The pathway taken by nerve impulse to accomplish reflex action is called reflex arc.

(i) Brain is protected by skull.
(ii) Brain is covered by three connective tissue membrane or meninges. Thuy are outer Duramater, middle Arachnoid membrane and inner Piamater. Meningeal membranes protect the brain from mechanical injury.
(iii) The brain is suspended in a special fluid environment called cerebrospinal fluid (CSF). It acts as shock absorbing fluid and protects the brain from damage when it is subjected to sudden jerk.

(i) 'Stimulus' refers to the changes in the environmental condition, that are detected by receptors present in the body. 
(ii) A receptor is a cell or a group of cells that receives the stimuli.
(iii) An effector is a part of the body which can respond to a stimulus.

The neurons may be of different types based on their structure and functions.

Structurally the neurons may be of the following types:
(i) Unipolar neurons
(ii) Bipolar neurons
(iii) Multipolar neurons
(i) Unipolar neurons:
i) Only one nerve process arises from the cyton which acts as both axon and dendron. 
ii) Unipolar neurons found in early embryos but not in adults.
(ii) Bipolar neurons:
i) The cyton gives rise to two nerve processes of which one acts as an axon while another as a dendron. 
ii) Bipolar neurons found in retina of eye and olfactory epithelium of nasal chambers.
(iii) Multipolar neurons:
i) The cyton gives rise to many dendrons and an axon. 
ii) Multipolar neurons found in cerebral cortex of brain.