9.6 - Effects of drugs on the nervous system

What are agonist & anatagonist drugs?

• Agonist drugs: stimulate a synapse

• Antagonist drugs: inhibit a synapse

How does nicotine affect synapses?

Agonist drug:

• has a similar structure to acetylcholine, so binds to cholinergic receptors on the postsynaptic membrane in the CNS & switches them on, just like acetylcholine

• this causes Na⁺ ion channels to open, which leads to depolarisation & an action potential, & causes the release of more neurotransmitters (e.g. adrenaline, dopamine & endorphins)

How does lidocaine affect synpases?

Antagonist drug:

• blocks voltage-gated Na⁺ ion channels in the postsynaptic membrane, meaning Na⁺ ions cannot enter the cell when acetylcholine binds

• action potentials cannot form in the postsynaptic cell, preventing depolarisation

  • in low doses, lidocaine acts locally as an anaesthetic & muscle relaxant by inhibiting sensory & motor neurones, preventing pain signal transmission

  • in high doses, lidocaine has more widespread effects & can be fatal

How does cobra venom affect synpases?

Antagonist drug:

• binds tightly & irreversibly to acetylcholine receptors in neuromuscular junctions, stopping the receptors from opening → stops skeletal muscles from contracting

• the victim is paralysed & killed if the toxin reaches the breathing muscles

How does tetrodotoxin affect synapses?

Antagonist drug:

• blocks voltage-gated Na⁺ ion channels, so there is no influx of Na⁺ ions into the postsynaptic neurone

• there is no depolarisation, so no action potential is formed, which can result in muscle paralysis

What are the different types of ways that agonist drugs can affect synapses?

• Mimic & stimulate the release of a neurotransmitter

• Open a neuroreceptor channel

• Inhibit the breakdown of an enzyme

What is one way that an antagonist drug can affect a synapse?

Block a neuroreceptor channel