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.
What happens in Reflex
Actions?
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.
Human Brain
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.
How
does Nervous Tissue cause Action?
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.
Immediate Response to
Stimulus
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
Phototropism:
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.
Geotropism:
Movement in response to the
pull of earth or gravity.
Roots always grow downwards (positive).
Shoots grow upwards and away
from earth (negative).
Hydrotropism:
The movement of a plant toward
or away from water.
E.g. Roots bend towards the moist
soil.
Chemotropism:
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.
HORMONES
IN ANIMALS (ENDOCRINE SYSTEM)
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
|
Endocrine Gland
|
Hormone
|
Functions & other info
|
|
Hypothalamus
|
Releasing hormones
|
Stimulates pituitary gland to release hormones. E.g. growth hormone
releasing factor stimulates pituitary gland to release GH.
|
|
Pituitary gland
|
Growth
hormone (GH)
|
Stimulates growth & development of the body.
Dwarfism: Deficiency of GH in childhood.
Gigantism: Overproduction of GH in childhood.
|
|
Thyroid gland
|
Thyroxine
|
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.
|
|
Pancreas
|
Insulin
|
Regulates blood sugar level.
Deficiency of insulin causes diabetes (sugar
level increases).
Such patients are given insulin injections.
|
|
Adrenal gland
|
Adrenaline
|
Prepares
body to cope with emergency situations.
|
|
Testes (in male)
|
Testosterone
|
Changes during puberty. Development of male sex organs,
behaviour etc.
|
|
Ovaries (in female)
|
Oestrogen
|
Changes during puberty. Development of female sex
organs, regulates menstrual cycle, etc.
|
.
