cerebellum importance

Posted comment on ´Woman of 24 found to have no cerebellum in her brain` by H. Thomson published in New Scientist 13th September 2014 issue 2986

SUMMARY

The author of this article described the published findings of a hospital in China who using a CAT scan diagnosed the reason for a female patient`s dizziness and nausea symptoms as the absence of her cerebellum. The site was instead filled with cerebrospinal fluid. Only 8 other patients in the world share the defect with most sufferers dying young.

The cerebellum is important for functions such as the control of voluntary movements, balance, and motor actions consisting of sequences such as speaking, throwing and makes up 10% of the brain`s total volume, but has 50 % of its neurons. Thomson reports that the cases of cerebellar agenesis highlight the adaptability of the brain with other areas particularly the cortex taking over important cerebellar functions. This particular patient reached adulthood, but reported moderate movement problems (walking steadily for most of her life, but first walking was late at the age of 7) and moderate speech problems (speech understandable by others at the age of 6, but with slurring).

COMMENT

Thomson reports that the case of the 24 year old suffering with cerebellar agenesis demonstrates the adaptability of the brain to counteract functional and structural deficiencies. The demands placed on plasticity with regards to the cerebellum are huge since the cerebellum has over 85 billion neurons and is functionally important for sequence activities such as motor movements and language. Its strength lies in its structure and connections to other brain areas and according to researchers its evolutionary development was faster than the neocortex.

The cerebellum consists of two lateral hemispheres separated by a midline region (the vermis) and is divided into lobules. The whole structure is covered with the cerebellar cortex. One of the things that make the cerebellum interesting from a neurochemical point of view is the long-term depression observed in the cerebellar cortical Purkinje cells as a result of learning. Although these cells as a result of motor input are activated during the learning process in conjunction with the sensory input via the cortex, cerebellar granular cell layer cells and parallel fibres, when the event is learnt and the stimulus re-encountered then the activation is at a lower level (long-term depression, LTD). In other situations paired activation would lead to stronger firing (long-term potentiation, LTP). Research on the Purkinje cells demonstrates that the LTD is a result of AMPA receptor binding and calcium ion entry, consistent with other examples of this type of long-term cellular effect.

The cerebellum has many connections to other brain areas and these are important for its various functions. The movements of the limbs in terms of direction, force and timing begin with activity in the sensorimotor cortex, then pons, lateral cerebellum and back to the motor cortex via the ventral lateral nucleus of thalamus. Connections to the posterior parietal cortex is involved in spatial awareness; superior temporal gyrus and language; posterior parahippocampal area and spatial memory; prefrontal cortex planning, reasoning and working memory; and connections from the brainstem to the limbic system for emotions just to name a few.

Cerebellar diseases correlate to functional and structural deficiencies and location of lesions of the cerebellum. For example, sensorimotor information is processed in lobules V, VI and VIII and therefore defects in these regions elicit sensory motor defects; visuo-spatial processing occurs in the left hemisphere and therefore lesions on the left have a greater effect on performance than those on the right. One disease, the cerebellar cognitive affective syndrome, is a condition that demonstrates the link between the cerebellum, cortex and limbic system and hence cognitive deficiencies can occur in areas such as working memory, visual processing, spatial processing, planning, attention, emotions. Hence, the cerebellum is not just associated with motor function, but also cognition.
In the case of cerebellar agenesis, it appears that deficiencies may be not as serious as one might think considering the cerebellum`s multifunctional, multiple connections nature and this is attributed to brain plasticity. Subject HC who suffered from cerebellar hypoplasia worked as a manual labourer and died at the age of 76. He did suffer however from slurred speech, visual problems (squint) and problems with his gait. In the case of subject HK (age 59) he was diagnosed when he experienced a sudden loss of hearing, but symptoms already existed (motor, oculomotor, late developing language and slurred speech, gait problems. HK also demonstrated neuropsychological deficiencies such as planning behaviour, visual, verbal and spatial memory, visuospatial perception and attention. Therefore, the symptoms of the 24 year old described by Thomson agree in general with the deficiencies of others suffering with the same defect. The age of these subjects support the hypothesis that other parts of the brain takes over the functions normally associated with the cerebellum since all managed to survive well into adulthood.
Since we´re chatting about the topic ……
…can we assume that information gathered from looking at the sequences of firing cells (Marrs-Albus theory of motor learning) within the cerebellar cortex representing sequences of motor actions can be applied to firing of cerebral cortical cells representing visual movement.
….if there are two groups of Purkinje cells, one linked to stimulation of the medulla in response to motor input and the other activated by the parallel fibres arising from the granular cell layer and activated by the pons and sensory input, that it is only the latter group that experiences LTD after learning because sensory input is only required for error monitoring and this can be reasonably satisfied by direct connection of the granular cell layer to the molecular layer. The motor sequence can continue without sensory input if necessary (e.g. tying shoelaces in the dark).

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