The changes in the environment to which living organisms respond are called stimuli.
Eg :- heat, cold, sound, smell, taste, touch, pressure, gravity, water etc.
Living organisms respond to stimuli in the form of body movements.
b) Coordination :-
For a proper response to a stimulus many organs in the body should work together. The working together of various organs in an organism to produce a proper response to a stimulus is called coordination.
i) In animals control and co ordination is done by the nervous system and endocrine system.
ii) In plants control and co ordination is done by chemical substances called plant hormones or phytohormones.
Coordination in animals :-
In animals control and co ordination is done by the nervous system
and endocrine system.
The nervous system consists of the brain, spinal cord and nerves.
a) Receptors :- are the sense organs which receive the stimuli and
pass the message to the brain or spinal cord through the sensory
Eg :-Photoreceptorsin the eyes to detect light.
Phonoreceptorsin the ears to detect sound.
Olfactory receptors in the nose to detect smell.
Gustatory receptorsin the tongue to detect taste.
Tangoreceptorsin the skin to detect touch.
b) Effectors :- are the muscles and glands which respond to the
information from the brain and spinal cord through the motor nerves.
c) Sensory nerves :- are nerves which carry information from the
receptors (sense organs) to the brain and spinal cord.
d) Motor nerves :- are nerves which carry information from the brain
and spinal cord to the effectors (muscles and glands).
Neuron is the structural and functional unit of the nervous system. It has a cell body called cyton containing a nucleus and cytoplasm. It has several branched structures called dendrites. It has a long nerve fibre called axon which is covered by a protective covering called Myelin sheath. The junction between two neurons is called synapse.
Messages pass through the nerve cell in the form of chemical and electrical signals called nerve impulse. The dendrites receive the information and starts a chemical reaction which produce electrical impulse which passes through the axon.
The brain is the main coordinating centre in the human body. It is
protected by the cranium. It is covered by three membranes called
meninges filled with a fluid called cerebrospinal fluid which protects
the brain from shocks.
The brain has three main parts. They are fore brain, mid brain and
i) Fore brain :- consists of the cerebrum and olfactory lobes. It is the
thinking part of the brain and controls voluntary actions. It controls
touch, smell, hearing, taste, sight, mental activities like thinking,
learning, memory, emotions etc.
ii) Mid brain :- controls involuntary actions and reflex movements of
head, neck, eyes etc.
iii) Hind brain :- consists of cerebellum, pons and medulla.
Cerebellum :-controls body movements, balance and posture.
Pons :-controls respiration.