Question

Identify the major neurotransmitters and briefly discuss their distribution in the nervous system.

Answer 1

Neurotransmitter is a chemical substance that acts as a mediator for the transmission of nerve impulse from one neuron to another neuron through a synapse.

ACETYLCHOLINE
Acetylcholine is a cholinergic neurotransmitter. It possesses excitatory function. It produces the excitatory function by opening the ligand-gated sodium channels .
Source
Acetylcholine is the transmitter substance at the neuro- muscular junction and synapse. It is also released by the following nerve endings:
1. Preganglionic parasympathetic nerve
2. Postganglionic parasympathetic nerve
3. Preganglionic sympathetic nerve
4. Postganglionic sympathetic cholinergic nerves:
i. Nerves supplying eccrine sweat glands
ii. Sympathetic vasodilator nerves in skeletal
muscle
5. Nerves in amacrine cells of retina 6. Many regions of brain.
Synthesis
Ach is synthesized in the cholinergic nerve endings. Synthesis takes place in axoplasm and Ach is stored in the vesicles. It is synthesized from acetyl coenzyme A (acetyl CoA). It combines with choline in the presence of the enzyme choline acetyltransferase to form Ach.   
Fate
Action of Ach is short lived. Within one millisecond after the release from the vesicles, it is hydrolyzed into acetate and choline by the enzyme acetylcholin­ esterase. This enzyme is present in basal lamina of the synaptic cleft.
Acetylcholine Receptors
There are two types of receptors through which Ach acts on the tissues namely, muscarinic receptors and nicotinic receptors. Reason for the terminology of these receptors is as follows: Poisonous substance from toadstools called muscarine, acts on a specific group of receptors known as muscarinic receptors; similarly, another substance called nicotine acts on a specific group of receptors known as nicotinic receptors but Ach acts on both the receptors.
Muscarinic receptors are present in all the organs innervated by the postganglionic fibers of the parasympathetic system and by the sympathetic cholinergic nerves. Nicotinic receptors are present in the synapses between preganglionic and post- ganglionic neurons of both sympathetic and para- sympathetic systems.
Nicotinic receptors are also present in the neuro- muscular junction on membrane of skeletal muscle.
NORADRENALINE
Noradrenaline is the neurotransmitter in adrenergic nerve fibers. It is released from the following structures:
1. Postganglionic sympathetic nerve endings
2. Cerebral cortex.
3. Hypothalamus
4. Basal ganglia
5. Brainstem
6. Locus ceruleus in pons 7. Spinal cord.
In many places, noradrenaline is the excitatory chemical mediator and in very few places, it causes inhibition. It is believed to be involved in dreams, arousal and elevation of moods.
DOPAMINE
Dopamine is secreted by nerve endings in the following areas:
1. Basal ganglia 2. Hypothalamus 3. Limbic system 4. Neocortex
5. Retina
6. Small, intensely fluorescent cells in sympathetic
ganglia.
Dopamine possesses inhibitory action. Prolactin inhibitory hormone secreted by hypothalamus is considered to be dopamine.
SEROTONIN
Serotonin is otherwise known as 5­hydroxytryptamine (5-HT). It is synthesized from tryptophan by hydroxyla- tion and decarboxylation. Large amount of serotonin (90%) is found in enterochromatin cells of GI tract. Small amount is found in platelets and nervous system. It is secreted in the following structures:
1. Hypothalamus
2. Limbic system
3. Cerebellum
4. Dorsal raphe nucleus of midbrain 5. Spinal cord
6. Retina
7. GI tract 8. Lungs
9. Platelets.
It is an inhibitory substance. It inhibits impulses of pain sensation in posterior gray horn of spinal cord. It is supposed to cause depression of mood and sleep . Serotonin causes vasoconstriction, platelet aggregation and smooth muscle contraction. It also controls food intake.
HISTAMINE
Histamine is secreted in nerve endings of hypothalamus, limbic cortex and other parts of cerebral cortex. It is also secreted by gastric mucosa and mast cells. Histamine is an excitatory neurotransmitter. It is believed to play an important role in arousal mechanism.
GAMMA­AMINOBUTYRIC ACID
Gamma-aminobutyric acid (GABA) is an inhibitory neurotransmitter in synapses particularly in CNS. It is responsible for presynaptic inhibition. It is secreted by nerve endings in the following structures:
1. Cerebral cortex 2. Cerebellum
3. Basal ganglia 4. Spinal cord
5. Retina.
GABA causes synaptic inhibition by opening potas-
sium channels and chloride channels. So, potassium comes out of synapse and chloride enters in .This leads to hyperpolarization, which is known as inhibitory postsynaptic potential (IPSP.