Effects of drugs on the nervous system

In a nutshell

Drugs can interact with the nervous system at different locations causing problems with the normal function of neurones. Drug interactions can stimulate and prevent postsynaptic generation of action potentials.

Synapses

The myelin sheath found on neurones prevents the physical interaction of drugs. However, the synapse is an exposed area at the end of the neurone where it is susceptible to drug interaction.

Molecules with similar shape to neurotransmitters can interact with postsynaptic receptors either blocking or stimulating an action potential. Additionally, molecules can interact with ion channels stimulating or preventing postsynaptic action potentials.

Lidocaine

Lidocaine is primarily used as a local anaesthetic in dentistry when the nerves in your mouth need numbing.

1.	Lidocaine works by interacting with voltage gated sodium ion channels. Following this interaction the $Na^+$ channels are unable to open preventing a postsynaptic action potential from being generated because depolarisation is blocked.
2.	Pain receptors must fire an action potential for the signal to be transmitted to the brain. Following lidocaine interaction the signal cannot be transmitted, therefore, pain cannot be processed.

When lidocaine is applied by dentists it also interacts with motor end plates reducing the control of the mouth leading to speech difficulty and "droopy" lips.

The potency of drugs is dependant on their ability to be metabolised. Lidocaine is rapidly metabolised in the liver making it a perfect anaesthetic for quick procedures like those that are carried out by dentists.

Nicotine

Nicotine is generally taken into the body through smoking. It has many effects on the nervous system and hormonal system.

1.	Nicotine has a similar shape to acetylcholine (ACh) and has the ability to bind to the same postsynaptic receptors as ACh. Following this interaction there is greater stimulation at synapses leading to a feeling of alertness.
2.	Nicotine also stimulates the release of dopamine. Dopamine itself is a neurotransmitter and interacts with receptors leading to the release of chemicals called endorphins. Endorphins cause a feeling of euphoria.

Nicotine addiction can develop very quickly because the brain associates this euphoric experience with the action of smoking. The pleasurable feeling caused by nicotine only lasts for short periods of time which contributes further to the development of a nicotine addiction.

Cobra venom

Cobra snakes have altered saliva which accommodates a range of proteins. Cobra snakes have fangs which allow them to pierce the skin of their victim and inject their modified saliva into the bloodstream.

1.	A protein found in cobra venom has the ability to interact with postsynaptic ACh receptors initiating postsynaptic action potentials.
2.	Acetylcholinesterase cannot interact and digest the cobra venom protein, therefore, the protein constantly interacts with ACh receptors. There is constant $Na^+$ depolarisation of the postsynaptic membrane leading to a loss of control over postsynaptic action potentials.

Injection of cobra venom leads to paralysis of muscles in the respiratory system and eventually death caused by suffocation.