Receptors and Sensory Transduction

This lesson is mapped to AQA 7402 Section 3.6.1 — survival and response / receptors as transducers (refer to the official AQA specification document for exact wording). Receptors are the front-end of the nervous system — the cells and structures that detect changes in the internal and external environment and convert (transduce) those changes into the electrical events covered in lessons 0–3. Every sensory experience, from the touch of a pencil to the colour of a sunset, begins with a transduction event at a specialised receptor cell.

This lesson covers the general principle of sensory transduction (stimulus → receptor → generator potential → action potential), the Pacinian corpuscle as the AQA model mechanoreceptor, the rods and cones of the mammalian retina as model photoreceptors, and the conceptual distinction between graded generator potentials and all-or-nothing action potentials. We close with the A-Level-depth extension of spatial and temporal summation at receptor fields, and with Sherrington's framework of receptors as the integrative gateway of the nervous system.

Key Definition: A sensory receptor is a specialised cell or cell organelle that detects a specific kind of stimulus (light, mechanical force, temperature, chemical) and converts that stimulus into a change in membrane potential — the generator potential.

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The General Principle of Sensory Transduction

All sensory receptors perform the same logical operation:

Stimulus → mechanical / chemical / electromagnetic change at the receptor → opening (or closing) of ion channels → change in membrane potential (generator potential) → if threshold reached, action potential(s) in the afferent sensory neurone

This is a transduction — energy in one form (mechanical, light, chemical) is converted into a common electrical currency that the nervous system can integrate, transmit, and interpret. The same stimulus reaching different receptor types is encoded differently: visible light triggers photoreceptors but does not stimulate touch receptors, despite both being capable of generating action potentials.

graph LR
    A["Stimulus<br/>(pressure, light, chemical, heat)"] --> B["Receptor protein / structure"]
    B --> C["Opening or closing of ion channels"]
    C --> D["Graded generator potential<br/>at receptor membrane"]
    D --> E["Threshold reached?"]
    E -->|yes| F["Action potential(s)<br/>in afferent sensory neurone"]
    E -->|no| G["Decays; signal lost"]
    F --> H["Conduction to CNS<br/>integration in cortex"]

    style B fill:#27ae60,color:#fff
    style D fill:#3498db,color:#fff
    style F fill:#e74c3c,color:#fff

Key Properties of Receptors

• Specificity — each receptor type responds optimally to one stimulus modality (its adequate stimulus). A photoreceptor responds to light; a mechanoreceptor responds to pressure. Other stimuli will produce responses only at extreme intensities (pressing the closed eye produces "phosphenes" — a mechanical activation of photoreceptors).
• Transducer function — energy of the stimulus is converted into electrochemical signal.
• Threshold sensitivity — every receptor has a minimum stimulus level below which no signal is generated. Below threshold, the generator potential does not reach the level required to trigger an AP in the afferent neurone.
• Adaptation — sustained stimulation often produces diminishing response over time. Pacinian corpuscles are rapidly-adapting (they signal changes in pressure rather than absolute pressure); muscle stretch receptors are slowly-adapting (they signal sustained posture).

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Generator Potential vs Action Potential

The most important conceptual move in this lesson is to distinguish the generator potential at the receptor membrane from the action potential in the afferent axon:

Feature	Generator Potential	Action Potential
Where	Receptor membrane (or specialised dendrite)	Axon of afferent neurone
Mechanism	Stimulus-gated ion channels (mechano-, photo-, chemo-, thermo-sensitive)	Voltage-gated Na⁺ and K⁺ channels
Size	Graded — proportional to stimulus intensity	All-or-nothing — fixed amplitude
Summation	Yes — spatial and temporal	No — discrete events
Propagation	Local — decays with distance	Regenerative — travels long distances without decay
Encoded information	Stimulus magnitude (amplitude)	Stimulus magnitude (frequency)

Generator potentials are the graded analogue input; action potentials are the digital output that travels to the CNS. The receptor is essentially an analogue-to-digital converter in which stimulus amplitude is translated into AP frequency. A stronger stimulus produces a larger generator potential, which fires the afferent axon at a higher rate (frequency code, as introduced in lesson 1).

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The Pacinian Corpuscle — Model Mechanoreceptor

The Pacinian corpuscle is the AQA model receptor for mechanical pressure. It is a 1–2 mm onion-like structure found deep in the dermis of the skin, in joint capsules, in the mesentery, and in the periosteum of bones. It responds to deep pressure and vibration in the 20–1,000 Hz range.

Structure

• A single sensory neurone ending (unmyelinated dendrite) surrounded by concentric lamellae of connective tissue.
• The lamellae are separated by viscous gel.
• The dendrite membrane contains stretch-mediated Na⁺ channels that open when the membrane is physically deformed.

Mechanism of Transduction

1. Pressure applied to the skin physically deforms the lamellae of the Pacinian corpuscle.
2. The deformation stretches the dendrite membrane at the centre of the corpuscle.
3. Stretching opens stretch-mediated Na⁺ channels in the dendrite membrane.
4. Na⁺ enters the dendrite down its electrochemical gradient.
5. The membrane is depolarised — this is the generator potential, graded with the magnitude of the stretch.
6. If the generator potential exceeds threshold, voltage-gated Na⁺ channels at the first node of Ranvier in the afferent axon open, and an action potential is generated.
7. The action potential propagates along the afferent sensory neurone to the spinal cord (typically entering via a dorsal root).

graph TD
    A["Pressure applied to skin"] --> B["Lamellae deform<br/>dendrite membrane stretched"]
    B --> C["Stretch-mediated Na⁺ channels open"]
    C --> D["Na⁺ enters dendrite<br/>generator potential"]
    D --> E["Graded with stimulus intensity"]
    E --> F["If above threshold:<br/>AP in afferent axon"]
    F --> G["AP frequency encodes pressure magnitude"]

    style B fill:#27ae60,color:#fff
    style D fill:#3498db,color:#fff
    style F fill:#e74c3c,color:#fff

Why the Lamellae?

The concentric lamellar structure acts as a mechanical high-pass filter: it transmits rapid pressure changes (vibrations) to the dendrite but, over a few hundred milliseconds, the viscous gel between lamellae redistributes the pressure load and the deformation at the central dendrite decays. This is the structural basis of the Pacinian corpuscle's rapid adaptation — it detects vibration and pressure changes, not sustained pressure. If you place your finger on a vibrating object, the Pacinian corpuscles fire vigorously; if you sit motionless on a chair for a few minutes, the corpuscles in your skin stop responding to the constant chair pressure.

Summation in the Receptor Field

Each afferent sensory neurone in the dermis serves a small receptive field of skin containing multiple receptor structures. Spatial summation across these receptors allows the nervous system to distinguish a sharp point (one corpuscle firing) from a broad pressure (many corpuscles firing) — this is the basis of two-point discrimination, used clinically as a test of dorsal-column function.

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Rods and Cones — Model Photoreceptors

The mammalian retina contains two principal photoreceptor types arranged in a ~125-million-cell mosaic at the back of the eye.

Rod Cells

• Approximately 120 million per human retina, concentrated in the peripheral retina (absent from the fovea).
• Contain the photopigment rhodopsin (opsin protein + 11-cis-retinal chromophore).
• Very sensitive to light — can respond to a single photon under fully dark-adapted conditions.
• Monochromatic — only one pigment, so no colour discrimination.
• Many rods converge onto a single bipolar / ganglion cell (convergent wiring) — this maximises sensitivity (spatial summation of weak signals) but reduces visual acuity.
• Specialised for scotopic vision (dim-light vision).

Cone Cells

• Approximately 6 million per human retina, concentrated in the central fovea.
• Three subtypes containing different opsin variants of the photopigment iodopsin — S-cones (peak ~420 nm, blue), M-cones (peak ~530 nm, green), L-cones (peak ~560 nm, red). Colour discrimination arises from the ratio of activity across the three types — the trichromatic theory (associated with Young and Helmholtz; paraphrasing their framework, not quoting them).
• Less sensitive than rods — require bright light to fire reliably.
• Trichromatic — colour vision.
• One cone in the central fovea synapses with one bipolar / ganglion cell (one-to-one wiring) — this maximises visual acuity at the cost of sensitivity.
• Specialised for photopic vision (bright-light vision).

Feature	Rods	Cones
Approximate number per eye	120 million	6 million
Distribution	Peripheral retina	Concentrated in fovea
Photopigment	Rhodopsin (one type)	Iodopsin (three types: S, M, L)
Sensitivity	Very high (single-photon)	Lower; bright light needed
Colour discrimination	None (monochromatic)	Yes (trichromatic)
Acuity	Low (many rods → one bipolar)	High (one-to-one in fovea)
Function	Scotopic (dim-light) vision	Photopic (bright-light) and colour vision

Mechanism of Photoreception

The remarkable feature of photoreceptors is that they hyperpolarise in response to light — they signal stimulus by reducing transmitter release, the opposite of most receptors. In summary:

1. In the dark, photoreceptors are depolarised (~−40 mV); cyclic GMP (cGMP) keeps Na⁺/Ca²⁺ channels open in the outer segment.
2. A photon strikes a rhodopsin molecule; 11-cis-retinal isomerises to all-trans-retinal.
3. The retinal change causes a conformational change in opsin, activating a G-protein cascade (transducin).
4. The cascade activates phosphodiesterase, which hydrolyses cGMP.
5. cGMP-gated Na⁺/Ca²⁺ channels close.
6. The photoreceptor hyperpolarises (toward −70 mV).
7. Hyperpolarisation reduces glutamate release at the photoreceptor's basal synapse onto bipolar cells.
8. The bipolar cell and downstream ganglion cells encode the change in glutamate release as a change in their own firing rate — and ganglion cell axons fire action potentials toward the brain.

Photoreceptors are remarkable in that the action potentials are not generated in the receptor itself, but in the ganglion cell several synapses downstream. The generator potential here is a hyperpolarising potential, and the encoding is by reduced transmitter release rather than direct AP firing — a useful A* discriminator.

A-Level Depth: Why Two Photoreceptor Types?

The duplex retina (rods + cones) reflects a trade-off impossible to solve with a single receptor:

• Sensitivity vs acuity: a single photoreceptor needs many photons to fire reliably; convergent wiring of many rods onto one ganglion cell increases the chance of summation, but blurs spatial detail.
• Range of light intensities: starlight is ~10⁻⁶ of midday sunlight. No single receptor can span this range; rods saturate in moderate light and cones do not fire below it.
• Colour vision: requires comparison of two or more pigments, which means at least two receptor types are required. Three is the mammalian standard.

Insects, birds, and many fish have four, five, or even more cone types — vertebrate evolution has not selected for trichromacy uniquely.

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Threshold and the Receptor as Filter

Every receptor has a threshold below which the generator potential is too small to trigger an AP in the afferent axon. This filters out background noise — the random thermal opening of ion channels would otherwise flood the CNS with spurious signals. Threshold also lets the receptor encode stimulus intensity:

• Sub-threshold stimulus → no AP. The signal is rejected.
• Just-threshold stimulus → occasional AP.
• Strong stimulus → high-frequency train of APs.

The CNS then interprets AP frequency as a measure of stimulus magnitude.

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A-Level Deep Dive

Spec mapping

This content sits in AQA 7402 Section 3.6.1 — survival and response, receptors as transducers; Pacinian corpuscle; rods and cones (refer to the official AQA specification document for exact wording). Examined directly on Paper 2.

Synoptic links

This lesson connects to:

1. AQA 7402 Section 3.2.3 — transport across membranes (course 2): the stretch-mediated Na⁺ channels of the Pacinian corpuscle and the cGMP-gated channels of photoreceptors are integral membrane proteins. The general principles of gated ion channels covered in section 3.2 underpin every receptor type.
2. AQA 7402 Section 3.6.2 — synaptic transmission (lesson 2 this course): photoreceptors signal via reduced glutamate release, an inverted version of the synaptic transmission covered in lesson 2 — a synoptic test of whether students understand transmitter release rather than memorise a sequence.
3. AQA 7402 Section 3.4.2 — protein synthesis (course 4): the rhodopsin / opsin / iodopsin proteins are synthesised in the photoreceptor cell body and inserted into the outer-segment membrane disks. Genetic mutations in opsin produce inherited colour blindness (X-linked recessive for L- and M-cone defects).

Mark-scheme literacy

Receptor questions split AO marks predictably: