Question

In: Anatomy and Physiology

An individual ingests a substance that acts as an agonist to muscarinic receptors. What effect do...

An individual ingests a substance that acts as an agonist to muscarinic receptors. What effect do you predict it will have on the individual?

Solutions

Expert Solution

Answer :

Muscarinic receptors : muscarinic acetyle receptors , or mAChRs, are (acetyle choline receptors).That from G protein-coupled receptors complexes in the cell membranesof certain neurons and other cells.Muscarinic receptors are so named because they are more sensitive to muscarine than to nicotine.

An acetylecholine receptor is an integral membrane protein that responds to the binding of acetylecholine, a neurotransmittar.

Classification :

Acetylecholine receptors clssified according to their "pharmacology",or according to their relative affinities and sensitivities to different molecules. Although all acetylecholine receptors,by definition,respond to acetylecholine, they respond to other molecules as well.

  • Nicotinic acetylecholine receptors (nAChR, also known as "ionotropic" acetylecholine receptors.) are particularly responsive to nicotine. The nocotine ACh receptor is also a Na+, K+  and Ca2+ ion channel.
  • Muscarinic acetylecholine receptors (mAChR,also known as "metabotropic" acetylecholine receptors) are particularly responsive to muscarine.

Nicotinic and muscarinic are two main kinds of "cholinergic" receptors.

mAChR:

In, contrast, the mAChR are not ion channels, but belong instead to the super family of G-pritein-coupled receptors that activate other ionic channels via a second messenger cascade. The muscarine cholinergic receptor activates a G-protein when bound to extracellular ACh. The alpha subunit of the G-protein activates guanylate cyclase (inhibiting the effect of intracellular cAMP) while the beta-gamma subunit activates the K-channels and therefore hyperpolarize the cell. This causes a decrease in cardiac activity.   

Muscarinic effects :

When ACh stimulates muscarinic receptors, some effects are

Eyes :

  • Miosis ( pupil constriction)
  • Laccrimation (tear secretion)
  • Accomodation for near vision (the lense of the eye becomes set for reading)
  • Decreasing intra-occular pressure(useful for glucoma)

Digestive tract :

  • Saliva secretion which breaks food down
  • Stomach acid secretion
  • Peristalsis

Other effects :

  • Decreased heart rate
  • Bronchial constriction (makes it hard to breath in asthmatics)
  • Enhanced urination (by relaxing the sphincture and contracting the bladder)

Again all these are parasympethetic effects from acetylecholine stimulaying the muscarinic receptors.


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