19. Hind-brain comprises of cerebellum, pons varolii and medulla oblongata. The cerebellum is the second largest part of brain that coordinates muscular activity of the body as well as maintains body equilibrium or posture. Involuntary actions like blood pressure, salivation and vomiting are controlled by the medulla in the hind-brain.
20. The spinal cord is also well protected by vertebral column as it runs within a canal located in the centre of each vertebrae. Spinal cord is a centre of many reflex activities apart from conducting sensory and motor impulses to and from the brain.
21. Reflex action is a nerve mediated automatic involuntary response to a stimulus acting on a specific receptor without consulting the brain. The pathway followed by reflex action is called a reflex arc.
22. Reflex arc can be schematically represented as follows :
23. Reflex actions play a vital role in checking over involvement of brain. The response is immediate and therefore have a survival value.
24. The nervous system generally operates through muscles. Muscles have contractile proteins, actin and myosin. The neurotransmitter sensitizes the chemoreceptor sites of membrane of muscle fibres. These proteins change shape during muscle contraction resulting in movement.
25. Plants do not have nervous system and muscles. Even then they exhibit well-coordinated and controlled movements.
26. Plants possess chemical coordination. They respond to stimuli by secreting chemical substances called as plant growth regulators. They either stimulate or retard growth. Five major types of plant growth hormones are auxins, gibberellins, cytokinins, abscisic acid and ethylene.
27. Auxin is synthesized at shoot apices in response to light. Unidirectional exposure to fight at shoot apex results in diffusion of auxin towards the shaded region of the shoot. This leads to more growth on the shaded side as compared to the illuminated region. So the stem bends towards the source of light.
28. Gibberellins promote'growth in stems and leaves. If they are applied to genetically dwarf plants like cabbage then it induces growth of stem.
29. Cytokinin promotes cell division, delays ageing and is required for differentiation of cells and tissues.
30. Ethylene is a gaseous hormone that promotes ageing and abscission of leaves and flowers. It is essential for ripening of fruits.
31. Abscisic acid is a growth inhibitor also referred as stress hormone as it overcomes stress conditions. It promotes ageing, causes abscission of leaves and flowers, checks excessive transpiration by causing stomatal closure and inhibits growth.
32. The stimulus oriented movement of plant or plant part is called tropism. The direction of movement is related to the direction of stimulus. These movements are always growth-oriented movements. They are of different types namely phototropism (growth movement in response to fight), hydrotropism (water), geotropism (force of gravity), thigmotropism (contact) and chemotropism (chemicals).
33. Roots are positively geotropic and negatively phototropic. Shoots are positively phototropic and negatively geotropic. Growth of pollen tube inside the style, ovary and ovule in response to the chemicals produced by them is an example of chemotropism. Thigmotropism is found in twines and tendrils. After initial contact with support the tendril shows less growth in the region of contact and more growth on the other side. The positive hydrotropic response of roots is stronger than their geotropic response.
34. Certain changes in the orientation of plant parts in relation to other parts caused by intrinsic or external stimuli are non-directional, growth independent movements. They are referred to as nastic movements.
35. Folding of leaves of touch me not plant or sensitive plant (Mimosa pudica or Chhui-Mui) on being touched is an example of nastic movement. The movement is directed neither towards nor away from the stimulus. This movement is due to change in the turgor pressure of cells.
36. In animals, apart from nervous coordination, chemical coordination also exists. It is maintained by hormones which are secreted by endocrine glands (ductless glands).
37. Hormones are chemical informational molecules that are required in minute quantities and are directly poured in the bloodstream. They act on a specific organ called target organ. So their site of production varies from site of action,
38. The various endocrine glands in our body are :
(ix) Testis in males and ovary in females
39. The secretion of hormone is regulated by feedback mechanism. It’s a regulatory mechanism in which presence of certain level of substance promotes or inhibits its further formation. For example, presence of excess glucose in blood, as after meals, stimulates (cells of islets of langerhans in pancreas to secrete insulin. Insulin promotes glucose absorption by individual cells, absorption and formation of glycogen in liver and muscles. This reduces glucose level in blood. As soon as the blood sugar comes to normal the pancreatic cells stops secretion of insulin.
40. Adrenaline hormone is secreted by adrenal gland in response to fear, anger, joy, cold and emotional stress. Its target organs include heart and arteries. This hormone increases blood supply to heart and skeletal muscles. It constricts arterioles. There is an increased rate of breathing. So it is called an emergency hormone.
41. Thyroxine secreted by thyroid regulates metabolism of carbohydrates, fats and proteins. Iodine is essential for the synthesis of thyroxine. Deficiency of iodine in our diet causes enlargement of thyroid gland resulting in swelling of neck, a condition known as goitre.
42. The anterior pituitary secretes a hormone called growth hormone. Its over-secretion during growing period causes gigantism and its under-secretion results in dwarfism.
43. Testis in males secrete a hormone testosterone and ovary secretes estrogen in females. These hormones are responsible for various changes in body of males and females respectively, during puberty.
44. Hypothalamus regulates the release of many hormones by secreting specific ‘release factor’. For example, during low levels of growth hormone, hypothalamus secretes ‘growth hormone-releasing factor’ which stimulates the pituitary to secrete more growth hormone
45. Table : Some important hormones and their functions