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I came across this question today, it says:

Nicotinic receptors stimulation is directly responsible for …....

I should fill the spaces by one of the following choices:

  1. Increasing the excitability of sympathetic postganglionic neurons.
  2. increasing the the force of stomach contraction.
  3. delaying the emptying of liquids from the stomach.
  4. decreasing the excitability of sympathetic postganglionic neurons.

The only article speaking about the nicotinic receptors in my physiology book says that they are cholinergic receptors found in the autonomic ganglia, i.e., they are found on the postganglionic neurons.

So when I tried to solve the question, I thought the upper three answers are correct. Specifically, because stimulating the receptors on the postganglionic neurons will increase their excitability, option 1 seems to make sense.

But if that postganglionic cell happen to be sympathetic it may provoke the emptying of the liquids from stomach to be delayed (choice 3)?

And, if it was parasympathetic the force of stomach contraction would also increase (choice 2)?

I am confused as all three answers 1) 2) and 3) seem to be correct and the question asks specifically for one correct answer. Could someone help me find the correct answer, but put simply please, as I'm just a first-year dental student.

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Short answer
1

Background
Nicotinic recepeptors (nAchRs) are widely distributed in the central and peripheral nervous system. Your question seems to focus on the peripheral nervous system. Taken from (Taylor, 2012):

In the periphery, nAchRs are found in:

  • Innervated skeletal muscle, especially postsynaptically in the motor end plate;
  • The presynaptic nerve ending in some motor neurons to control Ach release;
  • Ganglia at the postsynaptic dendrite and nerve cell bodies, and presynaptically to control Ach release.

Now in your answer the options 2 and 3 are incorrect, as muscarinic Ach receptors regulate gastric fluid secretion (Aihara, 2005) and contraction (Kitazawa et al., 2006). Given that nAchRs are excitatory receptors. In muscle cells, the binding of Ach leads to an opening of the associated ion channel, in turn producing a rapid increase in the cellular permeability of sodium and calcium ions, resulting in the depolarization and excitation of the muscle cell, thereby producing a muscular contraction (source: EMBL-EBI). This leaves only one opion left, namely 1). Since this was basically answering the question by exclusion, below Fig. 1 shows that answer 1) is indeed the correct one through a more positive approach - the top panel clrearly confirms that the sympathetic pathway deploys Acetylcholinergic neurons to stimulate its postganglionic adrenergic neurons.

Ach
Fig. 1. Ach in the peripehral nervous system. source: Austin Community College

References
- Ayhara, Gastroint, Liver Physiol (2005); 2886): G1199-207
- Kitazawa et al., Eur J Pharmacol (2007); 554(2-3): 212-22
- Taylor, Primer on the Autonomic Nervous System (3rd Ed. ( 2012}

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