Question

Messages are transmitted from one neuron to another via the synapse process. IN YOUR OWN WORDS: Define neurotransmitter, the four main categories, and their role in cell communication. Discuss one of the following synapse types: excitatory cholinergic, inhibitory GABA-ergic, or excitatory adrenergic.

Answer 1

Neurotransmitters are the chemical substances which are responsible for the transmission of an impulse through a synapse. Criteria for a neurotransmitter:

• It should be synthesized by presynaptic neurons and stored in the vesicles.
• It should be released on stimulation of the nerve.
• It travels very small distance between presynaptic membrane and postsynaptic membrane.
• It is associated with an enzyme or enzyme system for its inactivation.
• When applied intrinsically, it should mimic the effects of the nerve stimulation.
• Drug which modifies the response to Nerve stimulation should also modify the proposed transmitter action in a similar way.

Classes:

I. Acetylcholine (principal neurotransmitter released by cholinergic neurons in the parasympathetic nervous system and it is also the neurotransmitter for preganglionic neurons in sympathetic.

II. Biogenic amines:-

• Norepinephrine: It is an excitatory chemical, it is believed to be involved in dreams, arousal and elevation of mood. Noradrenergic neurons are involved in regulation of secretion of ADH, oxytocin etc in Hypothalamus. It is an inhibitory chemical jn thalamus, cerebral cortex, cerebellar cortex.
• Dopamine: It induces vomiting. It controls movements as it controls corpus striatum. It inhibits prolactin secretion.
• 5-HT: It regulates carbohydrate intake and hypothalamic releasing hormones. It inhibits pain.
• Histamine: It plays role in arousal and sexual behaviour, drinking, pain threshold and inflammation.

III. Amino Acids:

• Glutamate (excitatory): involved in motor coordination and learning.
• Aspartate (excitatory): chief excitatory transmitter of cortical pyramidal cells.
• GABA (Inhibitory): major inhibitory neurotransmitter in whole CNS
• Glycine (inhibitory): it produces direct inhibition in the spinal cord.

IV. ATP, Nitric oxide.

• ATP: provides energy.
• Nitric oxide: responsible for vasodilation.

Inhibitory GABAergic synapse.

Gamma aminobutyric acid is the major inhibitory neurotransmitter in the whole CNS.

Synthesis: it is formed by decarboxylation of glutamic acid by the enzyme glutamate decarboxylase. Pyridoxal phosphate, Vitamin B complex derivative is a cofactor for this enzyme. Removal: GABA is removed from the synaptic cleft chiefly by its reuptake phenomenon.

GABA receptors: GABA produces presynaptic inhibition by two types of receptors:

GABA-A receptors produce inhibition by increasing chloride conductance.

GABA-B receptorsproduce inhibition by increasing potassium conductance and by decreasing cAMP activity that decreases calcium influx.

GABA-C receptors are exclusively present in retina.