Ion Channels in Nervous Systems: Focus on Ligand-Gated Types
Cholinergic nicotinic receptors (nAChRs) are a crucial component of the nervous system, playing a significant role in neuromuscular transmission, parasympathetic neurotransmission, and the adrenal medulla's response to stress. However, their presence and functionality vary across different body parts and tissues.
In the neuromuscular junctions of skeletal muscles, nAChRs are predominantly found, facilitating fast synaptic transmission. This is essential for the smooth coordination of muscle movements. In contrast, in the central nervous system (CNS), nAChRs are present in certain brain regions but are absent or low in others. Muscarinic receptors tend to predominate at higher CNS levels.
Smooth muscles, cardiac muscles, and many internal organs typically lack nAChRs on their postsynaptic membranes. Instead, cholinergic signalling in these tissues is mediated mainly by muscarinic acetylcholine receptors. For example, parasympathetic innervation of the heart and smooth muscles operates via muscarinic receptors, not nicotinic ones.
Autonomic ganglia express nicotinic receptors on postganglionic neurons; however, effector organs innervated by postganglionic parasympathetic neurons usually do not have nAChRs but instead have muscarinic receptors. Other body parts without nAChRs include many parts of the sensory organs and tissues not directly involved in cholinergic neurotransmission or neuromuscular transmission.
Myasthenia gravis, a rare autoimmune disorder, affects the body by attacking cholinergic nicotinic receptors, leading to weakness and fatigue, especially in the face and neck muscles. Treatment strategies aim to restore communication lines between the brain and muscles, allowing the muscles to respond to commands again. Cholinesterase inhibitors, immunosuppressants, plasmapheresis, and, in severe cases, thymectomy are some of the methods used to manage the condition.
In conclusion, nAChRs are absent or not functional in smooth muscles, cardiac muscles, and parasympathetic effector organs, where cholinergic signalling is mediated instead by muscarinic receptors. Understanding the role and distribution of these receptors can provide valuable insights into various physiological processes and the pathology of diseases like myasthenia gravis.
In the physiological processes of the body, cholinergic signaling in smooth muscles, cardiac muscles, and many internal organs is predominantly mediated by muscarinic acetylcholine receptors, rather than nicotinic cholinergic receptors, which are typically absent or not functional in these tissues. Treatment for myasthenia gravis, a rare autoimmune disorder that affects the cholinergic nicotinic receptors, involves methods aimed at restoring communication lines between the brain and muscles, such as the use of cholinesterase inhibitors and immunosuppressants.
