Rani did not wear a helmet. Some people wear a helmet but don't fasten the strap. When they fall down from the bike, the helmet falls off and they are injured. Thus a helmet worn properly can save lives.

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.

Spinal Cord :

i) Spinal cord is a cylindrical structure lying in the neural canal of the vertebral column.
ii) It is also covered by meninges.
iii) It extends from the lower end of medulla oblongata to the first lumbar vertebra.
iv) The posterior most region of spinal cord tapers into a thin fibrous thread like structure called filum terminale.
v) Internally, the spinal cord contains a cerebrospinal fluid filled cavity known as the central canal.
vi) The grey matter of spinal cord is 'H' shaped.
vii) The upper end of letter 'H' forms posterior horns and lower end forms anterior horns.
viii) A bundle of fibres pass into the posterior horn forming dorsal or afferent root.
ix) Fibres pass outward from the anterior horn forming ventral or efferent root.
x) These two roots joins to form spinal nerves.
xi) The white matter is external and have bundle of nerve tracts.
xii) Spinal cord conducts sensory and motor impulses to and from the brain.
xiii) It controls reflex actions of the body.

A neuron typically consists of three basic parts:
Cyton, Dendrites and Axon.
(i) Cyton:
(a) Cyton is also called cell body or perikaryon.
(b) It has a central nucleus with abundant cytoplasm called neuroplasm.
(c) The cytoplasm has large granular body called Nissl's granules and the other cell organelles like mitochondria, ribosomes, lysosomes, and endoplasmic recticulum.
(d) Neurons do not have the ability to divide, as there is no centriole
(e) Several neurofibrils are present. in the cytoplasm that help in transmission of nerve impulses to and from the cell body.
(ii) Dendrites:
(a) These are the numerous branched cytoplasmic processes that project from the surface of the cell body.
(b) They conduct nerve impulses towards the cyton.
(c) The branched projections increase the surface area for receiving the signals from other nerve cells.
(iii) Axon:
(a) The axon is a single, elongated, slender projection.
(b) The end of axon terminates as fine branches which terminate into knob like swellings called synaptic knob.
(c) The plasma membrane of axon is called axolemma, while the cytoplasm is called axoplasm.
(d) It carries impulses away from the cyton.
(e) The axons may be covered by a protective sheath called myelin sheath which is further covered by a layer of Schwann cells called neurilemma.
(f) Myelin sheath breaks at intervals by depressions called Nodes of Ranvier.
(g) The region between the nodes is called as internode.
(h) Myelin sheath acts as insulator and ensures rapid transmission of nerve impulses.
(iv) Synapse:
(a) A junction between synaptic knob of axon of one neuron and dendron of next neuron is called synaptic junction.

(i) Nerve cells or neurons
(ii) Axon and Dendrites
(iii) Synapse
(iv) The chemical is Acetylcholine

(i) 'A' is Spinal cord.
(ii) (a) vertebral column (b) meninges
(iii) 31 pairs of nerves

(i) Spinal cord.
(ii) Central canal.
31 pairs of nerves are produced from spinal cord.

The brain is suspended in a special fluid environment called cerebrospinal fluid.
Functions:
(i) It acts as shock absorbing fluid and protects the brain from damage when it is subjected to sudden jerk.
(ii) The CSF supplies nutrients to the brain.
(iii) It collects and removes wastes from the brain.
(iv) It is also responsible for maintaining a constant pressure inside the cranium.