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THE CEREBELLUM

     The cerebellum is the second largest region of the human brain.  It coordinates motor output, measures body position and balance, and may have a variety of additional roles as well.  The most primitive cerebellums evolved in the first vertebrates.

     The existence of a cerebellum in hagfish and lampreys has been debated.   Some conclude that hagfish possess a very poorly developed cerebellum while others feel that this brain region is completely absent (Butler, 1996; Nieuwenhuys, 2002).  Lampreys possess a simple cerebellum, although they lack Purkinje fibers and cerebellar nuclei.  The cells of the cerebellar cortex are organized in a way similar to those of higher vertebrates with an outer layer of large cells, which may be the precursors to the Purkinje cells of gnathostomes and an inner layer of small cells (Ariens, p. 707; Murakami, 2005).  Lampreys possess a number of cerebellar connections which are also present in gnathostomes including bulbocerebellar, crossed and uncrossed tectocerebellar, cerebellotectal, cerebellotoral, and bello-tegmental connections and the tractus lobo-cerebellaris.   The vestibular nerve projects to the cerebellum in lampreys; in higher vertebrates this nerve enables the cerebellum to measure balance (Ariens, p. 708).

LAMPREY

LAMPREY CEREBELLUM

DRAWING

After the divergence of modern jawless fish, euconodonts, and euphaneropids, fossil jawless fish developed swellings in the region of the cerebellum. These became even larger in placoderms (Janvier, 2008).

     In gnathostomes, the cerebellum is larger than that of lampreys and is composed of an unpaired corpus and 2 lateral auricles.  The corpus cerebellum contains a pars anterior and pars posterior. (Ariens).  In the corpus, there are distinct molecular, Purkinje, and granular layers (Ariens, p. 720).  Trochlear fibers no longer cross the cerebellum and the trochlear nucleus is no longer within cerebellum as in lampreys (Ariens, p. 710).  Dorsal and ventral spinocerebellar tracts exist in cartilaginous fish, although they are largely uncrossed (Ariens, p. 722).  In gnathostomes, the cerebellum is involved in maintaining muscle tone, posture, and the integration of diverse stimuli.  (Hoar,Vol. IV, 1970, p. 54).  There is considerable variation in cerebellum of sharks and rays.

Gnathostome cerebellums express Pax6 while that of the lamprey does not and it is possible that the expansion of the cerebellum is linked to this transcription factor (Murakami, 2005).
CEREBELLUM CEREBELLUM
SHARK BRAINS
    Bony fish evolved a number of modifications of the cerebellum which are shared with tetrapods.  There is a region homologous to the ponto-bulbar body and tractus mesencephalo-cerebellaris and all fibers in the olivo-cerebellar tract cross the midline (Ariens, p. 736).  The cerebellar ventricle is reduced (Ariens, p. 729).  The cerebellum can vary in fish.   In the family Mormyridae, it can extend over the forebrain.  (Hoar,Vol. IV, 1970, p. 50; Webster, 1974, p. 265)

Evidence indicates the teleost fish cerebellum is involved in classical conditioning. Lesions in the cerebellum of teleost fish has produced decreased exploratory behavior, spatial cognition, and autonomic responses involving changing heartrate (Rodriguez, 2005).

PERCH

CEREBELLUM

LUNGFISH

LUNGFISH

     Amphibians possess a single deep cerebellar nucleus (Butler, 1996, p. 194).  Some amphibians have such a reduced cerebellum that it can be considered absent (Ariens, p. 741).

FROG

CEREBELLUM

FROG CEREBELLUM
      Amniotes  share a number of cerebellar features such as a second deep cerebellar nucleus  (Butler, 1996, p. 194), a fissure prima, a posterior fissure, and a pars lateralis (may be homologous to cerebellar hemispheres) (Ariens, p. 750-2).  The following illustration depicts the two deep cerebellar nuclei in the alligator brain (there are three deep cerebellar nuclei in mammals). CEREBELLAR NUCLEI

TURTLE

CEREBELLUM

CEREBELLUM

ALLIGATOR

CEREBELLUM

CEREBELLUM

CHICKEN

CHICKEN

CEREBELLUM
     There are a number of cerebellar changes which mammals share.   The cerebellar hemispheres are greatly expanded in mammals and include a central vermis (Romer p. 581-2, Weichert, 1970).  Mammals (including monotremes) develop an enlarged floccular region; a distinct parafloccular region; anterior, middle, and posterior lobes of the cerebellum; and the fissuras postpyramidalis, prepyramidalis, and uvulo-nodularis (Ariens, p. 775). In primitive mammals, including some insectivores, the cerebellum is visible from a dorsal view of the brain (Shoshani, 2006).Mammals also share the features of three cerebellar peduncles, cortico-ponto-cerebellar tracts (Ariens, p. 798), and three deep cerebellar nuclei: the medial (fastigial) nucleus, lateral (dentate) nucleus, and the nucleus emboliformis.  (Butler, 1996, p. 194).  The older regions of the cerebellum (the vermis, fastigal nucleus, archicerebllum) seem to have functions which are distinct from those of the neocerebellum (Parsons, 2000)

Large variations in the size of the cerebellum are known in mammalian groups, especially in bats, insectivores, whales, primates, and elephants (Weaver, 2005). Elephant brains possess a larger brain than humans, a more complex pattern of gyri and sulci than in humans (but not as complex as that of elephants), a relatively larger cerebellum (18.6% of the brain weight as opposed to 10.3% in humans), and large olfactory bulbs (which retain the olfactory ventricle) (Shoshani, 2006). The cerebellum is visible from the dorsal view of the brain which is a primitive condition retained from primitive mammals (Shoshani, 2006).

The size of the human cerebellum compared to the rest of the brain is less than would be expected of an ape of human size. The human cerebellum composes 11% of the total brain (9.7-15.5%), 14% in bonobos (12.9-14.5%), 16% in gorillas (14.9-17.3%), 11.8% (10.4-13.4%), and 13% in gibbons (10.7 to 14.6%) (Semendeferi, 2000).

Apes expanded their lateral cerebellar lobes compared to Old World monkeys. The lateral cerebellum expanded substantially in the ancestor of apes and humans. Given the importance of the cerebellum in both the planning and execution of motor tasks, visual-spatial skills, and learning, this cerebellar expansion may underlie some of the greater cognitive abilities of apes and humans (MacLeod, 2003). In early hominids, the cerebral hemispheres increased in size while the cerebellum remained relatively small. Neanderthals and Cro-Magnon 1 possess the largest known cerebrum: cerebellum size ration known in primates. In more recent humans, the size of the cerebellum compared to the rest of the brain has increased and the relative size of the cerebrum has decreased. The expansions in the cerebellum seem to allow greater complexity despite decreases in cerebral size (Weaver, 2005).

The regions of the primate brain which are typically used for vocalizations are still active in humans for activities such as laughing and crying. Human speech however, depends on novel brain activity patterns involving the frontal lobe, the basal ganglia, and the cerebellum. As a result, speech is the most complex motor ability in humans which can utilize up to 100 muscles. Damage to the cerebellum can result in speech deficits, such as ataxic dysarthria, suggesting that the cerebellum has roles in both the timing of speech and the formation of an internal verbal code prior to speech. One of the genes whose modification in human lineages seems to be critical in the acquisition of human speech is the FOXP2 gene. This gene is active during the fetal development of brain regions which include the cerebellum and mutations cause a number of neurological deficits, including speech problems (Ackermann, 2008).

A rare human condition known as dysequilibrium syndrome is caused by abnormal development of the cerebellum (especially the vermis) and variable abnormalities of the cerebrum. The individuals may display mild retardation and although they are capable of complex coordinated movements and are able to move on their hands and legs, they are unable to walk bipedally (Skoyles, 2006).

DRAWING DRAWING

OPOSSUM

CEREBELLUM

CEREBELLUM

CAT

CAT CEREBELLUM

PIG

PIG BRAIN

PIG BRAIN

SHEEP

CEREBELLUM

CEREBELLUM
MONKEY

HUMAN

CEREBELLUM

CEREBELLUM
CEREBELLUM CEREBELLUM