A cortical homunculus is a pictorial representation of the anatomical divisions of the primary motor cortex and the primary somatosensory cortex; it is the portion of the human brain directly responsible for the movement and exchange of sensory and motor information of the body.
It 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 in a physical form. 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.
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. 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 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.
This is the point-for-point correspondence of an area of the body to a specific point on the central nervous system. Typically, the area of the body corresponds to a point on the primary somatosensory cortex (postcentral gyrus).
This cortex is typically represented as a sensory homunculus which orients the specific body parts and their respective locations upon the homunculus. Areas such as the appendages, digits, and face can draw their sensory locations upon the somatosensory cortex.
Areas that are finely controlled, such as the digits, have larger portions of the somatosensory cortex, whereas areas that are coarsely controlled, such as the trunk, have smaller portions. Areas such as the viscera do not have sensory locations on the postcentral gyrus.
Wilder Penfield was a groundbreaking researcher and highly original surgeon. With his colleague, Herbert Jasper, he invented the Montreal procedure, in which he treated patients with severe epilepsy by destroying nerve cells in the brain where the seizures originated.
Before operating, he stimulated the brain with electrical probes while the patients were conscious on the operating table (under only local anesthesia), and observed their responses. In this way he could more accurately target the areas of the brain responsible, reducing the side-effects of the surgery.
This technique also allowed him to create maps of the sensory and motor cortices of the brain, showing their connections to the various limbs and organs of the body. These maps are still used today, practically unaltered.
Along with Herbert Jasper, he published this landmark work in 1951 as Epilepsy and the Functional Anatomy of the Human Brain. This work contributed a great deal to understanding the lateralization of brain function.
Penfield's maps showed considerable overlap between regions (for instance, the motor region controlling muscles in the hand sometimes also controlled muscles in the upper arm and shoulder), a feature that he put down to individual variation in brain size and localization; we now know that this is due to the fractured somatotropy of the motor cortex.