Unit 2: Chapter 3: Chemical Transmission At Autonomic Junction

Question 1

What are the main chemical transmitters at the synaptic junctions in the autonomic nervous system?

Answer 1

1. Acetylcholine.
2. Noradrenaline.

Question 2

Autonomic nerve fibers are divided into:

Answer 2

1. Cholinergic fibers.
2. Adenergic fibers.

Question 3

What are cholinergic receptors activated by?

Answer 3

Acetylcholine

Question 4

What are adrenergic receptors activated by?

Answer 4

Noradrenaline

Question 5

Acetylcholine

Answer 5

Chemical transmitter released by cholinergic fibers.

Question 6

Sites of cholinergic fibers:

Answer 6

1. All preganglionic autonomic fibers:
   - Preganglionic sympathetic and parasympathetic fibers.
   - Adrenal melluda
2. All postganglionic parasympathetic fibers.
3. Some postganglionic sympathetic fibers:
   - Secretory fibers to sweat glands.
   - Vasodilator to blood vessels of skeletal muscles.

Question 7

Where is acetylcholine synthesized?

Answer 7

In the Axilla am of the terminal endings of cholinergic nerve fibers by choline acetyl transferase.

Question 8

How do the terminal knobs help in the synthesis of acetylcholine?

Answer 8

Terminal knobs are rich in mitochondria to supply ATP required for the synthesis of chemical transmitters.

Question 9

Where is synthesized acetylcholine stored?

Answer 9

In small clear vesicles.

Question 10

When is acetylcholine released?

Answer 10

1. When an action potential spreads over the terminal fibers, the depolarization process increases the influx of Ca2+ into the nerve terminals.
2. Ca2+ ions causes exocytosis of vesicles and releases acetylcholine.

Question 11

Removal of acetylcholine

Answer 11

1. 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.
2. Choline produced is transported back into the nerve endings, where it is used again for synthesis of new acetylcholine.

Question 12

The removal of acetylcholine is divided into two parts:

Answer 12

1. True cholinesterase (specific).
2. Pseudocholinesterase (non specific).

Question 13

True cholinesterase (specific)

Answer 13

1. Great affinity for acetylcholine.
2. Present in high concentrations in:
   - Cholinergic nerve endings.
   - Synaptic area bound to collagen.
   - Cell membrane at cholinergic nerve terminal.

Question 14

Pseudocholinesterase (non specific)

Answer 14

1. Present in plasma.
2. Acts on acetylcholine that diffuses into surrounding fluids or in the plasma.

Question 15

Site of adrenergic fibers

Answer 15

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.

Question 16

Synthesis of noradrenaline

Answer 16

1. Synthesized in the terminal endings of adrenergic nerve fibers.
2. The noradrenaline is stored in granulated dark vesicles.
3. Bound to ATP and protein called: Chromogranin.

Question 17

Release of noradrenaline

Answer 17

When the nerve impulse reaches the terminal knobs of adrenergic fibers., noradrenaline is released by exocytosis.

Question 18

Removal of noradrenaline

Answer 18

It is removed within few seconds in three different ways:
1. Active re-uptake.
2. Diffusion away.
3. Destruction of enzymes.

Question 19

Active re-uptake

Answer 19

Active re-uptake into the adrenergic nerve endings themselves accounting for removal 50-80% of the secreted norepinephrine.

Question 20

Diffusion away

Answer 20

Diffusion away from the nerve endings into the surrounding body fluids and blood.

Question 21

Destruction of enzymes

Answer 21

1. Monoamine oxidase (MAO).
2. Catechol-O-methyl-transferase (COMT).

Question 22

Monoamine oxidase (MAO)

Answer 22

1. Located on the outer surface of mitochondria.
2. It is widely distributed, particularly in:
   - Adrenergic nerve fibers.
   - Brain.
   - Liver.
   - Kidneys.
3. Deamination of NE.

Question 23

Catechol-O-methyl-transferase

Answer 23

1. Present in all tissues but not not in adrenergic nerve endings.
2. Methylation of NE.