IMAT 2014 Q34 [Unidirectional Transmission]

The transmission of nerve impulses at a synapse is unidirectional.

Which of the features of the transmission of nerve impulses given below have a role in maintaining the unidirectional transmission?

  1. Exocytosis of neurotransmitter at the pre-synaptic membrane
  2. Neurotransmitter hydrolysing enzyme found on the post­-synaptic membrane
  3. Neurotransmitter receptors found on the post-­synaptic membrane
  4. Sodium gated channels on the post­-synaptic membrane

A. 2 and 4 only
B. 1, 2 and 3 only
C. 1, 3 and 4 only
D. 1 and 4 only
E. 1, 2 and 4 only

The answer to this question is choice C.

What is this question asking for?

  • At first glance, all of the statements seem to be a fitting association with synaptic transmission. But don’t be confused, and try to approach this question in a systematic way. What’s being asked in this question are the features that have a role in “maintaining” the unidirectional transmission of impulse. Thus, what you need to figure out is, which one of the statements are an odd-one-out in terms of being related to the directionality of the impulse.

With that being said, let’s rationalize each of the statements:

  1. “Exocytosis of neurotransmitter at the pre-synaptic membrane”: A neurotransmitter is a signaling molecule used by nerves to transmit impulses across a synpase. This feature is a correct association with unidirectionality of the the impulse because if you think about it, the action potential needs to be transmitted somehow to the post synaptic membrane to further propagate the impulse! This is achieved exactly by neurotransmitters, which are released to the synapse through exocytosis from the pre-synaptic membrane.

  2. “Neurotransmitter hydrolysing enzyme found on the post­-synaptic membrane”: This is the odd-one-out statement out of the rest. For clarity, an example of a hydrolysing enzyme found on the post synaptic membrane would be the acetylcholinesterase, which hydrolyses acetylcholine. After the impulse has been transmitted, there will be a need to terminate the signal and/or to prevent overactivation of receptors via the same acetylcholine that was used to transmit the prior impulse. This is achieved by a hydrolysing enzyme called acetylcholinesterase. If you analyze it, it’s not much related to unidirectionality of the transmission of impulse, but instead, it is more related to terminating the signal transmission.

  3. “Neurotransmitter receptors found on the post-­synaptic membrane”: The neurotransmitters that were released into the synapse need to be sensed by the post-synaptic membrane in order to propagate or continue the signal! This is not possible if there are no receptors that are able to even sense the neurotransmitters in the first place. Thus, this allows the continuation of the transmission of nerve impulse, and also contributes to the unidirection of this impulse.

  4. “Sodium gated channels on the post­-synaptic membrane”: After the neurotransmitter binds to the receptors on the post-synaptic membrane, sodium channels will now open, and if you can recall, sodium is needed to depolarize the membrane! With this depolarization, action potentials can now be transmitted onwards. Thus, this contributes as well to the unidirectionality of the transmission of impulse.

Thus, the only correct associations are statements 1, 3, and 4, making choice C as the correct answer.