1.Muscarinic Receptors (mAChRs):
Nature and Mechanism:
Muscarinic receptors are metabotropic receptors that work through G proteins.
They are slower than nicotinic receptors because they set off a cascade of cellular interactions rather than directly opening ion channels.
Subtypes:
There are five subtypes of muscarinic receptors (M1, M2, M3, M4, and M5).
Each subtype has a different distribution and role in the body, affecting various physiological functions.
Distribution and Function:
a.M1, M3, M5:
These are generally excitatory, found in the central nervous system, exocrine glands, smooth muscle, and other areas.
They can affect neural activity, glandular secretions, and smooth muscle contraction.
b.M2, M4:
These are generally inhibitory and are found in the heart (where they slow heart rate and reduce contractility) and the central nervous system.
Muscarinic receptors are targeted by various drugs to treat conditions like overactive bladder, gastrointestinal disorders, and some types of poisoning.
2.Nicotinic Receptors (nAChRs):
Nature and Mechanism:
Nicotinic receptors are ionotropic receptors that, when activated by acetylcholine or nicotine, directly open an ion channel, leading to rapid changes in cell membrane potential. This quick response makes them crucial in fast synaptic signaling.
Distribution and Function:
A.Neuromuscular Junction:
They are found in the neuromuscular junction of skeletal muscles where their activation leads to muscle contraction.
B.Central Nervous System (CNS):
They are involved in cognitive functions, arousal, reward, and other neural processes.
C.Autonomic Ganglia:
Both sympathetic and parasympathetic nervous systems contain nicotinic receptors that mediate neurotransmission.
Nicotinic receptors are targets for treating conditions such as Alzheimer's disease, nicotine addiction, and some types of pain. Additionally, they are the site of action for drugs like neuromuscular blocking agents used in anesthesia.
Distribution Summary:
Muscarinic: Found widely in the brain, heart, smooth muscles, and glands.
Nicotinic: Located at neuromuscular junctions, in the CNS, and in the autonomic ganglia.