Cholinergic receptors

Cholinergic Receptors

Cholinergic receptors are a type of neurotransmitter receptor that bind to the neurotransmitter acetylcholine. These receptors are integral to the functioning of the nervous system, playing crucial roles in both the central nervous system (CNS) and the peripheral nervous system (PNS). Cholinergic receptors are broadly classified into two main types: muscarinic receptors and nicotinic receptors.

Types of Cholinergic Receptors

Muscarinic Receptors

Muscarinic receptors are a subtype of cholinergic receptors that are G protein-coupled receptors. They are named after their selective activation by the alkaloid muscarine, which is derived from certain mushrooms. Muscarinic receptors are primarily found in the CNS and in various tissues of the PNS, including the heart, smooth muscles, and glands.

There are five subtypes of muscarinic receptors, designated as M1 through M5:

• M1 receptors are primarily located in the CNS and are involved in cognitive functions.
• M2 receptors are found in the heart and act to decrease heart rate and contractility.
• M3 receptors are located in smooth muscles and glands, mediating contraction and secretion.
• M4 and M5 receptors are less well understood but are believed to play roles in the CNS.

Nicotinic Receptors

Nicotinic receptors are ionotropic receptors that form ligand-gated ion channels. They are named for their activation by nicotine, a compound found in tobacco. Nicotinic receptors are found at the neuromuscular junctions of skeletal muscles, in the autonomic ganglia, and in the CNS.

Nicotinic receptors are further divided into two main types based on their location:

• Nicotinic muscle (Nm) receptors are located at the neuromuscular junction and are responsible for muscle contraction.
• Nicotinic neuronal (Nn) receptors are found in the autonomic ganglia and the CNS, where they influence neurotransmission.

Function

Cholinergic receptors are involved in a wide range of physiological processes. In the CNS, they are crucial for cognitive functions, including learning and memory. In the PNS, they mediate the actions of the parasympathetic nervous system, such as reducing heart rate, increasing glandular secretions, and contracting smooth muscles.

Clinical Significance

Cholinergic receptors are targets for various drugs used to treat conditions such as Alzheimer's disease, myasthenia gravis, and glaucoma. Antagonists of muscarinic receptors, such as atropine, are used to treat bradycardia and as pre-anesthetic medications. Nicotinic receptor agonists and antagonists are used in smoking cessation therapies and as muscle relaxants during surgery.

Also see

• Acetylcholine
• Neurotransmitter
• Parasympathetic nervous system
• Muscarinic receptor
• Nicotinic receptor