Biology: Support and movement system

Chapter 3 (6%)
Support and movement system 

Movements: The act of changing position or place by the entire body of an organism or by one or more of its parts is called movement.         

1.       Brownian movement: Zig-zag motion of molecules in the cytosol.(molecular level)

2.       Cyclosis: Streaming movement of cytoplasm. It ensures uniform distribution of materials within the cell.(cellular level)

3.       Amoeboid movement: Movement produced by temporary protoplasmic outgrowth, called Pseudopodia.

4.       Ciliary movement: movement triggered by very fine hair like structures called ‘Cilia’.

5.       Flagellar movement: Movement by using flagellum, a whip like structure found in some sponges and coelenterates.

Cyclosis, Amoeboid movement, ciliary movement and flagellar movement are examples of nonmuscular movements.    

Muscular Movement: Movement produced by Muscles, responsible for locomotion as well as movement of body parts. Organisms with the ability to move are called Motile. Locomotion is the movement of an organism as a whole resulting in displacement.

Muscle:Muscles are formed of specialised elongated cells called Muscle fibres. The properties of muscle fibres are electric excitability, contractility and elasticity.

There are three types of Muscles:

1.       Voluntary/ skeletal/striated muscles: These muscles exhibit striations, found attached with the skeleton by tendon. When examined under microscope, fibres of this muscle show prominent transverse striations (striated muscles) and they work under the will or conscious control of nervous system, hence are also called voluntary muscles.

2.       Involuntary/ smooth/ non-striated/visceral muscles: No striations and are smooth in appearance. They are not under the voluntary control of our nervous system. These muscles are found in the hollow internal organs like digestive tracts, blood vessels etc. hence called as visceral muscles.

3.       Cardiac muscles: these are the striated muscles of the wall of Heart. They are involuntary in action and work tirelessly till the end. They have inherent rhythmic contractility.

Structure of Striated (skeletal) Muscles:

The striated muscles form the body flesh and about 80% of the soft body tissue. Human body has around 650 skeletal muscles.Each muscle is enclosed in a dense connective tissue sheath called epimysium. It encloses numerous small muscle bundles called Muscle Fasciculi, each with a connective tissue covering called Perimysium. Each muscle fascicle is in turn formed of several Muscle fibres. Each Muscle fibre is covered with endomysium formed of loose connective tissue. At the junction of a muscle with a tendon, the endomysium, perimysium and epimysium are continuous with the fibres of the tendon.

Structure of a muscle fibre (myocyte):

Muscle fibres are the structural and functional unit of Muscle. Each muscle fibre is a long, unbranched and cylindrical fibre. It is covered with a thin elastic membrane called sarcolemma which encloses a multinucleated (syncytial) cytoplasm called sarcoplasm. In the sarcoplasm, numerous myofibrils are embedded. These are tightly packed in parallel bundles, separated by thin sheet of cytoplasm.

Myofibrils form the contractile apparatus of a muscle fibre. It has transverse striations in the form of alternate dark and light cross bands. Because of these striations, these muscles are called striated or striped muscles.

Light bands (I-band) are formed of thin actin filaments. Actin filaments are firmly attached to Z disc. The part of myofibril between two adjacent Z discs is known as Sarcomere.

Dark bands (A band) are formed of thick Myosin filaments. They are free at both ends and are thicker than I-bands. A band is also bisected at the mid-point by a thin paler line which is known as H zone. A narrow dark line passes through H- band. It is called M-line.

Myosin and Actin myofilaments are called as Primary and secondary myofilaments respectively.

Different types of proteins present in muscles:

1.       Contractile proteins: Actin and Myosin.

2.       Regulatory proteins: Tropomyosin and troponin. They regulate the interaction between myosin and actin.

Tropomyosins are long filaments that lay in the groove between two chains of actin molecules. With troponin molecules it covers the binding site on actin where myosin head aligns.

Troponins are small globular proteins molecule. Binding of Troponin to tropomyosin prevents the myosin heads from contacting actin and prevent contraction.

Mechanism of muscle contraction (Sliding filament theory):

Sarcomere is the unit of muscle contraction.

At rest, when muscle is relaxed, the actin and myosin filaments lie parallel to each other and actin partially overlapping at the two ends of myosin filament. The H zones remain wide.

During muscle contraction, the thin actin filaments slides over myosin filaments towards H zone till the H zone disappears. Hence the sarcomere shortens in size and also the whole muscle filament. This mechanism of muscle contraction was given by A.F.Huxley, H.E. Huxley and Jean Hanson and was called Sliding filament theory .

Here is what happens in detail.

1.       A nervous impulse arrives at the neuromuscular junction, which causes release of a chemical called Acetylcholine. The presence of Acetylcholine causes the depolarisation of the motor end plate which travels throughout the muscle by the transverse tubules, causing Calcium (Ca+) to be released from the sarcoplasmic reticulum.

2.       In the presence of high concentrations of Ca+, the Ca+ binds to Troponin, changing its shape and so moving Tropomyosin from the active site of the Actin. The Myosin filaments can now attach to the Actin, forming a cross-bridge.

3.       The breakdown of ATP releases energy which enables the Myosin to pull the Actin filaments inwards and so shortening the muscle. This occurs along the entire length of every myofibril in the muscle cell.

4.       The Myosin detaches from the Actin and the cross-bridge is broken when an ATP molecule binds to the Myosin head. When the ATP is then broken down, the Myosin head can again attach to an Actin binding site further along the Actin filament and repeat the 'power stroke'.

5.       This process of muscular contraction can last for as long as there is adequate ATP and Ca+ stores. Once the impulse stops the Ca+ is pumped back to the Sarcoplasmic Reticulum and the Actin returns to its resting position causing the muscle to lengthen and relax.

Stretched Muscle

                                                                                                                     

Fully Contracted Muscle

Prerequisites for muscle contraction:

1. Nervous stimulus for contraction.

2. ATP as a source of energy

3. Ca++ ions to expose binding site of actin molecules and initiate ATPase activity of myosin head.

Electrical and biochemical events during muscle contraction:

a.       Depolarisation of Sarcolemma to set up action potential.

b.      Action potential transferred to sarcoplasmic reticulum cause release of calcium ions.

c.       Calcium ions bind to troponin and tropomyosin and thus changes the 3D shape of troponin-tropomyosin-actin complex that makes the active site on actin filaments exposed.

d.      Calcium ions also act on myosin heads, activating them to release energy.

e.      Formation of actin-myosin complex: myosin heads bind to the active sites of actin filament forming cross-bridges.

f.        Sliding of actin filaments over myosin filaments and contraction of muscle.

g.       Repolarisation of sarcolemma and relaxation of muscles.

Energy for muscle contraction:

·         Energy for contraction is provided by hydrolysis of ATP by the enzyme myosin ATPase.

·         Creatine- phosphate ADP change to ATP 

·         When ATP consumption is more muscle fibres respire anaerobically (glycolysis) to replenish ATP. This produce Lactic acid as a by-product.

·         1/5^th of lactic acid is oxidised to CO2 and water.

·         4/5^th of it is changed into glycogen in the liver. (refer fig.3.7)

Cori Cycle

Lactic acid formed during anaerobic respiration in voluntary muscles is carried by blood to liver, where it is converted to glycogen. Glycogen releases glucose in to the blood which is again converted into glycogen in the muscles. This lactic acid-glycogen-glucose-glycogen cycle is called Cori’s cycle.

Motor unit: The set of muscle fibres innervated by all the axonal branches of a motor neuron constitutes a motor unit. The area of contact between a nerve and muscle fibre is known as Motor end plate or neuro-muscular junction. (fig. 3.10)

Threshold stimulus: for being stimulated to contract, the muscle fibre always requires a specific minimum intensity of the nerve impulse. Below this intensity, the stimulus fails to evoke contraction. This is called threshold stimulus.

Summation: When a muscle is stimulated by a single inadequate stimulus, no contraction occurs. But if two or more such below threshold stimuli are given in rapid succession, contraction is evoked. This addictive effect is called Summation.

All or none law: A muscle fibre fails to contract if the strength of stimulus is below threshold. But if the strength of stimulus is equal to or higher than the threshold stimulus, the muscle fibre will contract with maximum force irrespective of strength of stimulus. This is known as ‘All or none law’.

Muscle Twitch and Tetanus: A muscle fibre contracts only once on stimulated by a single nerve impulse or by single electric shock. This single isolated contraction of muscle fibre is called single muscle twitch. Immediately after a twitch, the muscle fibre relaxes.

But, if a muscle fibre is stimulated by a rapid succession, due to repeated brief stimuli, the muscle remains in a state of long contraction phase. Such a sustained contraction is called Tetanus. Almost all our daily activities are carried out by tetanic contraction.

Muscle Fatigue: Reduction in the force of contraction of muscle fibres after their prolonged contraction is called muscle fatigue. It may develop due to lactic acid accumulation, fall of pH, exhaustion of glycogen or exhaustion of ATP.

Refractory period: It is the time between two successive stimuli during which a muscle fibre fails in responding to the second stimulus after the first excitation.

Rigor Mortis: ATP is essential during muscle contraction for dissociation of actin and myosin association. After death, cells cannot synthesise ATP. Hence the actomyosin-ADP bonds fail to separate and muscle remains in a contracted state. This is called Death rigor or Rigor Mortis.

Antagonistic muscles: The set of muscles which contract to produce opposite movements at the same joint are called Antagonistic muscle. Eg: Biceps and Triceps.

Types of muscles on the basis of their function:

1.       Abductors and Adductors: Abductor muscles move the bone away from the middle line, while Adductor muscles moves the bone towards the middle line.

2.       Rotators: Rotators cause a part to rotate on its axis.

3.       Supinators and pronators: Contraction of supinators rotates the forarm and turns the palm upward, while pronators turn the hand or palm downward.

4.       Flexors and Extensors: contraction of Flexor muscle brings two bones closer while extensors increase the angle of a joint.

5.       Levators and depressors: levators raise the part while depressors lower the part.

6.       Invertors and evertors: Invertor muscle turn the sole inward and evertor muscle turn the sole outwards.

7.       Sphinctors: These muscles are arranged like a ring around the apertures. They open or close those apertures.

8.       Tensors: these muscles make a part tense or more rigid.

The skeletal muscles are attached with the bones by tendons. All the skeletal muscles have their origin on one bone and their insertion on the other bone. The broad end of muscle attached to the relatively fixed bone is called Origin. The opposite end of muscle which is attached to the movable bone is called Insertion. The thick part of muscle between the two ends is called belly. (Fig.3.12)

Red and white muscle fibres:

Red muscle fibre	White muscle fibre
Red muscle fibres are thin and smaller in size	White muscle fibre are thick and larger in size
They are red in colour as they contain large amounts of myoglobin	They are white in colour as they contain small amounts of myoglobin
They contain numerous mitochondria	They contain less number of mitochondria
They carry out slow and sustained contractions for a long period	They carry out fast work for short duration