Unit 2: Chapter 3: Chemical Transmission At Autonomic Junction Flashcards
What are the main chemical transmitters at the synaptic junctions in the autonomic nervous system?
- Acetylcholine.
- Noradrenaline.
Autonomic nerve fibers are divided into:
- Cholinergic fibers.
- Adenergic fibers.
What are cholinergic receptors activated by?
Acetylcholine
What are adrenergic receptors activated by?
Noradrenaline
Acetylcholine
Chemical transmitter released by cholinergic fibers.
Sites of cholinergic fibers:
- All preganglionic autonomic fibers:
- Preganglionic sympathetic and parasympathetic fibers.
- Adrenal melluda - All postganglionic parasympathetic fibers.
- Some postganglionic sympathetic fibers:
- Secretory fibers to sweat glands.
- Vasodilator to blood vessels of skeletal muscles.
Where is acetylcholine synthesized?
In the Axilla am of the terminal endings of cholinergic nerve fibers by choline acetyl transferase.
How do the terminal knobs help in the synthesis of acetylcholine?
Terminal knobs are rich in mitochondria to supply ATP required for the synthesis of chemical transmitters.
Where is synthesized acetylcholine stored?
In small clear vesicles.
When is acetylcholine released?
- When an action potential spreads over the terminal fibers, the depolarization process increases the influx of Ca2+ into the nerve terminals.
- Ca2+ ions causes exocytosis of vesicles and releases acetylcholine.
Removal of acetylcholine
- Once the acetylcholine has been secreted, most of it within a fraction of a second, is split into acetate ion and choline by the enzyme: acetylcholine esterase.
- Choline produced is transported back into the nerve endings, where it is used again for synthesis of new acetylcholine.
The removal of acetylcholine is divided into two parts:
- True cholinesterase (specific).
- Pseudocholinesterase (non specific).
True cholinesterase (specific)
- Great affinity for acetylcholine.
- Present in high concentrations in:
- Cholinergic nerve endings.
- Synaptic area bound to collagen.
- Cell membrane at cholinergic nerve terminal.
Pseudocholinesterase (non specific)
- Present in plasma.
- Acts on acetylcholine that diffuses into surrounding fluids or in the plasma.
Site of adrenergic fibers
Secreted from the adrenal melluda with adrenaline.
All postganglionic sympathetic fibers secrete noradrenaline except:
1. Secretory fibers to sweat glands.
2. Vasodilator cholinergic fibers to the blood vessels of skeletal muscles.
Synthesis of noradrenaline
- Synthesized in the terminal endings of adrenergic nerve fibers.
- The noradrenaline is stored in granulated dark vesicles.
- Bound to ATP and protein called: Chromogranin.
Release of noradrenaline
When the nerve impulse reaches the terminal knobs of adrenergic fibers., noradrenaline is released by exocytosis.
Removal of noradrenaline
It is removed within few seconds in three different ways:
1. Active re-uptake.
2. Diffusion away.
3. Destruction of enzymes.
Active re-uptake
Active re-uptake into the adrenergic nerve endings themselves accounting for removal 50-80% of the secreted norepinephrine.
Diffusion away
Diffusion away from the nerve endings into the surrounding body fluids and blood.
Destruction of enzymes
- Monoamine oxidase (MAO).
- Catechol-O-methyl-transferase (COMT).
Monoamine oxidase (MAO)
- Located on the outer surface of mitochondria.
- It is widely distributed, particularly in:
- Adrenergic nerve fibers.
- Brain.
- Liver.
- Kidneys. - Deamination of NE.
Catechol-O-methyl-transferase
- Present in all tissues but not not in adrenergic nerve endings.
- Methylation of NE.
