Sensory homunculus, cortical homunculus, Motor Homunculus are different ways to call a graphical representation of the anatomical divisions of the the portion (primary motor cortex and the primary somatosensory cortex) of the human brain directly responsible for the movement and exchange of sense and motor information of the rest of the body. We see the image below, courtesy of McGill university.

Sensory homunculus, cortical homunculus, Motor Homunculus

Sensory homunculus, cortical homunculus, Motor Homunculus

Always on McGill University website we find the explanation: “Dr. Penfield‘s experiments in stimulating the cortex enabled him to develop a complete map of the motor cortex, known as the motor homunculus (there are also other kinds, such as the sensory homunculus). The most striking aspect of this map is that the areas assigned to various body parts on the cortex are proportional not to their size, but rather to the complexity of the movements that they can perform. Hence, the areas for the hand and face are especially large compared with those for the rest of the body. This is no surprise, because the speed and dexterity of human hand and mouth movements are precisely what give us two of our most distinctly human faculties: the ability to use tools and the ability to speak.”

Another way to portray this map is with a 3D human body,  with disproportionately huge hands, lips, and face in comparison to the rest of the body. Because of the fine motor skills and sense nerves found in these particular parts of the body they are represented as being larger on the homunculus. A part of the body with fewer sensory and/or motor connections to the brain is represented to appear smaller.

Sensory homunculus: cortical homunculus, Motor Homunculus

Sensory homunculus: cortical homunculus, Motor Homunculus

This model shows what a man’s body would look like if each part grew in proportion to the area of the cortex of the brain concerned with its sensory perception. The cortical homunculus is a visual representation of the concept of “the body within the brain” that one’s hand or face exists as much as a series of nerve structures or a “neuron concept” as it does a physical form. This concept relates to many neuro-biological phenomena including “phantom limb” and “body integrity identity disorder”.

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