spatial coding: receptive fields and tactile...

Spatial Coding:Receptive Fields and Tactile Discrimination

Where is it?

What is it?

How strong is it ?

What must sensory systems encode?

Different sensory modalities are ‘represented’ in different lobes..

When the brain wants to keep certain types of information distinct, one major strategy is to put that information in different places.

This strategy is also evident within topographic sensory modalities, i.e. modalities in which a sensory surface is mapped to a sheet of neural tissue like the cortex, preserving near-neighbor relationships.

Right Occipital LobeMedial Surface

Calcarine Sulcus

Left Visual Field

Fixation Point

Superior

Inferior

Anterior

Inferior visual field

Superior visual field

Retinotopic Map of V1

Tonotopic Maps

Somatic

Sensory

Motor

Somatotopic Maps

Wilder Penfield

Dermatome: cutaneous area supplied by a single spinal nerve root

BCP for review of segmental organization

CN V

C2

T1L1 S1 S3

S4-5

Herpes Zoster (Shingles)Reactivated chicken pox virusfrom within cells of nerve root

Cutaneous Receptors

Sir Henry Head (1861-1940)

“Protopathic System”“Epicritic System”

Large Diameter Afferents (Aa or I, Ab or II)

All are mechanoreceptors

Encapsulated Receptors

Small Diameter Afferents (Ad or III, C or IV)

Free nerve endings

Mechanoreceptors (crude or non-discriminative touch)

Nociceptors (damaging mechanical, thermal, chemical)

Chemoreceptors („spicy‟)

Thermoreceptors (warm, cold)

Discriminative touch

Joint position sense (proprioception)

Serve different submodalities

100 m/sec = 225 miles/hour

„Fine‟ touch „Crude‟ touchPosition sense

Encapsulated receptors Free nerve endings

Differentially sensitive to pressure and local anesthetics

Dorsal column-medial lemniscus system

Anterolateral (spinothalamic) system

Information from the two classes of receptors (afferents) diverges into two distinct pathways in the CNS.

Dorsal root ganglion

Dorsal columns To brain

THALAMUS

(Discriminative Touch, Position Sense)

Dorsal Root Fiber

CORTEX

MEDULLA

SPINAL CORD

Dorsal Columns

VPL

DORSAL COLUMN

MEDIAL LEMNISCUS SYSTEM

SI

MIDBRAIN

PONS

Dorsal Column Nuclei

MedialLemniscus

I (A-alpha)

II (A-beta)

Dorsal root ganglion

Synapse

DecussationAnterolateral system

To brain

Dorsal Root Fiber

CORTEX

THALAMUS

SPINAL CORD

(Pain, Temperature, Low-acuity or "Crude" Touch)

ANTEROLATERAL SYSTEM

Intralaminar Nuc.

Posterior Grp.

SI, SII & Others

VPL

BRAIN STEMReticular

Formation

AnterolateralColumn

III (A-delta)

IV (C)

Be able to sketch these pathways

The Trigeminal (Vth) Nerve

What is the basis for this?

Two-Point Discrimination Thresholds (or Minimum Separable)

Small receptive fields = high spatial resolution or acuity.

Vallbo

(Receptive field: the patch skin where a stimulus will produce a response in the nerve.)

Skin resolution

Palm Finger Finger tip

0

0.3

0.6

0

40

80

120

Spatial re

solu

tion

Innerv

ation d

ensity

units/s

q. m

m.

1/m

m

Meissner’s corpuscles

Two-point discrimination

High receptive field density = high spatial resolution or acuity.

Differential Cortical Magnification of the Receptor Surface

S1

Regions of high spatial acuity have:

Small receptive fields

High receptive field density

Large representations in sensory cortex

Critical separation of activity in cortex

Cutaneous receptive fieldsA

C D

S

T

I

M

Receptive fieldB

ResponseSkin

Feed-Forward Inhibition

Feedback Inhibition

Lateral Inhibition

Recurrent collateral

Excitatory

synapseInhibitory synapse

STIM

peak

peak

trough

11

22

33

4

5

4

5

First order neurons Second order neurons

Lateral Inhibition

A similar diagram using feed-forward inhibition is in BCP

Lateral inhibition increases spatial contrast in the neural activity pattern

Which receptors mediate the two-point discrimination thresholds?

(neurospeak)

Physiological

Class

Adaptation Receptive Field Probable Morphology

PC (RA 1 ) Fast Large Pacinian corpuscle

RA (RA II) Fast Small Meissner’s corpuscle

SA I Slow Small Merkel’s disc

SA II Slow Large Ruffini’s ending

t

Raised dots

Move pattern

Record from all cells simultaneouslyA

Snapshot of spike activity at one instantReceptive fields

Stored spike activity

t

Record from one cell, assume it is identical to the othersB

Receptive field

Receptive field

Spike trains on

sequential sweeps

Step this way after each sweep

„Reconstruct‟ pattern for

a continuous sheet of

cells with identical

receptive fields

Merkel‟s receptors

Meissner‟s corpuscles

Pacinian corpuscles

Patch of skin with

a sheet of receptors

Merkel‟s disc

Meissner‟s corpuscle

Pacinian corpuscle

Information about the pattern is distributed among the receptor afferents.

Information about the pattern is transmitted in parallel over the receptor afferents.

Information about the pattern is transmitted by a population of receptor afferents.

Mueller’s “Law of Specific Nerve Energies”

“Labeled lines”

“Grandmother cells”

“Feature detectors”

Etc.

• Each type of sensory nerve ending, however stimulated (electrically, mechanically, etc.), gives rise to its own specific sensation

• Each type of sensation depends not upon any special character of the different nerves but upon the part of the brain in which their fibers terminate.

Does each receptor type mediate a familiar category of sensation? Does Mueller’s law hold?

Torebjörk, Vallbo and Ochoa

Stimulating

Shocks

Normal cutaneous sensation is the result of simultaneous activity in more than one class of afferent

Does a punctate stimulus on the skin cause a point of activity in the skin? The cortex?

Muscles

Joints, deep

tissuesSkin

3a 3b 1 2

S1

Hand area

Vibrating probe

Muscle receptors

Cutaneous

receptors

Deep tissue

receptors

Information from a point on the skin is distributed to a zone or zones of neural circuitry in the cortex.

Cortex

Light Source

Detector

Bone

Vibrating probe

Optical Imaging in Vivo

See BCP 3: page 328

Are neural representations in S1 fixed?

S1 representational expansion with use

Some Major Principles of Somatic-Sensory Spatial Coding

Spatial acuity varies inversely with receptive-field size

Lateral inhibition in the CNS enhances spatial contrast

Information from a point on the skin is “dissected” by differentclasses of receptors and transmitted in parallel channels that influence a region or regions of cortex

Spatial acuity varies directly with receptive-field (receptor) density

Spatial acuity varies directly with the cortical magnification

spatial coding: receptive fields and tactile...

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