The cortical homunculus is a pictorial representation of the anatomical divisions of the primary motor cortex and the primary somatosensory cortex. It presents the portion of the human brain directly responsible for the movement and exchange of sense and motor information of the rest of the body.
The resulting image is a disfigured human 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 or motor connections to the brain is represented to appear smaller.
Dr. Wilder Penfield used a similar image to depict the body according to the areas of the motor cortex controlling it in voluntary movement. Sometimes thought to be the brain’s map of the body, the motor homunculus is really a map of the proportionate association of the cortex with body members. It also reflects kinesthetic proprioception, the body as felt in motion.
For example the thumb, which is used in thousands of complex activities, appears much larger than the thigh with its relatively simple movement. This develops over time and differs from one person to the next. The hand in the brain of an infant is different from the hand in the brain of a concert pianist. The difference is due to differences in the functional organization of associated areas of the brain, which is in turn influenced by the muscular anatomy of the effector muscles of the hand.