Chapter 7: Acetylcholine Flashcards
Where are cholinergic neurons located?
In the peripheral nervous system (including neuromuscular joints) and at specific synapses within the sympathetic and parasympathetic nervous divisions of the autonomic nervous system.
How is acetylcholine synthesized?
Acetylcholine is synthesized from the precursors choline and acetyl coenzyme A. Choline comes from dietary fats and can also be produced in the liver. Acetyl coenzye A is made in all cells by the metabolism of sugars and fats. The enzyme choline acetyltransferase catalyzes the synthesis and transfers the acetyl group from acetyl coenzyme A to choline to make acetylcholine. The second product of this reaction is coenzyme A.
What controls the rate of acetylcholine synthesis?
- availability of precursors
2. rate of cell firing
How is acetylcholine inactivated?
Acetylcholine is inactivated by the enzyme acetylcholinesterase.
Where is the enzyme acetylcholinesterase (AChE) present?
- inside the presynaptic cell
- on the membrane of the postsynaptic cell
- at neuromuscular junctions
What does AChE do in the presynaptic cell?
metabolizes excess acetylcholine when too much is synthesized
What does AChE do on the membrane of the postsynaptic cell?
metabolizes molecules that have been released into the synaptic cleft
What does AChE do at neuromuscular junctions?
breaks down acetylcholine quickly after it causes a muscle contraction so that the muscle can relax and contract again
What are the products of acetylcholine breakdown?
- choline
2. acetic acid
Describe the anatomy of the cholinergic pathway in the parasympathetic branch of the autonomic nervous system.
In the parasympathetic branch, cholinergic preganglionic neurons send their axons to cholinergic ganglionic neurons which in turn send their axons to the target organ and release acetylcholine.
Describe the anatomy of the cholinergic pathway in the sympathetic branch of the autonomic nervous system.
In the sympathetic branch, cholinergic preganglionic neurons send their axons to cholinergic ganglionic neurons which in turn send their axons to the target organ and release norepinephrine.
Describe the anatomy of the cholinergic pathways in the brain.
- Cholinergic neurons in the basal forebrain cholinergic system (BFCS) innervate the cerebral cortex, hippocampus, amygdala and limbic system.
- Cholinergic neurons in the laterodorsal tegmental nuclei innervate the ventral tegmental area.
- Cholinergic cells in the pedunclopontine tegmental nuclei innervate the substantia nigra.
What are the cholinergic receptor subtypes?
- nicotinic receptors
2. muscarinic receptors
Where are nicotinic receptors located?
- muscle cells at neuromuscular junctions
- ganglionic neurons in the sympathetic and parasympathetic systems
- some neurons in the brain.
Describe how nicotonic receptors work.
Nicotinic receptors are ionotropic and are made up of five subunits arranged to form a channel. When acetylcholine binds to the receptor the ionic channel opens and sodium (Na+) and calcium (Ca2+) ions enter the cell producing a fast excitatory response. The excitatory response happens because of depolarization of the cell membrane. In the brain the neuron is more likely to fire and in a muscle cell it will respond to this depolarization by contracting. Thus, they mediate fast excitatory responses in both the CNS and PNS.