Classification of Neurotransmitters

Neurotransmitters are classified as excitatory, inhibitory, or modulatory, based on their role in nerve signaling.

1. Excitatory Neurotransmitters

   • These promote depolarization of the postsynaptic membrane, increasing the likelihood of an action potential.
     • Glutamate: Main excitatory neurotransmitter in the CNS
     • Aspartate: Excitatory amino acid in the CNS
     • Acetylcholine: Excitatory at the neuromuscular junction (nicotinic receptors)
     • Norepinephrine (Noradrenaline): Excitatory in the sympathetic nervous system and CNS
     • Epinephrine (Adrenaline): Excitatory in various CNS and peripheral pathways
     • Histamine: Excitatory in the brain; involved in arousal and wakefulness

2. Inhibitory Neurotransmitters

   • These cause hyperpolarization, making it less likely for the postsynaptic neuron to fire.
     • GABA (Gamma-aminobutyric acid): Primary inhibitory neurotransmitter in the CNS
     • Glycine: Inhibitory, especially in the spinal cord
     • Serotonin (5-HT): Often inhibitory; effects vary by receptor subtype

3. Neurotransmitters with Both Excitatory and Inhibitory Actions

   • These can act as either excitatory or inhibitory depending on the receptor subtype and location.
     • Dopamine: Can be excitatory (D1 receptors) or inhibitory (D2 receptors)
     • Acetylcholine: Excitatory at neuromuscular junctions; inhibitory in some CNS regions (muscarinic receptors)
     • Serotonin (5-HT): Effects depend on receptor subtype (can be both)
     • Nitric Oxide: Gaseous neurotransmitter with variable modulatory effects

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