In one breath
A receptor is a transducer: the ending of an afferent fibre, or a special cell, that turns one form of energy (its adequate stimulus) into action potentials. Receptors are classified by function (extero-, intero-, proprio- and teleceptors), by adequate stimulus (mechano-, thermo-, noci-, chemo- and photoreceptors) and by location (superficial, deep, visceral). The skin’s mechanoreceptors differ in structure and in how fast they adapt: the Pacinian corpuscle is the largest and senses vibration, and Merkel’s disk is the most superficial and adapts slowly.
Builds on: Sensation: general principles · Leads to: Receptor potential and properties of receptors · Pain: concepts, receptors and fibers · Itch and temperature sensation · Muscle spindle · Golgi tendon organ
What a receptor is
- A transducer: it converts some form of energy from the environment into action potentials in sensory neurons.
- It is the ending of an afferent nerve fibre. It may be part of the neuron itself, or a specialised cell that makes the neuron fire.
- Non-neural cells usually surround it. The receptor and those cells together form a sense organ.
- Each receptor is specific to one form of energy. The energy it is most sensitive to is its adequate stimulus: for the rods and cones of the retina, light.
Classification
Receptors are classified in three ways: by function, by adequate stimulus and by location.
By function
| Type | Detects | Where or example |
|---|---|---|
| Exteroceptors | External changes close to the body | Skin, subcutaneous tissue |
| Interoceptors | Internal changes | Baroreceptors, osmoreceptors |
| Proprioceptors | Body position in space | Muscles, tendons, joints |
| Teleceptors | Stimuli far from the body | Ear (distant sound) |
- Exteroceptors make up the cutaneous sense organs.
- Interoceptors: baroreceptors detect changes in blood pressure, and osmoreceptors changes in the osmolality of body fluids.
- Proprioceptors are mechanoreceptors. Sense of position also relies on kinesthesia, the sensation of how body parts move, which covers both movement and still posture.
By adequate stimulus
The forms of energy are touch–pressure, cold–warmth, pain, chemicals and photons. Sense organs in the skin, subcutaneous tissue and epithelia pick them up.
| Receptor | Adequate stimulus | Examples |
|---|---|---|
| Mechanoreceptors | Mechanical: touching, stroking | Pacinian, Meissner, Merkel, Ruffini, Krause |
| Thermoreceptors | Change in skin temperature | Warmth and cold receptors |
| Nociceptors | Harmful stimuli | Aδ-mechanical, C-polymodal |
| Chemoreceptors | Chemicals | Taste and olfactory receptors |
| Photoreceptors | Light (photons) | Rods and cones of the retina |
Mechanoreceptors fall into three groups:
- Expanded endings: Merkel’s disks and Ruffini endings.
- Encapsulated endings: Pacinian corpuscles, Meissner’s corpuscles and Krause’s end-bulbs.
- Naked nerve endings: the other two groups are not essential. Skin that has only naked endings can still give all four cutaneous modalities.
- Worth knowing, though not in your pages: the four cutaneous modalities are touch–pressure, cold, warmth and pain.
Thermoreceptors are of two kinds, warmth and cold receptors. Both work over a broad range of temperature change (see Itch and temperature sensation).
Nociceptors respond to stimuli that damage tissue or threaten to (see Pain: concepts, receptors and fibers).
| Nociceptor | Responds to |
|---|---|
| Aδ-mechanical | Fast, sharp pain, such as a prick; not heat or chemicals |
| C-polymodal | Many noxious stimuli, thermal and chemical included |
By location
- Superficial: in the skin, such as the touch and pressure receptors.
- Deep: in muscles, bones, tendons and similar tissues.
- Visceral: in the viscera, such as the visceral pain receptors.
- Worth knowing, though not in your pages: the Muscle spindle and the Golgi tendon organ are the best-known deep receptors.
Important mechanoreceptors
| Receptor | Where | Adapts | Senses |
|---|---|---|---|
| Pacinian | Skin, deep tissues | Rapidly | Vibration, fine touch, pressure |
| Meissner’s | Dermal ridges, hairless skin | Rapidly | Vibration, touch |
| Merkel’s disk | Epidermis | Slowly | Touch–pressure |
| Ruffini | Hairless and hairy skin | Slowly | Crude touch |
| Krause’s | Dermis | Rapidly | Touch, pressure |
| Golgi-Mazzoni | Tendons, muscles, skin | Rapidly | Probably touch–pressure |
| Receptor | Size | Capsule |
|---|---|---|
| Pacinian | 0.5–2 mm long, 0.7 mm wide | 20–70 lamellae |
| Meissner’s | 150 µm long, 50 µm wide | Single layer |
| Golgi-Mazzoni | 150–250 µm across | 10–15 lamellae |
Pacinian corpuscle
- Where: in the skin and deep tissues.
- Structure: concentric lamellae, 20–70 of them, layered like an onion.
- Size: the largest mechanoreceptor.
- Nerve supply:
- one Aβ primary afferent fibre, whose tip runs into the core of the lamellae;
- its cell body is in the dorsal root ganglion, and it enters the cord by the dorsal root;
- the fibre is myelinated; only its terminal, inside the lamellae, is unmyelinated;
- the first node of Ranvier usually lies inside the corpuscle.
- Function: rapidly adapting. It picks up high-frequency vibration, so it senses vibration and fine touch, and pressure as well.
- The transducer is the nerve ending, not the lamellae. Strip the lamellae away and pressure on the bare ending still makes a receptor potential (see Receptor potential and properties of receptors).
Draw it: the Pacinian corpuscle
Draw an oval of many concentric rings, like a cut onion. Bring a myelinated fibre in at one pole; let it lose its myelin inside and run up the core as a bare terminal. Mark the first node of Ranvier just inside the lamellae, and label the fibre “Aβ, cell body in dorsal root ganglion”. Write the numbers beside it: 0.5–2 mm × 0.7 mm, 20–70 lamellae.
Meissner’s corpuscle
- Where: the dermal ridges of glabrous (hairless) skin. It is a relatively large receptor.
- Structure: encapsulated, with a single-layered capsule. The myelinated afferent enters it, loses its myelin, and branches into a complex tangle inside.
- Function: rapidly adapting; senses vibration and contact (touch).
Merkel’s disks
- Where: the epidermis of both glabrous and hairy skin. This makes them the most superficial mechanoreceptor.
- Structure:
- the flattened tips of primary afferent axons; the axons are myelinated in the dermis but bare by the time they reach the epidermis;
- each ending branches into flat, cup-shaped disks, each pressed against a specialised cell, the Merkel cell, in a synapse-like contact;
- disk plus Merkel cell is the Merkel apparatus, or Iggo-dome receptor, so called because it raises the epithelium into a dome.
- Function: slowly adapting; senses touch–pressure.
- What the Merkel cells themselves do is uncertain. Their synapse-like contact with the disk suggests that they perform the first step of transduction.
- Worth knowing, though not in your pages: the step is mechano-electrical transduction (mechanical energy in, electrical signal out). Your book writes “electro-mechanical”, which puts the words the wrong way round.
Ruffini endings
- Where: both glabrous and hairy skin. Your book calls them the smallest mechanoreceptors.
- Structure: expanded endings of Aβ fibres. Beyond its myelinated part, the fibre’s bare terminals branch widely into an expanded, complex ending.
- Function: slowly adapting; usually senses crude touch.
- Worth knowing, though not in your pages: other books also credit Ruffini endings with sensing sustained pressure, stretch of the skin and joint rotation.
Krause’s end-bulb
- Where: the dermis, widely spread through the skin.
- Structure: encapsulated. The primary afferent, a myelinated Aβ fibre, enters the capsule and divides into bare branches; the capsule is a ring of modified cells wrapped round them.
- Function: rapidly adapting; senses touch and pressure.
Krause and Ruffini are not "cold" and "warmth" receptors
Worth knowing, though not in your pages: older books called Krause’s end-bulbs cold receptors and Ruffini endings warmth receptors. Your book gives both a touch role, which fits the modern view that temperature is sensed by naked nerve endings. Write touch and pressure for Krause’s, and crude touch for Ruffini.
Golgi-Mazzoni corpuscle
- Where: first found in tendons and muscles, and later in the skin too.
- Structure: encapsulated, with 10–15 lamellae; 150–250 µm across.
- Function: rapidly adapting. Its exact role is not known, but it probably helps to give touch–pressure sensation.
Hair follicle endings
- Afferent endings lie close to the hair follicles. Pressure on a hair distorts them, so bending a hair without touching the skin is enough to be felt as touch.
- There are three kinds of follicle, each with its own type of ending.
| Follicle | Adapts |
|---|---|
| Simple | Rapidly |
| Sinus | Both slowly and rapidly |
| Nonsinus | Slowly |
- Simple hair follicle: no erectile tissue. Many myelinated axons send bare (unmyelinated) terminals to it.
- Sinus hair follicle: belongs to thick hairs with erectile tissue round the base of the follicle, and has a rich nerve supply. It holds both slowly and rapidly adapting endings. These hairs are the vibrissae, or tactile hairs.
- Nonsinus hair follicle: its endings spray out like Ruffini endings. These follicles are mostly in the skin of the face.
Exam-answer skeleton: "Classify sensory receptors, describe the main mechanoreceptors, and say how receptors code the kind and strength of a stimulus" (long essay)
- Define a receptor; sense organ; adequate stimulus.
- Classify by function (extero-, intero-, proprio-, teleceptors), with examples.
- Classify by adequate stimulus (the five types, the three groups of mechanoreceptors, the two nociceptors) and by location.
- Pacinian corpuscle: structure, size, nerve fibre and function; draw it.
- The other mechanoreceptors and hair follicle endings: where each lies, how fast it adapts and what it senses (the comparison table).
- Transduction: receptor potential in the terminal, action potential at the first node of Ranvier (see Receptor potential and properties of receptors).
- Coding the kind of stimulus: specificity, and Müller’s doctrine of specific nerve energy.
- Coding the strength: action potential frequency, receptor recruitment and the Weber–Fechner law; adaptation.
Asked in exams
- Sensory receptors, and how they code what and how strong: Dec 2015, 10 marks