Control and Coordination Class 10 Notes
Control and Coordination
The changes in the environment (or environmental factors) to which the organisms respond and react are called stimuli (singular : stimulus). Environmental factors such as light, heat, cold, touch, taste, smell, sound, water current, force of gravity etc., act as stimuli to induce responses and reactions in all living organisms. Different organisms respond and react to stimuli differently. Most of the plants do not possess any special structures for the perception of external stimuli. Responses of different animals to ‘touch’ stimulus vary a great deal. Among animals, simple organisms respond to external stimuli by moving towards them (positive response) or away from them (negative response).
The working together of various organs (parts) of the body of an organism in a proper manner to produce proper reaction to a stimulus, is called coordination.
For proper control and coordination, higher animals, in fact, have evolved two systems – nervous system and endocrine system (hormonal system).
Coordination in Plants
Plants coordinate their responses against environmental stimuli by using hormones. The mode of action of hormones in plants is different from that in animals. In plants, the hormones coordinate their behaviour by affecting the growth of the plant. As a result, part of a plant shows movement. Thus, growth and movements in plants are regulated by both external and internal factors. Plants can not respond as quickly as animals because they lack nervous system. Instead, plants respond to various stimuli very slowly by revealing growth. Therefore, generally the response of the plants to various stimuli can not be observed immediately.
Phytohormones
Charateristics of Plant Hormones
(1) They can have a positive effect on a process and thus promote it or they may have a negative effect and cause inhibition.
(2) They are produced in small amounts.
(3) They act away from their site of production and reach there by simple diffusion.
(4) They are very specific in their action.
(5) The synthesis and action of phytohormones is greatly influenced by the external stimuli.
(6) They control and coordinate growth, development and responses to the stimuli.
So far five types of phytohormones have been identified.
Auxin (G : auxein to grow)
(1) It also occurs in the human urine, especially in person suffering from pellagra.
(2) Promotes elongation and growth of stems and roots.
(3) Promotes enlargement of many fruits.
(4) Promotes cell division in vascular cambium.
(5) Help in root initiation in cutting.
(6) Cause apical dominance (terminal or apical bud inhibits the development of lateral buds).
(7) Used in parthenocarpy (production of seedless fruits).
(8) Promote production of female flowers.
(9) Causes callus differentiation in tissue cultures.
(10) An inhibitory effect of auxin is on abscission of leaves and fruits. Leaves and fruits must produce auxin continuously to prevent the formation of the abscission zone which cuts off their nutrient and water supply and leads to leaf fall and fruit drop.
Eg. IAA (Indole acetic acid, natural auxin); 2, 4-D (2, 4-Dichlorophenoxyacetic acid), NAA (Naphthalene acetic acid); IBA (Indole butyric acid.)
Gibberellins
(1) The first gibberellin was isolated from the culture of a fungus Fusarium moniliforme.
(2) It stimulates stem elongation and leaf expansion.
(3) Used in the elongation of genetic dwarf (mutants) varieties of plants such as corn and pea.
(4) Help in breaking seed dormancy (inability of seed to germinate even in the presence of all favourable conditions.)
(5) Help in seed germination.
(6) Promotes flowering in some plants even under unfavourable condition (breaks bud dormancy).
(7) Promotes production of male flowers.
(8) Gibberellins, alongwith auxin, control fruit growth and development. Gibberellins are now used in India to increase the fruit size and bunch length in grapes.
E.g. Gibberellic Acid (GA) or Gibberellin A3 (GA3).
Cytokinin (Cytokinesis – cell division; CK)
(1) This group of hormones mostly act in conjunction with auxins.
(2) Promotes cell division and elongation.
(3) The ratio of cytokinins to auxins controls cell differentiation. When both are present in relatively equal quantities, cells divide but do not differentiate. If there is more cytokinin than auxin, shoot buds develop from a callus. If there is more auxin than cytokinin, roots develop.
(4) Cytokinins promote production of female plants.
(5) Help in secondary growth (growth in thickness).
(6) It suppress apical dominance (promotes lateral branches in the presence of apical bud).
(7) It can retard or delay ageing in plants (senescence) by controlling protein synthesis and mobilization of resources. Eg. Zeatin, Kinetin
Ethylene
(1) It is the only gaseous natural phytohormone.
(2) High concentrations of auxin induce ethylene formation.
(3) Promotes fruit growth and ripening.
(4) Accelerates abscission of leaves, flowers and fruits.
(5) Promotes production of female flowers.
(6) Cause ageing (senescence).
(7) Inhibits stem elongation and stimulates transverse expansion so that the stem looks swollen.
(8) Cause epinasty (more growth on upper surface).
Abscisic Acid (ABA)
(1) Inhibits growth, hence called antiauxins.
(2) Increase seed and bud dormancy, hence called antigibberellins.
(3) Reduce transpiration by closing stomata under water stress conditions, hence called stress hormone.
(4) Stimulate formation of abscission zone (zone of separation).
(5) cause senescence.
Plant Movement
The movements in plants are due to external stimuli. These are of three types :
(i) Tropic movements
(ii) Nastic movements
(iii) Tactic movements.
Tropic movements
Stems and roots show this type of movements. Stems bend towards light. Tendrils when touches a rough object, turn around the object. This type of movement is due to unilateral (one sided) stimulus. It causes differential growth on the two sides of the stem and tendril, which causes bending. The external stimuli which cause movement in plants are gravity, light, touch, water and chemical substances. The movements stimulated by them are geotropism, phototropism, thigmotropism, hydrotropism and chemotropism respectively.
(a) Geotropism : The stem moves away from the gravity and root grows towards the earth’s gravity. It can be shown experimentally. If a potted plant is placed horizontally for few days, stem apex bends upwards and becomes vertical. Similarly, its root bend downward towards the gravity.
Figure: The effect of gravity on root and stem
Figure : Seedling of maize kept in different positions
It can also be seen in seedling of maize in an earthen pot kept in different directions. Its radicle always move downwards towards gravity and plumule towards light. This type of bending in stem (plumule ) and root (radicle) of germinating seed is due to stimulus of gravity. Stem is negatively geotropic and root is positively geotropic.
This type of growth is due to auxins (chemical substances or hormones) produced by the shoot and root tips. These hormones diffuse to the region of response, i.e., in root on the lower surface causing inhibition of growth on the lower surface, and the upper surface of root grows rapidly, causing bending towards gravity of earth.
In geotropism, concentration of auxins in stem is more on the lower side, causing fast growth on this side due to whichstem bends upward. Thus, in roots more concentration of auxins stops growth and in stems promotes growth.
(b) Thigmotropism : Stimulus of contact with solid rough surface is called thigmotropism. It is visible in tendril and twinner. Tendril coils around the support as it touches that.
(c) Phototropism : Bending of stem towards source of light is called phototropism. Stems are positively phototropic and roots are negatively phototropic. It is due to unidirectional light source. It causes the stem apex to move towards the source of light and in root, away from it. It is due to unequal growth of the illuminated and non-illuminated (shaded) sides of the stem and root apex. In stems growth is more on the shaded side and in roots it is more on the illuminated side. In stems, the shaded side has more auxin concentrations, so it shows more growth.
Figure: Experiment to show effect of auxin on the growth of a plant in response to light (phototropism)
(d) Hydrotropism : Growth movement in plants in response to moisture is called hydrotropism. Roots grow towards water, so they are positively hydrotropic.
(e) Chemotropism : The movement of plant organs due to unilateral chemical stimulus is chemotropic, e.g., growth of pollen tube of pollen grain towards ovary through style.
Figure: Pulvinus Base (section) of Mimosa pudica
After sometime when stimulus diminishes, cells again become turgid and the leaf returns back to its normal position.
Nastic Movements
These are found in sepals and petals of a flower. It occurs due to differences in the rate of growth on two opposite sides of bifacial organs. Examples are opening and closing of sepals and petals of flowers. This movement is due to stimulus of light and temperature.
Seismonasty : Leaves of ‘touch-me-not’ plant (Mimosa pudica) drops rapidly when touched. It is due to turgidity of cells at the base. Here touch response is diffused affecting the entire leaf. Its leaves are bipinnately compound. Basal part of petiole of each leaf is swollen called pulvinus formed by parenchyma. The vascular strand of leaf passing through the petiole divides pulvinus into upper stable and lower sensitive part. In normal condition both parts remain turgid and thus, leaf remains erect. After touching the leaf, stimulus reaches the pulvinus. The upper half remains unaffected but cells of the lower half loose water and become flacid. Thus, leaf droops down due to loss of turgor pressure.
Tactic movements
In this type, entire plant or a part of it swims to the place of stimulus. It is found in bacteria, algae, spores, etc. The direction of movements depends upon light, temperature, oxygen and chemical substances.
Photoperiodism
It has been observed that some plants require a periodic exposure to light to induce flowering. It is also seen that such plants are able to measure the duration of exposure to light. For example, some plants require the exposure to light for a period exceeding a well defined critical duration, while others must be exposed to light for a period less than this critical duration before the flowering is initiated in them. The former group of plants are called long day plants while the latter ones are termed short day plants. The critical duration is different for different plants. There are many plants, however, where there is no such corelation between exposure to light duration and induction of flowering response; such plants are called day-neutral plants. It is now also known that not only the duration of light period but the duration of dark period is also of equal importance. Hence, it can be said that flowering in certain plants depends not only on a combination of light and dark exposures but also their relative durations. This response of plants to periods of day/night is termed as photoperiodism. The site of perception of light/ dark duration are the leaves. It has been hypothesised that there is a hormonal substance that is responsible for flowering. This hormonal substance migrates from leaves to shoot apices for inducing flowering only when the plants are exposed to the necessary inductive photoperiod.
Eg. (1) Long day plants : spinach, radish, sugerbeet, etc.
(2) Short day plants : sugarcane, tobacco, soyabean, chrysanthemum, etc.
(3) Day neutral plants : tomato, cucumber, sunflower, cotton, pea, etc.
Control and Coordination in Animals
Chart 1 : Endocrine and Nervous Control
Human Nervous System
Basic Functions of Nervous System
- It controls and coordinates the body functions so that various organs of the body operate harmoniously to ensure a smooth operating system.
- It receives sensory impulses from the sensory organs through afferent nerve fibres (sensory function), analyses, processes and integrates these informations and initiates the motor impulses which are carried to effector organs like muscles and glands to show response (motor function).
- It keeps the previous stimuli as the experiences or memory which guide the animal in future.
- It coordinates the visceral functions to maintain a homeostasis in the body fluids.
♦ Neurons are altogether absent in the sponges e.g., Sycon.
♦ In coelenterates (e.g. Hydra) diffused nervous system is present. It is formed of epidermal and gastrodermal nerve nets in which neurites or neurons are not differentiated into dendron and axon.
♦ In flatworms (e.g. Planaria, liver fluke, etc.) nervous system is of ladder type. It is formed of a primitive nerve ring and nerve cords. The latter are interconnected by transverse connectives.
♦ In annelids, nervous system is formed of a circumpharyngeal nerve ring and single, solid, ventral and ganglionated nerve cord.
Mode of Control of Nervous System
There are two control systems in the human body which control and coordinate the body functions : nervous system and hormonal system. Nervous control differs from the hormonal control in following characteristics :
- The nervous control is rapid.
- It takes place through electrical signals called nerve impulses.
- Nerve impulses travel in specific direction.
Parts of Nervous System
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 and ganglionated. CNS is formed of two parts : Brain upper and broader part lying in the head; and spinal cord – lower, long and narrow part running from beginning of neck to trunk.
- Peripheral Nervous System (PNS) : It is formed of long, thin, whitish threads called nerves which extend between CNS and body parts (muscles, glands and sense organs). It controls the voluntary functions of the body. It has cranial and spinal nerves.
- Autonomic Nervous System (ANS) : It is formed of nerve fibres extending upto visceral organs and controls the involuntary functions of visceral organs of body like heart beat, peristalsis etc. It is again formed of two systems: sympathetic and para-sympathetic nervous system, with opposing functions.
Structure of Neurons :
Each nerve cell consists of two parts :
(1) Cell body or Cyton
(2) Axon
Cytons of neurons vary in size from small to large. Their shape varies from round, or oval to pyramidal. In the centre there is present a nucleus which is usually centrally placed, large, spherical and has a prominent nucleolus. The cytoplasm of neurons contains Nissl’s granules. From the cyton, there are processes known as dendrites which are helpful in conveying impulses towards the cell body. From the cyton there extends a long process known as axon. Axon often branch. Axons are surrounded by myelin sheath which is a fatty substance. The amount of myelin varies considerably; in fact some small fibres have less or no myelin. They are known as unmyelinated. Outside the neuron there is an additional covering known as neurilemma sheath which is made up of Schwann cells. These cells lay down the myelin around the fibres. This myelin, laid down by neurilemma cells, is notched at regular intervals. These notches are known as Nodes of Ranvier. Neurons having two sheaths i.e., inner thick myelin (or medullated) sheath and outer thin neurilemma sheath are known as myelinated or medullated, while those having only neurilemma sheath are called as nonmyelinated or nonmedullated.
Figure: Neuron
Multipolar Neurons
In this type of neurons, cyton is branched at many points thus forming many poles. These are associated with many functions.
Each neuron is a separate structure, i.e., it does not have any cytoplasmic continuity with other neuron. Two neurons are connected only by passage of impulse across intervening junctions which is called synapse. The information passing through neurons is in the form of chemical and electrical signals called nerve impulse.
Figure : Synapse between two neurons
Chemical Transmitter : As there is no cytoplasmic continuity between neurons at a synapse, an impulse is passed from one neuron to another, when the first neuron liberates a chemical substance known as neurotransmitter, which excites a fresh nerve impulse in the next neuron. One example of chemical transmitter is acetylcholine.
Table 1 : Differences between Axon and Dendron
| S.NO | Characters | Axon | Dendron |
| 1 | Number | Always single | One or more in number |
| 2 | Structure | Formed of neuroplasm with only neurofibrils but no Nissl’s granules. | Formed of neuroplasm with both neurofibrils and Nissl’s granules. |
| 3 | Size | Long sized | Small sized |
Central nervous system
Central nervous system in human beings is highly developed. It lies along the middle line of the bony cavity. It consists of (i) brain and (ii) spinal cord.
The central nervous system consists of the brain and the spinal cord. The areas of the CN system where the cell bodies of neurons are situated, look grey and constitute the grey matter. Other areas containing only nerve fibres look white, due to presence of myelin around the myelinated fibres and constitute the white
matter.
- Brain : Brain is a soft, whitish and somewhat flattened organ. It is protected by the cranium, a body box in the skull. It is the highest coordinating centre in our body. In man, it is about 1200-1400 g in weight and has about 10,000 million neurons.
Figure : Diagram showing saggital section of the human brain
Meninges (Protective Coverings) :
Brain is covered by three protective membranes, called meninges (singular : meninx). These are pia mater, arachnoid mater and dura mater. Space between these membranes is filled by a fluid called cerebrospinal fluid. Meninges and cerebrospinal fluid protects the brain from mechanical shocks. This fluid helps in the nutrition of neurons and maintains a constant pressure inside the cranium in spite of fluctuations in the volume and pressure of blood in the cranial vessels.
Brain is broadly divided into three regions :
Each of these regions of brain consists of various centres. In most parts of the brain, the grey matter is situated on the surface while the white matter is located deep inside the brain.
(i) Forebrain
Forebrain consists of olfactory lobes, cerebrum and diencephalon.
(a) Olfactory lobes : These are a pair of small, solid, club-shaped bodies. They are fully covered by cerebrum. They receive impulses for smell.
(b) Cerebrum : Cerebrum is the largest part of the brain. It consists of two cerebral hemispheres joined by a thick band of nerve fibres called Corpus Callosum. Surface of cerebral hemispheres made up of grey matter is called Cerebral cortex. Cerebral cortex is made of bodies of billions of nerve cells. It becomes highly folded to increase area for accommodation of more neurons. The folds are called gyri (singular : gyrus) and depressions between them are called sulci (singular : sulcus). Deep and wide sulci are called fissures. Fissures divide each cerebral hemisphere into four lobes.
(1) Occipital lobe-region for visual perception.
(2) Frontal lobe- for muscular activities.
(3) Parietal lobe-for pain, touch, smell, temperature and conscious association.
(4) Temporal lobe-for auditory reception.
(c) Diencephalon : Diencephalon encloses a cavity called third ventricle. It consists of thalamus and hypothalamus. Thalamus serves as a relay centre for pain, sensory and motor impulses from spinal cord and medulla oblongata to cerebrum. It recognises sensory impulses of heat, cold, pain, light and pressure.
Hypothalamus possesses control centres for hunger, thirst, body temperature, sleep, sex, stress, etc. It is also an important endocrine gland which controls the activities of anterior pituitary.
(ii) Midbrain : It consists of two heavy fibre tracts, called cerebral peduncles or crura cerebri. These tracts connect forebrain to the hindbrain. It has two optic lobes (Corpora quadrigemina). These are centres for control of eye movements and hearing responses.
(iii) Hindbrain : Hindbrain consists of cerebellum, pons varolii and medulla oblongata.
(a) Cerebellum : Cerebellum, like cerebrum, is very large and well-developed. It consists of two large lateral cerebellar hemispheres and a median small vermis. It controls coordination and adjustment of movements (equilibrium) and posture.
(b) Pons Varoli : It is a cross-wise bundle of nervous tissue that lies on the antero-ventral side of medulla oblongata. It connects the cerebellum, medulla oblongata and cerebrum. Pons functions as relay centre among different parts of brain. It also possesses pneumotaxic area of respiratory centre.
(c) Medulla oblongata : It is the most posterior part of the brain and continues into the spinal cord. It encloses a cavity, called the fourth ventricle. It controls involuntary functions of the body such as heartbeat, rate of breathing, secretion of saliva, swallowing,coughing, sneezing, vomiting, blood pressure, etc.
Spinal Cord
Spinal cord is about 45 cm long cylindrical structure. It lies in the bony vertebral canal of the vertebral column. It starts from medulla oblongata and extends downwards. It contains a cavity called central canal. Central canal is the extension of fourth ventricle of medulla oblongata, and it also contains cerebrospinal fluid.
Like brain, it is also formed of grey matter and white matter. Grey matter is internal and white matter is external. A cross-section of spinal cord reveals that grey matter forms two horns-dorsal horn and ventral horn. Dorsal horn is joined by sensory nerve which brings impulses from sense organs to the spinal cord. From ventral horn arises motor nerves which carry messages from spinal cord to the organ concerned.
Peripheral Nervous System (PNS)
The peripheral nervous system consists of the nerves between the central nervous system (brain and the spinal cord) and various organs. Thus, peripheral nervous system carries impulses to and from the central nervous system. Each nerve of PNS is composed of many nerve fibres enclosed by a connective tissue sheath.
Peripheral nervous system consists of two sets of nerves :
- Cranial Nerves : These nerves either arise from or end into the brain. There are 12 pairs of cranial nerves, some of which are sensory while others are either motor or mixed nerves.
- Spinal Nerves : These nerves emerge from the spinal cord. Each spinal nerve is a mixed nerve having both sensory and motor fibres. There are 31 pairs of spinal nerves and one pair arises from each segment of the spinal cord.
(a) Cervical nerves (neck region) 8 pairs
(b) Thoracic nerves (thorax) 12 pairs
(c) Lumbar nerves (upper part of abdomen) 5 pairs
(d) Sacral nerves (lower part of abdomen) 5 pairs
(e) Coccygeal nerves (Tail region) 1 pair
Total spinal nerves 31 pairs
Autonomic Nervous System
The autonomic nervous system or ANS is auto-functioning or “self-governed”. It plays a major role in fine-tuning of body’s internal environment, and this, in a sense, allows our behaviour to proceed normally. Functionally, like the voluntary somatic nervous system, the autonomic nervous system achieves its control of smooth muscles, cardiac muscles and certain glands, by both feedback loops and antagonistic control, but without conscious control of CNS. Structurally, the ANS consists of two separate output systems, the sympathetic and the parasympathetic divisions.
Figure : Autonomic Nervous System
The neurotransmitter within the ganglion is acetylcholine for both sympathetic and parasympathetic nerves. However, the neurotransmitter between the terminal autonomic neuron axon and the target organ is different in the two antagonistic autonomic nervous systems. In the parasympathetic system, the neurotransmitter at the terminal synapse is acetylcholine, just as it is in the ganglion. In the sympathetic system, the neurotransmitter at the terminal synapse is either adrenaline or noradrenaline, both of which have an effect opposite to that of acetylcholine.
Table 1 : Functions of autonomic nervous system
| Organ Innervated | Function of Sympathetic system | Function of Parasympathetic system |
| Heart | Accelerates heart beat | Slows heart beat |
| Arteries | Constricts arteries and raises blood pressure | Dilates arteries and lowers blood pressure |
| Digestive tract | Slows peristalsis, decreases activity | Speeds peristalsis, increases activity |
| Salivary glands | Inhibits secretion | Stimulates secretion |
| Gall Bladder | Relaxes | Contracts |
| Gastric glands | Inhibits secretion | Stimulates secretion |
| Pancreas | Inhibits secretion | Promotes secretion |
| Intestinal glands | Inhibits secretion | Stimulates secretion |
| Liver | Promotes sugar release; decreases bile production | Promotes glycogen formation; increases bile production. |
| Urinary bladder | Relaxes bladder | Constricts bladder |
| Muscles in bronchi | Dilate passages, making breathing easier | Constricts passages |
| Muscles of iris | Dilate pupil | Constricts pupil |
| Muscles attached to hair | Cause erection of hair | Cause hair to lie flat |
| Sweat glands | Increases secretion | Decreases secretion |
Reflex Action and Reflex ARC
You must have experienced a sudden withdrawl of a body part which comes in contact with objects that are extremely hot, cold, pointed or animals that are scary or poisonous. The entire process of response to a peripheral nervous stimulation, that occurs involuntarily, i.e., without conscious effort or thought and requires the involvement of a part of the central nervous system is called a reflex action. The reflex pathway comprises at least one afferent neuron (receptor) and one efferent (effector or excitor) neuron appropriately arranged in a series. The afferent neuron receives signal from a sensory organ and transmits the impulse via a dorsal nerve root into the CNS (at the level of spinal cord). The efferent neuron then carries signals from CNS to the effector. The stimulus and response thus forms a reflex arc (as shown in figure 3 in the knee jerk reflex).
Figure Diagrammatic Presentation of Reflex Action (Showing Jerk Reflex)
Electro Encephalogram (EEG)
An instrument called electro encephalograph can record electrical activity of brain. The activity of the brain is recorded as electric potentials. Such a record is called Electro encephalogram (EEG). By placing two electrodes on the scalp and leading via suitable amplifier to link writing device, a record of four different types of waves is obtained. These waves named as alpha, beta, delta, and theta, vary in frequency. These waves give the characteristic activity of brain which is very useful for clinical purposes.
Hormones
Definition of Hormones
The term Hormone (Gr., hormaein = to excite or stir up, i.e., to arouse activity, or to set in motion) gives the idea of a biologically highly active organic substance. In 1903, Baylis and Starling extracted the first hormone from secretory cells of duodenal mucosa and named it secretin, because it was found to stimulate the pancreas for secretion. “Hormones are compounds released by certain cells to alter the metabolism of other cells near or far, reaching their target cells through blood stream, axoplasmic flow, or immediate intercellular space”.
Hormones are also called chemical messenger or informational molecules.
Characteristics of Hormones
(1) Hormones are not found in food. These are synthesized in the body itself by endocrine cells.
(2) More than fifty hormones have been discovered. There are mainly four categories of these organic compounds, viz proteins, steroids, amino acid derivatives and eicosanoids.
(3) Hormones are released by endocrine cells in response to body requirements. These are not stored in the body.
(4) Some hormones affect all cells of body. Mostly, however, a particular hormone affects cells only of a specific type in definite parts of body, and in a definite manner. This is called hormone specificity. The specific cells affected by a hormone, are called its target cells. Since all blood-borne hormones circulate in the whole body, the mechanism of taking in only specific hormones from blood resides in the target cells themselves. This proves that hormones carry coded informations which can be decoded only by target cells.
(5) Hormones do not directly take part in metabolic reactions of target cells. Instead, these influence synthesis and the activity levels of metabolic enzymes to alter the rates and form of metabolic processes. Thyroxine hormone, for instance, enhances the oxidation process in mitochondria of all cells, elevating A TP-formation.
(6) A few protein hormones differ somewhat in various animal species, but most do not possess this species specificity. Hormones of one mammalian type can, therefore, be pharmaceutically used for other types.
The Glands of Body
A gland is such a cell, or such a tissue or organ whose cells synthesize and secrete some compound. Several types of glands are found in vertebrate body. Unicellular glands occur here and there in epithelial layers. Multicellular glands are formed by foldings of epithelia. These are of three types.
(1) Exocrine glands (Gr., ex = out + krinein = to secrete): Their secretions are carried by their ducts and poured on the surfaces of concerned epithelia. These glands are, therefore, called duct glands. Salivary glands, digestive glands, sweat and sebaceous glands of skin, etc.
(2) Endocrine glands (Gr., endo = within + krinein = to secrete): Being disconnected from the epithelia of origin, these glands become ductless. Hence, their secretions (hormones) are released in ECF, diffuse into blood and circulate throughout the body. That is why, these glands possess dense networks of blood capillaries. Thyroid, parathyroid, adrenal, pituitary, pineal and thymus glands are endocrine. The endocrine glands which secrete only hormones are called holocrine glands.
(3) Mixed or Heterocrine glands : These are duct glands whose major part is exocrine, but some endocrine cells or tissues also occur. Pancreas is a well known example of such a gland, where the exocrine tissue secrete pancreatic juice and endocrine tissue secrete hormones. Gonads are also an example of mixed gland.
Figure : Various Endocrine Glands in Human
The Hypothalamus and Feedback Mechanism
Figure : Feed Back Control
Figure : Thyroid gland
Table 1 Hormones and their functions
| Endocrine glands and their location | Hormone secreted | Principal functions |
| Hyothalamus. It lies below thalamus. | Releasing factors eg TRH, GRH, PRH, ARH, MRH. Inhibitory factors eg GIH, PIH, MIH. | Function Stimulates the activity of target gland orInhibit the activity of target gland. |
| Pituitary Gland: It has three lobes. It is attached to the lower surface of the brain. | ||
| Anterior lobe | ||
| Growth Hormone (GH) or Somatotrophic Hormone (STH) | Controls the overall growth of the body, muscles and bones. Lack of this hormone (hypoactivity) causes dwarfness. Its excessive secretion from childhood (hyperactivity) causes excessive growth of long bones making the person very tall (gigantism). | |
| Adrenocorticotrophic Hormone (ACTH) | Controls the growth and function of adrenal cortex. Stimulates the adrenal cortex to secrete steroid hormones called glucocorticoids. | |
| Thyroid Stimulating Hormone (TSH) | Controls the growth and functioning of the thyroid gland. Stimulating the thyroid gland to produce thyroxine. | |
| Follicle Stimulating Hormone (FSH) | In males, it stimulates the process of spermatogenesis. In females, it stimulates the follicle cells in the ovaries to develop into mature eggs and also stimulates them to produce oestrogen. | |
| Luteinizing Hormone (LH) | In males, it stimulates the secretion of male hormone, testosterone, which in turn influences the appearance of secondary sexual characteristics. In females, it stimulates the secretion of estrogen and progesterone, which in turn influence the process of |
| Endocrine glands and their location | Hormone secreted | Principal functions |
| Prolactin Hormone (PRL) | It enhances mammary gland development and milk production in females after child birth. | |
| Middle Lobe | Melanocyte Stimulating Hormone (MSH) | Controls the production of melanin pigment in the skin which is responsible for skin colour. |
| Posterior Lobe | Oxytocin | Controls the vigorous contraction of uterine muscles at the time of child birth. It also helps in milk ejection from the mammary glands to provide nourishment for the newborn during feeding. |
| Vasopressin or Antidiuretic Hormone (ADH) | Controls reabsorption of water in kidney tubules. Disorder: Diabetes insipidus | |
| Pineal Gland It is attached to ventral side of the brain. It lies near the pituitary. | Melatonin | It has antigonadal function. It may delay the development of sexual organs in the body. |
| Serotonin | It decreases the diameter of blood vessels thus regulating blood pressure. | |
| Thyroid Gland It is situated in the neck region on the ventral side of the body. It has two lateral lobes, one on either side of the trachea joined by a narrow band called Isthmus. Fig. | Thyroxine Calcitonin | It stimulates the rate of cellular oxidation and metabolism. Hyperactivity of this hormone leads to high rate of metabolism, faster heart beat and high blood pressure, in turn leading to difficulty in sleeping and nervousness. Hypoactivity of this hormon |
| Parathyroid Gland These are four small oval bodies which lie embedded in the lobes of the thyroid gland. Fig. | Parathormone | Regulates calcium and phosphate levels in the blood. Disorders include: (i) Parathyroid tetany (ii) Kidney stones (iii) Bone softening |
| Thymus It has two lobes, one on either side of the trachea present behind the sternum. It is active during childhood and gradually disappears in adults. | Thymosin | It accelerates cell division, thus influencing the rate of growth. It stimulates proliferation of lymphocytes, thus, increasing resistance to infection. |
| ABDOMINAL REGION Adrenal Gland In human beings, a pair of adrenal glands are present, one on top of each kidney, hence, they are also called supra-renal glands. Each adrenal gland has an outer part called the cortex and an inner part medulla | ||
| Adrenal Cortex It is vitally important for life | Glucocorticoids (cortisone, cortisol) | Regulate the metabolism of protein, fats and carbohydrates in the body and the level of blood sugar. Disorder-cushing’s syndrome. |
| Mineralocorticoid (aldosterone) | It controls re-absorption of sodium in urinary tubules and maintains Na+ and K+ ratio in the extracellular and intracellular fluids. Disorder-Aldosteronism. | |
| Adrenal Medulla It helps the body to combat against stress or emergency conditions. | Adrenaline (Epinephrine) and Nor-adrenaline (Nor-epinephrine) | Both these hormones together control emotions, fear, anger, blood pressure, heartbeat, respiration and relaxation of smooth muscles. Comprises the sympathetico – adrenal system. |
| Pancreas It is a compound gland in the abdominal region located posterior to the stomach. It has an exocrine part which produces digestive juices. Its endocrine part, Islets of Langerhans, secretes hormones insulin & glucagon. | Insulin — secreted by b – cells | It regulates the conversion of glucose to glycogen. Hyposecretion of insulin causes diabetes mellitus in which glucose is present in excess in blood and is also excreted out along with the urine. |
| Glucagon secreted by a – cells | It regulates the conversion of glycogen and some non-carbohydrates back to glucose. Disorder : Raises the blood sugar level & cause elimination of sugar in urine. | |
| Ovaries These are a pair of organs present in the lower abdominal region in females. | Secrete eostrogen, progesterone, relaxin | Oestrogen : developes the females secondary sexual characters; progesterone: prepares uterus for receiving the fertilized egg and also for menstruation in the absence of pregnancy; relaxin secreted by ovary and placenta, helps in child birth. |
| Testis These are extra abdominal in position. The interstitial or Leydig cells present in testis produce the male hormone. | Testosterone | It controls the development of secondary sexual characteristics in males, such as enlargement of penis and scrotum, growth of facial and pubic hair, and enlargement of larynx that causes deepening of voice. Disorder : Eunuchoidism |
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