Somatic Sensation (The Bodily Senses)
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Transcript of Somatic Sensation (The Bodily Senses)
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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)
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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
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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
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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
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Dorsal root ganglion neurons differ in:
• 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
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Fiber diameter profile for different modalities
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The Sense of Touch
Jusepe de Ribera c. 1615-16
Norton Simon Museum Pasadena CA
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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
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The sense of touch is mediated by skin indentation
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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
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Mechanoreceptors detect tissue deformation(A) Lipid tension
(B) Structural protein linkage
(direct gating)
(C) Indirect structural link to TRP receptor
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Scanning EM of Fingerprints in the Skin
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Four types of mechanoreceptors in glabrous skin
Meissner’s corpuscles (RA1)
Merkel cells (SA1)
Ruffini endings (SA2)
Pacinian corpuscles (RA2)
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Touch receptors in the glabrous skin
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Meissner’s corpuscles border papillary ridges
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SA1 fibers innervate clusters of Merkel cells
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Distinct innervation patterns for touch receptors • Receptors in the superficial layers (Meissner’s
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)
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Nerve branch patterns define receptive fields
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Receptive fields determine spatial properties
• The receptive field of a sensory neuron 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
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Cutaneous Receptive Fields
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Receptive and perceptive fields coincide
A = RA1 fiber Tap or vibration
B, C = SA1 fiberPressure
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Receptors are organized in maps by dermatome
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Receptor Density in Skin
• Fingertip– RA = 141 /cm2 SA I = 70 /cm2
– PC = 21 /cm2 SA II = 9 /cm2
• Palm– RA = 25 /cm2 SA I = 8 /cm2
– PC = 9 /cm2 SA II =16 /cm2
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Receptive fields are smallest on the fingertips
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Receptive field size determines spatial resolution
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Two-point thresholds are smallest on the hand
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Two-point thresholds correlate with receptive field size
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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
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Other tests of spatial acuity of the hand
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Spatial Resolution … and Receptive Fields
20 x 20 pixel 60 x 60 pixel 400 x 400 pixel
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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
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Slow and rapid adaptation of touch receptors
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Neural firing rate codes stimulus intensity
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Merkel cells (SA1 fibers) signal shape and pressure
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SA1 fibers respond to probe curvature
2.9 mm probe 7.8 mm probe
Goodwin and Wheat, 2004
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Mechanoreceptors code surface curvature
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Merkel cells (SA1 fibers) are used to read Braille
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Ruffini endings (SA2 fibers) signal posture and movements
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Ruffini endings (SA2 fibers) respond to skin stretch
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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
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Meissner’s corpuscles sense texture
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Meissner’s corpuscles detect motion of a small dot
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RA spike trains code vibratory frequency
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RA1 and RA2 fibers detect low and high frequencies
RA1 40 Hz
RA2 200 Hz
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Tuning curves quantify vibratory threshold
RA1 fibers
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Vibration thresholds are frequency dependent
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Vibration amplitude not coded by firing rate
0
0.5
1
1.5
2
2.5
0 50 100 150 200 250 300 350 400Vibratory amplitude (µm)
PC1
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Populations code stimulus intensity
0
0.5
1
1.5
2
2.5
0 50 100 150 200 250 300 350 400Vibratory amplitude (µm)
PC1
PC2
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Rapidly-Adapting Receptor Function
• Meissner’s corpuscle (RA): – Motion
– Texture
– Edges
– Flutter (low-frequency vibration)
• Pacinian corpuscle (PC): – Vibration (tool use)
– Contact and release
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Threshold diversity extends dynamic range
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Touch receptor thresholds differ
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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
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Mechanoreceptors Sense Hand Actions
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Object properties modify grip and load forces
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Mechanoreceptor response properties
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DRGs Develop From Neural Crest Cells
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Neurotrophins Determine DRG Survival
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Neurotrophins Determine Receptor Type
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DRGs Express Neurotrophin Receptors
• trkA: free nerve endings (pain and temperature)
• trkB: cutaneous mechanoreceptors
• trkC: muscle spindles and tendon organs
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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)
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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