Biology · Nervous Systems, Senses & Movement

Sensory receptors, transduction and information coding

Three spikes and six spikes can have the same individual spike symbol while carrying different information about a selected condition. The observation window matters: a count without time is not a firing rate.

Cells, Membranes & Organisation supplies boundaries, ion gradients and transport. Gene Regulation & Cell Signalling introduces receptors and regulatory relationships; Homeostasis, Nutrition & Biological Organisation supplies coordinated physiology and energy accounts. This chapter reintroduces voltage and neural vocabulary before using them. Physics’s Electricity and Magnetism and Optics and Photonics are optional return paths for electrical and lens relations; Chemistry’s Solutions, Concentration & Solubility supports the distinction between ion amount and distribution.

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By the end, you can

Distinguish a physical or chemical stimulus, receptor response and neural code, including rate denominators and adaptation.

Detection changes the state of a receptor

Sensory transduction converts a change in the environment or body into a biological signal. A receptor may be a specialised region of a sensory neuron or a separate receptor cell connected to a neuron. Mechanical forces, chemical binding and absorbed light initiate different mechanisms; they do not all become current through the same protein.

A receptor potential is a graded electrical response in the receptor. A generator potential in a sensory neuron can influence its spike initiation; a separate receptor cell can alter transmitter release onto a sensory neuron. An action potential is a later event in some pathways, not a synonym for every electrical response.

The selected transduction table assigns four stimulus levels, receptor responses and later firing rates. Its depolarising response applies only to this model. Some photoreceptors hyperpolarise in light, and many receptors saturate, adapt or depend strongly on recent conditions. A receptor potential cannot simply be read as an action-potential amplitude.

A stimulus, graded response and later spike rate

A sensory receptor can be a part of a neuron or a separate connected cell.

Read the diagram in words

The four conditions assign stimulus levels 0/1/2/3, receptor changes 0/4/7/9 mV and later mean rates 0/20/40/50 spikes/s. The three columns have different meanings and units.

Selected routes and effects
Displayed structure	Assigned x coordinate	Assigned y coordinate
Selected stimulus	0	1
Graded receptor response	3	1
Receiving or generating neuron	6	1
Later spike pattern	9	1

Selected routes and effects
From	To	Selected relationship	Direction	Passage through	Selected condition	Incoming symbol	Released symbol
Selected stimulus	Graded receptor response	The receptor converts the selected input	Directed route	—	—	—	—
Graded receptor response	Receiving or generating neuron	Graded response influences the neural pathway	Directed route	—	—	—	—
Receiving or generating neuron	Later spike pattern	The selected pathway produces a spike record	Directed route	—	—	—	—

Assigned condition
Assigned condition	Stimulus level (Assigned stimulus units)	Graded depolarising change (mV)	Later mean spike rate (Spikes per second)
Condition 0	0	0	0
Condition 1	1	4	20
Condition 2	2	7	40
Condition 3	3	9	50

Model conditions: A constructed depolarising sensory route with four separate conditions. Intermediate values, mechanism constants and universality across receptors are unassigned.

Count spikes within the same window

Neural information can depend on which cells respond, the timing and rate of their spikes, and patterns across a population. A stimulus can recruit additional cells as well as change the behaviour of an already responsive cell. No single code is sufficient for every sense or task.

In the constructed record, condition A produces spikes at 20, 50 and 80 ms within a 100 ms window. Condition B produces six at 10, 25, 40, 55, 70 and 85 ms. Each glyph denotes the same selected spike event. The model differs in event number and timing, not a claim that stronger conditions double the size of one spike.

The counts give average rates of 30 and 60 spikes per second because 100 ms is 0.1 s. These averages do not reproduce the exact spacing of the events, nor predict how a different cell or a longer window responds. A rate is one summary of a specified record.

Same event symbols, different counts

Average rate is one summary and does not replace the full timing record.

Read the diagram in words

A has three spikes at 20/50/80 ms and mean rate 30 spikes/s. B has six at 10/25/40/55/70/85 ms and mean rate 60 spikes/s. Individual glyphs represent the same kind of selected event.

Time within the observation window
Minimum	Maximum	Axis ticks	Unit
0	100	0, 20, 40, 60, 80, 100	ms

Condition
Condition	Assigned spike times (ms)	Number of spikes	Window duration (s)	Count ÷ window duration (Spikes per second)
Condition A	20, 50, 80	3	0.1	30
Condition B	10, 25, 40, 55, 70, 85	6	0.1	60

Observation window: 0–100 ms
Selected event: One selected spike event

Model conditions: Two constructed records from a selected neural pathway. Spike times are assigned; waveform size, cellular mechanism and responses outside the window are unassigned.

A response can change while the stimulus remains

Adaptation is a change in a sensory response during sustained or recent stimulation. It can occur at receptor or later processing stages, and the mechanism depends on the system. A weaker response does not necessarily mean that the environmental stimulus disappeared.

The separate constructed adapting receptor has zero assigned stimulus at time zero, then a sustained level of two stimulus units at times 1, 2 and 3 s. Its depolarising responses are 8, 5 and 3 mV at those samples. The environmental input remains the same while the recorded response falls.

This is not a general rule that every receptor becomes silent, a measured time constant or a demonstration of learning. Adaptation and behavioural habituation can produce similar-looking changes at different levels. To identify the mechanism, measure the appropriate receptor, circuit and behavioural states instead of relying on one declining line.

Response falls during a sustained selected input

A smaller response need not mean that the stimulus has disappeared.

Read the diagram in words

At times 0/1/2/3 s, stimulus samples are 0/2/2/2 and response samples 0/8/5/3 mV. The two series require separate units. Only the sampled sustained-input comparison is quantitatively assigned.

Assigned sample time
Minimum	Maximum	Axis ticks	Unit
0	3	0, 1, 2, 3	s

Selected environmental input
Minimum	Maximum	Axis ticks	Unit
0	3	0, 1, 2, 3	Assigned stimulus units

Receptor response
Minimum	Maximum	Axis ticks	Unit
0	10	0, 2, 4, 6, 8, 10	mV

A sustained input can accompany a changing response
Sample time (s)	Assigned stimulus units	Receptor response (mV)
0	0	0
1	2	8
2	2	5
3	2	3

Model conditions: Four constructed samples of one adapting receptor, distinct from the spike-window and transduction-condition examples. No measured time constant, universal adaptation mechanism or learning outcome assigned.

Worked example

Use the denominator and preserve the changing level

Three events in 0.1 s give 3/0.1=30 spikes/s; six give 60 spikes/s. The time denominator is identical, so the assigned rate doubles. If six events were instead counted over 0.2 s, their average rate would be 30 spikes/s.

In the separate adaptation table, response falls from 8 to 3 mV while the sampled stimulus remains at two units. That supports a change in the constructed receptor response under sustained input. It does not establish a stronger or weaker external stimulus at the final sample.

Check your understanding

Is a receptor potential necessarily an action potential?
Show reasoning
No. It is a graded receptor response. Depending on the pathway, it affects spike initiation in that cell or transmitter release onto another cell.
What are the assigned average rates for three and six spikes in 100 ms?
Show reasoning
Thirty and sixty spikes per second, respectively. Divide by 0.1 s; the individual spike glyph does not become twice as large.
Does the adaptation record show that the stimulus has vanished?
Show reasoning
No. At times 1–3 s the stimulus stays at two units while receptor response falls from 8 to 3 mV. The record does not identify a universal mechanism or behavioural learning.

What to carry forward

Separate stimulus, receptor response, spike pattern and later interpretation, with the observation window and history kept explicit.

References

Resources supporting this lesson’s explanations and examples.

Biology 2eMary Ann Clark, Matthew Douglas and Jung ChoiOpen textbook · 2018Biology 2e 35.1 Neurons and Glial Cells 35.2 How Neurons Communicate 36.1 Sensory Processes 36.3 Taste and Smell 36.4 Hearing and Vestibular Sensation 36.5 Vision 38.1 Types of Skeletal Systems 38.4 Muscle Contraction and Locomotionopenstax.org
Neuroscience · second editionD. Purves, G. J. Augustine, D. Fitzpatrick and colleagues, editorsNeuroscience textbook · 2001Electrical Signals of Nerve Cells Synaptic Transmissionncbi.nlm.nih.gov
Piezo1 and Piezo2 are essential components of distinct mechanically activated cation channelsBertrand Coste and colleaguesOriginal mechanotransduction research · 2010pubmed.ncbi.nlm.nih.govRead source
Introduction to NeuroscienceValerie HedgesOpen neuroscience textbook · 2022Introduction to Neuroscience Touch: The Skin Spinal Reflexesopenbooks.lib.msu.edu