Neurotransmitters are the nervous system’s chemical messengers. AP Psychology emphasises how specific neurotransmitters influence behaviour and mental processes, including whether their effects are generally excitatory or inhibitory in neural circuits.

Core idea: chemical messages at synapses

This labeled synapse diagram shows neurotransmitter-containing vesicles releasing chemical signals into the synaptic cleft, followed by binding at receptor sites on the postsynaptic membrane. It visually anchors the direction of information flow (presynaptic → postsynaptic) and highlights that receptor binding is what changes the postsynaptic cell’s likelihood of firing. Source

Neurotransmitters do not “carry thoughts” directly; instead, they alter neural signalling by changing postsynaptic activity, shaping sensation, movement, mood, and learning.

Excitatory vs inhibitory effects

What “excitatory” and “inhibitory” mean in AP Psych

• Excitatory messages increase the chance the postsynaptic neuron will fire (often by promoting depolarisation).

• Inhibitory messages decrease the chance the postsynaptic neuron will fire (often by promoting hyperpolarisation).

This figure contrasts excitatory postsynaptic potentials (EPSPs) and inhibitory postsynaptic potentials (IPSPs) using membrane-potential-over-time plots paired with simple neuron diagrams. It also illustrates temporal and spatial summation, showing how multiple small excitatory inputs can add together, and how inhibitory input can reduce or cancel excitation. Source

• A key AP point: the same neurotransmitter can be excitatory or inhibitory depending on receptor type and the neural pathway.

Key neurotransmitters you must know

Acetylcholine (ACh)

• Common roles: muscle action, learning, attention, and memory.

• AP associations:

  • ACh-producing neuron deterioration is linked to Alzheimer’s disease.

  • At the neuromuscular junction, reduced effective ACh signalling can produce muscle weakness.

Dopamine

• Common roles: movement, motivation, reward, and learning (especially reinforcement-related learning).

• AP associations:

  • Undersupply in motor pathways is linked to Parkinson’s disease (tremors, decreased mobility).

  • Oversupply (or overactivity at certain receptors) is linked to schizophrenia-like symptoms in classic AP framing.

Serotonin

• Common roles: mood regulation, sleep, and appetite.

• AP associations:

  • Undersupply is commonly linked with depression (as a simplified exam-relevant relationship).

  • Because serotonin relates to sleep and appetite, dysregulation can affect both.

Norepinephrine (noradrenaline)

• Common roles: alertness, arousal, and attention, especially in response to challenge.

• AP associations:

  • Undersupply can be linked to depressed mood and reduced alertness.

  • Oversupply can be associated with anxiety-like hyperarousal (e.g., feeling keyed-up).

Glutamate

• Primary role: the brain’s major excitatory neurotransmitter; strongly involved in learning and memory.

• AP associations:

  • Oversupply can overstimulate the brain, contributing to migraines or seizures (via excessive excitation).

GABA (gamma-aminobutyric acid)

• Primary role: the brain’s major inhibitory neurotransmitter; helps regulate neural firing and overall calm/stability in networks.

• AP associations:

  • Undersupply is linked to seizures, tremors, and insomnia, reflecting too little inhibition.

Endorphins

• Role: natural, morphine-like neurotransmitters involved in pain control and pleasure.

• AP associations:

  • Released during stress and sometimes during vigorous exercise, helping reduce pain perception.

  • Low endorphin activity can be associated with increased pain sensitivity.

Substance P

• Role: involved in pain perception, especially signalling that supports the experience of pain.

• AP associations:

  • Higher activity in relevant pathways is associated with stronger pain signalling; lower activity can reduce pain transmission.

What to memorise for exam use

• Neurotransmitters can send excitatory or inhibitory messages.

• Know each named transmitter and its “headline” links:

  • ACh: muscle + memory (Alzheimer’s link)

  • Dopamine: movement + reward (Parkinson’s / schizophrenia links)

  • Serotonin: mood + sleep (depression link)

  • Norepinephrine: alertness/arousal (mood/anxiety links)

  • Glutamate: major excitatory (seizures/migraines if too much)

  • GABA: major inhibitory (seizures/insomnia if too little)

  • Endorphins: pain relief/pleasure

  • Substance P: pain signalling