A Touching Experience: Somatosensation

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Overview

I. Peripheral encoding in 2 pathways

II. Central Nervous System encoding in 2 pathways

III. Cortex: Basic organizational principles

IV. Cortex: Plasticity & its perceptual implications

Head lesioned the nerve in his own hand to feel how sensation returned as thenerve recovered. Found there were 2distinct recovery phases:

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Protopathic --> Temperature and Pain

Epicritic --> Non-painful touch, spatially and temporally

precise sensations

Henry Head: The Vincent Van Gogh of Somatosensory Neuroscience

Distinct Pathways :Peripheral Encoding of

Fine Temporal & Spatial Info* Defining features of receptive field properties: Firing propert

ies & receptive field size

* Slowly adapting vs rapidly

adapting: Distinction?

* Why is this distinction needed?

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0."

Peripheral Receptors:Slowly Adapting (SA), Merkel’s

* Merkel cells/disks, SA Type I-> Dense innervation of skin (100/cm2)-> Respond with SA discharge, sensitive to edges,

points and surface features-> Spatial rf size: 2-3 mm2* Perceptual abilitieswe need these for?‘2-Point Discrimination’

Images removed due to copyright reasons. Please see "Squire, Larry R., ed. Fundamental Neuroscience.2nd ed. Boston, MA: Academic Press, 2003, Figures 25-1 and 25-5. ISBN: 0126603030."

Peripheral Receptors:Slowly Adapting, Ruffini

* SA Type II (Ruffini corpuscles?)-> Only in humans-> 2-4 x less sensitive than SAI-> Respond to skin stretch

* Perceptual abilitieswe need these for?.

Image removed due to copyright reasons.Please see "Squire, Larry R., ed. Fundamental Neuroscience.2nd ed. Boston, MA: Academic Press, 2003, Figure 25-5. ISBN: 0126603030."

Peripheral Receptors: Rapidly Adapting/Meissner’s

Meissner’sCorpuscle

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Peripheral Receptors: Pacinian Corpuscles

Pacinian Corpuscle

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Peripheral Receptors: Anatomical/Structual Determinants

of Functional Capability

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Structural Determinants of Displacement Sensitivity & Spatial/

Temporal Specificity

Images removed due to copyright reasons.Please see "Squire, Larry R., ed. Fundamental Neuroscience.2nd ed. Boston, MA: Academic Press, 2003, Figure 25-5. ISBN: 0126603030."

Structural Determinants ofDisplacement Sensitivity &

Spatial/Temporal Specificity

Image removed due to copyright reasons.Please see "Squire, Larry R., ed. Fundamental Neuroscience.2nd ed. Boston, MA: Academic Press, 2003. ISBN: 0126603030."

Distinct Pathways :Peripheral Encoding of

Pain &Temperature

Paradoxical Cold Hot stimuli applied to a very discrete point

on the skin can be perceived as cold in some cases…Why?

Distinct Pathways : Perception is not only a

‘Labeled Line’

• What is a labeled line?

• The Coca-Cola Illusion…

OverviewI. Peripheral encoding in 2 pathways

II. Central Nervous System encoding in 2 pathways

III. Cortex: Basic organizational principles

IV. Cortex: Plasticity & its perceptual implications

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PNS vs CNS: Dorsal Root

* PseudounipolarNeuron

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* Dorsal RootGanlgion* PNS/CNSdivision point

Dermatomes

* Dorsal roots cover a distinct body region* 4 distinct ascending segments of the cord

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Entoumbences: Why?

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Image removed due to copyright reasons.Please see "Squire, Larry R., ed.Fundamental Neuroscience.2nd ed. Boston, MA: Academic Press,2003, Figure 25-9. ISBN: 0126603030."

Spinal Cord:Segregation of Gray & White Matter

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Segregation of Pathways forTouch & Pain

* Dorsal Column-Medial Lemniscus Pathway (DCML)& Anterolateral Pathway (AL)

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Spinal Cord:Segregation of Pathways for

Touch & Pain * Dorsal Column-Medial Lemniscus Pathway (DCML)

Image removed due to copyright reasons.Please see "Squire, Larry R., ed.Fundamental Neuroscience. 2nd ed. Boston, MA: Academic Press, 2003, Figure 25-11.ISBN: 0126603030."

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Location of DCN & Medial Lemniscus

* DC projections synapse in thedorsal column nuclei

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Spinal Cord:Segregation of Pathways for

Touch & Pain

Image removed due to copyright reasons.Please see "Squire, Larry R., ed. Fundamental Neuroscience.2nd ed. Boston, MA: Academic Press, 2003,Figure 25-11. ISBN: 0126603030."

Thalamus: Store Room of Relay& Modulatory Nuclei

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Somatosensory Pathways:Thalamic Relay Nuclei

* Ventrobasal complex is a collection of somatosensory thalamic nuclei with distinct properties

* Tactile inputs from body (DCML)synapse on the ventral posterior

lateral nucleus (VPL), facial tactile inputs synapse in VPM

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Somatosensory Pathways:Thalamic Relay Nuclei

* Ventrobasal complex is a collection of somatosensory thalamic nuclei with distinct properties

* Deep/proprioceptive inputsinput to regions surrounding the VPL, the ‘shell’ region, as opposed to the tactile’core’ region

Image removed due to copyright reasons. Please see "Squire, Larry R., ed. Fundamental Neuroscience. 2nd ed. Boston, MA: Academic Press, 2003, Figure 25-14. ISBN: 0126603030."

OverviewI. Peripheral encoding in 2 pathways

II. Central Nervous System encoding in 2 pathways

III. Cortex: Basic organizational principlesAreas, Maps, Columns and Magnification

IV. Cortex: Plasticity & its perceptual implications

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Somatosensory Pathways:Cortical Areas

* Areas 3b and 1 receive VP Nucleus Core input, and

3a and 2 VP Shell input* Convergent intracortical con

nections predict area 2 rf properties

Image removed due to copyright reasons.Please see "Squire, Larry R., ed.Fundamental Neuroscience. 2nd ed. Boston, MA: Academic Press, 2003, Figure 25-14. ISBN: 0126603030."

Somatosensory Pathways: Cortical Areas

* 4 key distinct areas in thePost-central gyrus:Area 3b = SI -> discrete tactile rf Area 1

-> Similar to 3b, more RA rf Area 3a deep & proprioceptive

Area 2 touch + proprioception

Please see "Moore, C. I., C. Stern, S. Corkin, B. Fischl, A. Gray, B. Rosen, and A. M. Dale. "Segregation of somatosensory activation in the human hand area using fMRI."Journal of Neurophysiology 84 (2000): 558-569."

Somatosensory Cortex:Cortical Maps

* Initially motor maps were inferred by Hughlings Jackson (‘Jacksonian March’)

•Initially motor maps were inferred by Hughlings Jackson (‘Jacksonian March’)

* Head (1920), tactile mapsInferred from crude lesion patients

•Penfield, electrical stimulation and in surgery patient reports

* Penfield and S. Corkin, well- described lesions post-surgery

Image removed due to copyright reasons.Please see "Squire, Larry R., ed. FundamentalNeuroscience. 2nd ed. Boston, MA:AcademicPress, 2003, Figure 25-15. ISBN: 0126603030."

Visual Cortex: Cortical Columns

Discovered & explicated by Mountcastle: Cellsin a vertical dimension across the layers of the

neocortex tend to have the same rf properties

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Somatosensory Cortex: Cortical Columns

* Orientation columns in V1

Image removed due to copyright reasons.Please see "Squire, Larry R., ed. Fundamental Neuroscience. 2nd

ed. Boston, MA: Academic Press, 2003. ISBN: 0126603030."

Cortical Isofrequency ColumnsAuditory Cortex

Image removed due to copyright reasons. Please see "Kilgard, M.P., and M. M. Merzenich. "Distrib

uted representation of spectral and temporal information in rat primary auditory cortex." Hearing Research 134 (1999), 16-28."

Somatosensory Cortex:Cortical Magnification

* Cortical Magnification Rule: Space devoted to a skin surface is inversely proportional to the size of the receptive field in that area (Sur et al., 1980)

Image removed due to copyright reasons.

Image removed due to copyright reasons.Please see "Squire, Larry R., ed.Fundamental Neuroscience.2nd ed. Boston, MA: Academic Press, 2003,Figure 25-15. ISBN: 0126603030."

Image removed due to copyright reasons.Please see "Sur, M, M. M. Merzenich, andJ. H. Kaas. "Magnification, receptive-field area, and "hypercolumn" size in areas 3b and1 of somatosensory cortex in owl monkeys."Journal of Neurophysiology 44 (1980): 295-311."

OverviewI. Peripheral encoding in 2 pathways

II. Central Nervous System encoding in 2 pathways

III. Cortex: Basic organizational principlesAreas, Maps, Columns and Magnification

IV. Cortex: Plasticity & its perceptual implications

Image removed due to copyright reasons.

Somatosensory Cortex:Adult Cortical Plasticity

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Somatosensory Cortex:Adult Cortical Plasticity

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Perceptual Consequences

* Phantom Limbs, reference zones

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Perceptual ConsequencesBeyond Phantom Limb?

* Increased spatial tactile resolution in ‘reference’ zones.First discoveredBy Teuber (founder of the Department)

* Possible cause?Image removed due to copyright reasons.

Effect of Massive Deafferentation: Spinal-Cord Injury

* Phantom body sensations

Effect of Massive Deafferentation:Cortical Plasticity

* Cortical activation maps to the phantom percept, but what’s the problem here? What’s the answer?

mage removed due to copyright reasons.Please see "Moore, C. I., C. E., Stern, C., Dunbar, S., Kostyk, A., Gehi, andS., Corkin. "Referred Phantom Sensations and Cortical ReorganizationAfter Spinal Cord Injury in Humans." PNAS USA 97 (2000): 14703-14708."

Effect of Massive Deafferentation: Cortical Plasticity

* Cortical activation maps to the phantom percept,Cortical Plasticity:Adaptive Effects

* Wang et al: Trained skin regions developbut what’s the problem here? What’s the answer?

Image removed due to copyright reasons. Please see "Moore, C. I., C. E., Stern, C.,Dunbar, S., Kostyk, A., Gehl, and S., Corkin."Referred Phantom Sensations and CorticalReorganization After Spinal Cord Injury inHumans." PNAS USA 97 (2000): 14703-14708."

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Cortical Plasticity:Adaptive Effects

Wang et al: Trained skin regions developoverlapping rf.

* String instrument players have expanded representations of the‘picking’ hand

Image removed due to copyright reasons.Please see "Wang, X., M. M. Merzenich, K. Sameshima, and W. M. Jenkins. "Remodeling of hand representation in adult cortexdetermined by timing of tactile stimulation."Nature 378 (1995): 71-75."

OverviewI. Peripheral encoding in 2 pathways

II. Central Nervous System encoding in 2 pathways

III. Cortex: Basic organizational principlesAreas, Maps, Columns and Magnification

IV. Cortex: Plasticity & its perceptual implications

Image removed due to copyright reasons.