Motor homunculus

A cortical homunculus is a physical representation of the human body, located within the brain. A cortical homunculus is a neurological "map" of the anatomical divisions of the body. There are two types of cortical homunculi: sensory and motor.

The primary motor cortex is located in the precentral gyrus, and handles signals coming from the premotor area of the frontal lobes. The primary sensory cortex is located in the postcentral gyrus, and handles signals coming from the thalamus. These signals are then transmitted from the gyri to the brain stem and spinal cord via corresponding nerves.

At the primary motor cortex, motor representation is arranged in an orderly manner, inverted. The toes are represented at the top of the cerebral hemisphere, while the mouth is represented at the bottom of the hemisphere, closer to the lateral sulcus. These representations lie along a fold in the cortex called the central sulcus. The homunculus is split in half, with motor representation for each side of the body represented on the opposite side of the brain. Some body parts may be controlled by partially overlapping regions of cortex.

The amount of cortex devoted to any given body region is proportional to how richly innervated that region is, not to the body region's physical size. Areas of the body with greater or more complex sensory or motor connections are represented as larger in the homunculus, those with fewer or less complex connections are represented as smaller. The resulting image is that of a distorted human body, with disproportionately huge hands, lips, and face.

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 really is a map of the proportionate association of the cortex with body members. It also reflects kinesthetic proprioception, the body as felt in motion.

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