Neurotransmitters are the body’s chemical messengers. They transmit signals across a tiny gap called the synapse, moving from one neuron to the next target cell, which can be another nerve cell, a muscle cell, or a gland. This continuous communication allows the brain and body to control countless functions, ranging from emotions and mood to muscle movement and organ functions like heart rate and breathing.
Classification by Function
Neurotransmitters are often grouped based on their effect on the receiving neuron. The three primary functional categories are excitatory, inhibitory, and modulatory. This classification helps understand how these chemical signals orchestrate complex activities within the nervous system.
Excitatory neurotransmitters increase the likelihood that the receiving neuron will generate an electrical signal. They make the neuron more prone to “firing.” In contrast, inhibitory neurotransmitters decrease the chance of the neuron firing, often having a calming effect on the nervous system. Modulatory neurotransmitters influence the effects of other chemical messengers, often affecting a larger number of neurons simultaneously over a slower timeframe.
Key Excitatory Neurotransmitters
Glutamate
Glutamate is the most abundant excitatory neurotransmitter within the central nervous system. It plays a significant role in cognitive functions, including thinking, learning, and memory formation. This neurotransmitter activates various neural systems, acting as a “gas pedal” for the brain’s activities.
However, excessive glutamate can lead to excitotoxicity, a condition where nerve cells become overstimulated and damaged. This overactivation can occur when glutamate levels are too high. Excitotoxicity is implicated in neurodegenerative diseases like Alzheimer’s disease, Parkinson’s disease, Huntington’s disease, and can contribute to neuron damage after events like a stroke.
Acetylcholine
Acetylcholine was the first neurotransmitter discovered and functions in both the central and peripheral nervous systems. In the peripheral nervous system, it is responsible for muscle contraction. This action is fundamental for every body movement.
In the central nervous system, acetylcholine influences arousal, attention, focus, memory, and motivation. Reduced levels of acetylcholine are associated with the memory loss and cognitive decline observed in Alzheimer’s disease. Medications called cholinesterase inhibitors work by blocking the enzyme that breaks down acetylcholine, thereby increasing its availability in the brain to help manage symptoms.
Key Inhibitory Neurotransmitters
GABA (Gamma-aminobutyric acid)
GABA is the primary inhibitory neurotransmitter in the brain, playing a central role in calming the nervous system. Its main function involves decreasing neuronal excitability, which helps regulate mood and anxiety levels. GABA achieves this by reducing the likelihood of nerve cells firing.
Low levels of GABA are associated with increased anxiety, irritability, and difficulties with concentration and sleep. Medications such as benzodiazepines, which include drugs like diazepam (Valium) and alprazolam (Xanax), enhance the effects of GABA by binding to its receptors and making them more responsive. This enhancement leads to sedative, anxiolytic (anti-anxiety), and muscle relaxant properties, making them useful for treating anxiety disorders and seizures.
Serotonin
Serotonin is a neurotransmitter that influences mood, sleep patterns, appetite, and digestion. It is found predominantly in the brain, intestines, and blood platelets. Serotonin contributes to feelings of well-being and happiness.
Insufficient serotonin levels in the brain are linked to conditions such as depression and anxiety disorders. Selective serotonin reuptake inhibitors (SSRIs) are a common class of antidepressants that work by blocking the reabsorption of serotonin by the transmitting neurons. This action increases the amount of serotonin available in the synaptic space, leading to prolonged effects on the receiving neuron and helping to improve mood and reduce symptoms.
Key Modulatory Neurotransmitters
Dopamine
Dopamine is a neurotransmitter involved in the brain’s reward system, motivation, pleasure, and fine motor control. It influences behaviors related to seeking pleasurable experiences. Dopamine also impacts focus, concentration, memory, and mood.
Dysregulation of dopamine signaling is associated with several neurological and psychiatric conditions. Low levels of dopamine are characteristic of Parkinson’s disease, leading to motor control difficulties. Conversely, imbalances in dopamine activity are implicated in conditions like schizophrenia and addiction, where drugs can hijack the brain’s reward system.
Norepinephrine (Noradrenaline)
Norepinephrine, also known as noradrenaline, functions as both a neurotransmitter and a hormone, involved in alertness, concentration, and the body’s “fight-or-flight” response. When the body perceives stress or danger, norepinephrine levels rise, preparing the individual to react by increasing heart rate, blood pressure, and blood flow to muscles. It also heightens attention.
Reduced norepinephrine neurotransmission can result in low energy levels, decreased alertness, and difficulties with attention. Imbalances in norepinephrine are linked to mental health conditions such as major depressive disorders and attention deficit hyperactivity disorder (ADHD), where dysregulation impacts focus and impulse control. Medications often aim to modulate norepinephrine levels to improve symptoms in these conditions.