Physiology chapters
Sensory receptors and somatic sensation
How receptors turn a stimulus into nerve signals, how intensity and type are coded, and how touch, vibration and position sense reach the cortex through the dorsal column and the thalamus.
- Each receptor responds best to one kind of stimulus and turns it into a receptor (generator) potential; bigger stimuli give more frequent action potentials.
- Receptors are rapidly adapting (Pacinian, Meissner: change and vibration) or slowly adapting (Merkel, Ruffini, muscle spindles: steady pressure and position).
- Touch, vibration and position travel in the dorsal column-medial lemniscus system to the VPL nucleus of the thalamus and on to the postcentral gyrus.
On this page
Overview
Sensation begins at a receptor, a structure that turns a stimulus (touch, stretch, heat, light, chemicals) into an electrical signal.
That signal travels along a sensory pathway to the thalamus and then to the cerebral cortex, where we become aware of it.
This page covers how receptors work, how the brain codes the type and strength of a stimulus, and the pathway for touch, vibration and position sense. Pain and temperature have their own page: pain and temperature.
Types of receptor
Receptors are grouped by the stimulus they respond to best.
| Type | Stimulus | Examples |
|---|---|---|
| Mechanoreceptors | Touch, pressure, stretch, vibration, sound | Skin receptors, muscle spindles, hair cells of the ear |
| Thermoreceptors | Warmth and cold | Free nerve endings |
| Nociceptors | Tissue damage (pain) | Free nerve endings |
| Chemoreceptors | Chemicals | Taste buds, smell, carotid body |
| Photoreceptors (electromagnetic) | Light | Rods and cones |
Each receptor is most sensitive to one type of energy, its adequate stimulus. A strong enough stimulus of another type can still fire it: pressing on the eye makes you "see" light.
The receptor potential
When the stimulus acts, ion channels in the receptor membrane open. The membrane depolarises a little. This is the receptor (generator) potential.
- It is graded: a stronger stimulus gives a bigger potential.
- It is local and does not spread far.
- When it reaches threshold at the first node of the nerve fibre, it starts action potentials.
The receptor potential is graded; the action potential is all-or-none. The stronger the receptor potential, the higher the frequency of action potentials.
Adaptation
When a steady stimulus continues, most receptors fire less and less. This is adaptation.
| Adaptation | Signals | Examples |
|---|---|---|
| Rapidly adapting (phasic) | Change and movement; vibration | Pacinian corpuscle, Meissner corpuscle, hair follicle receptors |
| Slowly adapting (tonic) | Ongoing state: steady pressure, position, pain | Merkel discs, Ruffini endings, muscle spindles, joint receptors, nociceptors |
The Pacinian corpuscle adapts within a fraction of a second because its onion-like layers of capsule soak up steady pressure. It is ideal for vibration.
Pain receptors adapt very little. This keeps warning us while the damage continues.
Sensory coding
The brain needs to know four things about a stimulus.
- Type (modality): decided by which pathway is stimulated, not by the stimulus itself. This is the labelled line principle. A pain fibre always gives pain, however it is stimulated.
- Strength (intensity): coded by the frequency of action potentials and by the number of receptors activated (recruitment).
- Location: decided by which receptors fire and their receptive fields. Small receptive fields (fingertips, lips) give precise localisation.
- Duration: shown by how long the receptors keep firing, and limited by adaptation.
Two-point discrimination
This is the smallest distance at which two touching points are felt as two. It is smallest on the fingertips and lips and largest on the back. It depends on the density of receptors and the size of their receptive fields, and it is carried in the dorsal columns.
Somatic sensations: touch, pressure, vibration and position
| Receptor | Location | Senses | Adaptation |
|---|---|---|---|
| Meissner corpuscle | Hairless skin (fingertips, lips) | Light touch, low-frequency vibration | Rapid |
| Pacinian corpuscle | Deep skin, around joints | High-frequency vibration, deep pressure | Very rapid |
| Merkel disc | Base of the epidermis | Steady touch, texture | Slow |
| Ruffini ending | Deep skin, joint capsules | Stretch of the skin, steady pressure | Slow |
| Muscle spindle and Golgi tendon organ | Muscles and tendons | Position and movement (proprioception) | Slow |
Proprioception is the sense of the position and movement of the body. It comes from muscle spindles, Golgi tendon organs and joint receptors. See spinal reflexes.
Nerve fibre types
| Fibre | Myelin | Speed | Carries |
|---|---|---|---|
| Aα (Ia, Ib) | Thick | Fastest (about 80–120 m/s) | Muscle spindle and tendon organ |
| Aβ (II) | Thick | Fast | Touch, vibration, pressure |
| Aδ (III) | Thin | Medium | Sharp (fast) pain, cold |
| C (IV) | None | Slowest (about 0.5–2 m/s) | Slow (burning) pain, warmth, itch |
Aβ fibres carry touch and vibration; Aδ carry fast pain; C fibres carry slow pain.
The dorsal column-medial lemniscus pathway
This pathway carries fine touch, vibration, two-point discrimination and conscious proprioception.
- First-order neuron: cell body in the dorsal root ganglion. The fibre enters the cord and ascends on the same side in the dorsal column (gracile for the legs, cuneate for the arms).
- Second-order neuron: in the nucleus gracilis or cuneatus of the medulla. Its fibre crosses in the medulla and rises as the medial lemniscus.
- Third-order neuron: in the ventral posterolateral (VPL) nucleus of the thalamus. It projects to the primary somatosensory cortex (postcentral gyrus, areas 3, 1, 2).
The pathway keeps a precise map of the body (somatotopic organisation) all the way to the cortex.
Touch from the face relays in the ventral posteromedial (VPM) nucleus via the trigeminal pathway, not the VPL.
The thalamus and sensory cortex
The thalamus is the relay station for almost all sensation (except smell). It sorts signals and sends them to the right part of the cortex. Crude awareness of pain and temperature is thought to begin at the thalamic level.
The primary somatosensory cortex (postcentral gyrus) works out exact location, fine detail and intensity. The association areas behind it (areas 5 and 7) put sensations together to recognise objects by touch (stereognosis).
Clinical relevance
Dorsal column loss
How it happens: vitamin B12 deficiency, tabes dorsalis, or a cord lesion.
What you see: loss of vibration and position sense; an unsteady gait that worsens in the dark; a positive Romberg test (falls with eyes closed).
Thalamic lesion
How it happens: a small stroke in the thalamus.
What you see: loss of sensation on the opposite side of the body; later, some patients develop severe burning pain on that side (thalamic pain syndrome).
Parietal cortex lesion
How it happens: a stroke affecting the sensory association areas.
What you see: astereognosis: the patient cannot recognise an object placed in the hand, even though basic touch is felt.
Questions test the adaptation of each receptor (Pacinian for vibration), what codes intensity (frequency) and type (labelled line), the fibre types, the three neurons of the dorsal column pathway and where they cross, and VPL vs VPM.
Quick revision
- Receptor potential: graded and local; action potential: all-or-none.
- Intensity: frequency of firing and number of receptors. Type: labelled line.
- Rapid adaptation: Pacinian, Meissner. Slow: Merkel, Ruffini, spindles, nociceptors.
- Pacinian corpuscle: high-frequency vibration.
- Aβ: touch. Aδ: fast pain. C: slow pain.
- Dorsal column: crosses in the medulla; relays in VPL; ends in the postcentral gyrus.
Written by The House Job from standard physiology references (Guyton and Hall Textbook of Medical Physiology; BRS Physiology).
FAQ
What is a receptor potential?
A graded change in membrane potential in a receptor caused by a stimulus. If it reaches threshold, it triggers action potentials in the sensory nerve.
Which receptor detects vibration?
The Pacinian corpuscle, which adapts very rapidly and responds best to high-frequency vibration.
How is stimulus intensity coded?
By the frequency of action potentials and the number of receptors activated.
Where does the dorsal column pathway cross?
In the medulla, after synapsing in the nucleus gracilis and cuneatus.