Neurotransmitters- What they are, Types & Functions

Neurotransmitters are chemical messengers that play a crucial role in transmitting signals across synapses in the nervous system. These signaling molecules transmit information from one neuron to another or from neurons to target cells (e.g., muscles or glands). Neurotransmitters can be classified based on various criteria, and each type has specific properties. Here’s a classification and an overview of the properties of neurotransmitters:

Classification of Neurotransmitters

1. Based on Chemical Structure:

   – Amino Acids:

     – Examples: Glutamate (excitatory), GABA (gamma-aminobutyric acid) (inhibitory), and glycine (inhibitory).

   – Monoamines:

     – Examples: Dopamine, norepinephrine, epinephrine, serotonin.

   – Acetylcholine:

     – Example: Acetylcholine (ACh).

2. Based on Function:

   – Excitatory Neurotransmitters:

     – Examples: Glutamate, acetylcholine.

   – Inhibitory Neurotransmitters:

     – Examples: GABA, glycine.

3. Based on Action:

   – Direct-Acting (Ionotropic) Neurotransmitters:

     – Bind directly to ion channels, causing rapid changes in membrane potential.

     – Examples: Acetylcholine, serotonin (5-HT3 receptor).

   – Indirect-Acting (Metabotropic) Neurotransmitters:

     – Activate G protein-coupled receptors, leading to a more prolonged response.

     – Examples: Dopamine serotonin (most receptors).

4. Based on Synthesis Location:

   – Neuronal Neurotransmitters:

     – Synthesized in the presynaptic terminal.

     – Examples: Acetylcholine, dopamine, serotonin.

   – Non-neuronal Neurotransmitters:

     – Synthesized outside neurons (e.g., in the adrenal gland).

     – Examples: Epinephrine, norepinephrine.

5. Based on Function in the Peripheral Nervous System:

   – Cholinergic Neurotransmitters:

     – Involve acetylcholine.

   – Adrenergic Neurotransmitters:

     – Involve norepinephrine and epinephrine.

Properties of Neurotransmitters

1. Synthesis and Storage:

   – Synthesized: Within the neuron’s cell body or terminal.

   – Stored: In vesicles in the presynaptic terminal.

2. Release:

   – Exocytosis: Neurotransmitters are released into the synaptic cleft in response to an action potential.

3. Binding to Receptors:

   – Specific Binding: Neurotransmitters bind to receptors on the postsynaptic membrane.

   – Ligand-Gated Ion Channels: Activation of ionotropic receptors directly opens or closes ion channels.

4. Action Termination:

   – Reuptake: Neurotransmitters are transported back into the presynaptic neuron.

   – Enzymatic Degradation: Neurotransmitters are broken down by enzymes in the synaptic cleft.

   – Diffusion: Neurotransmitters may diffuse away from the synapse.

5. Postsynaptic Response:

   – Excitatory Response: Results in depolarization of the postsynaptic membrane.

   – Inhibitory Response: Results in hyperpolarization of the postsynaptic membrane.

6. Modulation of Signal Strength:

   – Facilitation: Enhances the likelihood of postsynaptic excitation.

   – Inhibition: Reduces the likelihood of postsynaptic excitation.

7. Plasticity:

   – Synaptic Plasticity: Ability of synapses to undergo long-term changes in strength.

   – Long-Term Potentiation (LTP): Persistent strengthening of synapses.

   – Long-Term Depression (LTD): Persistent weakening of synapses.

8. Role in Diseases:

   – Imbalances: Dysregulation of neurotransmitters is associated with various neurological and psychiatric disorders.

   – Examples: Dopamine imbalance in Parkinson’s and schizophrenia.

Understanding the classification and properties of neurotransmitters is essential for comprehending the intricacies of neural communication and the modulation of physiological processes within the nervous system.