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Grade 10-11 (age 15-17)

high school Stimulus and Response — Nervous System

Nervous System & Action Potential

A neuron is a nerve cell that responds to a stimulus by making an electrical signal and passes that signal on to the next cell. When the membrane potential reaches threshold, an action potential fires, always at the same size, and when it falls short of threshold nothing fires at all; this is the all-or-none law. Within a neuron the action potential is conducted electrically along the axon, and at the synapse between neurons a neurotransmitter carries the excitation across chemically. In the diagram you can raise the stimulus from 1 to 10 and see an action potential appear the moment threshold is crossed, and see that a stronger stimulus adds more action potentials rather than bigger ones.

The Neuron, the Basic Unit of the Nervous System

⚡ Neurons Carry Signals
①A neuron (nerve cell) responds to a stimulus by making an electrical signal and passing it on
②The signal travels dendrites → cell body → axon → axon terminals
③At the synapse, the narrow gap between the axon terminal and the next neuron, a neurotransmitter carries the signal across

Action Potential and Threshold

3
💡 Changing the Stimulus
①With a stimulus of 4 or less, the membrane potential rises a little and falls back without reaching threshold (−55 mV)
②At a stimulus of 5 the membrane potential crosses threshold and an action potential shoots up to +30 mV
③A stronger stimulus leaves the peak at +30 mV but makes more action potentials in the same time
④This is the all-or-none law: nerves signal stimulus strength by the frequency of action potentials, not their size

Conduction and Transmission of Excitation

The Course of an Action Potential
resting (−70 mV) → depolarization (up to +30 mV) → repolarization → hyperpolarization (about −80 mV) → back to resting
Na⁺ channels open and Na⁺ flows in (depolarization), then K⁺ channels open and K⁺ flows out (repolarization), and the Na⁺–K⁺ pump restores the ion distribution

Conduction vs Transmission

Conduction and Transmission
ItemConductionTransmission
Where it happenswithin one neuron (along the axon)between neurons (at the synapse)
Howelectrical (the action potential spreads to the next patch of membrane)chemical (neurotransmitter)
Directioncan spread both ways from the point stimulatedone way (axon terminal → next neuron)
Speedfast (faster still with a myelin sheath, by saltatory conduction)relatively slow (the chemical has to cross the gap)

The Human Nervous System

Nervous System Classification

How the Human Nervous System Is Organized

Worked Examples

Example 1
A stimulus raises the membrane potential of a neuron with a threshold of −55 mV to −60 mV. Does an action potential fire?
1
An action potential fires only when the membrane potential reaches threshold or above.
2
−60 mV is below the threshold of −55 mV, so threshold was not reached.
▸ no
With the stimulus at 4 or less in the diagram, the curve turns back down below the threshold line. Short of threshold, no action potential forms at all.
Example 2
Two stimuli, both stronger than threshold but of different strengths, are given. How do the sizes of the resulting action potentials compare?
1
By the all-or-none law, once threshold is reached the action potential is always the same size.
2
A stronger stimulus leaves the peak the same but produces more action potentials in the same time.
▸ the two action potentials are the same size
Raising the stimulus to 5, 7 and 9 in the diagram gives 1, 2 and 3 action potentials, every one peaking at +30 mV. The brain judges how strong a stimulus is from that difference in number.

Summary

The All-or-None Law
membrane potential ≥ threshold → an action potential, always the same size / below threshold → no action potential
stimulus strength is signaled by how often action potentials fire, not by their size
CSAT-style problem, adapted
Which statement about a neuron’s action potential and signal transmission is INCORRECT?
①The resting membrane potential is about −70 mV.
②Depolarization happens as Na⁺ flows into the cell.
③If a stimulus falls short of threshold, no action potential fires.
④At a synapse, excitation is passed on by neurotransmitters.
⑤The stronger the stimulus, the larger the action potential.
▸ ⑤ The stronger the stimulus, the larger the action potential.
1
By the all-or-none law, once threshold is reached the action potential is always the same size.
2
Stimulus strength is signaled by frequency, not by the size of the action potential, so ⑤ is incorrect.
🎯 Exam Points
①A neuron's signal travels dendrites → cell body → axon → axon terminals
②The resting membrane potential is about −70 mV, threshold about −55 mV and the action potential peak about +30 mV
③All-or-none: once threshold is reached, the action potential is always the same size; a stronger stimulus raises its frequency
④Conduction is electrical movement within a neuron; transmission is one-way movement at the synapse by neurotransmitters
⑤In myelinated neurons, saltatory conduction makes conduction fast
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