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MEDULLA
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MEDULLA The human medulla controls a number of phenomena
such as heartrate, breathing, swallowing, hiccupping,
and vomiting. It possesses a number
of nuclei and tracts. Many of these
functions and the anatomical regions which give rise to them evolved early
in the history of vertebrates. In jawless fish, the medulla is a large region.
It composes up to half of the
length of the brain and it is the best developed part of the brain. It possesses a choroid
plexus and contains the nuclei of cranial nerves V through X (Hardisty, p. 312-3, Weichert, 1970,
p.617). In the lamprey medulla, the nucleus magnocellus
inferior includes a fasciculus solitarius,
like that of gnathostomes (Ariens,
p.522 ). Jawless
fish, gnatostome fish, and salamanders possess
giant cells (Mauthner’s cells) in the medulla. In bony fish and tetrapods,
the sulcus limitans
of the medulla which divides the dorsal sensory and ventral motor potions
of the medulla is less obvious (Ariens, p.338). In amniotes, the dorsal column nucleus in the
hindbrain is partially subdivided, although in reptiles it is not divided
to the degree observed in mammals (Pritz, 2002). The medulla of amniotes includes a superior
olive nucleus (although there are the beginnings of this nucleus in amphibians),
a true lateral lemniscus tract, a superior vestibular
nucleus of Beccari (the nucleus of Bechterew in mammals), a nucleus angularis
(the homolog of mammalian dorsal cochlear nucleus), a more lateral position
of visceral efferent nuclei, ventrolateral visceral
nuclei, and the nuclear masses and tracts of cochlear system which are
similar to those of mammals. There
are no longer any lateral line organs or nerves (which are present in
fish and some aquatic amphibians, Ariens, p.466-560). In mammals, part of the lateral lemniscus proceeds to the medial geniculate
nucleus of thalamus and from there to the auditory cortex. Mammals possess an external cuneate nucleus, a nucleus medialis,
and a nucleus of von Monkow. The nucleus of Bechterew
extends farther caudally than in reptiles (Ariens,
p.485-6, 502). In placental mammals,
the nucleus magnocellularis is positioned more ventrolaterally
and the
tuberculum acusticum and ventral
cochlear nucleus are enlarged (Ariens, p.495-6). In prosimians, there is a greater segregation of inputs from
the hand to the cuneate nucleus than those of
the foot to the gracile nucleus. The clustering of cells in the cuneate nucleus is similar between prosimians
and THE RETICULAR FORMATION The reticular formation represents the sum
of nuclei interspersed in the brainstem’s white matter which have roles
in regulating heart rate, breathing, sleep, motor movements, swallowing,
dreaming, and many other functions. All
vertebrates possess a reticular formation. (Webster,
1974, p. 256). Lampreys possess a number of nuclei in their
reticular formation, including mesencephalic,
superior, medius, and inferior nuclei (Butler,
1996, p. 169). In cartilaginous
fish, these nuclei are subdivided and additional nuclei are present. These nuclei include the nuclei reticularis ventralis, parvocellularis, magnocellularis,
gigantocellularis, pontis
caudalis, pontis oralis, locis coeruleus,
subcoeruleus, mesencephali
(cuneiformis), and a number of nuclei of the
raphe group. All these
nuclei are present in mammalian reticular formations (Butler, 1996, p.
169). Jawless fish possess neuromeres and reticulospinal neurons
(Murakami, 2005). |
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In amniotes, an additional nucleus is present,
the nucleus reticularis pedunculopontis
(which in mammals is subdivided into pars compacta
and pars dispersa regions). (Butler,
1996, p. 170). In mammals,
a number of additional nuclei of the reticular formation exist such as the
nuclei reticularis paramedianus,
ventralis, dorsalis, and lateralis. (Butler, 1996, p. 171). |
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HAGFISH |
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LAMPREY |
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SHARK |
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PERCH |
LUNGFISH |
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FROG |
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TURTLE |
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ALLIGATOR |
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OPOSSUM |
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CAT |
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SHEEP |
PIG |
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HUMAN |
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