Anatomy:

The cerebellum has long intrigued theorists and experimentalists with its near-crystalline anatomical layout. It is divided into a highly folded cerebellar cortex and the deep cerebellar nuclei. Input to the cerebellum flows from many brain areas into the input layer of cerebellar cortex, named granule cell layer. A copy of the input also goes directly to the deep cerebellar nuclei. Granule cells are extremely small and extremely numerous. They send excitatory projections upwards into cerebellar cortex, where they reach both Purkinje cells, the large projection cells of cerebellar cortex, and local inhibitory interneurons. This projection is very unusual in that each granule cell axon splits into exactly two branches that bifurcate at an angle of 180 degrees to run at opposite directions along cerebellar cortex for up to 4 mm. All granule cell axons run in the same direction, which is why this system is also called the parallel fiber pathway. Along their way, parallel fibers pierce through the flat dendritic trees of Purkinje cells, and make at most 1 synapse with each Purkinje cell. Before they bifurcate to form parallel fibers, however, the ascending granule cell axon makes several connections with a single Purkinje cell. In addition, granule cell axons also contact inhibitory interneurons in cerebellar cortex, named stellate cells and basket cells. These neurons in turn make inhibitory synapses with Purkinje cells, and with each other. The only way out of cerebellar cortex is given by Purkinje cell axons, which make inhibitory connections with neurons in the deep cerebellar nuclei. That's right: Cerebellar cortex acts by inhibition, which is the opposite from cerebral cortex. The deep cerebellar nuclei are much smaller than cerebellar cortex, but they are funneling all of cerebellar output (well, nearly all) back to the rest of the brain. This output reaches the spinal cord through the red nucleus and the cerebral cortex through the thalamus.

Physiology:

Electrical recordings have been obtained from the cerebellum since the 1960s, when Eccles et al. made it the focus of their research.  The cerebellum has since become a model system, in which the properties of single neurons are studied in detail, and are being interpreted with respect to system function.  The Purkinje cell in the cerebellar cortex in particular is one of the largest and most complex types of neurons in the brain.  It was one of the first neurons in which dendritic action potentials were discovered (by Llinas et al.), but curiously they were produced by calcium currents, not by the sodium action potential.  Purkinje cells also show plateau potentials, bistability, and bursting.  The functional significance of these properties is still not clear!  The cerebellum is also famous for the property of Long-term depression (LTD) in the parallel fiber – Purkinje cell pathway, which appears to be induced by conjunctive climbing fiber and granule cell activation.  In this system, the weakening of synapses is linked to learning motor behavior!  Recordings of Purkinje cells and deep nuclear neurons have also been obtained in awake behaving animals.  It was found that very fast and complex responses are tightly correlated to the timing of sensory stimuli and movement.  Unfortunately, the correlation with specific properties of the stimuli or movement is not so straight-forward, and is still being worked out.

Function:

The function of the cerebellum has been the topic of many theories and models, maybe more so than any other brain structure.  Early on around 1970, the work by Marr and Albus on possibilities of controlling movement execution based on learnt input patterns excited the whole field.  In fact, the property of LTD was found because of its theoretical prediction by Albus, a feat rarely achieved by theorists of neural function.  To this day researchers don’t fully agree on what the function of the cerebellum is in detail.  It is quite clear, however, that the cerebellum is an extremely fast processing machine, suitable for the on-line control of movement accuracy and error-correction.  It is also quite clear, that the activity of cerebellum is modified by learning.  In some sense, then, the cerebellum improves the control of behavior based on previous experience, and it using a lot of sensory inputs to perform this function.

Cerebellar Overview Chapter and Review Article Selection:

Albus JS. 1971. A theory of cerebellar function. Math.Biosci. 10:25-61.
Bloedel JR. 1992. Functional heterogeneity with structural homogeneity: How does the cerebellum operate? Behav.Brain Sci. 15:666-678.
Bloedel JR and Kelly TM. 1992. The dynamic selection hypothesis: A proposed function for cerebellar sagittal zones. In Llinás R and Sotelo C, editors. The Cerebellum Revisited. New York: Springer-Verlag. p 267-282.
Bower JM. 1997. Is the cerebellum sensory for motor's sake, or motor for sensory's sake:  the view from the whiskers of a rat? Prog.Brain.Res. 114:483-516.
Braitenberg V. 1967. Is the cerebellar cortex a biological clock in the millisecond range? Prog.Brain.Res. 25:334-346.
Braitenberg V, Heck D, and Sultan F. 1997. The detection and generation of sequences as a key to cerebellar function: Experiments and theory. Behav.Brain Sci. 20:229-77.
Eccles JC, Ito M, and Szentagothai J. 1967. The Cerebellum as a Neuronal Machine. New York: Springer-Verlag.
Hansel C, Linden DJ, and D'Angelo E. 2001. Beyond parallel fiber LTD: the diversity of synaptic and non-synaptic plasticity in the cerebellum. Nat.Neurosci. 4:467-475.
Houk JC. 1987. Model of the cerebellum as an array of adjustable pattern generators. In Glickstein M, Yeo C, and Stein J, editors. Cerebellum and Neuronal Plasticity. New York: Plenum Press. p 249-260.
Houk JC, Buckingham JT, and Barto AG. 1996. Models of the cerebellum and motor learning. Behav.Brain Sci. 19:368-383.
Houk JC and Gibson AR. 1987. Sensorimotor processing through the cerebellum. In King JS, editor. New Concepts in Cerebellar Neurobiology. A.R. Liss. p 387-416.
Ito M. 1984. The Cerebellum and Neural Control. New York: Raven Press.
Llinás R and Sugimori M. 1992. The electrophysiology of the cerebellar Purkinje cell revisited. In Llinás R and Sotelo C, editors. The Cerebellum Revisited. New York: Springer Verlag. p 167-181.
Marr D. 1969. A theory of cerebellar cortex. J.Physiol.(Lond.) 202:437-471.
Mauk MD, Medina JF, Nores WL, and Ohyama T. 2000. Cerebellar Function: Coordination, Learning or Timing? Current Biology 10:R522-R525.
Medina JF and Mauk MD. 2000. Computer simulation of cerebellar information processing. Nat.Neurosci. 3:1205-11.
Thach WT, Kane SA, Mink JW, and Goodkin HP. 1992. Cerebellar output: Multiple maps and modes of control in movement coordination. In Llinás R and Sotelo C, editors. The Cerebellum Revisited. New York: Springer-Verlag. p 283-300.
 
 
 

Web links:
Purkinje cell tutorial