Neural sensations
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Here is a comprehensive overview of sensory receptors and neural sensory modalities, drawn from Costanzo Physiology, Guyton & Hall, and Junqueira's Histology.
Types of Sensory Receptors
Receptors are classified by the stimulus that activates them. There are five fundamental classes:
| Type | Modality | Receptor Example | Location |
|---|---|---|---|
| Mechanoreceptors | Touch, audition, vestibular, blood pressure | Pacinian corpuscle, Meissner corpuscle, hair cells | Skin, organ of Corti, semicircular canals, carotid sinus |
| Photoreceptors | Vision | Rods and cones | Retina |
| Chemoreceptors | Olfaction, taste, O₂/CO₂ sensing | Olfactory receptors, taste buds, carotid body | Olfactory mucosa, tongue, carotid/aortic bodies |
| Thermoreceptors | Temperature | Cold receptors, warm receptors | Skin |
| Nociceptors | Pain, extreme temperature/pressure | Thermal nociceptors, polymodal nociceptors | Skin, deep tissues |
— Costanzo Physiology 7th Edition, p. 82
Cutaneous Receptors in Detail
The skin hosts both unencapsulated and encapsulated receptors:
Unencapsulated
- Free nerve endings — detect pain (nociception), temperature, and itch (pruritus); extend from papillary dermis into lower epidermal layers
- Merkel (tactile) cells — tonic receptors for sustained light touch and texture discrimination
- Root hair plexuses — detect hair movement
Encapsulated (all phasic mechanoreceptors)
- Meissner corpuscles — elliptical structures (30–75 × 50–150 µm) in dermal papillae; respond to light touch and low-frequency stimuli; most dense in fingertips and palms
- Pacinian (lamellated) corpuscles — large oval structures (~1 mm) deep in reticular dermis/subcutaneous tissue; detect pressure, coarse touch, and vibration
Below is a histological cross-section of both encapsulated receptors:
(a) Meissner tactile corpuscle (TC) in dermal papilla (DP), surrounded by epidermis (E) — detects light touch. (b) Pacinian corpuscle (PC) with its distinctive concentric lamellae among adipose tissue (A) — detects pressure/vibration. — Junqueira's Basic Histology 17e, p. 923–924
Sensory Transduction & Receptor Potentials
When a stimulus activates a receptor:
- The stimulus causes a change in ion channel permeability in the receptor membrane
- Ion flow creates a graded change in membrane potential — the receptor potential (also called generator potential)
- If the receptor potential is depolarizing and reaches threshold, an action potential fires in the sensory neuron
— Costanzo Physiology 7th Edition, p. 83
Receptor Adaptation: Phasic vs. Tonic
Receptors differ in how long they continue to fire during a sustained stimulus:
| Type | Also called | Behaviour | Examples |
|---|---|---|---|
| Phasic (rapidly adapting) | Rate / movement receptors | Fire at onset (and offset) of stimulus; go silent during sustained pressure | Pacinian corpuscles, Meissner corpuscles |
| Tonic (slowly adapting) | Static receptors | Fire continuously throughout the stimulus | Pain receptors, muscle spindles, Golgi tendon organs, baroreceptors, chemoreceptors |
The Pacinian corpuscle adapts by two mechanisms: (1) redistribution of fluid within its viscoelastic capsule, and (2) sodium channel inactivation (accommodation) in the nerve fiber itself. — Guyton & Hall Textbook of Medical Physiology, p. 586
Sensory Coding
The nervous system encodes several stimulus features simultaneously:
- Modality — via labeled lines: dedicated neural pathways for each modality (e.g., the visual pathway only responds to photons, never to touch or sound)
- Location — encoded by receptive fields; smaller receptive fields = finer spatial discrimination. Enhanced by lateral inhibition (surround inhibition sharpens stimulus boundaries)
- Intensity — encoded by: (1) number of receptors activated, (2) firing rate of neurons, and (3) recruitment of additional neural units
- Threshold — the minimum stimulus strength needed to generate a suprathreshold receptor potential
- Duration — reflected in sustained firing of tonic receptors
Sensory Pathway Overview
Sensory signals travel through a chain of neurons to the cortex:
- 1st-order neuron: sensory receptor → spinal cord or brainstem relay nucleus
- 2nd-order neuron: relay nucleus → thalamus (most pathways cross the midline here or earlier)
- 3rd-order neuron: thalamus → primary sensory cortex
- 4th-order neurons: in specialized areas (e.g., primary auditory/visual cortex)
Higher-order association areas integrate complex sensory information across modalities.
— Costanzo Physiology 7th Edition, p. 81–84; Guyton & Hall, p. 585–587; Junqueira's Basic Histology 17e, p. 922–924
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