Somatic Sensation (The Bodily Senses)
• Exteroception (perception of external events)
– Touch (active and passive)
– Thermal senses (heat and cold)
– Pain (external or internal events that damage or
harm the body)
• Proprioception (sense of oneself)
– Movements of the limbs
– Posture of the body
• Interoception (function of organ systems)
Dorsal root ganglion neurons mediate somatic sensation
• Detect mechanical, thermal or chemical signals
• Transduce stimulus energy into electrical signals
• Encode depolarization as a spike train
• Transmit encoded information to spinal cord or brainstem
Common properties of DRG neurons
• Sensory transduction occurs in the nerve
endings, not in the DRG or trigeminal cell bodies
• Sensory modality determined by the receptor
class expressed in the nerve terminals
• Distal axons of DRGs form the peripheral nerves
• Each peripheral nerve innervates a specific body
region
Common properties of DRG neurons
• Damage to a peripheral nerve produces
deficits in more than one sensory modality in
a specific body region
• Proximal branch of DRG neuron transmits
somatosensory information to the spinal cord
and brainstem from specific body region
Dorsal root ganglion neurons differ in:
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Receptor morphology and sensitivity
Diverse sensations mediated
Body region innervated
Axon conduction velocity and fiber diameter
Analysis of type(s) of sensory deficits and
localization to specific body region(s) are important
diagnostic tools
• Spinal and brainstem termination sites
• Ascending pathways to higher brain centers
• Sensitivity to neurotrophins during development
Fiber diameter profile for different modalities
The Sense of Touch
Jusepe de Ribera
c. 1615-16
Norton Simon Museum
Pasadena CA
The sense of touch
• Touch is the special sense by which contact with the body is
perceived in the conscious mind
• Touch allows us to recognize objects held in the hand and use
them as tools
• Touch enables the blind to perceive the three dimensional form
of objects, and to read Braille with their fingers
• Tactile information guides the skilled movements of the
surgeon, the sculptor, the musician, the pitcher, or the chef
The sense of touch is mediated by skin indentation
Mechanoreceptors mediate the sense of touch
• Mechanoreceptors in the skin provide information to the
brain about the size, shape, weight, and texture of objects
• They allow us to perceive whether objects appear hard or
soft, large or small, heavy or light in weight, smooth or
rough in texture
• Tactile acuity is highest on the glabrous skin of the hand
• The hand contains ~150,000 mechanoreceptors
innervated by ~25,000 myelinated nerve fibers traveling in
the median, ulnar and superficial radial nerves
Mechanoreceptors detect tissue deformation
(A) Lipid tension
(B) Structural protein linkage
(direct gating)
(C) Indirect structural link to
TRP receptor
Scanning EM of Fingerprints in the Skin
Four types of mechanoreceptors in glabrous skin
Meissner’s corpuscles (RA1)
Merkel cells (SA1)
Ruffini endings (SA2)
Pacinian corpuscles (RA2)
Touch receptors in the glabrous skin
Meissner’s corpuscles border papillary ridges
SA1 fibers innervate clusters of Merkel cells
Distinct innervation patterns for touch receptors
• Receptors in the superficial layers (Meissner’s
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•
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corpuscles or Merkel cells ) are smaller than those
in the deep layers (Pacinian corpuscles and Ruffini
endings)
Individual RA1 and SA1 fibers innervate multiple
Meissner’s corpuscles or Merkel cells (divergence)
Individual Meissner’s corpuscles are innervated by
2-5 RA1 axons (convergence)
Merkel cells within a ridge are innervated by
several SA1 axons
Pacinian corpuscles and Ruffini endings are
innervated by single axons (RA2 and SA2 fibers)
Nerve branch patterns define receptive fields
Receptive fields determine spatial properties
• The receptive field of a sensory neuron
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defines the spatial location where it
responds to stimuli of the appropriate energy
Neurons represent particular sensory spaces
Spatial position of a receptor within the
sense organ localizes the stimulus in space
Where we are touched is coded by which
specific fibers are activated
Receptive fields within a modality differ in
size depending upon function
Cutaneous Receptive Fields
Receptive and perceptive fields coincide
A = RA1 fiber
Tap or vibration
B, C = SA1 fiber
Pressure
Receptors are organized in maps by dermatome
Receptor Density in Skin
• Fingertip
• Palm
– RA = 141 /cm2
– PC = 21
/cm2
SA I = 70 /cm2
SA II = 9
/cm2
– RA = 25 /cm2
SA I = 8 /cm2
– PC = 9 /cm2
SA II =16 /cm2
Receptive fields are smallest on the fingertips
Receptive field size determines spatial resolution
Two-point thresholds are smallest on the hand
Two-point thresholds correlate with receptive field size
Two-Point Discrimination Threshold
• Reflects receptive field (RF) diameters of
Meissner’s corpuscle (RA) and Merkel cell
(SA I) afferents
• RF diameter correlates inversely with
innervation density
• Spatial acuity highest on densely innervated
body regions (fingertips, lips, toes)
• Central body map reflects innervation density
Other tests of spatial acuity of the hand
Spatial Resolution … and Receptive Fields
20 x 20 pixel
60 x 60 pixel
400 x 400 pixel
Why have multiple touch receptors?
• Different receptive field areas encode both fine
and broad spatial information
• Specialize for dynamic and static sensitivity
– Motion sensors
– Pressure sensors
• Different sensory thresholds extend range of
intensities encoded
Slow and rapid adaptation of touch receptors
Neural firing rate codes stimulus intensity
Merkel cells (SA1 fibers) signal shape and pressure
SA1 fibers respond to probe curvature
2.9 mm probe
7.8 mm probe
Goodwin and Wheat, 2004
Mechanoreceptors code surface curvature
Merkel cells (SA1 fibers) are used to read Braille
Ruffini endings (SA2 fibers) signal posture and movements
Ruffini endings (SA2 fibers) respond to skin stretch
Slowly-Adapting Receptor Function
• Merkel cell (SA I):
– Pressure
– Precision grip force
– Small object shape discrimination
– Braille reading and texture discrimination
• Ruffini ending (SA II):
– Whole hand grip
– Hand posture and skin stretch
– Large object shape discrimination
Meissner’s corpuscles sense texture
Meissner’s corpuscles detect motion of a small dot
RA spike trains code vibratory frequency
RA1 and RA2 fibers detect low and high frequencies
RA1 40 Hz
RA2 200 Hz
Tuning curves quantify vibratory threshold
RA1 fibers
Vibration thresholds are frequency dependent
Vibration amplitude not coded by firing rate
2.5
PC1
Spikes per cycle
2
1.5
1
0.5
0
0
50
100
150
200
250
Vibratory amplitude (µm)
300
350
400
Populations code stimulus intensity
2.5
PC1
Spikes per cycle
2
PC2
1.5
1
0.5
0
0
50
100
150
200
250
Vibratory amplitude (µm)
300
350
400
Rapidly-Adapting Receptor Function
• Meissner’s corpuscle (RA):
– Motion
– Texture
– Edges
– Flutter (low-frequency vibration)
• Pacinian corpuscle (PC):
– Vibration (tool use)
– Contact and release
Threshold diversity extends dynamic range
Touch receptor thresholds differ
Active and passive touch use the same receptors
• Active touch (touching: motor behavior)
– Subject controls body contact with external
objects or persons
– Stroke, tap, grasp, press, manipulate
• Passive touch (being touched)
– Response to an external stimulus
– Physical examination (describe sensation)
– Name objects
Mechanoreceptors Sense Hand Actions
Object properties modify grip and load forces
Mechanoreceptor response properties
DRGs Develop From Neural Crest Cells
Neurotrophins Determine DRG Survival
Neurotrophins Determine Receptor Type
DRGs Express Neurotrophin Receptors
• trkA: free nerve endings (pain and temperature)
• trkB: cutaneous mechanoreceptors
• trkC: muscle spindles and tendon organs
Neurological Tests of Touch
• Simple tactile tests
– Detection thresholds
– Point localization
– Vibration sense
– Two-point discrimination
• Complex tactile recognition
– Texture discrimination (rough-smooth)
– Grating orientation (horizontal-vertical)
– Stereognosis (object recognition by touch)
Somatosensory modalities
• The somatosensory system codes five
major sensory modalities:
1. Discriminative touch
2. Proprioception (body position and motion)
3. Nociception (pain and itch)
4. Temperature
5. Visceral function
• Senses external objects contacting the body
• Provides self-awareness of our bodies