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 New World monkeys, but not as complex as that of Old World monkeys and apes (Strata, 2003).

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).

HAGFISH

LAMPREY

SHARK

PERCH

LUNGFISH

FROG

TURTLE

ALLIGATOR

OPOSSUM

CAT

SHEEP

PIG

HUMAN