Topic 1.3 The Neuron and Neural Firing: explain the structure of the neuron, the action potential, synaptic transmission, and how neurotransmitters and drugs influence neural communication.

What this topic is asking

Topic 1.3 is the most detailed process topic in Unit 1. The College Board wants you to know the structure of a neuron, trace an action potential, explain the all-or-none and refractory principles, describe synaptic transmission, identify major neurotransmitters, and explain how agonists and antagonists (including drugs) change neural communication.

The structure of a neuron

Signals flow in one direction: dendrites to cell body to axon to terminals. The myelin sheath, a fatty insulating layer, lets the impulse jump along the axon faster; its degeneration (as in multiple sclerosis) slows communication.

The action potential

The all-or-none principle is heavily tested. Stimulus intensity is encoded by how often neurons fire and how many fire, not by the size of a single action potential.

Synaptic transmission

Neurons do not touch. They communicate across a tiny gap called the synapse:

1. The action potential reaches the axon terminal.
2. The terminal releases neurotransmitters into the synaptic gap.
3. Neurotransmitters bind to receptor sites on the receiving neuron's dendrites, exciting or inhibiting it.
4. Leftover neurotransmitter is cleared by reuptake (reabsorbed by the sending neuron) or broken down by enzymes.

Major neurotransmitters

You should know what several key neurotransmitters do:

• Dopamine: reward, movement, learning; excess is linked to schizophrenia, too little to Parkinson's.
• Serotonin: mood, sleep, appetite; low levels are linked to depression.
• Acetylcholine: muscle action, learning, and memory; loss is linked to Alzheimer's.
• GABA: the main inhibitory neurotransmitter; low levels are linked to anxiety and seizures.
• Glutamate: the main excitatory neurotransmitter; involved in learning and memory.
• Endorphins: natural painkillers that produce pleasure and reduce pain.

Agonists and antagonists

Most psychoactive drugs work by being agonists or antagonists. An antidepressant that blocks serotonin reuptake is acting as an agonist (more serotonin stays active); a drug that blocks dopamine receptors is acting as an antagonist. Knowing this lets you predict a drug's behavioral effect from how it changes a neurotransmitter.

The reason these mechanisms matter is that every thought, feeling, and movement ultimately reduces to neurons firing and communicating. Because the action potential is all-or-none, the nervous system cannot send a "louder" single signal; instead it encodes intensity through firing rate and recruitment of more neurons, which is why a bright light or a loud sound recruits more neural activity rather than bigger impulses. At the synapse, the precise balance of excitatory and inhibitory neurotransmitters determines whether the next neuron reaches threshold, so even small chemical changes, such as a drug blocking reuptake, can shift mood, attention, or movement. This is the bridge between biology and behavior that the rest of the course builds on.

Try this

Q1. State the all-or-none response in one sentence. [1 point]

• Cue. A neuron either fires a full-strength action potential or does not fire at all, regardless of how strong the stimulus is above threshold.

Q2. Explain how a reuptake-blocking drug could increase a neurotransmitter's effect. [2 points]

• Cue. Reuptake normally removes leftover neurotransmitter from the synapse; blocking it leaves more neurotransmitter available to bind receptors, acting as an agonist and strengthening the effect.