Overview
This lecture covers nicotinic cholinergic receptors, including their types, mechanisms, locations, and functions within the nervous system and muscles.
Cholinergic and Nicotinic Receptors
- Cholinergic receptors bind the neurotransmitter acetylcholine (ACh).
- Two main cholinergic receptor types: nicotinic and muscarinic; focus here is on nicotinic.
- Nicotinic receptors are ligand-gated ion channels activated by ACh or nicotine.
Types of Nicotinic Receptors
- There are two types: Nicotinic M (NM) and Nicotinic N (NN) receptors.
- NM ("M" for muscle) receptors are found on skeletal muscle at the neuromuscular junction.
- NN ("N" for neuronal) receptors are found on neurons and the adrenal medulla.
NM Receptor Locations and Functions
- NM receptors are located on the sarcolemma of skeletal muscle fibers at the neuromuscular junction.
- When ACh binds NM receptors, the channel opens, allowing sodium influx, causing depolarization and muscle contraction.
- All skeletal muscles (e.g., biceps, frontalis, extensor digitorum longus) use NM receptors for contraction.
- Clinical correlation: Myasthenia gravis is an autoimmune disease where antibodies block or destroy NM receptors, causing muscle weakness.
NN Receptor Locations and Functions
- NN receptors are found in all peripheral autonomic nervous system ganglia (both sympathetic and parasympathetic).
- Located on dendrites of postganglionic neurons in autonomic ganglia (e.g., sympathetic chain, celiac ganglion, submandibular ganglion).
- NN receptors are also on chromaffin cells in the adrenal medulla, where stimulation leads to secretion of epinephrine.
- Activation of NN receptors propagates action potentials in autonomic neurons.
- NN receptors are widespread in the central nervous system, but details not covered here.
Key Terms & Definitions
- Cholinergic receptor — a receptor that binds acetylcholine.
- Ligand-gated ion channel — a channel that opens in response to a specific molecule binding.
- Nicotinic receptor — a cholinergic, ligand-gated ion channel that responds to acetylcholine or nicotine.
- NM (Nicotinic M) receptor — a nicotinic receptor on skeletal muscle, mediating contraction.
- NN (Nicotinic N) receptor — a nicotinic receptor on neurons or adrenal medulla, mediating neural transmission or hormone release.
- Neuromuscular junction — the synapse between a motor neuron and skeletal muscle fiber.
- Myasthenia gravis — autoimmune disease blocking NM receptors, causing muscle weakness.
Action Items / Next Steps
- Review clinical implications of NM receptor dysfunction (e.g., myasthenia gravis).
- Study the anatomy of the autonomic nervous system ganglia where NN receptors are found.
- Become familiar with the mechanism of synaptic transmission and muscle contraction.