The cortical homunculus
Drag the electrode along the strip of cortex. Each point answers with a part of the body.
What the homunculus is
The cortical homunculus ("little man") is a drawing of the body laid along a strip of the cerebral cortex, sized by how much cortex each part gets. There are two. The motor homunculus lies on the precentral gyrus, the primary motor cortex at the back of the frontal lobe; the sensory homunculus lies just behind it, across the central sulcus, on the postcentral gyrus of the parietal lobe[1][2]. In both, the feet and legs are on the inner face of the hemisphere, inside the cleft between the two halves of the brain; moving outward and down come the hips, trunk, shoulders, arms and hands; the face, lips and tongue are lowest, just above the lateral sulcus[1]. established
The strip on this page is a cross-section through one hemisphere cut along the central sulcus, as if you were looking at the cut face from the front. The midline is on the left: the strip starts deep in the cleft between the hemispheres, climbs the inner (medial) wall, crosses the top of the brain and runs down the outer side to the lateral sulcus, near the ear. Beside it, each body part is drawn at the size it has in Penfield's drawing. Each hemisphere maps the opposite side of the body.
How Penfield made the map
Between 1928 and 1936 the neurosurgeon Wilder Penfield, at McGill University and the Montreal Neurological Institute, operated on 126 patients, many of them with epilepsy, under local anesthesia[3]. With the cortex exposed and the patient awake, he touched points with a weak electric current to find the area to remove and the areas to spare. Stimulating the precentral gyrus made a part of the body move; stimulating the postcentral gyrus made the patient report a sensation, typically tingling or numbness, in a part of the body. Penfield and his colleague Edwin Boldrey collected the responses, about 170 summary maps in all, into a 1937 paper in the journal Brain, and drew the first homunculus to show "the order and comparative size of the parts of the body" on the cortex[4][3]. In 1950 Penfield and Theodore Rasmussen published the better-known version, drawn as figures draped over a cross-section of the hemisphere, one for the motor strip and one for the sensory strip[5][3].
Why the hands and mouth are huge
Drag the electrode from end to end and the distortion is plain: the hand and fingers, the lips and the tongue take up much of the strip, while the whole trunk gets a short stretch. The motor cortex gives more space to muscles that make fine, agile movements, which need more motor units and therefore a larger patch of cortex[2]. The sensory map is distorted the same way: fingertips and lips are densely supplied with touch receptors and have large patches of cortex, so they can tell apart two points close together, while the back cannot[6].
A map with gaps
The homunculus has been a fixture of textbooks for more than 80 years, and its broad layout (feet medial, hands in the middle, face lateral) has held up. The details have not all held up. In 2023 Evan Gordon, Nico Dosenbach and colleagues used precision functional MRI on individual people and found that the motor strip is not one continuous body map. Regions for the foot, hand and mouth alternate with three "inter-effector" regions that are thinner, connect strongly to each other and to networks for planning action and controlling the body's internal state, and are active when several parts of the body must be coordinated. They called this interleaved system the somato-cognitive action network[7]. active
The figure on this page is the classic map, not this newer picture: it is what the 1937 and 1950 stimulation studies found, and what most courses still teach.
Two things to try
The sensory map's surprises. Switch to Sensory and move the electrode to the very start of the strip: in Penfield's drawing the genitals are represented deep on the medial wall, beyond the toes. Later imaging has questioned that placement: a 2005 functional MRI study of touch to the penis found its representation on the postcentral gyrus between the foot and the lower abdominal wall, in order with its neighbors on the body[8]. At the other end of the drawing, past the tongue and throat, come the organs of the abdomen.
Motor and sensory side by side. Put the electrode at the same position on both strips. The two maps line up roughly but not exactly: on the motor strip the face area begins after the neck and brow; on the sensory strip the head and neck sit between the trunk and the arm.
For where the two strips are on the whole brain, see the motor and somatosensory cortex on the labeled brain diagram; for the lobes they belong to, see Chapter 1.
Sources
- (2022). Anatomy and Physiology 2e, §14.2 Central Processing. OpenStax, Rice University. textbook
- (2022). Anatomy and Physiology 2e, §14.3 Motor Responses. OpenStax, Rice University. textbook
- (2017). A little man of some importance. Brain 140(11):3055–3061. doi:10.1093/brain/awx270. paper
- (1937). Somatic motor and sensory representation in the cerebral cortex of man as studied by electrical stimulation. Brain 60(4):389–443. doi:10.1093/brain/60.4.389. paper
- (1950). The Cerebral Cortex of Man: A Clinical Study of Localization of Function. Macmillan, New York (1950; the linked copy is the 1957 printing). book
- (2024). Lobes of the brain. institutional
- (2023). A somato-cognitive action network alternates with effector regions in motor cortex. Nature 617:351–359. doi:10.1038/s41586-023-05964-2. paper
- (2005). The sensory cortical representation of the human penis: revisiting somatotopy in the male homunculus. Journal of Neuroscience 25(25):5984–5987. doi:10.1523/JNEUROSCI.0712-05.2005. paper