<P> José del Castillo and Bernard Katz used ionophoresis to determine the location and density of nicotinic acetylcholine receptors (nAChRs) at the neuromuscular junction . With this technique, a microelectrode was placed inside the motor endplate of the muscle fiber, and a micropipette filled with acetylcholine (ACh) is placed directly in front of the endplate in the synaptic cleft . A positive voltage was applied to the tip of the micropipette, which caused a burst of positively charged ACh molecules to be released from the pipette . These ligands flowed into the space representing the synaptic cleft and bound to AChRs . The intracellular microelectrode monitored the amplitude of the depolarization of the motor endplate in response to ACh binding to nicotinic (ionotropic) receptors . Katz and del Castillo showed that the amplitude of the depolarization (excitatory postsynaptic potential) depended on the proximity of the micropipette releasing the ACh ions to the endplate . The farther the micropipette was from the motor endplate, the smaller the depolarization was in the muscle fiber . This allowed the researchers to determine that the nicotinic receptors were localized to the motor endplate in high density . </P> <P> Toxins are also used to determine the location of acetylcholine receptors at the neuromuscular junction . α - Bungarotoxin is a toxin found in the snake species Bungarus multicinctus that acts as an ACh antagonist and binds to AChRs irreversibly . By coupling assayable enzymes such as horseradish peroxidase (HRP) or fluorescent proteins such as green fluorescent protein (GFP) to the α - bungarotoxin, AChRs can be visualized and quantified . </P> <P> Nerve gases and liquor damage this area . </P> <P> Botulinum toxin (aka botulinum neurotoxin, BoNT, and sold under the trade name Botox) inhibits the release of acetylcholine at the neuromuscular junction by interfering with SNARE proteins . This toxin crosses into the nerve terminal through the process of endocytosis and subsequently interferes with SNARE proteins, which are necessary for ACh release . By doing so, it induces a transient flaccid paralysis and chemical denervation localized to the striated muscle that it has affected . The inhibition of the ACh release does not set in until approximately two weeks after the injection is made . Three months after the inhibition occurs, neuronal activity begins to regain partial function, and six months, complete neuronal function is regained . </P>

Describe the process of impulse transmission at a neuromuscular junction