Current Affairs 11th Class

Nervous system is divided into three parts -     Central nervous system (CNS) In all the vertebrates including man, CNS is dorsal, hollow and non-ganglionated while in invertebrates when present, it is ventral, solid, double and ganglionated. CNS is formed of two parts : (1) Brain - Upper and broader part lying in the head. (2) Spinal cord - Lower, long and narrow part running from beginning of neck to trunk. CNS is covered by 3 meninges and its wall has two type of matter. Types of matter : CNS of vertebrates is formed of two types of matter – (i) Grey matter : It is formed of cell-bodies, non-medullated nerve fibres, neuroglea, dendrites of association neurons and motor neurons. (ii) White matter : It is formed of medullated nerve fibres or myelinated axon of motor and sensory neurons, which appear white due to presence of medullary sheath. Meninges : The meninges are connective tissue membranes which surround the brain and spinal cord of CNS. In the fishes, there is only one meninx called meninx primitiva (piamater). In amphibians, reptiles and birds, the brain is covered by two meninges or membranes : inner pia-arachnoid and outer duramater. In mammals, CNS is covered by three meninges or membranes or cranial meninges. Brain meninges are continuous with spinal meninges     The three layers of cranial meninges in order from superficial to deeper duramater, arachnoid and piamater. Duramater is nonvascular, tough made up of fibrous connective tissue. Arachnoid mater made up of reticular connective tissue with collagen and elastin fiber, while innermost vascular piamater (nutritive) made up of loose aerolar connective tissue. Between dura and arachnoid mater presence of sub dural space (no CSF in mammals here), between Arachnoid and piamater presence of sub-arachnoid space (with CSF in mammals, CSF also found in ventricles and central canal). Between duramater and periosteum presence of epidural space. An extension of duramater between two cerebral hemispheres called falx cerebri. Tentorium, an extension of duramater between cerebrum and cerebellum. Cerebrospinal fluid : All the ventricles of the brain, central canal of spinal cord are continuous and lined by a columnar, ciliated epithelium, the ependyma. They contain lymph-like extracellular fluid called the cerebrospinal fluid (C.S.F.). This fluid is secreted by the choroid plexuses by filtration of blood. The choroid plexuses consist of loose connective tissue of pia mater covered internally by a simple cuboidal epithelium of secretory (glandular) nature. The cerebrospinal fluid slowly flows toward the fourth ventricle by secretion pressure and passes into the spinal cord. Some fluid escapes into the subarachnoid spaces through three pores a median aperture (of magendie) and a paired lateral aperture (of Luschka) in the roof of the fourth ventricle in the medulla. From the subarachnoid spaces, the cerebrospinal fluid is transferred to the blood of the venous sinuses. Nervous tissue is more...

(1) Coelenterata : True nerve cell or ganglion cells occur for the first time in coelenterates. They are derived from interstitial cells of epidermis, forming nerve net or nerve plexus below whole epidermis. A polar neurons are found in coelenterata. (2) Platyhelminthes : Nervous system of planarians marks the beginning of a centralized nervous system encountered in higher animals. That is made up of brain or cerebral ganglia, two lateral longitudinal nerve chords, numerous peripheral nerves and transverse commissures or connectives. This is sometimes called the ladder type of nervous system. In Nematoda (e.g. ascaris) these system made up of central nervous system, peripheral nervous system and rectal nervous system. Rectal nervous system more developed in male. Ascaris with dorsal, ventral, and lateral nerve cords. (3) Annelida : Nervous system well developed and concentrated. It consists of three parts : central nervous system, peripheral nervous system and sympathetic nervous system, central N.S. made up of Nerve ring and ventral nerve cord. Nerves are of mixed type, consisting of both afferent (sensory) and efferent (motor) fibres. (4) Arthropoda : The nervous system of prawn or arthropods is of the annelidan type. However it is somewhat larger and has more fusion of ganglia. It consists of (i) The central nervous system including brain connected with a ventral ganglionated nerve cord through a pair of circum-oesophageal commissures, (ii) The peripheral nervous system including nerves and (iii) The sympathetic nervous system. In arthropods like cockroach sympathetic nervous system also known as stomatogastric nervous system, made up of 4 gonglion and retro-cerebral complex. (5) Mollusca : In gastropodes (e.g. pila) consists of paired ganglia, commissures and connective uniting them and nerves running from these central organs to all parts of the body. It has various type of ganglia as cerebral, buccal, pleuro-pedal, supra intestinal and visceral etc. In palecypoda nervous system is greatly reduced due to sluggish and sedentary mode of life and there is little evidence of the brain. But in cephalopoda shows a high grade of organization attained only by some insects and arachnids among the other invertebrates. (6) Echinodermata : Echinodermates has simple and primitive type nervous system.  It has the form of a nerve net, consisting of nerve fibres and a few ganglion cells, all confined to the body wall except the visceral nerve plexus situated in the gut wall. At certain places the nervous tissue is concentrated to form distinct nerve cords. It is made up of (i) Superficial or ectoneural nervous system (ii) Hyponeural or deep nervous system (iii) Aboral or coelomic nervous system and (iv) Visceral nervous system.  (7) Hemichordata : Nervous system is of primitive type resembling that of coelenterates and echinodermates, with both dorsal and ventral nerve cord. (8) Chordates : Nervous system well developed and formed by ectoderm. It is formed by central nervous system, peripheral nervous system and autonomic nervous system.

Nervous system begins develop early in third week of development from ectoderm. Nervous tissue also develop from ectoderm except microglial cell, develop from mesoderm. The central nervous system of vertebrates includes the brain and the spinal cord. These are derived from a longitudinal mid-dorsal ectodermal thickening of the embryo, called the medullary or neural plate. This neural plate or neural groove is converted by fusion into a closed mid-dorsal longitudinal neural tube lying above the notochord. Histologically, the embryonic neural tube exhibits three zones of cells.         (1) Germinal layer : These are actively dividing cells lining the neural canal. They form the connective tissue lining of neural canal, called ependyma, and ventricles of brain. (2) Mantle layer : It consists of embryonic neurons or nematoblasts, forming the gray matter. (3) Marginal layer : It consists of nerve fibres, mostly surrounded by fatty myelin sheaths, and forms the white matter. Neurons and fibres are supported by a special connective tissue of ectodermal origin, the neuroglea, cells of which become increasingly abundant and diversified in higher vertebrates. Development of brain The anterior end of embryonic neural tube is already enlarged forming the embryonic brain, called encephalon. By differential growth and two constrictions, it is divided into a linear series of three primary cerebral vesicles, termed the forebrain, midbrain and hindbrain. These give rise to the three major divisions of the adult brain - (1) prosencephalon (forebrain), (2) mesencephalon (midbrain), and (3) rhombencephalon (hindbrain). These further become subdivided into 5 subdivisions. Prosencephalon divides into an anterior telencephalon and posterior diencephalon; the mesencephalon remain unchanged. The rhombencephalon divides into an anterior metencephalon and a posterior myelencephalon. Ultimately, telencephalon develops into cerebral hemisphere and basal ganglia and houses lateral ventricle. Diencephalon develops into thalamus, hypothalamus, and pineal gland and houses the third ventricle. Mesencephalon develops into mid brain and houses cerebral aqueduct. Metencephalon develops into pons and cerebellum; and myelencephalon develops into medulla oblongata, houses 4th ventricle. The area of neural tube inferior to myelencephalon gives rise to spinal cord.        

Nerve cells (= neurons) : Irritability is a basic characteristic of the “living substance”, i.e., the protoplasm. Consequently, every living cell becomes excited when stimulated. However, the nerve cells and muscle fibres are specialized excitable cells of body, capable of transmitting or conducting excitations along their membranes. Of these, muscle cells are further specialized for contraction while nerve cells are further specialized for receiving stimuli (as sensory or receptor cells) and transferring excitations from one to the other. A typical neuron consists of a nucleated cell body (= cyton, soma or perikaryon), five to seven short, slender and branched (= arborized) dendrites, and a single, relatively thicker and longer fibrous axon. The latter is terminally branched into short telodendria. Each telodendron bears a terminal knob or botton. Bottons of one neuron lie upon dendrites or cytons of adjacent neurons (figure), or upon muscle fibres or glands. Nerve fibres : Although, all parts of a neuron transmit excitations (= impulses), but the transmission is always unidirectional. The dendrites and cytons usually constitute the impulse receiving parts which receive impulses directly from receptors, or from other adjacent neurons. The axons are specialized as fibres conducting impulses away from the receiving parts. Thus, the reaction or response impulses are always carried to the effectors by axons. That is why, the term ‘nerve fibres’ is usually applied to the axons. The latter are 0.1 mm to one or more (upto 10) metres long and about 0.025 m thick on an average. Main properties of nervous tissue : The nervous tissue has two outstanding properties excitability and conductivity. (1) Excitability : It is the ability of the nerve cells and fibres to enter into an active state called the state of excitation in response to a stimulus. Excitation arises at the receptors on account of various stimuli such as light, temperature, chemical, electrical or pressure which constantly act on the organisms. (2) Conductivity : The excitation does not remain at the site of its origin. It is transmitted along nerve fibres. The transmission of excitation in a particular direction is called conductivity. Definition of nerve impulse : A wave of reversed polarity or depolarization (action potential) moving down an axon is called a nerve impulse. Mechanism of conduction of nerve impulse Most accepted mechanism of nerve impulse conduction is ionic theory proposed by Hodgkin and Huxley. This theory states that nerve impulse is an electro-chemical even governed by differential permeability of neurilemma to Na+ and \[{{K}^{+}}\]which in turn is regulated by the electric field. (1) Transmission of nerve impulse along the nerve fibre (i) Polarization (Resting membrane potential-RMP) : In a resting nerve fibre (a nerve fibre that is not conducting an impulse), sodium ions \[N{{a}^{+}}\] and \[C{{l}^{}}\]predominate in the extracellular fluid, whereas potassium ions \[({{K}^{+}})\] predominate in the intracellular fluid (within the fibre). Intracellular fluid also contains large number of negatively charged (anions) protein molecules. \[N{{a}^{+}}\] are 10 times more outside the neuron and \[{{K}^{+}}\] ions are 25 times more inside the cell. Thus more...

The hardened tissues of the body together form the skeleton (sclero = hard). Skeleton of invertebrates is most often secreted on the surface, forming a lifeless or dead exoskeleton. Whereas skeleton of vertebrates develops most often underneath the surface forming a living or growing endoskeleton. Three types of skeletons develop in vertebrates : (1) Epidermal/Horny exoskeleton : These include hard and horny of keratinized derivatives of epidermal layer of skin, such as claws, most reptilian’s scales, bird feathers and mammalian hairs, horns, nails and hoofs, etc. All living amphibians lack an exoskeleton. (2) Dermal/Bony skeleton : Dermal bony skeleton is derived from the dermis of skin. It includes bony scales and plates or scutes (osteoderms), finrays and antlers of fishes, some reptiles (crocodiles, turtles and tortoises) and mammals. In fishes, dermal scales become exposed due to wearing out of epidermis, and form exoskeleton. (3) Endoskeleton : Greater part of vertebrate skeleton lies more deeply, forming the endoskeleton. It develops from mesenchyme. Endoskeleton is formed by bones in vertebrates. Skeleton in different animals are as follows - Invertebrate - (i) Protozoa - No skeleton. (ii) Porifera – Calcarius spicules + silicious spicules + spongin fibre in mesenchyme. Spicules in porifera represent endoskeleton. (iii) Coelentrata - Calcareous (corals) and chitinous (perisarc). (iv) Helminth - No skeleton, cuticle present. (v) Annelida - No skeleton, cuticle present. In earthworm and ascaris is hydrostatic skeleton is found that is fluid is filled in coelom and form turgid skeleton. (vi) Arthropoda - Dead Chitinus exoskeleton, shed at intervals, called ecdysis or moulting. Cuticle made up of non chitinous outer epicuticle and chitinous, inner endocuticle. (vii) Mollusca - Calcarius shell, may be external or internal or absent. (viii) Echinodermata - Dermal calcareous plates are present. (ix) Hemichordates - Endoskeleton in form of proboscis skeleton, pygochord. In vertebrates exoskeleton may be epidermal or dermal. Vertebrates : In vertebrates dermal skeleton is formed by bones. Bone is the connective tissue with intercellular spaces filled with ossein matrix composed of 25% water, 25% protein fibers, 50% mineral salts. The inner most region is full of bone marrow having various types of cells. In mammals the bone is full of haversian canals. The bones are of following types - (i) Cartilage bones : The bones which are formed by the ossification of preexisting cartilage are called cartilage bones or replacing bones. e.g., vertebra, Girdles, limbs bones, basioccipital, supraoccipital, sphenoid, Incus, malleus, stapes. (ii) Membrane or dermal bones : The bones which are formed by independent ossification in connective tissue are called dermal, membrane or investing bones. e.g., Ribs, sternum, clavicle, Nasal, vomer, palatine, maxilla. (iii) Sesamoid bones : Ossification takes place on Ligament cotyloid bone of Rabbit and Tendons e.g., Patella, Pisiform. (iv) Pneumatic bones : Bones with hollow spaces containing air e.g., bones of bird, Frontal, sphenoid ethmoid, maxilla of human. (v) Irregular bones : Vertebrae are irregular bone. (vi) Flat bones : Cranial bone, scapula, Ribs. (vii) Short bones : Carpals and tarsals. Functions of endoskeleton : Chief function more...

Movement is one of the most important characteristics of living organisms. Nonliving objects do not move. If nonliving objects show movement, that is always due to some external force. For example, the cart is moved by the horse and the fan revolves by the energy of electric current. The movement of a nonliving object is, therefore induced (due to external force) while the movement of living things are autonomic (self sustained). Study of movement is called kinesiology (G. Kinein = to move, Logos = study). The movement of living systems are thus autonomic or active, that is effected by the organisms themselves without external influences. On the other hand the movement of nonliving systems are induced or passive, i.e., made to occur by external forces. Movement of animals are two main types muscular and non muscular. (1) Muscular movement : Muscular movement are found in the majority of animals brought about by sliding of myofilaments. Muscular movement are further divide into two kinds – Locomotion and movement of body parts. (i) Locomotion (locus = place + moveo = to move) : Locomotion is the movement of an animal as a whole from one place to another. Types of locomotion : Locomotion takes several forms such as walking (man), creeping (earthworm, lizard), cursorial (Horse, flightless birds),  hopping (frog, rabbit), running (dog, horse), flying (insects, birds) and swimming (fish, whale). Animals have suitable adaptations for their specific mode of locomotion. Adaptations for running, hopping, swimming and flying are respectively called cursorial, saltatorials, natatorial, and volant adaptations. Morphogenetic movement, i.e., the streaming of cells in the early embryo to form tissues or organs, may be considered a form of locomotion. Advantage of locomotion : Locomotion is helpful for animals as escape from predators, search of shelter, food and water, shift to favourable environment, reproduction, collect materials for nest building, locate suitable area for breeding and dispersal to new location. All forms of locomotion require energy to overcome two forces that tend to keep the animals stationary. These are friction and gravity. (a) Swimming : Water is a much denser medium than air so body modified for swimming in the form of buoyancy, fusiform body etc. Mode of swimming varies in animals. fishes swim by moving their body and tail from side to side. Whales and dolphins swim by undulating their body and tail up and down. Insects and 4-legged vertebrates use their legs as oars to push against the water. Cuttle fish and squid are jet-propelled, taking in water and squirting it out in bursts. (b) Locomotion on land : For walking, running, hopping and crawling on land, animal expends energy body to prevent falling down and move forward against gravity. Powerful muscles and strong skeletal support are more important for moving on land than a streamlined body. Creeping animals have their entire body in contact with the ground. Therefore, they make a considerable effort to overcome friction. (c) Flying : Gravity is a major problem in flight. Wings must produce enough lift more...

There are many articulations or joints present in the skeleton. Joint or articulations is a point of contact between bones. Joints are classified based upon their structure and the kinds of movements which they permit. Three main types of joints are - (1) Immovable joints (Synarthroses) : No joint cavity, no movement possible. These joints include - (i) Sutures : Found between skull bones, sutures are fixed or fibrous joints, articulating bones are held together by white fibrous tissue. (ii) Gomphoses : It is a type of fibrous joint in which cone shaped peg fits soket. Teeth in mandibles, and maxillary bones.     (iii) Syndesmosis : It is type of fibrous joint with more fibrous tissue than sutures. e.g., distal articulation between Tibia and fibula. (2) Imperfect joints (Amphiarthroses) slightly movable : Joints in which syanovial cavity is absent. Permit a small amount of movement. Fibrocartilage is placed between the bones. These are cartilaginous joints e.g., Pubic symphysis, between bodies of the vertebrae, between the manubrium and the body of sternum, sacroilliac joint in frog.     (3) Perfect joints (Diarthroses) freely movable : Syanovial cavity and ligaments are present. These are typical joints having articulate surface and syanovial capsule. Syanovial fluid act as a grease in the joint e.g., Joints of elbow, ankle, wrist, hip, knee. Articular cartilage covers the surface of articular bones. Articular cartilage of synovial joint is hyaline cartilage. Synovial joints are surrounded by tubular articular capsule. The articular capsule consists of two layers, outer fibrous capsule and inner synovial membrane. The synovial membrane secretes synovial fluid which lubricates and provides nourishment to articular cartilage. In old age stiffness of joints is due to the decrease in synovial fluid.     (i) Ball and socket joint : Also known as enarthroses. Ball of one bone articulate in socket of another bone. e.g., head of humerus and glenoid cavity of pectoral girdle, femur and acetabulum of pelvic girdle, joint between incus and stapes.     (ii) Hinge joint : Also known as gingulum. Movement is possible in one direction only. e.g., Joint of malleus and incus, knee joint, elbow joint, articulation joint of lower jaw, joint of phalanges of digits.     (iii) Pivot joint : Also known as rotatoria and helps in turning movement. One bone is fixed and second articulate. e.g., Atlas and axial of skull rotate with axis vertebra also known as atlanto axial joint.     (iv) Gliding joint : Also known as arthrodial, limited movement in all more...

(1) Axial endoskeleton : (Skull + Vertebral column + Sternum + Ribs) (2) Appendicular endoskeleton : (Girdle + Limb bones)       Axial skeleton (Human) It occupies the body's main longitudinal axis. It includes four structure : skull in the head, vertebral column in the neck, trunk and tail if present, sternum and ribs in the thorax. It form the upright axis of body and includes 80 (87 in children) bones are as follows in man -     (1) Skull (General structure) : It is anterior most axial skeleton. It is divisible into two main parts – (i) Chondrocranium                                         (ii) Splanchnocranium    (i) Chondrocranium : Chondrocranium is formed by (a) brain box or cranium proper and (b) two sense capsules - Orbit or optic capsule (eye) and  auditory or otic capsule (ear). (a) Cranium proper : It is a strong and firm bony box with a helmet-like covering over the brain, called vault of skull, and a relatively thicker and stronger floor of base upon which the brain rests. Its cavity is called cranial cavity. Size of cranial cavity averages 1475 cubic centimetres \[(c{{m}^{3}})\] in adult men. At about the middle of the floor of cranium, there is a large opening of cranial cavity called foramen magnum. The brain is connected to spinal cord at this foramen. Cranium proper of mammal has following distinct zone -
  • Occipital zone : Occipital zone has one supra-occipital bone on dorsal side, one basioccipital on ventral side and two exoccipital on both lateral side of foramen magnum. Foramen magnum is present in ventral side of skull, which fills on Ist atlas vertebra. Two occipital condyles forming dicondylic skull at the junction of supra and exo-occipital.
  • Parietal zone : In the dorsal side of cranium parietal zone has three bone, that is two parietal, one inter parietal and ventral side of cranium has 3 bone i.e. one basisphenoid with pituitary foramen and two alisphenoid bone.
  • Frontal : Frontal part of cranium has two frontal bone in dorsal side, each frontal bone has one process called supra orbital process of frontal. Two orbitosphenoid, one presphenoid bone in ventral side.
  • Ethmoidal : Ethmoidal part has one circular plate called cribriformplate. That plate is having two perforation for exit of I cranial nerve.
(b) Sense capsule : Chondrocranium contains two sense capsule.
  • Optic or orbital capsule
  • Otic or auditory capsule
Optic capsule : One pair of optic or orbital capsule are present in frontal zone of chondrocranium. It is made up of 7 pairs of bones which are - \[IPre\text{ }frontalIIPost\text{ }frontalIIIAnterior\text{ }orbital\] \[IVPosterior\text{ }orbitalV\text{ }\text{ }Infra\text{ }orbitalVI\text{ }\text{ }Supra\text{ }orbital\]                       \[VIILacrymal\] In frog optic capsules are absent but in place of optic capsule more...

(1) Characteristics of female's skeleton : In female's skeleton, (i) skull is lighter, (ii) shoulders are narrower (iii) sacrum is shorter but wider, (iv) pelvis in wider, has a broader front and larger bottom opening to facilitate child birth, and (v) coccyx is more movable than in male's skeleton. (2) Adaptations in skeleton for upright posture : Human skeleton shows many adaptive features for upright posture – (i) Foramen magnum is directed downward so that the head may rest vertically on the vertebral column. (ii) Four curves in the backbone keep the centre of gravity near the heels. This helps to maintain balance and makes walking erect on two legs much easier. (iii) Thorax is wider from side-to-side than from front-to-back. This helps to maintain equilibrium. (iv) Bowel-like pelvis supports the lower abdominal viscera. (v) Metacarpals form a wide palm and the pollex is opposable. This make the hand a grasping organ to work with it. (vi) Leg bones are stronger than the arm bones as the femur carry the entire weight of the body in locomotion.  (vii) Broad feet provide stability in the upright posture. (viii) The arches of the feet enable the body to move with a degree of springiness. (ix) Increased mobility of the neck to see all round. (x) Increased skeletal height provides greater range of vision. (3) Types of bones : Bones are divided into 4 categories regarding their size and shape – (i) Long bones, e.g., humerus of upper arm, radius and ulna of forearm, femur of thigh, and tibia and fibula of leg. (ii) Short bones, e.g., metacarpals of palm and metatarsals of foot, phalanges of fingers and toes. (iii) Flat bones, e.g., scapula of shoulder girdle, sternum, cranial bones. (iv) Irregular bones, e.g., vertebrae, carpals of wrist and tarsals of ankle. (4) Bone movement : Movements of bones occur only at the joints. The movements are brought about by contractions of skeletal muscles inserted onto the articulating bones by firm cords of white fibrous tissue called tendons. Cords of yellow elastic tissue, termed ligaments, stabilise the joints by holding the articulating bones together. (5) Disorder of skeleton and joints : Any violent movement, such as jump, fall or knock, may cause injury to the skeleton. The injury can be of 5 types - sprain, dislocation, fracture, arthritis and slipped disc. (i) Sprain : Sprain refers to injury to a joint capsule, typically involving a stretching or tearing of tendons or ligaments. Unfortunately, both these structures have much poorer regenerative power than bone, and once stretched often remain weak. Sprain is often considered a minor disorder, but it may become chronic. (ii) Arthritis or Aching Joints : Arthritis refers to inflammation of the joints. It is a common disease of the old age. Its common symptoms are pain and stiffness in the joints. It has many forms. Three more common forms are described here - osteoarthritis or degenerative arthritis, the rheumatoid arthritis and gout. (a) Osteoarthritis : Secretion of the lubricating synovial fluid more...

In the body of all the multicellular animals muscles are found. The movement of the body takes place by these muscles. If the muscles become weak, the functioning of the body become difficult. The muscles are capable of contraction and relaxation, hence these are elastic. A muscle can pull a part of the body by its contraction (shortening). It cannot push that part by relaxation (elongation). Hence, the muscles are typically arranged in antagonistic (opposing) pairs, one muscle moves a body part in one direction by its contraction and the other muscle moves that part in the opposite direction by its contraction. Of course, when one muscle contracts, its opposing muscle relaxes. The principle of antagonistic muscles is true of both vertebrate as well as invertebrate muscles. Animal movements depend upon interaction of muscles and skeleton. (1) Action of body muscles : As mentioned above, the body muscles are arranged in antagonistic (opposing) pairs. One muscle of a pair moves a body part in one direction and the other in the opposite direction. For example, the muscle named biceps brings the forearm toward the upper arm, and the muscle called the triceps moves the forearm away from the upper arm. When biceps contracts to cause movement, the triceps relaxes to allow that movement to occur and vice versa. Similar pairs of opposing flexor and extensor muscles occur at the wrist, ankle and knee. The type of movement that results from the contraction of a muscle depends entirely upon the way the muscle is attached to the levers of the skeleton. (2) Classification of body muscles : According to the type of motion they cause, the muscles are divided into the following types. 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. (i) 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.   more...



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