In human body about 640 muscles are present. Each muscle contains many fasciculi and each fasciculus bundle of muscle fibres.
Striated muscles are found attached to skeleton by means of tendons. Tendons are inelastic thick bands of white fibrous connective tissue, which give firm attachment to muscle with bone.
Location and structure of striated muscle :
The muscle which moves a body part usually do not lie in that but located in the upper part e.g. biceps and triceps that move forearm are located in the upper arm. At any joint two types of bones are present i.e. stationary and movable. The end of muscle attached to stationary bone is called origin while the opposite end attached to movable bone is called insertion. The middle thick part of muscle is called belly. All the fibres in a muscle do not extend from end to end and there is a maximum concentration on the middle. Thus, large muscles are most often fusiform in shape.
Types of striated muscles :
On the basis of movements striated muscles are of three types :
i) Prime movers (agonist) - Bring initial movement of part, e.g. Biceps.
ii) Antagonists - Bring the action opposite to that of prime movers, e.g. Triceps.
iii) Synergists - Assist prime movers e.g., Brachialis assist Biceps.
Working of skeletal muscles :
Generally muscles work in pair and produce opposite action e.g. biceps (flexors) bring flexion and triceps (extensors) bring extension of elbow joint. The muscles which bring opposite action, are called antagonistic. If one member of a pair is capable of bending the joint by pulling of bones, the other member is capable of straightening the same joint also by pulling e.g. Biceps and triceps of upper arm are antagonistic to each other.
In antagonistic pair of muscles, one member is much stronger than the other, e.g., the biceps which flex the arm are stronger than the triceps which extend it.
The fundamental characteristic of muscle is contraction. Therefore, muscle can only pull and not push the bone.
The response or contraction of striated muscles is quick and for short duration. Therefore these muscles are prone to fatigue. These muscles are neurogenic i.e. need repeated stimulus from Central Nervous System (CNS).
Some important antagonistic muscles :
1. Flexor : On contraction results in bending or flexion of a joint e.g. Biceps.
Extensor : On contraction results in straightening or extension of a joint e.g. Triceps.
2. Abductor : It moves body part away from the body axis e.g. deltoid muscle of shoulder moves the arm away from the body.
Adductor : It moves body part towards the body axis e.g. Latissimus dorsi of shoulder moves the arm away near to the body.
3. Pronator : To turn the palm downward backward.
Supinator : To turn the palm upward forward.
4. Levator : Raises a body part.
Depressor : Lowers the body part.
5. Protractor : To move forward.
Retractor : To move backward.
6. Sphincters : Circular muscles present at inner side wall of anus, stomach etc. for closure and opening.
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| (a) Flexion |
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| (b) Extension |
II. Involuntary movements :
These movements are not under the controls of the will. It is under the control of Autonomic Nervous System (ANS).
1. Movements of visceral muscles :
The visceral muscles are present in the walls of visceral organs. These muscle cells are elongated, spindle shaped, non striated, uninucleate and involuntary. Visceral muscles are arranged in longitudinal and circular layers. These are neurogenic and on receiving motor impulses from Autonomous Nervous System., these muscle layers show alternate contraction and relaxation. These muscles control the involuntary movements of visceral organs. Visceral muscles do not get fatigued because their contractions are slow and for the longer duration, so that they are able to do work continuously.
For e.g; Contraction of urinary bladder, Constriction and dilation of blood vessels, Peristaltic movements of alimentary canal, Gastric movements of stomach, etc.
2. Movements of Cardiac muscles :
Cardiac muscles control movements of heart i.e. contraction and relaxation of heart. They are present in the wall of heart, i.e. myocardium. Cardiac muscles cells are elongated, cylindrical, striated, uninucleate, involuntary and connected to each other by intercalated discs. These muscle cells are arranged in the form of network. These muscles are myogenic because the impulse of contraction originates in the muscle node itself. Once stimulated these muscles show continuous, rhythmic contraction and relaxation. These muscles are immune to fatigue due to longer duration of relaxation than contraction period.