Somatosensation + Pain - cutaneous receptors

Structural components of sense organs which may play an important role in:
- protection
- conduction
- concentration
- analysis
- sensitisation
- inhibition
BUT not directly involved in transduction processes

Sense of limb positions

Sense of limb movement

Touch
Temperature
Pain
Body position

Epidermis and dermis

Mechanoreceptors
Thermoreceptors
Nociceptores

Mechanical stimulation
Produce depolarising receptor potential
Type of stimulation they respond to best depends on the accessory structures

Specific topographic location of sensory info assigned to a sensory neuron
Each receptor responds only to stimulation within its receptive field
Density of receptors in given part of body determines how well sensory system can resolve the detail of stimulation in that area

Using a fine stimulus probe
Recordings are made from single sensory axons

Slowly adapting type 1 afferents (SAI)
Rapidly adapting afferents (RAI)
Rapidly adapting Pacinian corpuscles (PC)
Slowly adapting type 2 afferents (SAII)

End in Merkel cells
Give tonic response to steady stimulus

End in Meissner's corpuscles
Respond to onset of stimulus

End in Ruffini endings
Give tonic response to steady stimulus

Glabrous (hairless) skin of hand

Receptor treats each period of the waveform as a new stimulus

Onset of stimulus
Sinusoidal mechanical stimuli with single AP for each cycle

Number of sensory fibres that are active
NOT the frequency of firing

Amplitude of vibration

Small receptive field
Several hotspots
Branched innervation
Meissner's corpuscles
Merkel cells

Large receptive field
Single hotspot
1:1 innervation
Pacianc corpuscle
Ruffini endings

Rapidly adapting
Deep receptor
Fire at onset and offset of stimulus
Best responds to 300Hz vibration
Help to perceive events through an object held in the hand

20-70 concentric layers of cellular membrane (lamellae) alternating with fluid-filled spaces, with a terminal in the middle

Distributed widely: connective tissue in muscles, periosteum of bones and mesentery of abdomen
~350 in finger and ~800 in palm

RFs have central zone of maximal sensitivty surrounded by large continuous surface on fingers/palm

Respond to onset and offset of a step pulse
Receptor potential usually increases then decays (adapts) rapidly (in response to onset and offset)
De-sheathed nerve endings (no lamellae) = more prolongued responses (slowly adapting)

Rapidly adapting
Surface receptor
Branched innervation - up to 20 converge to form a single afferent
Density ~150/cm2
Most sensitive to low frequency vibrations (10-50 Hz)
Can respond to skin indenetations of <10micrometers

Small RFs with several hotspots
Fingertips: 2-3mm in diameter
Palm: 10mm in diameter

Minimum intensity of stimulation required to generate AP in nerve
Eg. increase skin indentation until AP is generated to find threshold

Change responsiveness of:
RAI
RAII

Solid line = threshold of human vibration sensitivity
Higher dashed line = sensitivity function of Meissner's corpuscles (RAI)
Dashed line with large dip = sensitivity function of Pacinian corpuscle (RAII)

Reduces effectiveness of Meissner's corpuscles because they lie close to the surface

Reduces effectiveness of PC

Reduces effectiveness of Meissner's corpuscles

Slowly adapting
Superficial receptor
Single afferent nerve fibre branches to innervate up to 90 endings
Provide sharpest resolution of spatial pattern
Sensitive to points, edges, curvature
Can resolve spatial detail of 0.5mm
10 times more sensitive to dynamic stimuli
Spike discharge largely invariant (very good at discrimination)

Oval-shaped mechanoreceptors
Associated with SAI somatosensory nerve fibres
Not covered (shelled) so respond to pressures over long periods

Densely present in fingertips

Small, highly localised RFs

Mechanosensitive channel called Piezo2
Pressure opens channel and depolarises cell
NET released by cell excites nerve terminal

SAI nerve terminal

SAI nerve terminals responsible for rapid and dynamic response to mechanical stimuli
Merkel cells responsible for sustained response

Merkel cells usually respond to touch with inward current (in mice)
This is abolished when Piezo2 is knocked out - no sustained response

Perceived stimulus magnitude

Dorsal root ganglion
Abeta, Adelta and C-fibre low threshold mechanoreceptors

Slowly adapting
Deep receptor
Respond to most skin stretching in fingers
Important for generating awareness of finger and hand position
Involved in perception of object motion and shape of grasped objects

Large field with central zone of maximal sensitivity
Depending on location, excited by stretch of skin in specific directions (arrows)

Most numerous receptors
Distributed preferentially on distal half of fingertips

Less common
Distributed more uniformly on hand
Little differentiation on distal and proximal regions

Fingertips

120/cm2

50/cm2

Smallest discriminable distance between 2 points of contact

RF size and desnity of mechanoreceptors varies

Hands and fingers > back

Compass test devised by Weber

Hand immobilised
Receptive field of single receptor on finger stimulated by rotating drum with raised dots (braille patterns)
Dots cross medial, central and lateral portions of RF
Spaital event plots used to represent sequential APs discharged by individual receptors

Merkel cell
Meissner cell
Ruffini endings
Pascinian corpuscle

Able to differentiate between dots and blank space when spacing of dots exceeds RF diameter

RFs larger than the dot spacing

Unencapsulated
Do not end on/near specialised tissue

Thermoreceptors
Nociceptors

Simultaneous activation of many receptors - spatial summation

Dermis
Skeletal muscles
Liver
Hypothalamus

Trpv1
Trpm8

Ca2+ and Na+
Channel activated by hot temp (>43C) and capsaicin
In all mammals but not birds

Respond to menthol and cold temp (<28C)

Sub-population of C-fibres

Adelta and C-fibres

1

Occurs because cold receptors also express Trpv1
Happens when heat (>45C) is applied to small regions of the skin to a single cold spot
Occurs because activity in cold fibre experienced as cold regardless of the physical nature of the stimulus - example of labelled line coding

Modulate firing as a function of temp
At constant temp = tonic discharges
Cold and warm receptors fire continuously at low rates when skin is at normal temp of 34C
Firing rate does not increase/decrease monotonically if gradually warmed/cooled
Each thermal receptor class has peak firing at preferred skin temp
Code for skin temp involves comparing relative activity of different populations of thermal receptors and nociceptors

Mechanical
Thermal
Polymodal

First, sharp pain
Second, dull, burning pain

Block C-fibres

Block Adelta fibres

Requires strong, painful tactile stimuli to respond (eg. pinch)
Excited by sharp objects that penetrate/squeeze/pinch skin
Mediate sensations of sharp/pricking pain
Firing rates increase with destructiveness of mechanical stimuli

Excited by temp extremes and strong mechanical stimuli
One group excited by noxious heat, other by noxious cold

Respond to destructive mechanical, thermal and chemical stimuli
Activated by noxious mechanical stimuli (eg. pinch/puncture), noxious heat/cold and by irritant chemicals applied to skin
Stimulation results in slow, burning pain
Provide major sensory innervation of the tooth pulp

C-fibre tactile afferents

Only in hairy skin

As stroke velocity incresaes, there is a non-linear relationship with AP firing
Similar non-linear relationship when pps asked to rate pleasantness as stroke velocity increases
Positive correlation between CT output and pleasantness of stimulation