Receptors (A-level only)
AQA
A-Level
Receptors are transducers: they convert the energy of a stimulus into the electrical energy of a nerve impulse. Each type is specific, because only one form of energy can open its particular channel proteins. A Pacinian corpuscle, found deep in the skin and around joints and tendons, contains stretch-mediated sodium ion channels that open only when the membrane is physically deformed, so it responds to pressure and vibration and is completely insensitive to light, sound or chemicals. When pressure is applied, sodium ions enter and the membrane depolarises, and that local depolarisation is the generator potential. A generator potential is graded: a greater or faster pressure change deforms more channels, admits more sodium ions and produces a larger generator potential.
16
Objectives
12
Exam Tips
21
Pitfalls
23
Key Terms
21
Mark Points
Subtopics in this area
The Pacinian corpuscle should be used as an example of a receptor to illustrate that: receptors respond only to specific stimuli stimulation of a receptor leads to the establishment of a generator potential.
The basic structure of a Pacinian corpuscle.
Deformation of stretch-mediated sodium ion channels in a Pacinian corpuscle leads to the establishment of a generator potential.
The human retina in sufficient detail to show how differences in sensitivity to light, sensitivity to colour and visual acuity are explained by differences in the optical pigments of rods and cones and the connections rods and cones make in the optic nerve.
Receptors (A-level only) Revision Guide
Learning Objectives
What you need to know and understand
- Explain why a Pacinian corpuscle responds to pressure but not to light, in terms of its channel proteins.
- Describe how stimulation of a receptor produces a generator potential.
- Distinguish a generator potential from an action potential in terms of size, grading and threshold.
- Suggest why a measured reaction time is longer than the time predicted from nerve conduction speed alone.
- Describe the structure of a Pacinian corpuscle, naming the lamellae and the sensory neurone ending.
- Locate the stretch-mediated sodium ion channels within the corpuscle.
- Explain how the layered structure transmits an external pressure to the neurone membrane.
- Link each named structure of the corpuscle to its role in producing a generator potential.
- Describe, in five steps, how pressure on a Pacinian corpuscle produces a generator potential.
- Explain why the channels involved are described as stretch-mediated rather than voltage-gated.
- Predict the effect of a greater pressure on the size of the generator potential and on the frequency of action potentials.
- Interpret a trace of membrane potential against time for two different pressures applied to a corpuscle.
- Explain why rods give high sensitivity but low visual acuity, using their pigment and their connections.
- Use an absorption spectrum for the three cone types to explain how a given colour is perceived.
- Contrast the connection of rods and cones to bipolar neurones and link each to visual acuity.
- Predict how vision changes in dim light, justifying the prediction using rhodopsin and iodopsin.
Marking Points
Key points examiners look for in your answers
- one mark for receptors being specific because only one type of stimulus opens their channels
- one mark for the Pacinian corpuscle responding to pressure or vibration through stretch-mediated sodium ion channels
- one mark for sodium ions entering and depolarising the membrane to give a generator potential
- one mark for the generator potential being graded, so a larger stimulus gives a larger generator potential
- one mark for an action potential being produced only when the threshold is reached
- one mark for a single sensory neurone ending at the centre of the corpuscle
- one mark for concentric rings or layers of connective tissue lamellae surrounding it
- one mark for gel or fluid separating the lamellae
- one mark for stretch-mediated sodium ion channels in the membrane of the sensory neurone ending
- one mark for linking deformation of the lamellae to stretching of the neurone membrane
- stating that there are stretch-mediated sodium ion channels in the membrane
- increased pressure deforming the membrane, the lamellae or the sodium ion channels
- the sodium ion channels opening
- sodium ions diffusing in, or moving in down an electrochemical gradient
- depolarisation, or the inside becoming less negative, leading to a generator potential
- cones are the photoreceptors involved in colour vision
- each type of cone contains a different optical pigment, absorbing a different range of wavelengths
- colour perception depends on the relative stimulation of the three cone types and the resulting impulses sent to the brain
- many rods connect to one bipolar neurone, providing spatial summation and high sensitivity but low visual acuity
- one cone connects to one bipolar neurone, so two stimulated cones are resolved as two separate points, giving high acuity
- rhodopsin is broken down by light and is sensitive at low light intensities, enabling rods to function in dim light
Examiner Tips
Expert advice for maximising your marks
- 💡Always write sodium ions or Na⁺, never just sodium, because mark schemes cap answers that omit the ion.
- 💡Use the word transducer, or the phrase converts one form of energy into a nerve impulse, when defining a receptor.
- 💡In reaction time questions, the delay at the receptor, at the synapses and in the muscle all count as separate reasons.
- 💡Label a diagram with three things: the single sensory neurone ending, the lamellae, and the stretch-mediated sodium ion channels.
- 💡Link every structural feature you name to a function, because structure-only answers rarely gain full marks.
- 💡State where the corpuscle is found, deep in the skin or in joints and tendons, if the question asks for location.
- 💡Structure your answer logically: channels present, deformation, channels open, sodium ions diffuse in, depolarisation.
- 💡Write Na⁺ or sodium ions every single time to ensure biological accuracy.
- 💡Remember that a generator potential is a graded response, whereas an action potential is all-or-nothing.
- 💡Use the term 'red-sensitive cone' rather than 'red cone' to accurately describe the receptor's function.
- 💡When reading an absorption spectrum, quote approximate percentage values at the wavelength given to support comparisons.
- 💡Answer acuity questions by discussing neuronal connections (wiring) and sensitivity questions by discussing pigments and spatial summation.
Common Mistakes
Pitfalls to avoid in your exam answers
- calling the generator potential an action potential, when the generator potential is graded and local and the action potential is all-or-nothing
- saying the receptor creates the impulse, rather than converting stimulus energy into a change in membrane potential
- claiming a stronger stimulus gives a bigger action potential instead of a higher frequency of action potentials
- writing sodium rather than sodium ions, which caps the marks available on these questions
- forgetting that the time taken for the channels to open contributes to measured reaction time
- describing many neurone endings inside one corpuscle instead of a single one
- calling the lamellae layers of skin rather than connective tissue within the corpuscle
- placing the sodium ion channels in the lamellae rather than in the neurone's cell surface membrane
- omitting the channels altogether, so the structure cannot be linked to the generator potential
- labelling the central ending as myelinated, when the ending inside the corpuscle is not
- writing sodium instead of sodium ions, which is biologically inaccurate as the atom and ion have different properties
- saying the channels are voltage-gated, when in the corpuscle they are stretch-mediated and opened by deformation
- describing sodium ions being actively transported in, when they diffuse down the electrochemical gradient
- jumping straight to an action potential without the generator potential and the threshold
- describing hyperpolarisation or potassium ions leaving, which belongs to repolarisation
- writing 'red cones' and 'green cones' rather than 'red-sensitive' and 'green-sensitive' cones, which lacks precision
- writing signals or messages instead of impulses or action potentials
- saying rods have low acuity because there are fewer of them, rather than because many share one bipolar neurone
- claiming a single cone detects the colour orange, when colour perception arises from the relative stimulation of three cone types
- confusing sensitivity with acuity, so summation is incorrectly offered as the reason for sharp vision
- stating that rhodopsin is broken down by low light, when it is broken down by light and is sensitive at low light intensities