title: Corticalization of motor control in humans is a consequence of brain scaling in primate evolution.
abstract: Control over spinal and brainstem somatomotor neurons is exerted by two sets of descending fibers, corticospinal/pyramidal and extrapyramidal. Although in nonhuman primates the effect of bilateral pyramidal lesions is mostly limited to an impairment of the independent use of digits in skilled manual actions, similar injuries in humans result in the locked-in syndrome, a state of mutism and quadriplegia in which communication can be established only by residual vertical eye movements. This behavioral contrast makes humans appear to be outliers compared with other primates because of our almost total dependence on the corticospinal/pyramidal system for the effectuation of movement. Here we propose, instead, that an increasing preponderance of the corticospinal/pyramidal system over motor control is an expected consequence of increasing brain size in primates because of the faster scaling of the number of neurons in the primary motor cortex over the brainstem and spinal cord motor neuron pools, explaining the apparent uniqueness of the corticalization of motor control in humans.
authors.affiliation: Instituto de Ciências Biomédicas, Universidade Federal do Rio de Janeiro, Rio de Janeiro CEP 21941-902, Brazil.|Instituto Nacional de Neurociência Translacional, Ministério de Ciência e Tecnologia, São Paulo 03023-900, Brazil.
authors.forename: Suzana
authors.initials: S
authors.lastname: Herculano-Houzel
authors.affiliation: Department of Psychology, Vanderbilt University, Nashville, Tennessee, 37240-7817.
authors.forename: Jon H
authors.initials: JH
authors.lastname: Kaas
authors.affiliation: Instituto D'Or de Ensino & Pesquisa (IDOR), Rio de Janeiro 22281-100, Brazil.
authors.forename: Ricardo
authors.initials: R
authors.lastname: de Oliveira-Souza
mesh_terms: D000818:Animals; D005075:Biological Evolution; D001933:Brain Stem / Q000166:cytology / Q000502:physiology; D002452:Cell Count; D006801:Humans; D009043:Motor Activity / Q000502:physiology; D009044:Motor Cortex / Q000166:cytology* / Q000502:physiology; D009474:Neurons / Q000166:cytology* / Q000502:physiology; D011323:Primates / Q000502:physiology; D013116:Spinal Cord / Q000166:cytology / Q000502:physiology
keywords: cortical expansion; motor control; motor cortex; motor neurons; number of neurons