Spatial Coding: Receptive Fields and Tactile Discrimination
What must sensory systems encode? How strong is it? What is it? Where is it?
When the brain wants to keep certain types of information distinct, one major strategy is to put that information in different places. Different sensory modalities are represented in different lobes.. 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.
Retinotopic Map of V1 Left Visual Field Superior Right Occipital Lobe Medial Surface Fixation Point Inferior Calcarine Sulcus Inferior visual field Superior visual field Anterior
Tonotopic Maps
Somatotopic Maps Motor Somatic Sensory Wilder Penfield
Dermatome: cutaneous area supplied by a single spinal nerve root BCP for review of segmental organization
C2 CN V T1 L1 S1 S3 S4-5
Herpes Zoster (Shingles) Reactivated chicken pox virus from within cells of nerve root
Cutaneous Receptors
Sir Henry Head (1861-1940)
Epicritic System Protopathic System
Encapsulated Receptors All are mechanoreceptors Discriminative touch Joint position sense (proprioception) Large Diameter Afferents (Aa or I, Ab or II) Free nerve endings Serve different submodalities Mechanoreceptors (crude or non-discriminative touch) Chemoreceptors ( spicy ) Thermoreceptors (warm, cold) Nociceptors (damaging mechanical, thermal, chemical) Small Diameter Afferents (Ad or III, C or IV)
Encapsulated receptors Free nerve endings Position sense Fine touch Crude touch Differentially sensitive to pressure and local anesthetics 100 m/sec = 225 miles/hour
Information from the two classes of receptors (afferents) diverges into two distinct pathways in the CNS. Dorsal columnmedial lemniscus system Anterolateral (spinothalamic) system
SI CORTEX SI, SII & Others CORTEX Intralaminar Nuc. VPL Medial Lemniscus THALAMUS MIDBRAIN PONS Posterior Grp. VPL BRAIN STEM THALAMUS Reticular Formation MEDULLA Dorsal Column Nuclei Dorsal Columns Anterolateral Column SPINAL CORD Dorsal Root Fiber I (A-alpha) II (A-beta) SPINAL CORD Dorsal Root Fiber III (A-delta) IV (C) DORSAL COLUMN MEDIAL LEMNISCUS SYSTEM (Discriminative Touch, Position Sense) ANTEROLATERAL SYSTEM (Pain, Temperature, Low-acuity or "Crude" Touch) Be able to sketch these pathways Dorsal columns To brain To brain Synapse Dorsal root ganglion Dorsal root ganglion Anterolateral system Decussation
The Trigeminal (Vth) Nerve
Two-Point Discrimination Thresholds (or Minimum Separable) What is the basis for this?
Vallbo Small receptive fields = high spatial resolution or acuity. (Receptive field: the patch skin where a stimulus will produce a response in the nerve.)
Innervation density Spatial resolution Skin resolution 0.6 Two-point discrimination units/sq. mm. 1/mm 0.3 0 120 80 Meissner s corpuscles 40 0 Palm Finger Finger tip High receptive field density = high spatial resolution or acuity.
S1 Differential Cortical Magnification of the Receptor Surface Regions of high spatial acuity have: Small receptive fields High receptive field density Large representations in sensory cortex
Critical separation of activity in cortex
A Skin Cutaneous Receptive field Response receptive B fields S T I M C D
Lateral Inhibition Feed-Forward Inhibition Feedback Inhibition Recurrent collateral
Lateral Inhibition 1 Excitatory synapse 1 Inhibitory synapse S T I M 2 3 4 2 3 4 peak trough peak 5 5 First order neurons Second order neurons Lateral inhibition increases spatial contrast in the neural activity pattern A similar diagram using feed-forward inhibition is in BCP
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
A Record from all cells simultaneously Receptive fields Raised dots Snapshot of spike activity at one instant Move pattern t B Record from one cell, assume it is identical to the others Stored spike activity Receptive field t
Receptive field Spike trains on sequential sweeps Patch of skin with a sheet of receptors 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
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? Stimulating Shocks Torebjörk, Vallbo and Ochoa 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 Skin Joints, deep tissues
Vibrating probe Hand area S1 3a 3b 1 2 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.
Optical Imaging in Vivo Vibrating probe Detector Light Source Bone Cortex See BCP 3: page 328
Figure 2. Stability of topographic maps over time in the anesthetized animal Chen, L. M. et al. J. Neurosci. 2005;25:7648-7659 Copyright 2005 Society for Neuroscience
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 Spatial acuity varies directly with receptive-field (receptor) density Spatial acuity varies directly with the cortical magnification Lateral inhibition in the CNS enhances spatial contrast Information from a point on the skin is dissected by different classes of receptors and transmitted in parallel channels that influence a region or regions of cortex