Table of Contents
Overview of Skin Senses
Skin senses (also called somatosensory or cutaneous senses) are all forms of sensation that originate from receptors in the skin and underlying tissues. They inform the nervous system about:
- Mechanical influences on the skin (touch, pressure, vibration, stretch)
- Potentially damaging stimuli (pain)
- Temperature changes (heat and cold)
- The position and movement of body parts (through deep receptors in muscles, tendons, joints – closely connected to skin sense, but usually grouped as “proprioception”)
Together, these senses are crucial for protection, orientation in space, and fine control of movements.
Structure of the Skin in Relation to Senses
For skin senses, only the aspects of skin structure that matter for sensation are important:
- Epidermis (outer layer)
- Contains some free nerve endings (important for pain and temperature).
- No blood vessels.
- Dermis (corium, middle layer)
- Richly supplied with blood vessels and nerve endings.
- Contains many specialized sensory receptors for touch and pressure.
- Subcutis (subcutaneous tissue, deeper layer)
- Mainly fat and connective tissue.
- Contains receptors for deep pressure and vibration.
Sensory nerve fibers enter from below and end in the various receptors distributed across these layers.
Types of Sensory Receptors in the Skin
Skin senses depend on different receptor types. These can be grouped in several ways.
By Stimulus Type
- Mechanoreceptors – respond to mechanical deformation: touch, pressure, stretch, vibration.
- Thermoreceptors – respond to temperature changes.
- Nociceptors – respond to potentially damaging stimuli; basis of pain sensation.
By Adaptation Speed
- Rapidly adapting (phasic) receptors
- React strongly when a stimulus begins or ends.
- Signal changes (onset/offset, movement over skin, vibration).
- Poor at signaling constant, unchanging pressure.
- Slowly adapting (tonic) receptors
- Continue to fire during a constant stimulus.
- Provide information about ongoing pressure or stretch.
Important Receptor Types (Examples in Human Hairy and Glabrous Skin)
Names are less important than the ideas that different receptors are specialized for particular aspects of touch and pressure:
- Merkel cells / Merkel disks (slowly adapting)
- Near the boundary between epidermis and dermis.
- Small receptive fields (respond to stimuli in a very small area).
- Fine discrimination of shape, edges, and texture at low pressure (e.g., reading Braille).
- Meissner’s corpuscles (rapidly adapting)
- Located just under the epidermis, especially in hairless (glabrous) skin such as fingertips, lips.
- Small receptive fields.
- Sensitive to light touch and low-frequency vibration; important for detecting slipping objects.
- Ruffini endings (slowly adapting)
- In deeper dermis.
- Large receptive fields.
- Respond to skin stretch; help with detecting joint movements and hand shape.
- Pacinian corpuscles (rapidly adapting)
- Deep in dermis and subcutis.
- Very large receptive fields.
- Extremely sensitive to high-frequency vibration and deep pressure.
- Hair follicle receptors
- Wrapped around hair roots.
- Respond when hairs are moved.
- Important for detecting light touch and air movements over the skin.
- Free nerve endings
- Branching, unencapsulated endings in epidermis and superficial dermis.
- Many serve as nociceptors and thermoreceptors.
Each receptor type is connected to specific types of nerve fibers, which determine the speed and character of the transmitted signal.
Touch, Pressure, and Vibration
Touch (Tactile) Sensation
Touch sensations arise mainly from mechanoreceptors:
- Light touch, fine discrimination
- Detected especially by Merkel cells and Meissner’s corpuscles.
- These are dense in regions used for fine manipulation: fingertips, lips, tongue, face.
- Very small receptive fields here allow us to distinguish two closely spaced points – this is called tactile acuity.
- Two-point discrimination
- Minimum distance at which two separate touches are perceived as two, not one.
- Very small on fingertips; very large on the back.
- Reflects density and receptive field size of receptors and their neural processing.
Pressure
- Gentle continuous pressure
- Mainly detected by slowly adapting receptors (Merkel, Ruffini).
- Signal the “presence” and magnitude of pressure, not just its onset.
- Deep pressure
- Activates deeper receptors (Pacinian corpuscles, Ruffini endings).
- Important for sensing how strongly an object is grasped.
Vibration
- Pacinian corpuscles and Meissner’s corpuscles are especially sensitive to vibration:
- Low-frequency vibration (e.g., roughness when a finger is moved over a textured surface) – Meissner.
- High-frequency vibration (e.g., tool use, electric motors) – Pacinian.
- Vibration sensing helps detect surface textures and tool-mediated contact (feeling with a tool as if it were an extension of the hand).
Temperature Senses (Thermoreception)
Thermoreception in the skin is handled by free nerve endings:
- Cold receptors
- More numerous than warm receptors in many skin areas.
- Activated by cooling in a certain temperature range (roughly from just above painful cold to neutral skin temperature).
- Warm receptors
- Activated by warming over a specific range (below painful heat).
- Thermal “spots”
- Cold and warm receptors are not distributed uniformly; some points on the skin are particularly sensitive to cold, others to warmth.
- Explains why some tiny skin areas can feel temperature change more clearly than neighboring areas.
- Adaptation
- Thermoreceptors adapt: a constant temperature leads to reduced firing.
- The system is especially good at detecting changes (cooling or warming).
Extreme temperatures (too cold, too hot) strongly activate nociceptors, which are responsible for pain.
Pain Senses (Nociception)
Pain is not only a skin sense but the skin is densely supplied with nociceptors:
Nociceptors
- Most are free nerve endings in skin and deeper tissues.
- Different nociceptors respond preferentially to:
- Mechanical damage (e.g., pinch, cut).
- Extreme temperatures (heat or cold beyond safe range).
- Chemical signals (from damaged cells or inflammatory mediators).
Fast and Slow Pain
Different nerve fibers carry pain signals:
- Fast (first) pain
- Conducted by relatively fast, thinly myelinated fibers.
- Sharp, well-localized, pricking pain.
- Warns quickly about immediate danger (e.g., touching a hot stove).
- Slow (second) pain
- Conducted by very slow, unmyelinated fibers.
- Dull, throbbing, burning, or aching pain.
- Often poorly localized.
- Associated with tissue damage and inflammation.
Protective Role of Pain
- Pain leads to rapid protective reactions (withdrawal reflex, avoidance).
- It also motivates rest and protection of injured body parts.
- Absence of pain perception (rare inherited conditions) leads to repeated injuries and can be life-threatening.
Spatial Representation: Dermatomes and Receptive Fields
Receptive Fields in the Skin
- Each sensory neuron receives input from a limited skin area – its receptive field.
- Small receptive fields → high spatial resolution (e.g., fingertips).
- Large receptive fields → low spatial resolution (e.g., back).
Overlapping receptive fields and central processing allow the brain to:
- Localize stimuli on the body surface.
- Distinguish complex patterns (e.g., shapes, letters drawn on the skin).
Dermatomes
- The body surface is organized into dermatomes:
- Skin areas innervated mainly by sensory fibers of a single spinal nerve segment.
- Dermatomes are important for medical diagnosis:
- Damage to a spinal nerve or spinal cord segment leads to sensory loss in the corresponding dermatome.
- Patterns of pain or numbness can indicate where a lesion is located.
Differences Between Body Regions
Skin senses are not the same everywhere:
- High-resolution areas
- Fingertips, lips, tongue, parts of the face.
- very high receptor density,
- small receptive fields,
- large representation in the brain’s sensory cortex.
- Low-resolution areas
- Back, upper arms, thighs.
- fewer receptors, larger receptive fields.
Functional consequences:
- Hands and face are highly specialized for exploration, manipulation, and communication (e.g., touch, facial expressions).
- Trunk skin is more for protective and general sensory functions.
This uneven representation is reflected in the sensory homunculus (disproportionately large hands and face compared to trunk), but the detailed processing of that belongs to broader nervous system chapters.
Integration of Skin Senses in Perception and Behavior
Skin receptors provide raw information; the nervous system integrates this:
- Combining signals from mechanoreceptors, thermoreceptors, and nociceptors allows:
- Recognition of object properties (hard/soft, smooth/rough, warm/cold, painful/safe).
- Adjustment of grip force and movement during handling.
- Interactions with reflex pathways:
- Painful or extreme temperature stimuli trigger rapid withdrawal before conscious perception is fully formed.
- Emotional and social aspects:
- Gentle touch (e.g., stroking, holding) can modulate stress, bonding, and well-being.
- Certain low-threshold, slowly conducting skin fibers are especially active during pleasant social touch, connecting skin senses to emotional centers in the brain.
Skin senses therefore link the body surface not only with protective reflexes and movement control, but also with higher-level perception, emotion, and social behavior.