Key Notes: Locomotion & Movement - NEET PDF Download

• Movement → Significant features of living beings.
• Streaming of protoplasm → Amoeba (a simple form of movement) 
• Movement of cilia, flagella and tentacles → in many organisms. 
• Human beings → can move limbs, jaws, eyelids, tongue, etc. 
• Movements that result in a change of place or location. → Such voluntary movements are called Locomotion.
• Walking, running, climbing, flying, swimming 
• All Locomotion Are Movements But All Movements Are Not Locomotion. 
• Methods of Locomotion → vary with animal’s habitats and the demand of the situation
• Locomotion → Generally for search of food, shelter, mate, suitable breeding grounds, favourable climatic conditions or to escape from enemies/predators. 
• Cells of the human body exhibit three main types of movements, namely, Amoeboid, Ciliary And Muscular.
• Immune cells macrophages and leucocytes in blood → exhibit amoeboid movement. → It is effected by pseudopodia formed by the streaming of protoplasm → Cytoskeletal elements like microfilaments are also involved in amoeboid movement. 
• Ciliary movement occurs in most of our internal tubular organs which are lined by 

Ciliated Epithelium

• The coordinated movements of cilia in the trachea help us in removing dust particles and some of the foreign substances inhaled along with the atmospheric air.
• Passage of ova through the female reproductive tract is also facilitated by the ciliary movement. 

Movement of our limbs, jaws, tongue, etc, require muscular movement. The Contractile Property of Muscles are effectively used for locomotion and other movements by human beings and majority of multicellular organisms. Locomotion requires a perfect coordinated activity of muscular, skeletal and neural systems. 

Muscle

• A specialised animal tissue 
• Mesodermal origin.
• About 40-50 per cent of the body weight of a human adult.
• Have special properties : excitability, contractility, extensibility and elasticity.
• Based on their location, three types of muscles are identified : 

(i) Skeletal  

(ii) Visceral

(iii) Cardiac 

i) Skeletal Muscles

Structure of Skeletal Muscle

⇛ closely associated with the skeletal components of the body. 

⇛  a striped appearance → called Straited Muscles.

⇛ activities are under the voluntary control of the nervous system,

⇛ Voluntary Muscles

⇛ Primarily involved in locomotory actions and changes of body postures. 

ii) Visceral Muscles

⇛ Located in the inner walls of hollow visceral organs of the body like the alimentary canal, reproductive tract, etc. 

⇛ Not have any striation & are smooth in appearance. → also called Smooth MUSCLES (Nonstraited 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. 

iii) Cardiac Muscles

⇛ muscles of heart. 

⇛ Many cardiac muscle cells assemble in a branching pattern to form a cardiac muscle. 

⇛ cardiac muscles are striated. 

⇛They are involuntary 

Skeletal Muscle in Detail

• 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 of 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. 

• 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 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 polymerised protein. 

• Many monomeric proteins called Meromyosins constitute 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

• Best explained by the sliding filament theory 

• 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 along with 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. It is clear from the above steps, that 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 P1 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. 

• The reaction time of the fibres can vary in different muscles. 

• 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. 

• On the other hand, 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.