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:

• 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

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 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 fiberPressure

Receptors are organized in maps by dermatome

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

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

0

0.5

1

1.5

2

2.5

0 50 100 150 200 250 300 350 400Vibratory amplitude (µm)

PC1

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

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