The nervous system is built from roughly 86 billion neurons — cells that receive, integrate, and transmit information. Understanding one neuron’s firing is the foundation for everything from reflexes to memory. This lesson traces a signal through a single neuron and across the gap to the next.
Parts of a neuron
A neuron has three working parts. Dendrites are branching fibers that receive messages and carry them toward the cell body. The cell body (soma) integrates the incoming signals. The axon is the long fiber that sends the message away to other neurons or to muscles and glands. Many axons are wrapped in a fatty myelin sheath that speeds transmission; when myelin is damaged (as in multiple sclerosis), signals slow or misfire.
[Diagram — see the figure in the print workbook.]
The action potential and the all-or-none law
At rest, a neuron holds a slight negative charge inside — the resting potential — maintained by ions across the membrane. When enough excitatory signals push the neuron past its threshold, it fires an action potential: a brief electrical impulse that sweeps down the axon as channels let positive ions rush in (depolarization).
Firing obeys the all-or-none law: a neuron either fires at full strength or not at all — there is no “half firing.” A stronger stimulus does not make a bigger action potential; it makes the neuron fire more often. After firing, a brief refractory period must pass before the neuron can fire again.
[Diagram — see the figure in the print workbook.]
“All-or-none” is a favorite trap. A more intense stimulus never makes a larger action potential — it increases how frequently the neuron fires.
The synapse and neurotransmitters
Neurons do not touch. Between the axon terminal of one and the dendrite of the next is a tiny gap, the synapse. When the action potential reaches the terminal, it triggers the release of chemical messengers — neurotransmitters — that cross the gap and bind to receptors on the next neuron, either exciting or inhibiting it. Leftover neurotransmitter is reabsorbed in a process called reuptake.
Each neurotransmitter has characteristic jobs; too much or too little is linked to specific problems.
| Neurotransmitter | Main role | Linked to (imbalance) |
|---|---|---|
| Dopamine | Reward, movement, motivation | Too little: Parkinson’s; oversupply linked to schizophrenia |
| Serotonin | Mood, sleep, appetite | Low levels linked to depression |
| Acetylcholine (ACh) | Muscle movement, learning, memory | Loss linked to Alzheimer’s |
| GABA | Major inhibitory messenger; calms | Too little: anxiety, seizures |
| Glutamate | Major excitatory messenger; memory | Oversupply: overstimulation, migraines |
| Endorphins | Pain relief, pleasure | Mimicked by opioid drugs |
| Norepinephrine | Alertness, arousal | Low levels linked to depressed mood |
The neurotransmitters the exam asks about most.
Drugs: agonists and antagonists
Drugs work by hijacking the synapse. An agonist mimics or boosts a neurotransmitter’s effect (it may imitate the transmitter or block reuptake so more stays in the synapse). An antagonist blocks or reduces its effect (often by filling receptor sites so the real transmitter can’t bind). Many psychoactive drugs act this way — caffeine and cocaine are stimulants; alcohol and opioids are depressants.
Agonist = “A go” (turns the signal ON / up). Antagonist = “anti” (turns it OFF / down).
Worked example 1
A person takes a drug that blocks acetylcholine receptors on muscle cells. Predict the effect and name the drug's category.
Acetylcholine normally binds those receptors to trigger muscle contraction. A drug that blocks the receptors prevents ACh from acting, so the muscles cannot be activated — the person would experience muscle weakness or paralysis.
Because it reduces the neurotransmitter’s effect by occupying receptor sites, the drug is an antagonist. (This is roughly how the poison curare works.)
Common trap: Reuptake vs. release
Release sends neurotransmitter into the synapse; reuptake pulls the leftovers back into the sending neuron to be recycled. A drug that blocks reuptake (like an SSRI antidepressant) leaves more neurotransmitter in the synapse — an agonist-like effect, not a blocking of the message.
If a stem says a drug “blocks the reuptake of serotonin,” the result is more serotonin activity — watch for the double negative.
Hack: Trace the arrow: Dendrite, Soma, Axon, Synapse
Memorize the one-way street: signals come in through dendrites, get summed in the soma, travel out along the axon, and jump the synapse as neurotransmitters. Almost every neuron question is answerable if you know where on this street the action is happening.
For neurotransmitters, learn them in pairs of extremes: GABA (calms) vs. glutamate (excites); too little dopamine (Parkinson’s) vs. too much (schizophrenia symptoms). Contrast fixes them in memory.
