- A chemical substance present in neuron.
- Transmits nerve impulse from one to another neuron.
- Thus also called a mediator for impulse transmission.
- Transmission occurs through synapse.
- Discovered first by Otto Loewi in 1921.
- Was an Austrian scientist who practically came out with this discovery.
Criteria for Neurotransmitters
- Various substances are categorized as neurotransmitters.
- Following criteria should be met for being a substance neurotransmitter.
- Must be present in a neuron.
- Produced by a neuron.
- Released by a neuron.
- Must act on target site after release.
- Must produce biological affect.
- Must be inactivated after showing affect.
- Classified into different types on the different basis.
- Some of the types are as follows.
A) Based on chemical nature
- Substances with different chemical nature are identified as neurotransmitters.
- Depending on it, they are kept in three groups. They are:
a) Amino acids
- Involved in fast synaptic transmission that falls in this group.
- Inhibitory and excitatory in function.
- GABA, glycine, glutamate, aspartate falls in this group.
- Modified amino acids.
- Shows slow synaptic transmission.
- Also inhibitory and excitatory in action.
- Noradrenaline, adrenaline, dopamine, serotonin and histamine falls into this group.
- Acetylcholine like neurotransmitters does not belong to above group.
- Formed from choline and acetyl co-enzyme A.
- Presence of choline acetyltransferase is required.
- Nitric oxide, a soluble gas also acts as neurotransmitters.
B) Based on function
- Some neurotransmitters are related to excitation only.
- Some cause inhibition of post synaptic neuron.
- Depending on it, they are kept in two groups.
a) Excitatory neurotransmitters
- Conducts impulse from presynaptic to post synaptic neuron.
- Release does not cause development of action potential in post synaptic neuron.
- Though it is released by pre synaptic neuron terminal.
- But, it causes change only in resting membrane potential i.e., slight depolarization.
- This change occurs by opening of sodium channels in the post synaptic membrane.
- This results in influx of sodium ions from ECF.
- This is called excitatory post synaptic potential (EPSP).
- Thus, results in development of action potential in initial portion of post synaptic neuron.
- Acetylcholine and noradrenaline falls into this group.
b) Inhibitory neurotransmitters
- Inhibits the impulse conduction from pre to post synaptic neuron.
- Potassium channels opens in the post synaptic membrane in result of its release in pre synaptic axon.
- This happens as the action potential arrives on the axon.
- This finally leads to potassium ions efflux.
- Thus, hyperpolarization occurs which is called inhibitory post synaptic potential (IPSP).
- Action potential won’t generate in the post synaptic neuron in IPSP development.
- Gamma-amino butyric acid (GABA) and dopamine falls in this group.
Transportation and Release
- Cell body of the neuron produces it.
- Carried through axon and stored in axon terminal.
- Stored in small packets called vesicles.
- Stimulus influences these vesicles to open.
- Thus, in synaptic cleft neurotransmitters are released.
- Receptors on the surface of post synaptic cell are specific.
- Thus, binding occurs after they are released.
- G proteins, protein kinase or ligand-gated receptors are some examples.
- After neurotransmitters function completely, they are inactivated.
- Four mechanisms deactivate them.
- Diffuses out to the area where it cannot show its function.
- Specific enzymes present may destroy them.
- Astrocytes (macrophages) engulf them and thus removed.
- Reuptake into the axon terminal removes them also.
Neurotransmitters and Their Types