Tetrodotoxin is a protein toxin from puffer fish that blocks the opening of voltage-gated sodium channels at quite low doses. Assume a researcher applies this drug to just the postsynaptic pair of two neurons (the presynaptic neuron we will assume is the mechanically-sensitive spinal neuron described above, and the post-synaptic cell is a relay neuron receiving input from the first cell in the spinal column). Draw the response of the post-synaptic cell to three stimuli of different intensity (gentle, medium, and intense). Be sure to label all axes.

Although it is not possible for me to draw this for you, there are a few concepts that I can write and this might help you to be able to do the drawing or, at least, understand the concept.

Tetrodotoxin is a toxic substance produced by puffer fish that in certain amounts can become completely lethal. However, in recent years TTX has been also found to be useful for the treatment of several conditions, including problems with the bladder, blockage of pain, and other medical circumstances.

What TTX does, basically, is to establish a blockade, particularly of sensory neurons, preventing action potentials from generating, and much less from propagating between firing neurons. The process by which this happens is that TTX will not allow sodium ion channels to open, and since sodium cannot pass into the cell, the action potential cannot be generated. Thus, in large amounts, TTX can generate death as the neuronal cells are unable to produce any action potentials.

However, in this particular case we are talking about a therapeutic dosage. So, when the researcher applies it to the postsynaptic neuron, this means that even though neurotransmitters are being released by the presynaptic neuron, to generate an action potential post-synapse, the post-synaptic neuron will not respond, will be incapable of propagating the action potential, because its sodium channels will not open.

However, as said, since the dosage is minimum and at therapeutic levels, it has been found that depending on the level of stimulus sent by the presynaptic neuron, the effects of TTX can be overcome, even if not completely and what is achieved is a controlled, less intense, excited response from the post-synaptic neuron. If the stimulus is small, or gentle, then TTX´s effect will overcome the capacity of the stimulus to force the neuron´s channels to open. But when the medium and intense stimulus (releasing a much higher dose of ACh into the synaptic space) takes place, research has shown that ACh may interact with TTX and produce a moderated response from the postsynaptic neuron. This moderation is what is being sought for in conditions as bladder afferent responses to distension.