Download Cortical Representation of Somatosensation

TSM8: SOMATOSENSATION – CORTICAL REPRESENTATION
30/09/08
LEARNING OUTCOMES
Describe the concept of a receptive field
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A receptive field describes the area over which a stimulus will evoke a response in a particular sensory
receptor
Receptive fields of primary afferents (sensory receptors) are always excitatory, typically circular in
shape and are smallest where the greatest level of sensitivity is required e.g. in the fingertips
They often overlap to allow more accurate location of stimuli
Primary afferents e.g. of fine touch ascend through the dorsal columns to the gracile and cuneate
nuclei where they converge to form a somatotopic map of the body
o These nuclei are also said to have receptive fields consisting of a central excitatory zone
surrounded by an outer inhibitory zone
o Receptors in adjacent skin areas can produce lateral inhibition by using interneurones to
suppress the output of neighbouring primary afferents
o This mechanism provides a sense of contrast and spatial detail
Describe the representation of somatosensation in the cerebral cortex
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All sensory information eventually reaches the primary somatosensory cortex (PSSC) located in the
post-central gyrus of the forebrain
Three key organisational structures are found in the PSSC:
o Somatotopic mapping – the sensory homunculus has a ‘point-to-point’ representation of the
whole body with sensitivity distortion to particular areas e.g. fingertips, lips
o Modality differentiation – skin sensory information reaches ‘medial’ areas of the gyrus
(Brodmann’s 1/3b) whereas muscle and joint information reaches ‘lateral’ areas (2/3a)
o Columnar organisation – neurones representing aspects of particular bodily areas or sensory
modalities run in columns perpendicular to the cortical surface
Describe the cortical basis for the perception of touch and object recognition
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PSSC neurones have highly complex receptive fields with various mechanisms (such as lateral
inhibition) for distinguishing finer characteristics of sensation e.g. direction of stroke across a skin area
These mechanisms allow feature extraction which is crucial in stereognosis
The PSSC is connected to several other areas of the brain which modify response and carry out higher
brain functions e.g. learning, memory
o The SII cortex is located in the superior bank of the lateral sulcus and has a bilateral
somatotopic map; it is involved in learning and its response is affected by mood and context
o The posterior parietal cortex is located posterior to the PSSC and integrates information from
various sensory modalities to produce coherent sensation of an object