SOMATOTOPIC MAPS: MAPS OF THE BODY

Rebecca Im (OHS)

‘The brain is the citadel of the senses; this guides the principle of thought.’ – Pliny the Elder

The brain is arguably the most central organ of the human body. It is the one with the most complexity, functioning as a network between a hundred billion neurons which form ever-changing connections at synapses.1 These neurons transport electrical impulses that allow vital communication and coordination with other cells of the body.2 Neurons in the somatosensory cortex (part of the brain that receives and processes sensory information from the entire body3) are organised correspondingly with the location of touch receptors on the skin surface, preserving neighbourhood relations. For instance, sensory neurons which receive impulses from the feet are located adjacently to the sensory neurons which receive impulses from the legs.4 This internal presentation of the body is the somatotopic map5: a fundamental feature for precise motor control and spatial awareness.6 Somatotopic maps are not simply linear transformations of the body surface; specific body parts are mapped at different scales. The scale at which distinct body parts are mapped depends on their sensory importance – which is also directly reflected by the density of their surface receptors on the skin. Therefore, the section of the somatosensory cortex receiving information from the fingers will be greater than the section receiving impulses from the back. Currently, it is unknown when, specifically, the mammalian brain creates this organisation. The opposing groups of thoughts are either prenatal or postnatal development. One animal study7 showed evidence that these ‘maps’ may appear from birth and therefore could be genetically hardwired. Functional MRIs were used to indicate neural responses to the tactile stimulation of different body parts (the face, hands and feet) of nine macaques. The ages of these animals ranged from newborn (11 days) to juvenile (961 days). The results showed that, regardless of the macaque’s age, their large-scale body maps were all indistinguishable from each other – activity detected in the somatosensory cortex corresponded with the location of the body part stimulated. It is important to emphasise that M1 (below), at just eleven days old, displayed an identical large-scale somatotopic organisation to the others, confirming the presence of such ‘mapping’ from a new-born and even prenatal stage.

(Each pair of egg-shaped regions represents data from one monkey. Red - head, green - hands, blue - feet) https://www.pnas.org/content/116/49/24861 However, this same study also showed some sections of the somatosensory cortex were less responsive in the new-born macaque relative to the older ones. For instance, the finer-scale differentiation of body parts (eg. Individual fingers) increased with the age of the macaque. We could thereby conclude that while a somatotopic scaffold is present from birth, the map is refined postnatally through experience-driven modifications (during early development). Whether the development mainly occurs prenatally or postnatally, both aspects are equally important in ensuring final motor and somatosensory function. The prenatal process of producing a ‘scaffold’ of the somatotopic map is driven by genetic factors, as well as feedback from spontaneously generated neural activity. Deprivation of such stimulation can lead to permanent alteration of any somatotopic organization and function8, which could explain why prenatal birth notably increases the risk of developing motor and somatosensory dysfunction. This dysfunction in somatotopic arrangement can also appear in stroke patients. For instance, a 54-year-old patient who suffered a stroke in the right side of their body was left with severe distortion of their somatotopic sensory maps9. After the patient was tested with detailed tactile input on their left hand, they displayed incorrect localisation of the point of the stimuli. Further future in-depth study of stroke patients’ somatotopic maps in the affected area may help to reveal the true reasons for their various functional deficits. Interestingly, the somatotopic layout does not restrict brain remapping.10 For instance, following a hand amputation, the brain region that had usually received information from the hand will adapt to process information from other body parts. It was previously assumed that this deprived region would only process information from body parts that had neighboured the hand. However, it has been proved that proximity between brain regions does not limit brain remapping. This is due to native brain regions of these body parts already have varying levels of overlap with the brain region of the hand.

It is that idea that, in any scenario our brain will adapt to the changes and form new pathways and connections, that stays true to the theme of ‘maps’.

1 Phillips, H (2006) Introduction: The Human Brain https://www.newscientist.com/article/dn9969introduction-the-human-brain/ 2 Newman, T (2017) All you need to know about neurons https://www.medicalnewstoday.com/ articles/320289#In-a-nutshell 3 Penfield, W., and Rasmussen, T. (1950) The Cerebral Cortex of Man: A Clinical Study of Localization of Function. New York: Macmillan 4 Neuroskeptic (2019) Innateness of Body Maps https://www.discovermagazine.com/mind/innatenessof-body-maps 5 Wilson. S., and Moore, C. (2015) S1 somatotopic maps http://www.scholarpedia.org/article/S1_somatotopic_ maps 6 Dall’Orso, S., Steinweg, J., Allievi, A. G., Edwards, A. D., Burdet, E., & Arichi, T. (2018). Somatotopic Mapping of the Developing Sensorimotor Cortex in the Preterm Human Brain. Cerebral cortex (New York, N.Y. : 1991), 28(7), 2507–2515. https://doi. org/10.1093/cercor/bhy050 7 Acracro, M,. Schade, P,. Livingstone,. M (2019) Body map proto-organization in newborn macaques Pittsburgh, Pa: PNAS https://www.pnas.org/ content/116/49/24861 8 Larroque B, Ancel PY, Marret S, Marchand L, André M, Arnaud C, Pierrat V, Rozé JC, Messer J, Thiriez G, Burguet A, Picaud JC, Bréart G, Kaminski M; (2008) EPIPAGE Study group. Neurodevelopmental disabilities and special care of 5-year-old children born before 33 weeks of gestation (the EPIPAGE study): a longitudinal cohort study. Lancet. 9 Birznieks, I., Logina, I., & Wasner, G. (2012). Somatotopic mismatch following stroke: a pathophysiological condition escaping detection. BMJ case reports https://www.ncbi.nlm.nih.gov/pmc/ articles/PMC4543302/ 10 Hahamy, A,. and Makin, T (2019) Remapping in Cebrebral and Cerebella Cortices Is Not Restricted by Somatotopy J Neurosci . https://www.jneurosci.org/ content/39/47/9328#ref-53