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Homunculus: Somatosensory and Somatomotor Cortex

    Homunculus

    Homunculus Sensory & Motor Image 

    Motor Cortex
    • Each cerebral hemisphere includes primary motor cortex that is located just anterior to the central sulcus (a.k.a., precentral gyrus) and extends down to the sylvian fissure. This area is histologically known to be Brodmann's Area 4.
    • The topographical representation of the homunculus is arranged in an anatomical fashion and represents the contralateral side. This means that the primary cortex in the right cerebral hemisphere represents motor activity on the left side of the body and vice-versa.
      • It is important to recognize that the density of receptors for various parts of the body are not the same which is why the homunculus represents different sizes as it extends over the cortex.
    Somatosensory Cortex
    • Each cerebral hemisphere includes somatosensory cortex that is located just posterior to the central sulcus (a.k.a., postcentral gyrus) and extends down to the sylvian fissure. This area is histologically known to be Brodmann's Areas 1, 2, 3.
    • The topographical representation of the homunculus arranged in an anatomical fashion and represents the tactile representation of the contralateral side. 
    • It is important to recognize 2 main things about the somatosensory cortex:
      • That the density of receptors for various parts of the body are not the same which is why the homunculus represents different sizes as it extends over the cortex.  
      • The surface area of the anatomical body part also does not influence the amount of the cortex dedicated to that body part, but rather reflects the density of cutaneous tactile receptors dedicated to that body part.  For example, the lips make up a small surface area compared to other body parts but yet has a greater density of receptors compared to the shoulder or forearm.
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    Editors & Reviewers

    Editors:

    • Anthony J. Busti, MD, PharmD, FNLA, FAHA 
    • Dylan Kellogg, MD

    Last Updated:  September 2015