The cerebellum adjusts to changes in sensorimotor relationships, possibly functioning as in the Marr-Albus theory: Strong inputs from a single climbing fiber serve as a teaching signal to change the strength of impulses from the corresponding group of parallel fibers.
Four principles of cerebellum function have been identified. They include: feedforward processing, divergence and convergence, modularity, and plasticity.
These cells receive excitatory input from mossy fibers that originate from pontine nuclei.
The cerebellum is a region of the brain that plays an important role in motor control. It may also be involved in some cognitive functions such as attention and language, and in regulating fear and pleasure responses, but its movement-related functions are the most solidly established. The cerebellum does not initiate movement, but it contributes to coordination, precision, and accurate timing.
It receives input from sensory systems of the spinal cord and from other parts of the brain, including the cerebral cortex, and integrates these inputs to fine-tune motor activity. Because of this fine-tuning function, damage to the cerebellum does not cause paralysis, but instead produces disorders in fine movement, equilibrium, posture, and motor learning.
The cerebellum differs from most other parts of the brain, especially the cerebral cortex, in regards to the ability of signals to move unidirectionally from input to output. This feedforward mode of operation means that the cerebellum cannot generate self-sustaining patterns of neural activity, in contrast to the cerebral cortex. However, the cerebellum can receive information from the cerebral cortex and processes this information to send motor impulses to the skeletal muscle.
In terms of anatomy, the cerebellum has the appearance of a separate structure attached to the bottom of the brain, tucked underneath the cerebral hemispheres. The surface of the cerebellum is covered with finely spaced parallel grooves, in striking contrast to the broad irregular convolutions of the cerebral cortex. These parallel grooves conceal the fact that the cerebellum is actually a continuous thin layer of tissue (the cerebellar cortex), tightly folded in the style of an accordion.
Within this thin layer are several types of neurons with a highly regular arrangement, the most important being Purkinje cells and granule cells. This complex neural network gives rise to a massive signal-processing capability, but almost all of its output is directed to a set of small, deep cerebellar nuclei lying in the interior of the cerebellum.
In addition to its direct role in motor control, the cerebellum is also necessary for several types of motor learning, the most notable one being learning to adjust to changes in sensorimotor relationships.
Several theoretical models have been developed to explain sensorimotor calibration in terms of synaptic plasticity within the cerebellum. Most of them derive from early models formulated by David Marr and James Albus, which were motivated by the observation that each cerebellar Purkinje cell receives two dramatically different types of input.
It receives input from thousands of parallel fibers, each individually very weak. However, each cerebellar Purkinje cell also gets input from one single climbing fiber, which is so strong that a single climbing fiber action potential will reliably cause a target Purkinje cell to fire a burst of action potentials.
The basic concept of the Marr-Albus theory is that the climbing fiber serves as a teaching signal, which induces a long-lasting change in the strength of synchronously activated parallel fiber inputs. Observations of long-term depression in parallel fiber inputs have provided support for theories of this type, but their validity remains controversial.
Insights from Cerebellar Dysfunction
The strongest clues to the function of the cerebellum have come from examining the consequences of damage to it. Animals and humans with cerebellar dysfunction show, above all, problems with motor control. They continue to be able to generate motor activity, but it loses precision, producing erratic, uncoordinated, or incorrectly timed movements.
A standard test of cerebellar function is to reach with the tip of the finger for a target at arm's length. A healthy person will move the fingertip in a rapid straight trajectory, whereas a person with cerebellar damage will reach slowly and erratically, with many mid-course corrections.
Deficits in non-motor functions are more difficult to detect. Thus, the general conclusion reached decades ago is that the basic function of the cerebellum is not to initiate movements, or to decide which movements to execute, but rather to calibrate the detailed form of a movement.
The comparative simplicity and regularity of the cerebellar anatomy led to an early hope that it might imply a similar simplicity of computational function. Although a full understanding of cerebellar function remains elusive, at least four principles are identified as important: 1) feedforward processing, 2) divergence and convergence, 3) modularity, and 4) plasticity.
Feedforward processing: Refers to the unidirectional
movement of signals through the system from input to output, with very little
recurrent internal transmission. This means that the cerebellum, in contrast to
the cerebral cortex, cannot generate self-sustaining patterns of neural
activity. Signals enter the circuit, are processed by each stage in sequential
order, and then leave.
Divergence and convergence: The 1000 or so Purkinje
cells belonging to a microzone may receive input from as many as 100 million
parallel fibers, and focus their own output down to a group of less than 50 deep nuclear cells. Thus, the cerebellar
network receives a modest number of inputs, processes them very extensively
through its rigorously structured internal network, and sends out the results
via a very limited number of output cells.
Modularity: The cerebellar system is functionally divided
into independent modules. All modules have a similar internal structure, but with different inputs and outputs. The output of one module does not appear to
significantly influence the activity of other modules
Plasticity: The synapses between parallel fibers and
Purkinje cells, and the synapses between mossy fibers and deep nuclear cells,
are both susceptible to modification of their strength. The influence of each
parallel fiber on nuclear cells is adjustable. This arrangement gives
tremendous flexibility for fine-tuning the relationship between the cerebellar
inputs and outputs.