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THE CEREBRUM 2
The cerebrum of mammals shares a number of
characteristics. In mammals, the
septum contains the lateral septal nucleus,
medial septal nucleus, nucleus of the diagonal band of Broca, septofimbrial nucleus, and
a triangular nucleus. (
Many of the cells which form part of the
neocortex circuits specific to mammals also exist in other
amniotes (reptiles and birds) (Karten, 1997). Compared to other amniotes, the cells of the
developing neocortex in mammals require more
time to complete cell divisions and undergo more divisions during a prolonged
neurogenetic period (Kornack, 1998).
Certain calcium binding proteins (parvalbumin,
calbindin, and calretin)
are expressed only by certain cells and in distinct layers of the mammalian
neocortex. Placental and marsupial mammals share this fundamental
architecture. The distribution
of these calcium binding proteins is significantly different in monotremes (
The cerebral cortex of therian
mammals possesses 6 cellular layers in the neopallium
(Romer p. 592-4). Insectivores possess a layer corresponding to human layer I, another corresponding to layer II, III, and IV and a third which corresponds to V and VI. In higher placental mammals, II and III are separate from IV. In primates there are subdivisions of layer III and in humans there are subdivisions of layer II (Hill, 2005). Striate (named for a band in layer IV of cortex) and extrastriate
regions exist in the visual cortex (Butler, 1996, p. 385).
In placental mammals, the cerebrum increased in size. Its expansion over the midbrain ends the “midbrain exposure” typical of other groups of vertebrates. Most placental mammals further increase the surface area of the cerebrum by developing gyri and sulci. This wrinkling of brain tissue to increase surface area is not unique to the cerebrum: a number of brain areas have increased their size through folding such as substantia gelatinosa, nucleus laminaris, dentate nucleus , inferior and superior olivary nuclei, lateral geniculate nuclei and cerebellar cortex. Folding of the cerebrum exists in some marsupials. While the majority of placental mammals possess gyri and sulci on the surface of the cerebrum, smooth cerebrums are known in some rodents, insectivores, and prosimians (Ariens, p.1518). The presence of the lipid lysophosphatidic acid (LPA) to the developing brain of a mouse converts its smooth cerebral cortex to a gyrated one, resembling the brain surface of higher mammals (Price, 2004).
A corpus callosum developed in placental mammals to connect the two cerebral hemispheres (although some marsupials have a dorsal corpus callosum; Ariens, p.1466). In placental mammals, the hippocampus is positioned more laterally (Ariens, p.1479).
|While the pattern of gyri and sulci may vary between mammalian groups, there are some folds and grooves which are shared. All mammals share a dentate gyrus, a fissure hippocampus, and a fisura rhinalis (Ariens, p.1413, 1517). The sulcus vallaris of monotremes and marsupials may be homologous to the sulcus genualis in primates and other placental mammals. Therian mammals possess a presylvian sulcus and placental mammals possess the following sulci: intercalates, rostralis, occipito-temporalis, suprasylvian, coronal, ansate (which contributes to the central sulcus in primates), and an interparietal (Simian) sulcus (Ariens, p.1522-50).|
| Primates possess a central sulcus which may or may not appear in other placental groups
(Romer, p. 595). Other gyri
and sulci in primates include the calcarine,
superior, temporalis inferior, postlateralis
(lunatus), and interparietalis
(lateralis) (Butler 1996; Ariens,
p.1523, 1548, 1666). Apes and
Mammals typically possess 4 lobes of the cerebral cortex (Hartman, 1933, p. 284). These lobes can then be divided into a number of regions, often referred to with the term “cortex”. The olfactory cortex contains an anterior olfactory nucleus, the piriform complex, an olfactory tubercle, a cortical amygdale, and an entorhinal cortex (Butler, 1996, p. 268). In monotremes, a motor cortex exists in addition to an area of combined motor and sensory cortex. Marsupials have a posterior visual cortex and a lateral auditory cortex. (Prosser, 1973, p. 690).
All mammals seem to share at least 20 isocortex areas, including the primary visual cortex, the primary somatosensory cortex, and the primary motor cortex (Kaas, 1995). In all
mammals, V1 projects to V2 and from there to several regions medial and
lateral to V2 (Kaas, 1995). Therian
mammals share a secondary somatosensory area
S2 and a central somatic sensory area (Lende,
1964). The external cuneate
nucleus is separated from the cuneate-gracile
nuclear complex (Johnson, 1994). Therians modified the dorsal cochlear nucleus to form an outer
molecular layer, a layer of granule and fusiform
cells, and a core of mixed cell types (Johnson, 1994). Eutherian mammals
share M2 primary and secondary motor fields (Kaas,
1995). Platypuses do not possess
additional sensory areas despite their perception of electrical stimuli
areas of the fronto-parietal cortex homologous
to those found in higher primates, although some prosimian
areas are further divided in higher primates.
Prosimians possess areas which represent
the entire body and other areas which represent only partial areas (Fogassi, 1994). A comparison
of the brains of a prosimian and
cortex 1 has a complete mirror image in anthropoids (monkeys and apes)
(Johnson, 1994). Most therians have
some degree of separation between the claustrum
and the cerebral cortex. In primates
these structures are completely separated (Johnson, 1994). In comparison to closely related rats, naked
mole rats possess a greatly enlarged somatasensory
cortex which occupies the majority of the cerebral cortex usually devoted
to vision. Almost 1/3 of the somatosensory
cortex is devoted to interpreting sensations detected by the incisors
The number of cerebral
areas have increased differently in the various lineages of mammals.
Owl monkeys possess over 20 visual areas, 10
auditory areas, and 15 sensorimotor areas. Macaques may possess more than 30 visual areas
Partial replications of the somatosensory
cortex exist in a number of species, for example, the duplication of the
hand area in Nycticebus
(slow loris) or whiskers in Didelphis (Johnson, 1994). The accessory olfactory bulb has been lost in
The increase in the size of brain regions processing vision contributed to the increase in encephalization of primates compared to other mammals and of anthropoid primates compared to prosimians (Kirk, 2006). In primates, about half of the tectopetal projections are ipsilateral unlike the ancestral state in which all projections are contralateral (Johnson, 1994). The external granular layer of the dorsal cochlear nucleus is lost in embryonic development in apes (Johnson, 1994). Humans involve the anterior cingulate cortex (ACC) in cognitive tasks. The ACC of non-apes lack clusters of spindle cell pyramidal neurons. Orangutans possess some clusters, gorillas possess more, chimps possess even more, and humans possess the greatest number (Uddin, 2004).
Broca’s area is a region of the left motor cortex which controls the muscle movements which are essential for human speech. In nonhuman primates, stimulation of Broca’s area moves muscles of larynx (Butler, 1996, p. 419). Stimulation of the area homologous to Broca’s region in chimps results in vocalizations (Manger, 2005).
axons from the motor cortex proceed to the nucleus
ambiguus (the site of laryngeal motor neurons). There are no cortico-ambigual
connections known in
The prefrontal cortex is enlarged in all the higher apes. Some cerebral asymmetries are known in the brains of chimps although they do not display the same difference in handedness observed in humans (Hill, 2005).
the first ape in the fossil record, possessed a larger brain than
Although the human brain is about 3 times the size of that apes, ape brains would be expected to reach comparable sizes if their brains grew at the same rate of the human brain after birth. The rate of growth of the human brain after birth is similar (although slightly higher) to that observed in chimp brains after birth but the period of rapid growth lasts considerably longer in humans. (Passingham, 1975). The human neocortex is 3.2 times bigger than that of chimps and the size of the human cerebellum 2.8 times the size of the cerebellum of chimps (Passingham, 1975).
Brain size increased in australopithecines but the external morphology of the brain remained apelike (Falk, 1984). Cerebral reorganization in hominids predated the increase in size according to A. afarensis and A. africanus endocasts (Holloway, 1992, Holloway, 2003). The Hadar endocast does not show an expansion of parietal and occipital regions (Falk, 1984). In H. habilis, the cerebrum significantly increased in size in its frontal lobes and parietal lobes. The cerebrum had become taller but not longer and the pattern of gyri and sulci was like humans and unlike apes (Tobias, 1987). Frontal lobes were narrower in archaic Homo sapiens. Modern humans have developed the parietal lobes. The occipital region was shortened and rotated under the parietal and temporal regions (Bruner, 2004). Broca’s area and Wernicke’s areas were enlarged and the structure of the meninges was like that of modern humans. The development of the inferior parietal lobule was like humans and unlike apes (Tobias, 1987).
The encephalization quotient (the ratio of brain size to body size) of Australopithecines is intermediate between that of modern apes and that of species of the genus Homo. Humans have an EQ of 7.4-7.8, that of a chimp is 2.2-2.5, gorilla 1.5-1.8, gibbon 1.9-2.7, old world monkeys 1.7-2.7, capuchin monkeys 2.4-4.8 (and white fronted capuchin 4.8), squirrel monkey 2.3; marmoset 1.7; whales 1.8 and bottlenose dolphin 5.3 (Roth, 2005).
The relative size of the frontal lobe compared to the rest of the brain is slightly higher in higher apes and humans compared to gibbons. It is not higher in humans compared to other higher apes. In humans, the frontal lobes compose 37% (35-38.5%) of the cerebral hemispheres while this value is 35% in bonobos (33.8-37%), 35% in common chimpanzees (33.3-36.2%), 35.1% in gorillas (34.9-35.3%), 36% in orangutans (35.3-37.1%) and 28.4% in gibbons (27.2-29.6%) (Semendeferi, 2000).
In individuals affected by microcephaly, the brain may only reach a third of its normal size (about the size of that found in early hominids) and the gyral pattern is less complex than normal. These individuals have reduced cognitive ability but are otherwise normal. There are two genes whose mutations are known to cause microcephaly.
spindle-like microcephaly associated ASPM is
a large protein which interacts with microtubules and is expressed in
areas where new neurons are produced.
Its homolog in flies is known to function in the organization of
microtubules during cell division. Microcephalin (MCPH1) is related to topoisomerase II-binding
protein and BRCA1. It regulates
chromosome condensation in mitosis and DNA repair. Homologs exist in
bilateran animals (Ponting, 2005). In ape lineages, both of these genes have
undergone positive selection unlike that of other mammalian lineages,
suggesting that they have contributed to brain growth in apes (Ponting,
Neanderthal brain shape was consistent with that of archaic humans while the change in modern human brains has been caused by growth of the parietal lobe (Bruner, 2003). The brain mass of neanderthals was only slightly less than that of modern humans. Representatives of Pleistocene humans actually had a larger brain mass than a sample of modern humans, perhaps due to a decrease in body size since the Pleistocene (Ruff, 1997).
Sleep is only known in animals with vision and which can focus their vision. In many animals, sleep requires that the eyelids are closed and the animal awakes when the eyelids are open.
Some memory consolidation in humans can only occur during sleep.
Although sleep is known in cephalopod mollusks, in the vertebrate lineage it probably originated in fish (Kavanau, 2005).
Cave organisms, tuna, and many sharks don’t
sleep. Most fish do. Sleep may have originated in sharks or bony
fish (Kavanau, 2004).
REM sleep is known in marsupials and throughout placental mammal
groups. It seems not to occur in
reptiles, amphibians, and the echidna (Seigel,
1995). While reptiles do have some of the characteristics of behavioral
sleep, they lack some of the diagnostic features observed in mammals (Nicolai,
2000). Fish can show a behavioral sleep. In turtles, sleep can be associated
with changes in electrical activity (slow wave sleep) like those in mammals
(Prosser, 1973, p. 692).
Sleep occurs only in animals with the ability to focus their vision in both vertebrates and invertebrates. Many animals must stop visual input to the brain by closing their eyelids in order to sleep (Kavanau, 2005). REM sleep reinforces memory (Kavanau, 2005).