Homunculus

The homunculus, also known as the little man inside the brain, usually describes the distorted human figure reflecting the relative space human body parts occupy on the somatosensory cortex and the motor cortex. Thus, there are two diagrams, one reflecting the senses and the other reflecting the motor or physical movement controlled by the brain.

The diagram below (from Wikipedia) shows the visual representation. Reference also the page on brain anatomy.

	Personal Experience We found out about this concept from our neurologist about 8 months after Peter's stroke. He showed us the diagram and used it to explain that the action of moving fingers involved so much more brain power than lifting up an arm. This explained two things for us: First, it helped us to understand why different functions were recovering more slowly than others Second, we now understood why the exercises that focused on fine motor skills were more challenging and tired Peter out so much. We've not seen this information readily available with other stroke resources and think that it's an important part of the puzzle to explain what's happening, how and why.

Sensory Homunculus

Homunculus or "little man" is located on the cerebral cortex of the brain. The sensory homunculus is found on the somatosensory or primary sensory cortex, immediately behind the primary motor cortex. Neurons in the central sulcus, located just in front of the somatosensory cortex, can identify the area of the body being stimulated by the information they receive from somatic receptors in the skin and from proprioceptors in skeletal muscles. These muscle receptors transmit information related to locomotion and position.

A particular body region is represented on the cortex with an area that is proportional to the number of touch receptors in the body part, and not by its size. It should be noted that the neurons on the primary cortex are not laid down in the shape of a human, but the relative size of the areas is indicative of how sensitive the skin of that area is to touch.

The sensory homunculus in humans has a very large face, tongue and fingertips, reflecting the amount of sensory input that comes from these features.

Motor Homunculus

Dr Wilder Penfield was a leader in brain research. His experiments in stimulating the cortex made him develop a complete map of the motor cortex, which was known as the motor homunculus. The areas assigned to various body parts on the cortex are proportional not to their size, but to the complexity of the movements that they can perform.

The areas for the hand and face are especially large on this map compared with those for the rest of the body. This is something to be expected because the speed and agility of human hand and mouth movements give us the fantastic abilities to use tools and to speak.

The motor homunculus is a representation of the human body depict it according to the areas of the motor cortex controlling voluntary movement. Each component is reflected in proportion to the amount of brain matter dedicated to facilitating movement. 

Just as the sensory homunculus, the motor 'little man' appears 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 cortex develops over a period of time and is different for different people. The hand in the brain of a newborn is different to the hand in the brain of a concert pianist. You may be able to flex and extend the end of your thumb fairly easily, but there are a few people that can make similar movements with any of their other fingers. This difference is due to differences in the functional organization of associated areas of the brain.

It is important to note that the parts of the body that perform the finest movements take up most of the space when the motor cortex is mapped. And the parts that perform gross movements are known to only take up little space. So a pictorial motor homunculus appears gross with a huge hand, thumb, face, mouth and tongue and small legs and feet.

This reflects our two most astonishing evolutionary adaptations - language and the human hand. Our thumbs allow us to perform extremely precise movements and handle tools. The ability of our ancestors to talk and to touch their thumbs to each finger in turn is quite possibly responsible for us not swinging from the trees.

Stroke and Motor Coordination

There are a range of diseases such as stroke that decrease one's ability to perform tasks that require fine motor skills.

Stroke impairs fine motor coordination skills making it difficult for patients to write or to use a computer mouse and to do a refined use of the small muscles controlling the hand, fingers, and thumb. As discussed previously, the parts of the body that perform the finest movements take up the most space while mapping the motor cortex. These fine motor skills thus take longer to come back in stroke patients. Gross movement in the affected hand may be possible; fine movement of the fingers may or may not return.