Acetylcholine

Acetylcholine (ACh) is a neurotransmitter released by certain nerve cells in both the central and peripheral nervous system. It is notably the chemical that motor neurons release at the neuromuscular junction to stimulate muscle contraction, and it also serves as a neurotransmitter in various brain circuits and the autonomic nervous system.

• At the <u>neuromuscular junction</u> (nerve-to-skeletal muscle synapse), ACh is released from the motor neuron and binds to nicotinic acetylcholine receptors on the muscle fiber, causing muscle contraction. This is a classic high-yield example of ACh in action.
• In the autonomic nervous system: ACh is released by all preganglionic neurons (both sympathetic and parasympathetic), and by parasympathetic postganglionic neurons onto target organs (where it binds to muscarinic ACh receptors). This often appears in questions about heart rate (ACh slows it via parasympathetic input, for instance).
• ACh is broken down by the enzyme <u>acetylcholinesterase (AChE)</u> in the synaptic cleft. Therefore, inhibitors of AChE (like certain nerve gases or Alzheimer's medications) cause ACh to accumulate, leading to prolonged muscle contraction or heightened parasympathetic effects - an important concept for pharmacology questions.

• Frequently tested via the <u>acetylcholinesterase</u> angle: e.g., a question about organophosphate poisoning (nerve gas) - expect discussion of AChE inhibition leading to excess acetylcholine and cholinergic crisis (salivation, muscle spasms, etc.).
• Identification: If asked "Which neurotransmitter is used at the neuromuscular junction?" - the answer is acetylcholine. Also, classic clues like a motor end-plate diagram or a description of a neurotransmitter that triggers skeletal muscle contraction all point to ACh.
• Receptor subtypes: Exams may ask about differences between ACh receptors - <u>nicotinic ACh receptors</u> are ligand-gated Na+ channels (ionotropic) found on muscle and autonomic ganglia, whereas <u>muscarinic ACh receptors</u> are G-protein-coupled (metabotropic) found on parasympathetic target organs. Recognizing which is involved in a given scenario is often key.