Neurotransmitters glutamate and GABA: VCE Psychology Unit 3

What this dot point is asking

VCAA wants you to explain how neurotransmitters carry a signal across the synapse from one neuron to the next using a lock-and-key process, and to distinguish the excitatory action of glutamate (central to learning and memory) from the inhibitory action of GABA (which regulates how easily the postsynaptic neuron fires).

The study design also asks you to compare neurotransmitters with neuromodulators such as dopamine and serotonin. That comparison, and the roles of the two neuromodulators, are on our neuromodulators page.

The answer

A neurotransmitter is a chemical molecule, released by a neuron, that carries a message across the synapse to the next neuron and changes how likely that neuron is to fire.

Synaptic transmission and the lock-and-key process

A signal travels along a neuron as an electrical impulse, but neurons are not physically joined. Between them is a tiny gap called the synapse (or synaptic cleft). To bridge it, the electrical message becomes a chemical one.

1. The electrical impulse reaches the axon terminals of the sending (presynaptic) neuron.
2. This triggers synaptic vesicles to release neurotransmitter molecules into the synapse.
3. The molecules drift across the gap and bind to receptor sites on the dendrites of the receiving (postsynaptic) neuron.

This binding follows a lock-and-key process: each neurotransmitter has a specific shape (the key) that fits only a matching receptor (the lock). A molecule cannot activate a receptor it does not fit, which is why a single neurotransmitter produces a specific effect rather than a general one.

Excitatory and inhibitory effects

When a neurotransmitter binds, it has one of two broad effects on the postsynaptic neuron.

• An excitatory effect makes the postsynaptic neuron more likely to fire its own impulse (it depolarises the membrane).
• An inhibitory effect makes the postsynaptic neuron less likely to fire (it hyperpolarises the membrane).

Whether a neuron fires depends on the balance of excitatory and inhibitory signals it receives at any moment. Healthy functioning depends on this balance being maintained.

Glutamate: the main excitatory neurotransmitter

Glutamate is the brain's primary excitatory neurotransmitter. When it binds to a postsynaptic receptor it increases the likelihood that the neuron will fire.

Glutamate is essential for learning and memory. Repeated, strong glutamate signalling between two neurons strengthens the connection between them, the cellular process underlying long-term potentiation (the lasting strengthening of a synapse that is thought to form the biological basis of new memories). Without adequate glutamate signalling, the synaptic strengthening that encodes learning cannot occur.

GABA: the main inhibitory neurotransmitter

GABA (gamma-aminobutyric acid) is the brain's primary inhibitory neurotransmitter. When it binds to a postsynaptic receptor it makes that neuron less likely to fire, so its role is to regulate postsynaptic activation and keep neural activity from spiralling out of control.

By dampening excessive firing, GABA prevents the nervous system from becoming over-aroused. Adequate GABA activity is associated with calm and stability; insufficient or dysfunctional GABA leaves the nervous system over-excited, which is linked to heightened anxiety. This is why GABA dysfunction features in the biological explanation of anxiety disorders such as specific phobia.

Why the balance matters

Glutamate and GABA work as opposing forces: glutamate accelerates neural firing while GABA puts on the brakes. Most neural circuits rely on both. Too much excitation (or too little inhibition) can produce over-arousal and, in extreme cases, seizures; too little excitation impairs the signalling needed for learning. Coordinated mental processes and behaviour depend on this push and pull staying in balance.