Stimulus and Response — Nervous System

Nervous System & Action Potential

Neurons are nerve cells that create and carry electrical signals. If a stimulus is at or above threshold, an action potential fires at fixed size; below threshold it does not, the all-or-none law. Within one neuron conduction is electrical, and across a synapse transmission is chemical via neurotransmitters. Raise the stimulus slider and check when an action potential appears past threshold.

Basics of the Nervous System — Neurons

⚡ Neurons = Electrical Signal Carriers

①Neurons (nerve cells) generate and transmit electrical signals

②Signal flow: dendrite → soma → axon → axon terminals

③At the synapse, chemicals (neurotransmitters) carry the signal to the next neuron

Action Potential and Threshold

Stimulus strength3

💡 Observe the Experiment

①Try raising the stimulus above 5 — action potential fires!

②Above threshold, it always fires with the same magnitude

③This is the "all-or-none" principle — fires or not, no in-between

Conduction vs Transmission

Action Potential Process

rest(-70mV) → depolarization(+30mV) → repolarization → hyperpolarization → rest

Na⁺ in → K⁺ out → Na⁺-K⁺ pump restores

Conduction vs Transmission

ChartNerve Conduction vs Transmission

Item	Conduction	Transmission
Definition	within a single neuron	between neurons
Mode	electrical (action potential)	chemical (neurotransmitters)
Direction	bidirectional possible (mostly one-way)	one-way only (vesicle → receptor)
Speed	fast (myelinated → faster)	relatively slower

Structure of the Nervous System

Nervous System Classification

Human Nervous System

Worked Examples

Example 1

A neuron has a threshold of −55 mV. A stimulus raises its membrane potential to −60 mV. Does an action potential occur?

An action potential occurs only if the membrane potential reaches the threshold (all-or-none).

−60 mV is below (more negative than) the −55 mV threshold, so the threshold is not reached.

▸ No, it does not occur

All-or-none: below threshold gives no action potential; at or above, it is always the same size. A stronger stimulus does not enlarge the spike.

Example 2

The same neuron is stimulated twice, both above threshold. How do the sizes of the first and second action potentials compare?

By the all-or-none law, any above-threshold stimulus produces an action potential of the same maximum size.

A stronger stimulus does not change the spike "size"; instead it increases the "frequency" (number of spikes).

▸ The two action potentials are the same size

Stimulus strength is encoded by frequency (spike count), not amplitude — the heart of the all-or-none law.

Summary

All-or-None Principle

stimulus ≥ threshold → action potential fires (constant magnitude)

Below threshold: no response; at/above: maximum response

CSAT-style

Which statement about a neuron’s action potential and signal transmission is INCORRECT?

①The resting membrane potential is about −70 mV

②Depolarization occurs as Na⁺ flows into the cell

③No action potential occurs if the stimulus is below threshold

④At a synapse, excitation is passed by neurotransmitters

⑤The stronger the stimulus, the larger the action potential

▸ ⑤ The stronger the stimulus, the larger the action potential

By the all-or-none law, any above-threshold stimulus gives an action potential of the same size.

Stimulus strength is conveyed by frequency, not size, so ⑤ is incorrect.

🎯 Exam Points

①Neuron structure: dendrite → soma → axon → terminal

②Resting -70mV, threshold -55mV, action potential +30mV

③All-or-none: fires at constant magnitude above threshold

④Conduction (within neuron, electrical) vs transmission (between, chemical)

⑤With myelin → saltatory conduction → faster

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