A cortical homunculus is a pictorial representation of the anatomical divisions of the primary motor cortex and the primary somatosensory cortex, i.e., the portion of the human brain directly responsible for the movement and exchange of sensory and motor information of the body.
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.
There are two types of homunculus: sensory and motor. Each one shows a representation of how much of its respective cortex innervates certain body parts. The primary somesthetic cortex (sensory) pertains to the signals within the postcentral gyrus coming from the thalamus, and the primary motor cortex pertains to signals within the precentral gyrus coming from the premotor area of the frontal lobes. These are then transmitted from the gyri to the brain stem and spinal cord via corresponding sensory or motor nerves.
The reason for the distorted appearance of the homunculus is that the amount of cerebral tissue or cortex devoted to a given body region is proportional to how richly innervated that region is, not to its size. The homunculus is like an upside-down sensory or motor map of the contralateral side of the body. This can be seen, since the upper extremities such as the facial body parts and hands are closer to the lateral sulcus than lower extremities such as the leg and toes. The decussation in the pathways also result in right gyri to receive input from the left side of the body and vice versa.
The resulting image is a grotesquely 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 and/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.
It plays a central role in phantom limb phenomena and their opposite such as the disappearance of body members from conscious perception with certain types of brain damage. Like the sensory homunculus, the motor homunculus looks distorted. For example the thumb, which is used in thousands of complex activities, appears much larger than the thigh with its relatively simple movement.
The motor homunculus 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.
Penfield's motor homunculus, which is usually shown as a canonical, 2D diagram is in actuality an oversimplification that does not show the original data Penfield collected from patients undergoing surgery for epilepsy. This simplification suggests that lesions of the motor cortex will give rise to specific deficits in specific muscles. This is a misconception that is not necessarily the case: lesions produce deficits in groups of synergistic muscles. This difference is critical as it suggests that the motor cortex functions in terms of overall movements as entire libraries rather than how to move individual muscles, allowing a great deal more complexity.
See also 
- Marieb, E., Hoehn, K. Human Anatomy and Physiology. 7th Ed. 2007. Pearson Benjamin Cummings: San Francisco.
- Saladin, Kenneth (2007). Anatomy and Physiology: The Unity of Form and Function. McGraw Hill. p. 544-546