7. CONTROL AND COORDINATION
In organisms, many movements are responses to changes in the environment or to use changes in environment. E.g.
- Plants grow out into the sunshine.
- Children get pleasure and fun out of swinging.
- Buffaloes chew cud for better digestion.
- Falling light on eyes or touching a hot object cause responses.
Such movements are carefully controlled and coordinated by specialised tissues. Also, they are connected to recognition of various events in the environment.
In animals, control and coordination are provided by nervous and muscular tissues.
Nervous tissue is made up of a network of nerve cells (neurons). It conducts information (electrical impulses) from one part of the body to another.
Some nerve cells have specialised tips called receptors. They are usually located in sense organs. They detect information from environment. E.g. gustatory receptors detect taste, olfactory receptors detect smell.
General perception of taste is jointly created by tongue (taste) and nose (smell). That’s why if nose is blocked or we have cold, there is a difference in taste of foods.
Transmission of nerve impulse
Information from receptors → dendritic tip of nerve cell → sets off a chemical reaction → generation of electrical impulse → impulse to cell body → axon → axonal end releases some chemicals → chemicals cross the gap (synapse) → generation of electrical impulse in dendrite of the next neuron → impulses deliver to muscles cells or gland.
Reflex actions are sudden unconscious actions of the body in response to a stimulus in the environment. E.g.
o Withdrawal of hand when we touch a flame.
o Blinking of eyes when light falls on them.
Here there is no thinking as it needs an immediate response.
Thinking needs complicated interaction of many nerve impulses from many neurons.
In brain, thinking tissue sits in the forward end of the skull. It receives signals from all over the body and thinks about before responding to them.
If thinking part is to instruct muscles to move, nerves must carry this signal back to different parts of the body. It takes much time and prevents quick response. E.g. when we touch a hot object, it would take much time to withdraw the hand if thinking tissue is involved.
The pathway of impulses in a reflex action is called reflex arc. It includes receptor, sensory neuron, CNS, motor neuron & effector (muscle or gland).
Nerves from all body parts meet in spinal cord on their way to the brain.
Reflex arcs evolved as efficient ways of functioning in the absence of true thought processes in animals. However, after complex neuron networks have evolved, reflex arcs continue to be more efficient for quick responses.
Sequence of events (reflex arc) of focussing bright light on eyes are given below:
Receptor → Sensory neuron → Brain → Motor neuron → Eye → Eye muscle contracts.
Brain & spinal cord constitute the central nervous system (CNS). They receive information from all parts of the body and integrate it.
Brain is protected in a fluid-filled balloon (shock absorber) inside the bony box (cranium).
Spinal cord is protected in vertebral column or backbone.
Brain is the main coordinating centre of the body. It involves more complex mechanisms & neural connections for complex processes such as thinking.
Spinal cord contains nerves which supply information to think about.
Brain sends messages to muscles to control voluntary actions such as writing, talking, moving, clapping etc.
Communication between CNS and other parts of the body is facilitated by the peripheral nervous system (PNS). It consists of cranial nerves (from brain) and spinal nerves (from spinal cord).
Brain has 3 regions: forebrain, midbrain & hindbrain.
Forebrain is the main thinking part of the brain. It has the following regions:
o Sensory regions: which receive sensory impulses of hearing, smell, sight etc.
o Association areas: They interpret sensory information by associating with information from other receptors and previously stored information. Thus, a decision is made to respond. This information is passed to the motor areas which control the movement of voluntary muscles.
o Centre of hunger: It gives sensations such as feeling full when food is eaten.
In our body, there are involuntary actions between simple reflex actions and thought-out actions. We do not have thinking control on them. E.g. salivation, heartbeat etc. They are controlled by mid-brain and hind-brain.
Medulla in hindbrain controls involuntary actions such as blood pressure, salivation, vomiting etc.
Cerebellum in hindbrain controls voluntary actions like walking, riding a bicycle, picking up a pencil etc. It also maintains the posture and balance of the body.
When a nerve impulse reaches the muscle, the muscle fibres (muscle cells) move by changing their shape so that they shorten.
Muscle cells have special proteins that change their shape and arrangement in response to nervous electrical impulses. This gives the muscle cells a shorter form.
Voluntary muscles: The muscles attached to skeleton. They can be moved as we decide.
Involuntary muscles: The muscles found in visceral organs. They are not under our control.
Plants have no nervous system or muscles. But they can respond to stimuli.
Like animals, plants show 2 types of movement:
o Dependent on growth: E.g. When a seed germinates, root goes down and stem comes up.
o Independent of growth: E.g. When we touch the leaves of a chhui-mui (the ‘sensitive’ or ‘touch-me-not’ of Mimosa family), they quickly fold up and droop.
In sensitive plant, movement happens at a point different from the point of touch. It means information about touch is communicated. Plants use electrical-chemical means to conduct information from cell to cell.
In animals, some muscle proteins help to change the shape of cells. But plant cells change the shape (swelling or shrinking) by changing amount of water in them.
Some plants (e.g. pea) have tendrils to climb up or fence. They are sensitive to touch.
When tendrils contact with a support, the touching part does not grow as rapidly as the part away from object. So, tendril circles around and clings to the object.
Plants respond to stimuli slowly by growing in a direction. Because of the directional growth, it appears as if the plant is moving.
Environmental triggers such as light, gravity etc. change the directions of plant growth. They are called tropic (directional) movements. These can be towards the stimulus (positive), or away from it (negative).
Types of tropic movements
It is the tropic movement in response to light.
Shoots respond by bending towards light (positive).
Roots respond by bending away from light (negative).
It can be proved by the following activity.
- Fill a conical flask with water and cover its neck with a wire mesh having 2-3 freshly germinated bean seeds.
- Keep the flask in a cardboard such that its open side faces light coming from a window.
- After 2 or 3 days, it is noticed that the shoots bend towards light and roots away from light.
- Now turn the flask so that shoots are away from light and roots towards light. Leave it for a few days.
- The old parts of the shoot and root have no noticeable change in direction. But new growth parts show change in direction. i.e., shoot bends towards light and roots bend away from it.
Movement in response to the pull of earth or gravity.
Roots always grow downwards (positive).
Shoots grow upwards and away from earth (negative).
The movement of a plant toward or away from water.
E.g. Roots bend towards the moist soil.
The movement of plant towards or away from chemicals.
E.g. growth of pollen tubes towards ovules.
Controlled movements can be either slow or fast. E.g.
o Sensitive plant quickly moves in response to touch.
o Sunflowers slowly move in response to day or night.
o Growth-related movements of plants are slower.
In animal bodies also have controlled directions to growth. E.g. growth of arms & fingers.
For fast responses to stimuli, information must be transferred very quickly. Electrical impulses are an excellent means for this. But it has some limitations:
o They reach only the cells having connection with nervous tissue.
o Once an impulse is generated and transmitted, the cell will take some time to reset its mechanism to generate a new impulse. So, cells cannot continually create and transmit electrical impulses.
So most multicellular organisms use chemical (hormone) communication between cells.
In this, stimulated cells release a hormone and it diffuses around the original cell. Other cells detect hormone using special molecules on their surfaces. Then they recognise information and transmit it.
This is slower process but can reach all cells and can be done steadily and persistently.
Plant hormones can coordinate growth, development and responses to the environment. They are synthesised at some places and diffuse to the area of action.
Some Plant Hormones
o Auxin: Regulates growth in plants. When plants detect light, auxin, synthesised at the shoot tip, helps the cells to grow longer. When light is coming from one side of the plant, auxin diffuses towards the shady side of the shoot. As a result, the cells on the shady side grow longer. Thus, the plant bends towards light.
o Gibberellins: Help in the growth of the stem.
o Cytokinins: Promote cell division. They are present in greater amount in areas of rapid cell division, such as fruits and seeds.
The above hormones help in promoting growth.
o Abscisic acid: Inhibits growth. Causes wilting of leaves.
Electrical impulses can instruct only few tissues to prepare for an activity. So, in animals, there is another way of control & coordination called Endocrine system.
It includes endocrine glands and their secretions called hormones (chemical signals). They can reach all cells and provide wide-ranging changes. E.g. Adrenal glands secretes adrenaline.
Adrenaline prepares body to cope with emergency situations. E.g. E.g. a scary animal such as squirrels prepare to fight or run away. It needs more energy.
Though fighting & running are different, both have some common preparations.
Adrenaline is secreted into blood and carried to different body parts. As a result, the following events occur:
o Heart beats faster to supply more oxygen to muscles.
o Blood to the digestive system and skin is reduced by contracting muscles around small arteries in these organs. This diverts the blood to skeletal muscles.
o Breathing rate increases due to the contraction of diaphragm and rib muscles.
These responses prepare the body to deal with the situation (fighting and running).
The timing and amount of hormone released are regulated by feedback mechanisms. E.g. if the blood sugar level increases, it is detected by cells of pancreas and produce more insulin. As the blood sugar level falls, insulin secretion is reduced.
SOME IMPORTANT HORMONES AND THEIR FUNCTIONS
Functions & other info
Stimulates pituitary gland to release hormones. E.g. growth hormone releasing factor stimulates pituitary gland to release GH.
Growth hormone (GH)
Stimulates growth & development of the body.
Dwarfism: Deficiency of GH in childhood.
Gigantism: Overproduction of GH in childhood.
Regulates carbohydrate, protein & fat metabolism for balanced growth.
Iodine is essential for synthesis of thyroxin. Iodised salts provide iodine.
Deficiency of iodine causes goitre. Swollen neck is the main symptom.
Regulates blood sugar level.
Deficiency of insulin causes diabetes (sugar level increases).
Such patients are given insulin injections.
Prepares body to cope with emergency situations.
Testes (in male)
Changes during puberty. Development of male sex organs, behaviour etc.
Ovaries (in female)
Changes during puberty. Development of female sex organs, regulates menstrual cycle, etc.