Human Physiology I · Neurophysiology
Graded Potentials, Threshold, and Action Potential Initiation
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In 30 seconds
Graded potentials are small, variable-size changes in membrane potential that arise when channels open in response to a stimulus and that decay with distance from their origin. When summed graded potentials depolarize the Trigger zone Region richest in voltage-gated Na⁺ channels Full entry → (Axon hillock Swelling where axon joins the cell body Full entry →) to Threshold V_m (≈ −55 mV) that triggers an action potential Full entry →, Voltage-gated sodium channels Na⁺ channels that open on depolarization Full entry → open and an action potential fires. The action potential is all-or-none — a stronger stimulus cannot enlarge it, only fire it more often — so stimulus strength is coded by firing frequency, not amplitude.
Why this matters
Summation of graded potentials underlies how local anesthetics and many drugs shift excitability: anything that reduces Na⁺-channel availability raises the effective threshold, making it harder for summed inputs to fire. In the lab, intracellular recordings show excitatory and inhibitory postsynaptic potentials summing at the trigger zone, and nerve conduction studies rely on the all-or-none response. Clinical values, diagnostic criteria, and protocols vary by institution and jurisdiction; these notes support education and do not replace clinical instruction or supervision. Urgent symptoms require evaluation by qualified clinicians or local emergency services.
The college version
1. Graded potentials: local, variable, decremental
Graded potentials (also local potentials) are voltage changes whose amplitude is proportional to stimulus size — a bigger stimulus opens more channels and makes a bigger change. They are decremental: as current leaks out through the membrane, the signal shrinks with distance, so they are short-range. Two classes are receptor potentials (in sensory receptors, from light, pressure, or chemicals) and postsynaptic potentials (at synapses, when neurotransmitter opens channels on the receiving cell).
2. Depolarizing vs. hyperpolarizing graded potentials
A Depolarizing graded potential V_m less negative (usually Na⁺ in) Full entry → makes V_m less negative, usually by Na⁺ (or Ca²⁺) entering; it is excitatory, moving the cell toward firing. A Hyperpolarizing graded potential V_m more negative (Cl⁻ in or K⁺ out) Full entry → makes V_m more negative, usually by Cl⁻ entering or K⁺ leaving; it is inhibitory. Whether a stimulus excites or inhibits depends entirely on which ion channels it opens.
3. Summation and the trigger zone
Graded potentials add. Temporal summation Rapid repeated inputs add at one site Full entry → adds inputs arriving at one site in rapid succession, before earlier ones decay. Spatial summation Simultaneous inputs from many sites add Full entry → adds inputs arriving at the same time from different sites. Both act at the trigger zone, the region richest in voltage-gated sodium channels — usually the axon hillock, where the axon meets the cell body. If the summed depolarization reaches threshold (≈ −55 mV), an action potential is initiated.
How it works
- A stimulus opens channels and makes a graded, decremental voltage change.
- Excitatory and inhibitory graded potentials converge on the trigger zone.
- Temporal and spatial summation combine their effects.
- If net depolarization reaches threshold, voltage-gated Na⁺ channels open.
- Na⁺ rushes in, driving positive feedback that fires a full action potential.
- Stronger stimuli raise firing frequency, not amplitude.
Common confusions
| Do not confuse | With | Difference |
|---|---|---|
| Graded potential | Action potential | Variable, decremental, short-range vs. all-or-none, self-propagating |
| Receptor potential | Postsynaptic potential | In a sensory receptor vs. on a postsynaptic neuron |
| Depolarizing graded potential | Hyperpolarizing graded potential | Toward threshold vs. away from threshold |
| Temporal summation | Spatial summation | Same place over time vs. different places at once |
| Threshold | Resting membrane potential | Firing voltage (≈ −55 mV) vs. resting baseline (≈ −70 mV) |
Memory aids
"Grade it, add it, fire it." Graded potentials are graded (size matches stimulus), they add up (summate) at the trigger zone, and if the total reaches threshold the neuron fires an all-or-none action potential.
Quick review
Topic Recap
Graded potentials are local, variable, decremental voltage changes arising in receptors (receptor potentials) or at synapses (postsynaptic potentials). Depolarizing and hyperpolarizing inputs sum temporally and spatially at the axon hillock/trigger zone. If the summed depolarization reaches threshold, voltage-gated sodium channels open and trigger an all-or-none action potential, with stimulus strength encoded as firing frequency rather than amplitude.
Knowledge Check
- Why do graded potentials shrink as they travel from their origin?
- A weak depolarizing input does not reach threshold. What two forms of summation could still make the neuron fire?
- What ion movements produce a hyperpolarizing graded potential?
- State the All-or-none principle Action potentials fire fully or not at all Full entry → and what it implies about coding stimulus strength.
- Where is the trigger zone, and why is it the site of Action-potential initiation Opening of the Na⁺ positive-feedback loop Full entry →?
Answers and Rationales
- Local current leaks out through the membrane, so amplitude falls with distance (Decremental conduction Amplitude falls with distance Full entry →). Rationale: graded potentials are passive, short-range signals.
- Temporal summation (rapid repeated inputs at one site) or spatial summation (simultaneous inputs from many sites). Rationale: both raise net depolarization at the trigger zone.
- Cl⁻ entering or K⁺ leaving makes the inside more negative. Rationale: both add negative charge inside or remove positive charge.
- An action potential fires at full amplitude or not at all; so stronger stimuli are coded by higher frequency, not larger amplitude. Rationale: all-or-none removes amplitude as an information channel.
- The trigger zone is the axon hillock (initial segment), with the highest density of voltage-gated Na⁺ channels and lowest threshold. Rationale: threshold is reached most easily where Na⁺ channels are densest.
Eli explains
The same idea, in plain words
Explain it like I’m 10
Graded potentials are like ripples from tapping a bathtub: a hard tap makes a bigger ripple, and the ripple fades as it spreads. A bigger stimulus opens more ion channels and makes a bigger local voltage change, which shrinks as it travels.
Now add a lever at the tub's edge that flips only when a wave arrives hard enough. Single fading ripples never reach it, but if you tap several places at once or tap very fast, the ripples add up until one arrives big enough to flip the lever — which flips completely or not at all.
Where it stops being exact: water ripples add simply, but graded potentials add with plus and minus signs — depolarizing inputs push toward firing, hyperpolarizing inputs push away, and they cancel. "Bigger" also has a strict meaning: graded potentials grow with the stimulus, but a triggered action potential is always the same size.
Simple Example
A light touch opens a few sodium channels in a sensory ending, producing a small local depolarization (a receptor potential); a firmer touch opens more channels and makes a larger one. Both fade as they spread. If several rapid touches arrive together and their effects sum to threshold at the trigger zone, the neuron fires.
Worked example
- A stimulus opens ligand-gated or mechanically gated channels in a local patch, producing a receptor potential (sensory) or postsynaptic potential (synaptic).
- Ions flow along their electrochemical gradients — Na⁺ in for depolarization, or Cl⁻ in / K⁺ out for hyperpolarization — shifting the local voltage.
- The charge spreads as local currents, but current leaks back out, so amplitude falls with distance (decremental conduction).
- Graded potentials sum in time (temporal) and space (spatial) as they converge on the axon hillock/trigger zone.
- If net depolarization reaches threshold (≈ −55 mV), voltage-gated Na⁺ channels open in a positive-feedback rush of Na⁺ entry — action-potential initiation.
- The resulting action potential obeys the all-or-none principle: it always fires at full amplitude. A stronger stimulus cannot enlarge it; instead it raises firing frequency — the nervous system's code for stimulus strength vs. frequency.
Key takeaways
- High yield: Graded potentials vary in size and decay with distance; action potentials are all-or-none and do not decay.
- High yield: Threshold is where enough Na⁺ channels open that Na⁺ entry becomes self-amplifying (positive feedback).
- High yield: Stimulus intensity is coded by action-potential frequency, not size.
- Depolarizing graded potentials = Na⁺ (or Ca²⁺) entry; hyperpolarizing = Cl⁻ entry or K⁺ exit.
- Temporal summation = same place, rapid succession; spatial summation = same time, different places.
- The axon hillock/trigger zone has the highest Na⁺-channel density and lowest threshold.
Study tools & related lessonsYou’ll learn to · Key vocabulary · Related
You’ll learn to
- Define graded potentials and explain how their size varies with stimulus and decays with distance (decremental conduction).
- Distinguish depolarizing from hyperpolarizing graded potentials and the ion movements behind each.
- Explain temporal and spatial summation and how they bring the trigger zone to threshold.
- Contrast graded potentials with the all-or-none action potential and relate stimulus strength to firing frequency.
Key vocabulary
- Graded potential
- Local, variable-size voltage change proportional to stimulus
- Local potential
- Synonym for graded potential, near its origin
- Receptor potential
- Graded potential in a sensory receptor
- Postsynaptic potential
- Graded potential on a postsynaptic cell
- Depolarizing graded potential
- V_m less negative (usually Na⁺ in)
- Hyperpolarizing graded potential
- V_m more negative (Cl⁻ in or K⁺ out)
- Decremental conduction
- Amplitude falls with distance
- Temporal summation
- Rapid repeated inputs add at one site
- Spatial summation
- Simultaneous inputs from many sites add
- Trigger zone
- Region richest in voltage-gated Na⁺ channels
- Axon hillock
- Swelling where axon joins the cell body
- Threshold
- V_m (≈ −55 mV) that triggers an action potential
- Voltage-gated sodium channels
- Na⁺ channels that open on depolarization
- All-or-none principle
- Action potentials fire fully or not at all
- Action-potential initiation
- Opening of the Na⁺ positive-feedback loop
- Stimulus strength vs. frequency
- Stronger stimuli coded as higher firing rate
- Graded vs. action potentials
- Variable, decremental, short-range vs. all-or-none, self-propagating
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