In one breath
A stimulus makes a receptor produce a local, graded depolarisation that does not travel, the receptor (generator) potential; once it reaches about 10–15 mV, an action potential fires in the sensory nerve. In the Pacinian corpuscle, pressure opens stretch-sensitive Na⁺ channels in the bare nerve terminal, and the action potential starts at the first node of Ranvier. All receptors share a set of properties: specificity, adequate stimulus, adaptation, acuity, intensity coding, the Weber–Fechner law, the law of projection and the doctrine of specific nerve energy.
Builds on: Sensory receptors and their classification · Synapses and synaptic transmission · Leads to: Afferent fibers, dorsal root and spinal laminae · Ascending tracts: dorsal column and anterolateral system · Somatosensory cortex
Receptor potential
- A stimulus is a change in energy. When one acts on a receptor, the potential across the receptor membrane changes.
- The change is usually a depolarisation that stays where it is made: it is not propagated. In this it resembles an EPSP.
- It is called the receptor potential, or the generator potential.
- Its size is graded: a stronger stimulus gives a proportionately larger receptor potential. At about 10–15 mV, an action potential fires in the sensory nerve.
| Feature | Receptor potential | Action potential |
|---|---|---|
| Size | Graded with the stimulus | Fires at a threshold |
| Spread | Local, not propagated | Conducted along the nerve |
| Site in Pacinian | Unmyelinated terminal | First node of Ranvier |
Worth knowing, though not in your pages: an action potential is all-or-none and is followed by a refractory period. A receptor potential has neither, so receptor potentials can summate.
Draw it: receptor potential and action potential
Draw two traces, one above the other, with time along the bottom. On the upper trace, “receptor potential in the terminal”, draw three humps of rising height for three stimuli of rising strength, and a dashed line at about 10–15 mV. Only the humps that cross the line get spikes on the lower trace, “action potentials in the sensory nerve”. Put more spikes, closer together, under the tallest hump.
The Pacinian corpuscle as the model
- It is large, so it is the receptor in which the receptor potential has been studied best.
- The lamellae are not needed. The unmyelinated nerve terminal is what makes the receptor potential.
- The receptor potential gives rise to the action potential at the first node of Ranvier, which lies inside the lamellae.
Draw it: where the action potential starts
Draw three Pacinian corpuscles side by side, each with a pressure arrow and a recording beneath it.
- A, intact: a receptor potential, then action potentials.
- B, lamellae stripped off: pressing the bare terminal still gives a receptor potential and action potentials.
- C, first node of Ranvier blocked: no action potential.
Worth knowing, though not in your pages: in C the receptor potential itself is unchanged, because the terminal still makes it.
Not the lamellae, not the terminal
MCQs ask where a Pacinian corpuscle generates its action potential. The answer is the first node of Ranvier. The unmyelinated terminal makes only the receptor potential, and the lamellae make neither.
How the receptor potential arises
- Pressure on the corpuscle deforms the lamellae.
- The deformation opens stretch-sensitive Na⁺ channels in the nerve terminal, which is the true receptor.
- Na⁺ flows in and depolarises the terminal: this is the receptor potential.
- At the first node of Ranvier, the receptor potential sets off an action potential.
- The stronger the pressure, the larger the receptor potential and the higher the frequency of action potentials.
A slip in your book: action potentials do not grow
Your book says the “magnitude and frequency” of the action potentials rise with stimulus strength. Only the frequency does. Every action potential in a fibre is the same size, so a stronger stimulus is signalled by faster firing and by more receptors firing (recruitment, below). Write “frequency” in the exam and you are right by any book.
Properties of receptors
| Property | In short |
|---|---|
| Specificity | Each receptor answers one kind of stimulus |
| Adequate stimulus | The energy it is most sensitive to |
| Adaptation | Firing falls during a steady stimulus |
| Acuity | How precisely a stimulus is localised |
| Intensity | Faster firing; more receptors recruited |
| Weber–Fechner law | Sensation ∝ log of stimulus intensity |
| Law of projection | Felt at the receptor, wherever stimulated |
| Specific nerve energy | The brain area reached sets the sensation |
Specificity and adequate stimulus
- Specificity: each receptor is stimulated by one type of stimulus. Pain receptors respond to painful stimuli, and touch receptors to touch.
- Adequate stimulus: the form of energy to which a receptor is most sensitive. For the rods and cones, it is light.
- Worth knowing, though not in your pages: a strong enough stimulus of another kind can still fire a receptor, but the sensation is always the receptor’s own. Pressing on the closed eye makes you see flashes of light.
Adaptation
- Keep a stimulus of constant strength on a receptor, and the frequency of action potentials in its sensory nerve falls. This is adaptation, also called desensitisation.
- Receptors are divided by how fast they adapt:
| Type | Adapts | Examples |
|---|---|---|
| Phasic | Rapidly | Touch and pressure receptors |
| Tonic | Slowly, or not at all | Baroreceptors; pain receptors |
- The tonic baroreceptors are those of the carotid sinus and the aortic arch.
- Not every touch receptor is phasic: Merkel’s disks and Ruffini endings adapt slowly (see Sensory receptors and their classification).
- Worth knowing, though not in your pages: phasic receptors signal change, such as movement or vibration. Tonic receptors keep reporting a steady stimulus, so the brain stays informed of the blood pressure, and of tissue damage for as long as it lasts.
Acuity
- The precision with which a stimulus is localised.
- It depends on the number of receptors in the area stimulated: the more receptors, the finer the acuity. The dimension of sensation with the same name is in Sensation: general principles.
Intensity
- A weak stimulus fires only the receptors close to the site and those with a low threshold.
- A stronger stimulus makes those receptors fire faster, and also fires receptors farther away from the site. Bringing in more receptors in this way is receptor recruitment.
- So stimulus strength is coded in two ways: by the firing rate and by the number of receptors active.
Weber–Fechner law
- The magnitude of the sensation is proportional to the logarithm of the stimulus intensity.
- Worth knowing, though not in your pages: so each tenfold rise in the stimulus adds about the same step to the sensation. This is how one sense can cover a very wide range of stimulus strengths.
Law of projection
- Wherever a sensory pathway is stimulated on its way to the cortex, the sensation is felt at its receptors, where the pathway begins.
- So the sensation is referred to the nerve endings, not to the point that was actually stimulated.
- Worth knowing, though not in your pages: a knock on the ulnar nerve at the elbow (the “funny bone”) is felt as tingling in the ring and little fingers.
Applied: phantom limb
After an amputation, the patient may still feel pain or itching in the limb that is gone. The pathway that once served the limb can still be stimulated, and by the law of projection the brain places the sensation where that limb’s receptors used to be.
Worth knowing, though not in your pages: irritated nerve ends in the stump are a common source of that stimulation.
Doctrine of specific nerve energy
- The sensation a stimulus evokes depends on which area of the brain its pathway finally activates.
- Stimulate a touch fibre anywhere between the receptor and the cortex, and the sensation is touch.
- This works because each pathway is specific and separate all the way from the sense organ to the cortex.
- What activates the pathway, and at what point, makes no difference: the sensation is always the one its receptor is built for.
- Johannes Müller (1801–1858), who taught physiology at Bonn, first described it, so it is called Müller’s doctrine. He also showed how blood capillaries relate to secreting glands, discovered chondrin and glutin, and studied digestion.
- Worth knowing, though not in your pages: the modern name for the same idea is the labelled line principle. Each fibre carries one modality, and the modality felt is fixed by where in the CNS the fibre ends.
Projection tells "where"; specific nerve energy tells "what"
The law of projection is about where a sensation is felt: at the receptor, wherever the pathway is stimulated. The doctrine of specific nerve energy (the labelled line) is about what is felt: the modality of that pathway, however it is stimulated. A phantom-limb itch is projection. Seeing light when the eye is pressed is specific nerve energy.
Sensory unit and receptive field
- A sensory unit is one sensory axon together with all its peripheral branches.
- Its receptive field is the area from which a stimulus makes that unit respond.
- The fields of neighbouring units usually overlap a little.
- Recruitment of sensory units: a stronger stimulus activates receptors over a larger area. Units around the stimulated spot join in, not only those in direct contact with it. This is the receptor recruitment described under intensity.
Exam-answer skeleton: "Properties of receptors" (short note)
- Define a receptor as a transducer, in one line.
- Specificity and adequate stimulus, with an example of each.
- Adaptation: phasic and tonic receptors, with examples.
- Acuity, and what it depends on.
- Intensity: faster firing and receptor recruitment.
- Weber–Fechner law.
- Law of projection, with phantom limb.
- Müller’s doctrine of specific nerve energy.
Asked in exams
- Properties of receptors: Jul 2011, 2 marks · Jan 2021, 5 marks
- The receptor (generator) potential: Jan/Feb 2022, MCQ · Jan/Feb 2022, 2 marks
- Law of projection, and phantom limb: Jan 2023, MCQ
- Labelled line principle: Apr 2023, MCQ