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MIDBRAIN
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In humans, the midbrain is an important region
for a diversity of types of processing ranging from muscle contraction
to vision and hearing. It is less
prominent in mammals than other vertebrates since the development of the
cerebrum has adopted some of the functions (such as association) which
are performed in the midbrains of lower vertebrates.
The midbrain is composed of three general regions: the tectum,
the tegmentum, and the isthmus. Hagfish possess a nucleus of the superior
raphe, an interpeduncular nucleus in the isthmus region of the midbrain,
and distinct nuclei in the tegmentum region.
(Butler, 1996, p. 207, 213-6).
The retina projects to the optic tectum in hagfish as in gnathostomes
(Butler, 1996, p. 244) and a prominent choroid plexus is present in the
midbrain of jawless fish (Weichert, 1970, p.621). |
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HAGFISH |
LAMPREY |
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Gnathostome midbrains share a number of regions
such as a cuneiform nucleus, intercollicular nucleus, red nucleus, locus
coeruleus, and a substantia nigra. (Butler,
1996, p. 207-17). The tectum is better developed in gnathostomes
and there is an increase in basal region of midbrain. The midbrain of gnathostomes
affects body position through connections with cerebellum and medulla and
the primitive ependymal region of the midbrain is absent (Ariens, p.1191-3
). In lower vertebrates, there is an optic ventricle
in the superior colliculi which fuses across the midline. (Weichert, 1970,
p.621). |
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In bony fish and tetrapods, the red nucleus
forms part of a rubrospinal tract and a nucleus isthmus exists (Butler,
1996, p. 207-17). A number of commissures
exist in the midbrain and the tori semicirculares are better developed in
bony fish and tetrapods (Ariens, p.1195). |
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PERCH |
LUNGFISH |
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In tetrapods, reticular regions exist in
the midbrain and the optic tectum covers tori semicirculares. Afferent fibers lead to the optic tectum even
in blind amphibians (Ariens, p.1196). |
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FROG![]() |
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Amniotes possess a pedunculopontine nucleus
(Butler, 1996, p. 216-7) and a homolog of the large-celled region of the
nucleus ruber (Ariens, p.1197). Ipsilateral
sensory projections from midbrain travel to the pallium ( |
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TURTLE |
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In mammals, the primitive optic tectum forms
the superior colliculi and the primitive tori semicirculares form the
inferior colliculi (Ariens, p.1202). The
inferior colliculus is exposed on the surface of the midbrain rather than
being deep to the optic tectum (Webster, 1974, p. 259) and a fissure splits
superior and inferior colliculi (Weichert, 1970, p. 621).
Only mammals possess cerebral peduncles.
(Webster, 1974, p. 259) There
are increased connections between the superior colliculus and diencephalon,
an increase in the medial longitudinal fasciculus, the red nucleus is
more prominent, and a pedunculo-tegmental system exists (Ariens, p.1202). The size difference between superior and inferior
colliculi (the superior colliculus is the larger of the two) is greater
in humans than in In most placental mammals, the expansion
of the cerebrum and cerebellum result in the covering the midbrain, ending
the “midbrain exposure” which is typical of primitive mammals and non-mammals. |
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OPOSSUM |
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CAT |
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SHEEP |
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PIG |
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MONKEY |
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HUMAN |
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