Treenig hjärna

Treenig hjärna är en modell av hjärnan föreslagen på 1960-talet av Paul D. MacLean. Modellen syftar till att förklara hur evolutionen har påverkat indelningen av den mänskliga hjärnan.

Modellen beskriver hjärnan i tre separata delar. Den treeniga hjärnan består av reptilhjärnan, det limbiska systemet eller däggdjurshjärnan och slutligen neocortex. MacLean menade att ju längre ut en del av hjärnan är desto mindre vital och mer avancerad är den. Idag är dock MacLeans modell att betrakta som föråldrad.^[1]^[2]

Tre delar

Reptilhjärnan

Reptilhjärnan inkluderar hjärnstammen och lillhjärnan. Att den kallas "reptilhjärnan" beror på att en reptils hjärna till största delen består av hjärnstammen och lillhjärnan. Den kontrollerar musklerna, balans och autonoma funktioner som omedveten kontroll av hjärta, lungor, mage och lever.

Det limbiska systemet

Det limbiska systemet utgörs bland annat av amygdalan, hypotalamus och hippocampus. Man brukar säga att känslor och instinkter kommer från det limbiska systemet. Detta är dock en grov förenkling. Om man stimulerar det limbiska systemet med en svag elektrisk spänning upplevs känslor.

Neocortex

Neocortex är den yttersta delen av hjärnan. Här uppfattas och tolkas alla våra sinnen. Motoriken styrs från neocortex. Skillnaden mellan motoriken som styrs i reptilhjärnan och neocortex är att reptilhjärnan styr grova rörelser medan neocortex sköter fina och exakta rörelser i till exempel fingrarna. Här finner man även logik, talförmåga och abstrakt tänkande.

Källor

^ Patton, Paul (25 december 2024). ”One World, Many Minds: Intelligence in the Animal Kingdom”. Scientific American. http://www.sciam.com/article.cfm?id=one-world-many-minds. Läst 29 december 2008. ”The traditional ideas about sequential brain evolution appeared, for example, in the late neuroscientist and psychiatrist Paul D. MacLean's triune brain model, formulated in the 1960s. MacLean's model promoted the belief that the human brain contains a “reptilian complex” inherited from reptilian ancestors. Beginning in the 1980s, the field of comparative neuroanatomy experienced a renaissance. In the intervening decades evolutionary biologists had learned a great deal about vertebrate evolutionary history, and they developed new and effective methods of applying Darwin's concept of the tree of life to analyze and interpret their findings. It is now apparent that a simple linear hierarchy cannot adequately account for the evolution of brains or of intelligence.”
^ Butler, Ann; Hodos, William. ”Comparative Vertebrate Neuroanatomy: Evolution and Adaptation”. ”The extensive body of work in comparative neurobiology over the past three decades unequivocally contradicts this theory. First, homologs of the limbic cortical areas that MacLean considers to have been first present in early mammals have been found in non-mammalian vertebrates. Second, homologs of neocortical structures and of dorsal thalamic nuclei have also been found in nonmammals. Third, MacLean's observations on the behavioral differences between mammals and nonmammals are oversimplified and ignore the elaborate social and parental behaviors of some non-mammalian vertebrates, including birds and a variety of ray-finned fishes”

[1] Patton, Paul (25 december 2024). ”One World, Many Minds: Intelligence in the Animal Kingdom”. Scientific American. http://www.sciam.com/article.cfm?id=one-world-many-minds. Läst 29 december 2008. ”The traditional ideas about sequential brain evolution appeared, for example, in the late neuroscientist and psychiatrist Paul D. MacLean's triune brain model, formulated in the 1960s. MacLean's model promoted the belief that the human brain contains a “reptilian complex” inherited from reptilian ancestors. Beginning in the 1980s, the field of comparative neuroanatomy experienced a renaissance. In the intervening decades evolutionary biologists had learned a great deal about vertebrate evolutionary history, and they developed new and effective methods of applying Darwin's concept of the tree of life to analyze and interpret their findings. It is now apparent that a simple linear hierarchy cannot adequately account for the evolution of brains or of intelligence.”

[2] Butler, Ann; Hodos, William. ”Comparative Vertebrate Neuroanatomy: Evolution and Adaptation”. ”The extensive body of work in comparative neurobiology over the past three decades unequivocally contradicts this theory. First, homologs of the limbic cortical areas that MacLean considers to have been first present in early mammals have been found in non-mammalian vertebrates. Second, homologs of neocortical structures and of dorsal thalamic nuclei have also been found in nonmammals. Third, MacLean's observations on the behavioral differences between mammals and nonmammals are oversimplified and ignore the elaborate social and parental behaviors of some non-mammalian vertebrates, including birds and a variety of ray-finned fishes”

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