Neurons & Neural Firing
- Label the parts of a neuron and describe how a signal travels through one
- Explain the action potential, the all-or-none principle, and the role of the synapse
- Connect specific neurotransmitters to their behavioral effects
The anatomy of a neuron
A neuron is the basic cell of the nervous system, built to receive, integrate, and pass along signals. Dendrites are branching fibers that receive incoming messages. The cell body (soma) integrates those inputs. The axon carries the outgoing signal away from the cell body, often wrapped in a fatty myelin sheath that speeds transmission — its breakdown is the basis of multiple sclerosis. At the axon’s end, terminal branches hand the message to the next cell. Neurons never touch: a tiny gap called the synapse separates them.
The action potential and the all-or-none principle
At rest, a neuron holds a slightly negative charge inside (the resting potential). When incoming signals push it past its threshold, the neuron fires an action potential — a brief electrical impulse that shoots down the axon as channels let positive ions rush in (depolarization). Firing follows the all-or-none principle: a neuron fires at full strength or not at all, like a gun trigger. A stronger stimulus does not make a bigger impulse — it makes the neuron fire more often. After firing, a brief refractory period must pass before the neuron can fire again.
Neurotransmitters: chemical messengers
When the action potential reaches the terminal, the neuron releases neurotransmitters into the synapse, which bind to receptors on the next neuron’s dendrites — either exciting or inhibiting it. Leftover molecules are cleared by reuptake back into the sending neuron. Each transmitter has signature effects: dopamine (reward, movement — too little is linked to Parkinson’s, excess to schizophrenia symptoms), serotonin (mood, sleep — low levels linked to depression), acetylcholine (ACh) (muscle movement and memory — depleted in Alzheimer’s), GABA (the major inhibitory transmitter, low in anxiety and seizures), and endorphins (natural painkillers, the source of "runner’s high").
A neuroscientist gives a patient a drug that blocks the reuptake of serotonin. Using your knowledge of neural firing, explain the likely effect on the patient’s synapses and mood.
- 1.Identify the normal role of reuptake: after serotonin is released into the synapse, the sending neuron reabsorbs the leftover molecules, ending the signal.
- 2.Determine what blocking reuptake does: the serotonin cannot be reabsorbed, so it lingers in the synapse and keeps stimulating the receiving neuron’s receptors.
- 3.Connect serotonin to behavior: serotonin regulates mood, and low serotonin activity is associated with depression.
- 4.Predict the outcome: with more serotonin active in the synapse for longer, mood is likely to improve — this is exactly how SSRI antidepressants work.
A researcher applies a weak stimulus and then a very strong stimulus to the same neuron. Both push the neuron past its threshold. How will the neuron’s action potentials differ?
Do not confuse the synapse (the gap between neurons) with the myelin sheath (the fatty insulation on the axon). And remember the all-or-none rule: stimulus intensity changes firing frequency, never the strength of an individual action potential.
An older adult begins experiencing tremors and difficulty initiating movement, and testing reveals reduced activity of a key neurotransmitter. Which neurotransmitter deficiency best fits these symptoms?
For neurotransmitter items, memorize one disorder or behavior per transmitter: dopamine–movement/reward, serotonin–mood, ACh–muscles/memory, GABA–anxiety (inhibitory), glutamate–excitatory, endorphins–pain. A single anchor per transmitter is usually enough to earn the point.
Answer the 2 checkpoints as you read.
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