Question

3. Saxitoxin prevents sodium channels from opening. Membrane Potential - - - - - - - - Time 4. Neostigmine competitively inhibits acetyelcholinesterase (the enzyme that breaks down the neurotransmitter acetylcholine). This means the neostigmine gives the neurotransmitter more time to interact with receptors on the post-synaptic cell before the neurotransmitter is broken down by enzymes. Membrane Potential Time

Answer 1

3. SAXITOXIN :

Saxitoxin is a potent neurotoxin that is produced by dinoflagellates and is commonly associated with red tides

Saxitoxin is most commonly produced by Alexandrium tamarense, Gymnodinium catenatum and Pyrodinium bahamense

The consumption of shellfish contaminated with saxitoxin leads to paralytic shellfish poisoning

Saxitoxin potency lies in its ability to irreversibly bind to and block Na+ channels.as a consequence Na+ ions are unable to pass and nerve impulse cannot be transmittted. This effect result in the inability of muscle to contract

The suppression of muscle contraction leads to respiratory paralysis

NORMAL EVENTS EFECTS OF SAXITOXIN Na+ . Ca2+. Nat Ca 2+ Toxin blocks channel Sodium channel Calcium Sodium channel channel ROOOO bOQO Calcium channel • DOOOO DOO 800 Impulse propagates Axon Impulse cannot propagate Axon

In the above figure saxitoxin bind to the sodium or calcium channel of the nerve axon membrane preventing the passage of these ions through the cell membrane thus blocking the transfer of the nerve impulse. a single saxitoxin molecule is enough to block one sodium channel. the two guanidine sub structure present in the saxitoxin must be protonated

m

Signal Nervenzelle außen Nervenzelle innen Na-Kanal 00000000 Carbamoyltoxin (STX)

model of how saxitoxin bind to the surface of excitable nerve cell membrane (above figure)

4.Neostigmine

Neostigmine is a parasympthomimetic agents that act as a reversible acetylcholinesterase inhibitor

The drug inhibits acetylcholinesterase which is responsible for the degradation of acetylcholine

Neostigmine indirectly stimulates both nicotinic and muscarinic receptors which are involved in muscle contraction

MECHANISM OF ACTION

Acetylcholine is metabolized by an enzyme called acetylcholinesterase that hydrolyzes acetylcholine in the neuromuscular junction

Neostigmine is an oxydiaphoretic inhibitor of the acetylcholinesterase enzyme,which means that it binds and inhibits via acid transferring (or binding to the anionic site of the enzyme creating a covalent bond )

Neostigmine inhibits acetylcholinesterase, which is the enzyme that metabolizes acetylcholine into choline and acetic acid . this allows acetylcholine to build up at the neuromuscular junction and over come the competitive inhibition of nondepolarizing blocking drugs

NEURON Acetate Acetyl- choline Choline 289999997 199999 10.000 Echothiophate Edrophonium Neostigmine Physostigmine INCREASED INTRACELLULAR RESPONSE

figure indicates the neostigmines inhibitory action