How receptors work: generator potentials and sensory adaptation – General Principles of Reception and Response in Animals (Senior 6 Biology) | Ganzaa

General Principles of Reception and Response in Animals

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Senior 6 · Biology

General Principles of Reception and Response in Animals

5 lessons

Lesson 2 of 5

How receptors work: generator potentials and sensory adaptation

25 min

Why you stop feeling your shirt

When you pull on a shirt in the morning you feel the cloth on your shoulders. Ten minutes later you feel nothing, although the shirt has not moved. The cloth is still pressing on thousands of touch receptors, yet they have almost stopped sending impulses. To understand this, and to understand how any receptor turns a stimulus into a signal the brain can use, A-level biologists study one receptor in detail: the Pacinian corpuscle, a pressure receptor found deep in the skin, especially of the fingertips, palms and soles.

Definition

Resting potential: the potential difference across the membrane of a neurone or receptor cell when it is not being stimulated, about $-70\,\mathrm{mV}$ (inside negative). Unit 6 explains how it is kept.

Generator potential (receptor potential): a local depolarisation of a receptor's membrane caused by a stimulus. Its size is graded: a stronger stimulus gives a larger generator potential.

Threshold: the level of depolarisation that must be reached before an action potential (nerve impulse) starts.

Sensory adaptation: the decrease in the response of a receptor, that is, in the frequency of impulses it starts, when a stimulus continues unchanged.

The Pacinian corpuscle

How the corpuscle produces impulses:

At rest, the membrane of the nerve ending has a resting potential, and its stretch-mediated sodium ion channels are closed.
Pressure deforms the lamellae, which press on and stretch the membrane of the nerve ending.
The stretch opens the sodium ion channels. Na⁺ ions diffuse into the ending, down their electrochemical gradient.
The inside becomes less negative: a generator potential. Gentle pressure opens few channels and gives a small generator potential; firm pressure opens more and gives a larger one.
If the generator potential reaches threshold at the first node of Ranvier, an action potential starts there and travels along the sensory neurone to the central nervous system. Below threshold, no impulse is sent.

Coding the strength of a stimulus

Action potentials are all-or-nothing: once threshold is reached, every impulse in a given neurone has the same size. A receptor therefore cannot signal "stronger" with bigger impulses. Instead:

a stronger stimulus gives a larger generator potential, which starts impulses at a higher frequency (more impulses per second); this is frequency coding;
a stronger stimulus also reaches the threshold of more receptors, so more sensory neurones carry impulses; this is recruitment.

From receptor to perception

The receptor only detects. Sensory neurones carry impulses to the spinal cord and brainstem, relay neurones pass them on, mostly through the thalamus, to a sensory area of the cerebral cortex, and only there is the stimulus perceived. The brain reads four things from the incoming impulses:

Feature of the stimulus	How it is coded
Type (modality): light, sound, touch	Which receptors and which pathway carry the impulses, and which area of cortex receives them
Strength (intensity)	Frequency of impulses in each neurone, and the number of neurones recruited
Position on the body	Which receptors are stimulated; each sensory neurone serves a small patch of skin, its receptive field
Duration	How long impulses continue, which adaptation can shorten

Worked example 1: reading a nerve recording

A recording electrode on the axon of a Pacinian corpuscle counts 18 action potentials in 0.30 s while a probe presses on it. A firmer press gives 33 action potentials in 0.30 s. (a) Calculate the impulse frequency for each press. (b) Explain the difference.

(a) Gentle press: $\frac{18}{0.30\,\mathrm{s}} = 60\,\mathrm{Hz}$ (60 impulses per second). Firm press: $\frac{33}{0.30\,\mathrm{s}} = 110\,\mathrm{Hz}$ .
(b) The firmer press deforms the lamellae more, opens more stretch-mediated Na⁺ channels and gives a larger generator potential. The larger, longer depolarisation brings the first node of Ranvier back to threshold sooner after each impulse, so impulses are started more often. The size of each impulse does not change.

Sensory adaptation

If a stimulus continues unchanged, most receptors send fewer and fewer impulses: they adapt. Receptors differ in how fast they do this.

	Phasic (rapidly adapting) receptors	Tonic (slowly adapting) receptors
Response to a steady stimulus	Burst of impulses at the start (and often at the end), then few or none	Impulses continue, at a slowly falling frequency, as long as the stimulus lasts
Signals	Change: the start, the end and vibration	Steady state: how strong the stimulus still is
Examples	Pacinian corpuscles; receptors around hair follicles; olfactory receptors	Muscle spindles and other proprioceptors; most nociceptors; Merkel discs in the skin

How a Pacinian corpuscle adapts. Under steady pressure the viscous gel flows and the lamellae slip, so the pressure is spread around the capsule and the nerve ending is no longer deformed. The stretch-mediated channels close, and sodium channels that have just opened become inactivated for a short time. The generator potential falls below threshold and impulses stop, although the pressure is still there. When the pressure is removed, the lamellae spring back, briefly deform the ending again, and a short burst of impulses signals "off".

Why adaptation is useful.

It stops the central nervous system being flooded with impulses about unchanging, unimportant stimuli, such as clothes, a steady background noise or the smell of your own home.
It keeps receptors sensitive to new changes, which are the ones most likely to matter: a sudden touch, a new smell, a predator moving.
Tonic receptors do not adapt quickly, and for good reason: the body must keep knowing the position of its limbs and keep warning of continuing tissue damage.

Worked example 2: identifying receptor types from data

A constant stimulus was applied for 3 s to two receptors, A and B, and the impulse frequency was recorded (illustrative data).

Time after stimulus began / s	0.1	0.5	1.0	3.0
Receptor A / Hz	140	20	0	0
Receptor B / Hz	64	60	56	48

Percentage fall over the 3 s. A: $\frac{140 - 0}{140} \times 100 = 100\%$ . B: $\frac{64 - 48}{64} \times 100 = 25\%$ .
Identify. A adapts within 1 s, so it is phasic; it behaves like a Pacinian corpuscle. B still fires at 75% of its first frequency after 3 s, so it is tonic; it behaves like a muscle spindle.
Evaluate. A is suited to signalling the start of a stimulus or vibration; B is suited to reporting a continuing state, such as the stretch of a muscle while you hold a heavy bag.

Warning

Common mistakes

"A stronger stimulus gives bigger impulses." No: the generator potential is bigger, but impulses are all-or-nothing. Strength is coded by frequency and by the number of neurones.
"Adaptation means the receptor is tired." Adaptation is a normal, built-in property of the receptor, not fatigue or damage.
Adaptation is not habituation. Adaptation happens in the receptor. Habituation is a form of learning in the central nervous system: an animal stops responding to a repeated, harmless stimulus, even though its receptors still fire. Pigeons in a market that stop flying away from passing people show habituation.

BioME: "Sensory Receptor Adaptation: Tonic vs. Phasic Receptors". Watch for how the impulse pattern of each receptor type changes during a steady stimulus.

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Practice: generator potentials and adaptation

Define generator potential, and state one way in which it differs from an action potential.
Describe how pressure on a Pacinian corpuscle causes Na⁺ ions to enter the nerve ending.
A probe gives 12 impulses in 0.25 s. Calculate the frequency in Hz.
Explain why a very light touch may produce a generator potential but no nerve impulses.
Give two ways in which the nervous system codes the intensity of a stimulus.
Distinguish between phasic and tonic receptors, with one example of each.
Suggest why it is an advantage that nociceptors (pain receptors) adapt only slowly.
A receptor fires at 80 Hz at the start of a stimulus and at 52 Hz after 2 s. Calculate the percentage fall, and say whether it behaves more like a phasic or a tonic receptor.

Show answersHide answers

A local depolarisation of a receptor membrane caused by a stimulus. Unlike an action potential it is graded (its size depends on the stimulus strength), it is not all-or-nothing and it does not travel along the axon.
Pressure deforms the lamellae, which stretch the membrane of the nerve ending; stretch-mediated sodium ion channels open; Na⁺ diffuses in down its electrochemical gradient.
$\frac{12}{0.25} = 48\,\mathrm{Hz}$ .
The touch opens only a few channels, so the generator potential is too small to reach threshold at the first node of Ranvier; no action potential starts.
The frequency of impulses in each sensory neurone, and the number of receptors and neurones stimulated (recruitment).
Phasic receptors respond strongly at the start of a steady stimulus and then stop (Pacinian corpuscle, hair follicle receptor). Tonic receptors keep firing, at a slowly falling frequency, while the stimulus lasts (muscle spindle, nociceptor).
Pain warns of continuing damage. If pain receptors adapted quickly, a person might keep using an injured limb or stay in contact with something harmful, making the damage worse.
$\frac{80 - 52}{80} \times 100 = 35\%$ . It still fires at 65% of its first rate after 2 s, so it behaves like a tonic receptor.

Key points

A stimulus opens ion channels in a receptor; Na⁺ enters and causes a graded generator potential.
If the generator potential reaches threshold, all-or-nothing action potentials start in the sensory neurone.
Stimulus strength is coded by impulse frequency and by recruitment of more receptors; the type of stimulus is coded by the pathway.
Sensory adaptation reduces the response to an unchanging stimulus: fast in phasic receptors, slow in tonic receptors. It protects the CNS from overload and keeps receptors sensitive to change.

Sources and credits

7.4.2: Sensory Processes and 7.4.3: Somatosensation, from Biology 2e by OpenStax (Mary Ann Clark, Matthew Douglas, Jung Choi), via LibreTexts Biology, licensed under CC BY 4.0. Used for receptor potentials, the Pacinian corpuscle and the coding of stimuli. Adapted by Ganzaa.
Photo: Histology of a Pacinian corpuscle, Mikael Häggström, CC0 1.0, via Wikimedia Commons.
Video: "Sensory Receptor Adaptation: Tonic vs. Phasic Receptors", BioME, embedded from YouTube.

Figures 1 and 2, the tables, the worked examples (with illustrative data) and the practice questions were written by Ganzaa.

1Why you stop feeling your shirt

2The Pacinian corpuscle

3Coding the strength of a stimulus

4From receptor to perception

5Sensory adaptation

Why you stop feeling your shirt

The Pacinian corpuscle

Coding the strength of a stimulus

From receptor to perception

Sensory adaptation