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Control And Coordination



Every organism must somehow become aware of what is going on around it and accordingly perform actions for its survival. Apart from actions which provide adjustments to the external environment, there are so many activities going on inside our body of which we are unaware. All such actions have to be properly timed and coordinated. Such coordination occurs by two agencies, namely the nervous system and the hormonal system in animals. Chemical substances called phytohormones regulate plant responses.

Plants respond to light, touch, gravitational force and other stimuli. Growth and movements in plants are regulated by both external and internal factors. The function of control and coordination in plants is performed by chemical substances known as plant hormones or phytohormones. Phytohormones are of different types, namely auxins, gibberellin, cytokinin, abscisic acid (ABA) and ethylene.

Auxins were first discovered to be chemical substances and as hormones by F.W. Went in 1928.

  • Some of the synthetic compounds related to auxins are indole-3-butyric acid (IBA), a and b naphthalene acetic acid (NAA), phenylacetic acid (PAA), 2,4-dichlorophenoxy acetic acid (2,4-D), etc.
  • Auxins stimulate cell elongation by modifying the osmotic equilibrium followed by increase in cell wall plasticity and extension and finally cause an increase in wall synthesis.
  • Pruning of trees and hedges is actually done to remove apical buds so that lateral buds may be relieved of their suppression. It results in dense growth of hedges.
  • Artificial application of IAA and NAA induces parthenocarpy.

Gibberellins: Gibberellins are an important class of plant hormones which are produced in the apical regions of shoots and roots. They play a major role in controlling stem elongation for most plants.

  • The important function of gibberellin is to cause de novo synthesis of the enzyme amylase in the aleurone layer of the endosperm of cereal grains during germination. This enzyme brings about hydrolysis of starch to form simple sugars, which are then translocated to the growing embryo to provide energy source.
  • Gibberellins were first isolated from, and named after, a fungus called Gibberella fujikuroi, which infected rice plants. Infected plants became tall and spindly eventually giving rise to poor rice crops. This condition was termed bakanae or ‘foolish seedling disease’.

Cytokinins: The first naturally occurring cytokinin to be chemically identified was from young maize (Zea mays) grains in 1963. Hence, it was named zeatin.

  • Cytokinins are a class of plant hormones that promote cell division. These substances are derivatives of the purine, adenine. Cytokinins are produced in actively growing tissues such as embryos, developing fruits and roots.
  • Cytokinins delay senescence of detached leaves.

Ethylene: Ethylene is the only gaseous hormone. It is a colourless unsaturated hydrocarbon gas which is lighter than air.

  • Most ethylene-induced effects result from ethylene in the air, the effects of ethylene can be contagious.

Abscisic Acid: It is produced mainly in mature leaves. Besides, it is also synthesised in stems and fruits and seeds and then transported to the rest of the plant through the vascular tissue, mainly the phloem. It accelerates senescence in leaves. ABA application to leaf causes the treated areas to become yellow—an effect opposite to that of cytokinin.

The induction of flowering in response to the relative length of the daily light and dark period is called photoperiodism.

  • Short-day plants flower during early spring or fall, when the nights are relatively longer and the days are relatively shorter.
  • Long-day plants flower mostly in summer, when the nights are relatively shorter and the days are relatively longer.
  • Day-neutral plants flower irrespective of the length of the day.
  • Phytochrome is a plant pigment in the leaves of plants that detects the day length and generates a response.
  • Phytochrome is a photoreceptor for red or far-red light and is involved with a number of development processes, such as flowering, dormancy, leaf formation and seed germination.

Nervous System


Importance of Nervous System: The nervous system is the master controlling and communicating system of the body. Nerve-net system is the first and simplest form of nervous system found in cnidarians. The need of a nervous system in our body is for the following major functions:

  1. It keeps us informed about the outside world through the sense organs.
  2. It enables us to remember, to think and to reason out.
  3. It controls and harmonises all voluntary muscular activities.
  4. It regulates involuntary activities such as breathing or the beating of the heart, without our thinking about them.

Neurons Neurons or nerve cells represent structural and functional units of nervous system.

  1. A neuron has main cell body called cyton, from which project out a number of processes called dendrites. One such process is very long and is called axon.
  2. The cell body contains a nucleus. The dendrite receives messages from the organs and transmits them through the cell body into the axon. The axon finally transmits the message either into the muscles (to contract) or to a gland (to secrete).
  3. The axon of one neuron branches towards its end and lies very close to the dendrites of another neuron. This point is called a synapse, where the messages get transmitted.

Nerves A nerve is formed of a bundle of nerve fibres (axon) enclosed in a tubular sheath.

The medullary sheath of the axon acts like an insulation and prevents mixing of impulses in the adjacent fibres. Three kinds of nerves are found as follows:

  1. Sensory nerve contains only sensory fibres bringing impulses from the sense organs to the brain or spinal cord, e.g. optic nerve of the eye.
  2. Motor nerve contains only motor fibres carrying impulses to muscles or glands from the brain or spinal cord, e.g. nerves to the muscles of the eye ball.
  3. Mixed nerve is one which carries both sensory and motor fibres, e.g. the nerve supplying the tongue.

Main Divisions of the Nervous system The human nervous system has the following two main sub-divisions:

  1. The central nervous system (CNS) consisting of brain and spinal cord.
  2. The peripheral nervous system (PNS) consisting of nerves given off by the two parts of the central nervous system.

Central Nervous System

  1. Brain: The human brain is one of the largest organs of the body. It lies well-protected within its bony covering called cranium. Brain contains about 100 billion neurons. The adult brain weighs about 1.35 kg and constitutes about 2% of the total body weight.

It is protected by three membranous coverings, the meninges which continue backwards on the spinal cord. They are as follows:

  1. Durameter: the outermost tough fibrous membrane.
  2. Arachnoid: the thin delicate middle layer giving a web-like cushion.
  3. Piamater: the innermost highly vascular membrane, richly supplied with blood.

The space between the covering membranes is filled with a watery fluid called cerebrospinal fluid which acts like a cushion to protect the brain from shocks.

Parts of a brain: 
Brain is divisible into three major divisions, namely forebrain (prosencephalon), midbrain (mesencephalon) and hindbrain (rhombencephalon).

  1. The forebrain consists of cerebrum, the largest part of the human brain and diencephalon.
  • Cerebrum forms about 85% of total mass of the brain. It consists of two large cerebral hemispheres. The two hemispheres are connected by a bridge of nerve fibres called the corpus callosum.
  • Cerebrum of human brain is more developed in comparison to others. The cerebrum has sensory areas where information is received from the sense organs.
  • Occipital lobe is the region for visual perception, temporal lobe for auditory perception, parietal lobe for touch, smell, temperature and conscious association, and frontal lobe for muscular activities.
  • Diencephalon contains epithalamus, thalamus and hypothalamus. Hypothalamus is the control centre for hunger, thirst, fatigue, anger and body temperature. It links the nervous system to the endocrine system and exercises a regulatory control on the functioning of endocrine glands by secreting neurohormones.
  1. Midbrain: The midbrain consists of optic lobes and cerebral peduncles. It controls muscle tone and some motor activities.
  2. Hindbrain: The hindbrain consists of three centres, namely cerebellum, pons and medulla oblongata.
  • Cerebellum is associated with regulation and coordination of movements, posture and balance.
  • Medulla oblongata is the regulating centre for swallowing, coughing, sneezing and vomiting.
  1. Spinal Cord: Spinal cord is a cylindrical structure. It provides link between the brain and the rest of the body. It begins in continuation with medulla oblongata and extends downwards, enclosed within vertebral column. It is also surrounded by meninges. Spinal cord conducts impulses to and from the brain and controls most of the reflex activities.

Peripheral Nervous System The peripheral nervous system is divided into somatic and autonomic divisions.

  • The somatic nervous system consists of the following two types of neurons:
  1. Sensory or afferent neurons convey impulses to the CNS from sense organs (receptors) located throughout the body.
  2. Motor or efferent neurons carry impulses from the CNS to effectors such as muscles or glands.
  • The autonomic nervous system (ANS) contains only motor neurons and it regulates individual organ function and homeostasis. ANS has two subdivisions, namely sympathetic and parasympathetic. Sympathetic nervous system is concerned with preparing the organism to withstand abnormal conditions while para-sympathetic system is more concerned with re-establishing normal condition.
  • Cranial nerves and spinal nerves are the main constituents of PNS. Twelve pairs of cranial nerves are present in amniotes and ten pairs in anamniotes. They are
    (i) olfactory,
    (ii) optic,
    (iii) oculomotor,
    (iv) trochlear,
    (v) trigeminal,
    (vi) abducens,
    (vii) facial,
    (viii) auditory,
    (ix) glossopharyngeal,
    (x) vagus,
    (xi) spinal accessory and
    (xii) hypoglossal. Thirty-one pairs of spinal nerves are present in humans.

Reflex Action A reflex is a spontaneous, involuntary, nerve-mediated activity produced at the unconscious level by stimulating specific receptors.

  • Each reflex is produced by the flow of nerve impulses along a specific nerve pathway called the reflex arc.
  • The reflex arc comprises some specific receptors, afferent neurons from them to the central nervous system, efferent neurons from the latter to specific muscle fibres or gland cells and a varying number of intermediate neurons conducting impulses from the afferent to the efferent neurons.
  • Pathway of a reflex arc is


  • The simplest type of reflex action is knee-jerk reflex.

Electroencephalograph An instrument called electroencephalograph can record electrical activity of brain. The activity of the brain is recorded as electric potentials. Such a record is called electroencephalogram (EEG).

Sensory Organs
Sense organs are such parts of the body which enable us to know about the conditions of the environment, i.e. light and dark, heat and cold, odours, sounds, etc. The major sense organs are eyeeartonguenose and skin.

The Sense of Sight: Eye

  • Each eye is in the form of an eye ball, which is lodged in the eye socket in the skull. The eyes are provided with movable eyelids which protects the outer surface of eye balls. Eyes are also provided with tear glands which produce tears that wash the eyes to keep them clean and to lubricate their surface.
  • The wall of each eyeball is made up of three layers, namely cornea, choroid and retina.
  • Retina is made up of photosensitive cells, the distribution of which is not uniform. At the back of the eye, in line with the centre of the lens and pupil, there is the most sensitive region of the retina. This is known as the yellow spot or fovea centralis. It has the maximum number of sensory cells. Just below this region is the blind spot. It is called so because there are no sensory cells here. This is the point at which the optic nerve leaves the eye.
  • If a person, who uses concave lenses, removes his spectacles, then the image of the object will be formed in front of retina. Reduction in the elasticity of eye lens with age may result in presbyopia.
  • The compound eye of an insect forms coloured images.

The Sense of Hearing and Balance: Ear Ears are statoacoustic organs meant for both balancing and hearing. The ear of human adult consists of three structural and functional divisions, namely the external ear, middle ear and inner ear.

  • External ear consists of pinna, auditory canal and tympanic membrane. Middle ear consists of three tiny bones called malleus, incus and stapes in a correct sequence from outside to inside.
  • The structure of the inner ear is concerned with the balance in humans.
  • Semicircular canals in the ear of vertebrate are responsible for maintenance of balance when the organism is in motion.
  • In mammals, the nerve impulse for hearing originates in cochlea.
  • A person going up to 10,000 feet high in a hot balloon may develop severe pain in the ear because of blocked eustachian tube.

The Sense of Taste: Tongue The sense of taste is located in the taste buds of the tongue. A taste bud is an ovoid group of sensory and supporting cells. The sensory cells end in hair-like processes and have nerve fibres extending from their bases. The taste hairs project into the outer taste pore located on the surface of the epithelium. They are located chiefly only on the upper surface of the tongue.

  • There are four fundamental tastes in man such as sweet, salt, bitter and sour.

The Sense of Smell: Nose The sense of smell is located in the delicate epithelial layers of the nasal chamber. The sense cells for smell have hair-like projections which respond to particles dissolved in the mucous secretion of the nose. The impulse from these cells is transmitted to the brain by the olfactory nerve. The number of odours we can smell seems to be endless.

The Sense of Touch: Skin
 Skin is chiefly made up of the following two layers:

  • Epidemis: It is the upper layer of the skin. Epithelial cells are protective in nature. The innermost cells have the dark pigment called melanin which gives colour to the skin. Epidermis has sensory nerve cells of hot and cold.
  • Dermis: It is the second or lower layer which is much thicker, made up of connective tissue, is elastic and tough. Dermis contains sweat glands. Secretion of sweat helps in cooling the body when hot. Hairs are found in dermis layer which provide protection. Oil glands (sebaceous glands) are also found in dermis which pour their secretion on the hair and keep the skin and hair soft.

Endocrine System


A gland is an organ whose cells are specialised for producing a particular secretion. Sometimes, a highly specialised single cell constitutes a gland. There are two types of glands in all vertebrates including man. They are glands with ducts (exocrine glands) and glands without ducts (endocrine glands).

  1. Exocrine glands discharge their secretions through the ducts to their target organse.g. salivary glands, sweat glands and digestive glands.
  2. Endocrine glands discharge their secretions directly into blood stream. Chemical substances secreted by the endocrine glands are called hormones.
  3. The characteristic of hormones are as follows:
  • They are specific chemical messengers
  • They are secreted by endocrine glands
  • They are poured directly into the blood and carried by blood circulation
  • They act on specific tissue/organ called target organ
  1. The major endocrine glands in the human body are pituitarythyroidparathyroidadrenalsislets of Langerhans and gonads.
  2. Some of these endocrine glands perform dual functions. For example, as a duct gland, the secretion of pancreas is poured into the duodenum for digestion and as an endocrine gland, the islets of Langerhans of pancreas secrete certain hormones and in addition to producing sperms and ova, and the gonads (testes and ovaries) release certain sex hormones. Hormones are involved in the regulation of several functions, such as growth, metabolic activities and reproduction.

Pituitary Gland The pituitary gland is a small projection which hangs from the base of the mid-brain. It is popularly called the master gland because it seems to control all other endocrine glands.

Pituitary gland has two distinct lobes, the anterior pituitary and posterior pituitary.


Human Endocrine System


Hormones from anterior pituitary

  1. Growth hormone promotes the growth of long bones and muscles during life, when the body is growing. It is also called somatotropin.
  2. Oversecretion (hyper or more) of this hormone in childhood leads to gigantism, which means an abnormal condition of overgrowth.
  • Growth hormone production is stopped in adults. If for any reason growth hormone production starts for the second time, it will not promote growth any more. But it will increase the thickness of lower jaw, hands and feet, giving a gorilla-like appearance. This condition is known as acromegaly.
  • The deficiency (hypo or less) of growth hormone in childhood results in dwarfism.
  1. Thyroid stimulating hormone (TSH) activates thyroid to secrete thyroxin.
  2. Gonad stimulating (gonadotropic) hormones regulate the activities of the testes and ovaries.
  3. Adrenocorticotropic hormone (ACTH) regulates the activity of adrenal cortex.

Hormones from posterior pituitary: The posterior pituitary stores and releases hormones into the blood.

  1. Vasopressin, also called antidiuretic hormone (ADH), constricts blood vessels with rise in blood pressure. It also acts on the kidney increasing the reabsorption of water from the kidney tubules. Deficiency of ADH causes diabetes insipidus in which urination is frequent and copious, resulting in loss of water from the body and the person becomes thirsty.
  2. Oxytocin stimulates vigorous contractions of the uterus in a pregnant mother, leading to the birth of the baby. Thyroid Gland The thyroid is a bilobed structure situated in front of the neck just below the larynx. The two lobes are joined by a narrow isthmus. It secretes two hormones thyroxine and calcitonin.
  • Thyroxine contains iodine. It enhances the metabolic rate, promotes body growth and tissue differentiation.
  • Failure of thyroid secretion produces cretinism in the young age. Cretinism is a condition which affects the growth of children showing dwarfism and mental retardation.
  • Myxoedema is a condition that affects an adult if his/her thyroid does not function properly. In this condition, the person becomes sluggish with swelling of the face and hands.
  • Goitre is a swelling of the neck due to enlargement of the thyroid gland. The gland enlarges in an attempt to increase the output of hormone. This is due to the insufficient quantity of iodine in food.

Parathyroid Gland These are the two small pairs of glands embedded in the back of the thyroid. Parathyroids secrete parathormone.

  • Parathormone increases the blood calcium level by increasing the mobilisation of bone calcium and the renal reabsorption of calcium from the urine.
  • Oversecretion may lead to the depletion of calcium and phosphorus from the bones making them soft.
  • Deficiency of parathormone lowers the blood calcium and consequently produces sustained muscle cramps or tetany.

Adrenal Glands The adrenal glands are like caps, above the kidneys. Each adrenal gland consists of two parts, namely adrenal medulla and adrenal cortex.

  • Adrenal medulla secretes adrenaline and noradrenaline. Adrenaline is a hormone which prepares the body to meet any emergency situation, for ‘fight’, i.e. to face danger or for ‘flight’, to run away from it and ‘fear’.
  • Adrenal cortex secretes three groups of steroid hormones.
  1. Mineralocorticoids such as aldosterone increase retention of Na+ in the body and elimination of K+ from the body.
  2. Glucocorticoids such as cortisol regulate the metabolisms of carbohydrates, proteins and fats.
  3. Sex corticoids develop external male sex characters.
  • Hyposecretion from adrenal cortex causes Addison’s disease. Symptoms are loss of energy, skin pigmentation, loss of weight, nausea, hypoglycemia, sensitivity to cold and pain, increased susceptibility to infections, etc.
  • Hypersecretion of adrenal cortex causes Cushing’s Syndrome. Symptoms are obesity, hyperglycemia, osteoporosis, weakness, salt and water retention.

Adrenal virilism is caused in women by over-secretion of sex corticoids, producing beards, moustaches and the male voice. Men with adrenal virilism develop some feminine characteristics, such as enlargement of breasts.

Pancreas is both a duct gland and a ductless gland. As a ductless gland, it has special groups of hormone-secreting cells called islets of Langerhans, which are scattered in the entire gland.

The islets produce three hormones, namely insulin, glucagon and somatostatin from three different kinds of cells called betaalpha and delta cells, respectively.

 lowers the blood sugar by increasing the utilisation of glucose and storage of glucose as glycogen in the tissue. Insulin deficiency produces diabetes mellitus with high blood sugar, excretion of sugar in the urine and increased urinary volume.

 causes the breakdown of glycogen into glucose, which in turn is released into the blood to maintain glucose levels within a homeostatic range. Glucagon production is stimulated when blood glucose levels fall, and inhibited when they rise.

Somatostatin inhibits secretion of insulin and glucagons.

Testicular interstitial cells secrete testosterone. It stimulates growth and functions of secondary male sex organs, and the development of male external sex characters.

Failure of testosterone secretion produces eunuchoidism with infantile male sex organs and undeveloped external sex characters.

The Graafian follicle and corpus luteum of ovary secrete estrogens and progesterone, respectively. Estrogens stimulate the growth of female secondary sex organs. Progesterone controls pregnancy changes.

Control of Hormonal Secretion
The amount of hormone released by an endocrine gland is determined by the body’s need for the particular hormone at any given time. The product of the target tissue exerts an effect on the respective endocrine gland. This is called feedback mechanism. This may be positive (secrete more) or negative (secrete no more).

Example includes hypothalamus of the brain which releases a hormone (TSH-RH
thyroid stimulating hormone releasing hormone) which orders the anterior pituitary to release thyroid-stimulating hormone (TSH). This TSH stimulates thyroid to release thyroxin. If the level of thyroxin in blood increases, it affects the pituitary so as to inhibit release of TSH. If the level of thyroxin had become still higher, then the inhibition by thyroxin would take place, both at the level of pituitary and hypothalamus to inhibit the release of TSH-RH. When the level of thyroxin falls in the blood, the thyroid gets stimulated to secrete more of it.

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