There are many types of receptor found throughout the animal kingdom. Receptors are like “biochemical locks”, which have corresponding “biochemical keys” to fit them.
Nicotinic Receptors are one type of receptor which require Nicotinic Chemicals as their biochemical keys.
In humans Nicotinic Receptors are found in the Central Nervous System (Brain) & in the Peripheral Nervous System (rest of the body). (picture left)
These receptors form part of large Nicotinic Nerve 
Networks which control many bodily functions.
Networks are made up from millions of nerve fibres. These nerve fibres have synapses at their ends and communicate with other nerve fibres and cells via Synaptic Junctions (pictures right). Synaptic Junctions are made up of Transmitter Synapses and a Receiver Synapses.
Transmitter synapses make & release nicotinic chemicals, the “biochemical keys” (mentioned above)
These “biochemical keys” travel across the gap (illustrated in the pictures right) and fit into the nicotinic receptors, the “biochemical locks”, located on the surface of the receiver synapse, at the other side of the synaptic junction.
When the key fits the lock, a micro-electrical impulse is generated inside the receiver synapse and passes along the length of the nerve fibre causing an effect at the other end.
As you can see from the picture (above top left), these nerve impulses travel along nerve fibres throughout the body causing their effects.
Nicotinic nerve pathways in the Central Nervous System, play an important part in regulating cognitive function & how alert we feel. In the Peripheral Nervous System they are essential for normal organ functioning e.g. the Heart & Circulation, the Lungs, the Liver, the Kidneys, the Gut etc.
The chemical Nicotine is a constituent of tobacco smoke.
Part of the Nicotine molecule (picture right), has the same
shape as the bodies own natural Nicotinic chemicals, so Nicotine is like having a duplicate key cut, were even though the top part of the key looks different, the bottom part still fits the lock.
Nicotine molecules when absorbed from cigarette smoke, compete with the bodies own natural Nicotinic chemicals, and these “duplicate Nicotine keys” fit the locks, in the same way as the bodies own natural nicotinic chemicals. By doing so, they stimulate the nicotinic nerve networks.
Smoking is perceived as a pleasurable experience partly because Nicotine, stimulates Nicotinic Central Nervous pathways in the brain.
The Peripheral Nervous pathways are also stimulated , e.g. Nicotine speeds up the heart rate and increases blood pressure.
Ongoing exposure to an outside supply of Nicotine from cigarette smoke, results in the body reducing production of its own nicotinic chemicals. This is because the body thinks there are enough nicotinic chemicals in the nerve network.
With a reduced level of natural nicotinic chemicals, the bodies’ nicotinic nerve networks become dependent on this outside supply of Nicotine for normal physiological function and for a type of perceived pleasure.
This dependancy or addiction varies according to the amount you smoke. The more you smoke, the more it will deplete the body of its natural nicotinic chemicals, and the more you will have to smoke to maintain a comfortable level of Nicotine in the body.
When the level of Nicotine drops below “comfort level”; Craving results and you feel like smoking an other cigarette.
Problems associated with stopping smoking