The Five Senses and Beyond: The Encyclopedia of Perception - Jennifer L. Hellier 2017
In mammals, and particularly in humans, the cerebral cortex receives sensory information from and sends out motor information to the head and body. These processes are performed by the primary sensory and motor cortices of the brain. The primary somatosensory cortex is the region of the brain directly responsible for the exchange of sensory information of the body, while the primary motor cortex is responsible for voluntary movement. To be able to determine the location of the sensory or motor signals, the brain has a pictorial map of the head and body “on” these cortices. It is a map in the sense that certain parts of the body’s sensory or motor information are connected to certain neuron groups located in a specific region of the corresponding cortex. This anatomical map is called a cortical homunculus, or homunculus, which is Latin meaning “little man.” Somatotopy is another term that is used to describe the point-for-point connection of a body region to a specific part of the cerebral cortex.
In the late 1940s to the early 1950s, Canadian neurosurgeon Wilder Graves Penfield (1891—1976) was a leader in his field as he was performing brain surgery on humans with epilepsy to destroy the neurons that were primarily involved in generating seizures. To ensure that he did not damage important sensory or motor functions for the patient, Penfield stimulated the motor and sensory cortices while the person was under local anesthesia but awake to answer questions about what was felt during the stimulation. From this information, Penfield was able to map out regions of the sensory and motor cortices that were connected to the different parts of the limbs and organs of the body. Penfield, along with Canadian psychologist and neurologist Herbert Henri Jasper (1906—1999), published these cortical homunculi in 1951 and again in 1954 (second edition) in their book titled Epilepsy and the Functional Anatomy of the Human Brain. These maps are still used today without much change from the originals.
Location and Orientation of the Homunculi
There are two types of homunculi: sensory and motor. Each type has a homunculus on the left and the right cerebral hemispheres; thus, there are four total homunculi—two sensory and two motor. The primary somatosensory cortex is the postcentral gyrus (the ridges of the cerebral cortex), and the primary motor cortex is the precentral gyrus. The maps wrap along these gyri from the medial (toward the midline) to the lateral (toward the side) surfaces. It is important to note that because the afferent and efferent pathways cross over (decussate) either in the spinal cord or brainstem, the right postcentral and precentral gyri contain information for the left side of the body and vice versa.
For the sensory homunculus, the most medial and deep portion of the postcentral gyrus maps to the genitals. Just superior to that are the toes and foot. At the gyrus’s bend, the leg is represented. From the medial superior surface moving laterally the following are mapped: hip, trunk, neck, head, shoulder, arm, elbow, forearm, and wrist. The hand and fingers come next, but these are connected to a much larger area of the sensory cortex than any other previously described body part. This is because humans have significantly large numbers of sensory receptors in their hands and fingers, which helps produce discriminative touch. The specific mapping continues: hand, little finger, ring finger, middle finger, index finger, and thumb. Now on the lateral surface superior to the temporal lobe, the next body regions are recorded: eye, nose, and face. As with the hands and fingers having a largely mapped region of the sensory cortex, so do the lips. This is because humans have many sensory receptors around the mouth that are mainly used for sensing taste, temperature, and proprioception (position in space). Thus, the following are represented: upper lip, lips, and lower lips. Nearing the final downward mapping are the teeth, gums, tongue, and pharynx. The very last body region that is demarcated, where the postcentral gyrus meets the temporal lobe, is the intraabdominal. It is important to note that the viscera are not mapped to the postcentral gyrus.
The motor homunculus is mapped relatively similar to the sensory homunculus, but the proportions are slightly different based on the amount of controlled movements a body regions has. For instance, the hand and fingers are mapped to almost twice the region that they were in the sensory cortex. This is because humans have significantly large numbers of motor fibers in their hands and fingers, which give them finely controlled movements that are needed to play a piano, knit, write, or throw a ball.
Jennifer L. Hellier
See also: Discriminative Touch; Phantom Pain; Somatosensory Cortex; Somatosensory System
Kell, Christian A., Katharina von Kriegstein, Alexander Rösler, Andreas Kleinschmidt, & Helmut Laufs. (2005). The sensory cortical representation of the human penis: Revisiting somatotopy in the male homunculus. Journal of Neuroscience, 25(25), 5984—5987.
Saladin, Kenneth S. (2012). Anatomy and physiology. New York, NY: McGraw-Hill.