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THE BRAIN
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The human brain can answer calculus problems,
compose music, coordinate the muscles for a dance routine, remember the
junior prom, and generate anger towards the driver who abruptly entered
your lane. The human brain is composed
of six major regions, each of which contains important subregions. The brain includes spaces through which cerebrospinal
fluid passes, regions which secrete hormones, and 12 cranial nerves.
How could the most complex structure in the known universe evolve?
Brains, including those of the human lineage, have evolved over
hundreds of millions of years through many transitional steps. Cnidarians (jellyfish, coral, Hydra ), only achieve
the tissue level of organization and also have the primitive condition
of radial symmetry. Rather than
having right and left halves, there is more than one plane of symmetry
around a central axis. In these radially
symmetrical animals, the nervous system consists of a diffuse nerve net. Since sensory structures are simple and equally
distributed on all sides of the body, there is no need for a concentration
of nervous tissue (that is, a brain) in any one part of the body. |
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A
group of jellyfish called the ctenophores (comb jellies) are elongated and
possess a gravity-detecting statocyst at one end
of the body. There is some evidence
that ctenophores are the sister group of bilaterans. There are a few examples of a concentration
of neural tissue in cnidarians. Marginal
ganglia of Scyphomedusae
show a slight centralization and in ctenophores the ganglion is associated
with a sense organ (aboral) at one pole of the
body.(Beklemishev, vol. 2 p. 79-80).
In cnidarian larvae, nervous tissue may
concentrate in anterior end of the body (Hyman). Hydra and bilaterans
both express homeogenes of the Prd
group, nuclear orphan receptors COP-TF, msh (or the Antennapedia
group) and thrombospondin in the developing head
or neural tissue (Miljkovic-Licina, 2004). |
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All animals with organs have bilateral symmetry:
the only way to divide them into two equal halves is to divide them into
right and left halves. Sense organs
tend to be located at one end, the end that is usually propelled forward. With sense organs concentrated at the front
end of early bilaterans, nervous tissue began
to accumulate there to process this sensory input, thus forming a brain. |
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The most primitive flatworms (the group called
Acoela) are the most primitive bilateral animals.
Their nervous system consists only of a diffuse nerve net, like that
of cnidarians. |
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Higher flatworms possess the first central
nervous systems. In addition to a
primitive nerve net, they possess longitudinal nerve cords and a cerebral
ganglion (“brain”), although it is not well developed. The brain develops around a statocyst, homologous to the statocyst
of ctenophores. These first “brains” developed at the end of the animal
which was becoming specialized to deal with sensory information. This brain has little connection with or control
over the diffuse nerve net throughout the rest of the body (Beklemishev, vol. 2, p. 50-1; Raikova,
2000). There are cranial nerves which
consist of short pairs of nerves which stretch from the brain to the pharynx
and regions of the head (Rieger, from Harrison,
1991). |
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In the brain of urochordates,
there exists a neural gland which may be homologous to the pituitary gland
(Hickman 714). As larvae they possess
a dorsal nerve cord and a cerebral eye (Hickman, 716). In larvae, there are a group of cranial nerves
(2 to 5) proceeding from the brain (Beklemishev,
vol. 2, p. 137-8). A vesicle in the
tunicate brain may be homologous to the saccus
vasculosus present in the brains of many fish (Svane, 1982). There
is no trace of a telencephalon or an olfactory
region (Nieuwenhuys, 2002).
Of the 335 cells in the tuncate brain,
70% are a group of glial cells called ependymal
cells (Nieuwenhuys, 2002). |
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AMPHIOXUS BRAIN |
In Amphioxus, there are paired nerves leaving
the CNS from the anterior end (the brain) but towards the posterior end
they begin to alternate. Although
lancelets possess a definite brain, the CNS is actually wider in the middle
of the body than at the anterior end (Willey, p. 82). The first two cranial nerves carry sensory information
only (as in vertebrates) and innervate the tip of the head. The brain includes both a cerebral vesicle and
a medulla-like posterior region. There
are no ventricles (Willey, p. 90-1). In lancelets as in vertebrates, a diffuse nerve
net exists along the digestive tract which may be a derivative of the
ancestral nervous system (Beklemishev, vol.
2, p. 137) |
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AMPHIOXUS BRAIN![]() |
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Although hagfish are craniates, they are
the most primitive craniates. Their
brains lack a cerebellum and a pineal complex.
Lamprey brains more similar to those of gnathostomes. The
majority of fiber tracts in lampreys have homologs
in gnathostomes (Nieuwenhuys,
2002). The brain of a larval lamprey
is depicted in the adjacent photo. |
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SEGMENTATION Many animals possess segmented regions of
their body, including the nervous system.
Flatworms are the most primitive animals which possess brains and
there is some evidence of compartmentalization of the flatworm (planarian)
brain determined by differing patterns of gene expression (Marshal, 2003). The vertebrate nervous system is segmented,
although different segmentation patterns are observed in different parts
of the nervous system. In the diencephalon, distinct prosomeres
are more obvious through the analysis of molecular differences between
cells rather than by an obvious macroscopic segmentation.
In contrast, the hindbrain forms a series of macroscopic rhombomeres and the spinal cord is obviously segmented with
its metameric repetition of nerves. The nervous system of invertebrates is often
segmented as well, although it is not obvious to what extent the segmentation
of vertebrate and invertebrate brains is related (Mazet,
2002). Note
the segments (rhombomeres) in the hindbrain
of the chick. |
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The vertebrate brain begins its development
as a tube whose separate regions differentiate into the brain regions
found in the adult. |
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Are
the divisions of vertebrates and advanced invertebrates homologous? In other words, were some of the divisions of
the brain of advanced vertebrates inherited from invertebrate ancestors? Molecular and developmental evidence supports
the conclusion that the brains of bilateran
animals are homologous organs which are derived from a common ancestral
structure (Reichert, 2001). Both
vertebrates and higher invertebrates (such as Drosophila)
have the common feature of a brain which is divided into a forebrain,
midbrain, and hindbrain with an important midbrain/hindbrain boundary
characterized by the expression patterns of Otx, Hox, and Pax2/5/8
genes (Hirth, 2003).
The brain of urochordates seems to be
divided into three regions: a forebrain/midbrain (as indicated by the
expression of Hroth, the homolog
of vertebrate Otx),
an anterior hindbrain (as indicated by the tunicate homolog of vertebrate
Pax genes), and
a posterior hindbrain (as indicated by the expression of a tunicate Hox gene) (Wada, 1998). Gene expression patterns (such as those
of AmphiFoxB
and AmphiSim)
indicate that the cerebral vesicle of Amphioxus
is organized into regions homologous to the vertebrate diencephalon
and midbrain. Posterior to this,
there is a region of the neural tube which expresses Hox genes, as does the vertebrate
hindbrain, although it does not form obvious rhomobmeres. Nevertheless, there are segmented blocks of
tissue revealed by the expression pattern of
AmphiFoxB which may represent an early stage
in rhombomere evolution in which the signals for segmentation
originated from surrounding tissues rather than retinoic acid gradients
in the nervous tissue as in vertebrates (Mazet,
2002). The lancelet hindbrain also reveals segmentation in the expression
pattern of islet1 (a gene important in developing
motor neurons). Islet1 expression also reveals that Amphioxus may have homologs of the pineal
gland and adenohypophysis (Jackman,
2000). In coelomate animals,
the gene Evx
has also been recruited into the development of the nervous system. In vertebrates (but not Amphioxus), the role of Evx in the nervous system is augmented to include expression
in the midbrain-hindbrain boundary (MHB) which is an important organizing
region for the vertebrate brain. (Ferrier, 2001). Pax6 expression patterns help to mark the
diencephalon-mesencephalon border in lampreys
and gnathostomes (Derobert,
2002). The midbrain-hindbrain boundary
is marked by the expression patterns of a number of additional genes such
as Otx-2, Wnt-1, FGF8, En (2,5,8),
and Pax(2,5,8) (Hall, 1999). Hox genes are involved
in the segmentation of the hindbrain.
For example, Hoxb-1 is
produced only in rhombomere 4 while Hoxa3 is
expressed at the boundary between rhombomeres
4 and 5 (Hall, 1999). In vertebrates, the boundary of the diencephalon and mesencephalon includes
a structure known as the subcommissural organ
(SCO) which secretes products important in the development of the nervous
system, including the spinal cord. A
protein secreted here, SCO-spondin, is homologous
with F-spondin secreted from the floor plate
and thrombospondin. The
SCO-spondin gene is present in all chordates
and there is some evidence that it exists in more basal deuterostomes
as well. The infundibular
organ in Amphioxus may be homologous to the subcommissural
organ of vertebrates (Gobron, 1999). Hagfish possess a cerebrum, diencephalon, midbrain, and medulla, as do all vertebrates.
Lampreys and all vertebrates possess a cerebellum.
Mammals possess a pons. The brains of vertebrates do not seem to be
unrelated structures, but rather homologous organs which are modified
versions of a common ancestral plan. |
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Note
that the brains of the following diverse organisms are composed of the
same regions. Also note the gradual increase in the overall size of the
brain through the taxonomic groups. |
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HAGFISH |
LAMPREY |
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SHARK |
PERCH![]() |
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LUNGFISH |
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FROG |
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TURTLE |
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OPOSSUM PIG |
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MODEL OF HUMAN BRAIN![]() |
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Placental
mammals studied to date share the production of new neurons in adults
in the dentate gyrus of the hippocampus and
the subventricular zone lining the lateral ventricles (Bernier,
2002). |
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