Neurotransmitter
A chemical messenger produced and stored in neurons. It is released into the synaptic gap by the presynaptic neuron and helps transmit signals between neurons by binding to receptors on the postsynaptic neuron.
Agonist
A substance that enhances or mimics the action of a neurotransmitter. It binds to the same receptors, amplifying the neurotransmitter’s effects.
Antagonist
A substance that inhibits the action of a neurotransmitter. It blocks or reduces neurotransmitter activity by binding to the receptors, dampening or stopping the signal.
Nicotine acts as an agonist for acetylcholine by stimulating its receptors. This activation increases neural signaling in the brain, leading to heightened focus, faster reaction times, and a sense of alertness or wakefulness.
Neurotransmission Process
Neurotransmission involves both electrical and chemical mechanisms.
- An electrical impulse travels along a neuron’s axon until it reaches the synapse.
- At the synapse, the process becomes chemical, causing neurotransmitters to release into the synaptic gap.
- These chemicals bind to receptors on the postsynaptic neuron, altering its electrical potential.
- The neurotransmitter can either be excitatory, promoting the continuation of the signal, or inhibitory, preventing the signal from progressing.
Neurons and Synapses
Neurons are the fundamental units of the nervous system, consisting of three main parts:
- Cell body: Contains the nucleus and essential cellular components.
- Dendrites: Receive signals from other neurons.
- Axon: Transmits signals away from the cell body to the synapse, where communication occurs.
- Synapses are the junctions where the axon of one neuron communicates with the dendrites of another.
Neurotransmitter Interaction: The activity of neurotransmitters can be modulated by various chemicals:
- Excitatory neurotransmitters: Increase the likelihood of the postsynaptic neuron firing an electrical impulse.
- Inhibitory neurotransmitters: Decrease the likelihood of the postsynaptic neuron firing.
Remember, Agonists and antagonists influence the strength or suppression of these signals.
ExampleGlutamate is an excitatory neurotransmitter, like pressing the "gas pedal" in a car. It increases brain activity, making the brain more alert and responsive. Higher glutamate levels make the brain more excitable, while lower levels slow things down.
GABA (gamma-aminobutyric acid) is an inhibitory neurotransmitter, like pressing the "brake pedal" in a car. It decreases brain activity, calming and relaxing the brain. Higher GABA levels promote calmness, while lower levels can lead to anxiety or overstimulation.
Key Neurotransmitters
Serotonin:
- Role: Regulation of mood, emotion, and aggression.
- Type: Inhibitory neurotransmitter.
- Example: Low serotonin levels have been linked to increased impulsivity and aggression.
Dopamine:
- Role: Associated with reward, motivation, and pleasure.
- Type: Can be both excitatory and inhibitory, depending on the receptor.
- Example: High dopamine activity in certain brain areas is linked to romantic attraction and addiction.
Acetylcholine:
- Role: Critical for learning, memory, and muscle control.
- Type: Excitatory neurotransmitter.
- Example: Decline in acetylcholine levels is associated with Alzheimer’s disease.
Antonova et al (2011)
Aim: to see whether acetylcholine (ACh) aids spatial memory, and it can be slowed down by scopolamine, since scopolamine is an antagonist for acetylcholine.
Method: injected with scopolamine or a saline (placebo) and completed a virtual maze. Participants came back 3-4 weeks later and got injected with the solution they did not receive in the first round and repeated the using randomized double blind crossover design.
Sample: 20 men with average age of 28
Results: Scopolamine led the participants to make more errors in the maze than people who received the placebo, and it reduced activity in the hippocampal area
Conclusion: Scopolamine reduced activity in the hippocampal area, demonstrates a correlation between ACh and spatial memory.
Adrenaline:
- Role: Plays a role in wakefulness and arousal. It increases the heart rate
- Type: Excitatory neurotransmitter.
- Example: Adrenaline has been linked to one's fight or flight response.
Research Studies
The Effect of Serotonin on Prosocial Behavior
Case studyCrockett et al. (2010)
Aim: To investigate how serotonin influences moral decision-making and prosocial behavior.
Participants: 30 healthy volunteers.
Method: Experimental, repeated measures design with double-blind and counterbalanced conditions.
Procedure: Participants were administered either a dose of citalopram (an SSRI) or a placebo. They were presented with moral dilemmas modeled on the "trolley problem":
- Impersonal scenarios: Participants chose whether to pull a lever to divert a runaway trolley, sacrificing one person to save five.
- Personal scenarios: Participants chose whether to push someone onto the tracks to stop the trolley, a direct and emotionally aversive action.
Results: In personal scenarios, participants under citalopram were less likely to choose harm, reflecting increased prosocial tendencies. Impersonal decisions were unaffected.
Conclusion: Serotonin enhances prosocial behavior by reducing the acceptability of causing direct harm to others.
Dopamine and Parkinson’s Disease
Case studyFreed et al. (2001)
Aim: To assess the effect of dopamine-producing neuron transplants on Parkinson’s disease symptoms.
Participants: 40 patients with severe Parkinson’s disease, aged 34–75.
Method: Experimental, independent measures design.
Procedure: The experimental group received transplants of dopamine-producing neurons into the putamen, a brain region affected by Parkinson’s. The control group underwent sham surgery. PET scans and clinical observations assessed brain changes and symptom improvement over a year.
Results: Younger participants in the transplant group showed significant symptom reduction (28%), linked to increased dopamine production in the putamen.
Conclusion: Dopamine is critical in managing Parkinson’s symptoms, especially in younger patients.
Dopamine and Romantic Love
Case studyFisher, Aron, and Brown (2005)
Aim: To investigate the neural basis of romantic love and its association with dopamine activity.
Participants: 17 individuals "intensely in love" (mean age: 21; mean duration: 7 months).
Method: Experimental, repeated measures design using fMRI scans.
Procedure: Participants viewed images of their romantic partner and neutral acquaintances in alternating sequences while undergoing fMRI scans. Brain activity patterns were compared.
Results: Photos of romantic partners triggered increased activation in dopamine-rich brain regions linked to reward and motivation.
Conclusion: Romantic love is associated with dopamine activity, reinforcing its role in reward processing.
Critical Thinking
Complex Interactions:
Neurotransmitter Systems: Neurotransmitters do not operate in isolation. They interact with one another, influencing and modulating each other’s effects.
ExampleSerotonin might affect dopamine pathways, making it difficult to pinpoint the exact role of a single neurotransmitter in complex behaviors like decision-making or aggression.
Brain Regions: The effects of neurotransmitters also depend on where they act in the brain.
ExampleDopamine’s role in the prefrontal cortex may influence decision-making, while in the striatum, it is more closely linked to reward and habit formation.
Ecological Validity of Research:
Artificial Conditions: Many studies artificially manipulate neurotransmitter levels (e.g., through drugs or dietary controls). While laboratory research helps establish causal links, its ecological validity is low and does not fully replicate the complexity of natural neurotransmission.
TipReal-world factors such as stress, social interactions, or genetic predispositions often play a role on behavioral effects.
Alternative Explanations:
Social and Cognitive Factors: Behaviors influenced by neurotransmitters, such as aggression or prosocial acts, are also shaped by life experiences, cultural norms, and situational contexts.
ExampleSerotonin may lower aggression, but a person’s upbringing or immediate environment could override this biological tendency.
Ethical Considerations:
- Human Studies: Ethical dilemmas arise in studies that manipulate neurotransmitters or involve invasive procedures.
- Administering SSRIs or reducing serotonin levels must be carefully monitored to avoid long-term harm to participants.
- Animal Studies: Research on animals (e.g., injecting neurotransmitters into the brain) provides valuable insights but raises ethical questions about animal welfare, particularly in experiments involving distress or harm.
Consent and Risks: In human studies, ensuring informed consent and minimizing risks is paramount, especially when the research involves vulnerable populations, such as those with mental health disorders.
Measurement Challenges:
- Indirect Measures: Neurotransmitter activity is often inferred from behavior or brain imaging rather than directly measured, which can limit the precision of findings.
- For instance, fMRI scans identify areas of activation but do not directly quantify neurotransmitter levels.
- Time Delays: Changes in behavior may result from long-term processes initiated by neurotransmitter activity.
- Research often struggles to account for these delayed effects, leading to potential oversimplification of cause-and-effect relationships.
