Definition and Criteria
Neurotransmitters are endogenous chemical messengers that allow neurons to communicate with one another, as well as with muscles and glands. To be classified as a neurotransmitter, a chemical must be synthesized within the neuron, released into the synaptic cleft upon electrical stimulation, and possess a specific mechanism for inactivation or removal.
Classification and Function
Neurotransmitters are broadly categorized by their chemical structure and their effect on the postsynaptic cell:
• Amino Acids: These mediate fast signaling. Glutamate is the primary excitatory neurotransmitter in the brain, while GABA (γ-aminobutyric acid) and Glycine are the main inhibitory neurotransmitters.
• Biogenic Amines (Monoamines): These play critical roles in regulating mood, attention, and motor control. Key examples include Dopamine (reward and movement), Serotonin (mood and sleep), Norepinephrine (alertness and stress), and Histamine (wakefulness). Acetylcholine is a small molecule transmitter essential for muscle contraction and memory.
• Neuropeptides: These are larger molecules, such as Oxytocin and Vasopressin, that often act as neuromodulators, influencing complex behaviors like social bonding and stress over longer durations.
Functionally, these chemicals can be excitatory (promoting electrical activity), inhibitory (suppressing activity), or modulatory (adjusting the sensitivity of neurons to other signals).
Mechanism of Action
Neurotransmission involves three key steps:
1. Release: An electrical signal (action potential) triggers calcium influx, causing vesicles to fuse with the presynaptic membrane and release neurotransmitters into the synaptic cleft.
2. Receptor Binding: Neurotransmitters bind to specific receptors on the target cell. Ionotropic receptors are ligand-gated ion channels that produce rapid electrical changes (milliseconds). Metabotropic receptors (G-protein-coupled receptors) initiate slower, intracellular signaling cascades that modulate cell function over seconds to minutes.
3. Termination: The signal must be stopped to maintain physiological balance. This is achieved through diffusion, enzymatic degradation (e.g., Acetylcholinesterase breaking down Acetylcholine), or reuptake into cells via specific transporters.
Dysregulation of these chemical systems is central to many neurological conditions, including Parkinson's disease, Alzheimer's disease, and depression.
