NOTICE: Posting schedule is irregular. I hope to get back to a regular schedule as the day-job allows.

Friday, April 1, 2011

My name is...

Cerebellum.  *THE* Cerebellum.

*not* the CEREBRUM.


The name of this structure is often confused cerebellum is the small "wrinkly" structure at the posterior (back), ventral (bottom) of the brain.  Put your hand on the back of your head, move down the the ridge of bone that is the lowest part of the skull, right above the neck.  Cerebellum lies directly underneath, it is protected by this bony ridge because damage to the cerebellum affects *all* of your ability to move and coordinate movement. 

The cerebrum, by the way, is essentially the rest of the brain - Occipital, Parietal, Frontal and Temporal lobes - excepting the brainstem and basal nuclei of the brain.

Cerebellum is unusual in being a complete brain structure (in fact, often named as a major lobe of the brain) that has a single function, and is present with that same function in all vertebrates (creatures with a bony spinal cord and distinct multi-lobed brain).  The function, is coordination of movement and sensation.

The picture at the right shows the brainstem (the subject of the next blog) and cerebellum.  H is anterior lobe of cerebellum, I is posterior lobe, and the area underneath G is the flucculonodular lobe.  The latter receives conenctions from inner ear (semicircular canals - for balance) and the visual system.  It is largely responsible for balance and walking movements.

The central portions of the cerebellum receive inputs from the position sensors on muscles and joints throughout the body, as well as the inner ear, then sends projections back to the spinal cord and motor cortex to control the flow of signals to the muscles.  The outer or lateral parts of the cerebellum are involved in movement planning.  The inputs come exclusively from the brain (and not the spinal cord or rest of the body) - in particular, the Parietal Lobe and projects back to thalamus to control movements that are about to occur.

In all, the operation of the cerebellum is not particularly well understood by persons outside a very limited field of researchers.  Most of what we know comes from studies of patients with damage to the cerebellum.  Cerebellar damage results in balance problems, inability to coordinate limb movements, problems with hand-eye coordination, and inability to *plan* a movement without actually *making* the movement.

There is one thing that neuroscientists all know the cerebellum for:  Purkinje cells.  There neurons have the most elaborate dendritic "arborization" - so named for its resemblance to the branches on a very large, very dense tree.  The drawing at left by the neuroanatomist Santiage Ramon y Cajal shows the two dense Purkinje cells at left, and the long horizontal parallel fibers (top) of the granule cells (large cell in center bottom).  The extremely high numbers of synapses possible (billions) due to the long parallel fibers and dense dendritic regions of the Purkinje cells likely contributes to the ability of the cerebellum to integrate inputs from all of the sensory neurons responsible for sensing body position with the motor control and planning regions of the brain.  It has been estimated by some that the cerebellum contains 10x more synapses than the rest of the brain, spinal cord and body, combined.

Next up:  Brainstem, then deep brain nuclei, then spinal cord.  We're almost through with the survey of brain regions, then will go back to fill in some more specific functions of the brain, followed by brain disorders and diseases.  So stay tuned for more of The Lab Rats' Guide to the Brain!

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