The oldest part of the cerebellum is the flocculonodular lobe, which has connections with the vestibular apparatus and is particularly concerned with balance and eye movements.

The anterior lobe has connections with the spinal cord and is concerned with coordination of the musculature.

The posterior lobe developed greatly when primates evolved and adopted the upright posture; at the same time the forebrain expanded and there are important connections between these two.

The cerebellum is concerned with the control of movement, integrating signals from different parts of the nervous system and generating error signals that allow adjustments to be made to achieve the desired objective of the movement.

In cerebellar disease, voluntary movements can become grossly exaggereated and unsuccessful attempts to correct them result in a tremor that occurs during voluntary movement.

The cerebellum receives copies of the messages (motor commands) sent down the corticospinal tract.

The cerebellum compares these commands, with feedback information (information from proprioceptors, vision, touch, hearing) and decides if the command is being accomplished, and whether there are errors.

Information about any errors is returned to the cortex, so that it can adjust the command signal.

The first pathway - a copy of the command message issued by the motor cortex - involves cortico-pontine and ponto-cerebellar neurones.

The sifting of feedback signals to extract information about errors is done by Purkinje cells, the principal cell of each cerebellar module.

Cerebellar Connections

The pontine fibres that feed into the cerebellum are known as 'mossy fibres' because of the ultrastructure of their terminals.

Mossy fibres supply the cerebellar cortex with information from many areas of the cortex, concerned with the position of the body, information from the visual and auditory fields, and the position of objects that are touching the skin.

A second input to the cerebellum arises in the inferior olive (in the medulla) and these fibres climb around the Purkinje cells and are known as 'climbing fibres'. The two inputs to the cerebellum are compared within the cerebellar cortex, so as to provide the cortex with information about errors in movement. The climbing fibres may also be concerned with selecting appropriate movements while learning motor skills.

The communication of error signals to the cortex depends on the output of Purkinje cells, through the deep cerebellar nuclei to the cortex (with synapses in the red nucleus and the thalamus).

The cerebellar cortex has three layers -

• a surface molecular layer of parallel fibres, consisting of the axons of granule cells and the dendrites of Purkinje cells
• the innermost granular layer, consisting of the cell bodies of the granule cells are densely packed in the innermost layer
• the Purkinje cell layer, one cell in thickness, between them, and is the main output pathway from the cerebellar cortex.

The Cerebellar Cortex is where comparisons are made between the command signal and the feedback signals from proprioceptors, touch, vision and hearing.

A copy of the command signal is carried by ponto-cerebellar neurones whose axons are called 'mossy fibres', on account of the structure of their terminations on granule cells.

Granule cells in the cerebellar cortex have axons that pass towards the surface of the molecular layer and bifurcate, sending their collaterals in opposite directions within the molecular layer; there are enormous numbers of parallel fibres in the molecular layer.

Parallel fibres make contact with the dendrites of Purkinje Cells in an arrangement that appears like the wires running between telegraph poles or pylons; but each group of Purkinje cells makes contact with a slightly different set of parallel fibres.
The synapses between the parallel fibres and Purkinje cells are excitatory and glutamatergic.

Purkinje cells can therefore extract information from the very large number of parallel fibres in the molecular layer, and pass it to the deep cerebellar nuclei, where they release GABA and have inhibitory effects.

Inputs to Purkinje Cells Purkinje cells have two types of input:

• the parallel fibres of the molecular layer (the axons of granule cells), and
• climbing fibres which originate in the inferior olive and have a very strong excitatory influence on the Purkinje cells.

The parallel fibres of the cerebellar cortex arise from granule cells and are excited by axons originating in the pontine nuclei, that carry a copy of the motor command sent to the motoneurones.
These axon terminals on granule cells are called mossy fibres, because of the tufted appearance of their synapses, and they also have axon collaterals that end on neurones of the deep cerebellar nuclei.

Climbing fibres. The name 'climbing fibers' is used because of the way in which the axon terminals wrap themselves around the Purkinje cell dendrites. Climbing fibres arise from the contralateral inferior olivary nucleus and excite Purkinje cells

Climbing fibres arise from the contralateral inferior olivary nucleus and excite Purkinje cells AND an associated cluster of neurones in the deep cerebellar nuclei. Also Mossy fibres have excitatory axon collaterals on the deep cerebellar nuclei.
In contrast, the Purkinje input to the deep cerebellar nuclei is inhibitory.

From a functional point of view, the cerebellum is divided into hundreds of independent modules, all of which have a similar basic structure, but each is capable of handling information from different sources, and relays the module output to a different destination.

Each module consists of

• a small group of neurones in the inferior olive that give rise to climbing fibres,
• a spatially discrete group of Purkinje cells and
• a few neurons in a deep cerebellar nucleus on which the Purkinje cells synapse.

The essential function of each module is to provide feedback, mainly to the cortex, concerning errors in movement.

Divergence and Convergence
While the number of input cells and output neurones in the cerebellum may be relatively small, there is considerable divergence in the input system, resulting in millions of parallel fibres within each module. This is the consequence of the massive numbers of granule cells present in the granular layer.

There is also considerable convergence on to the output system. Each Purkinje cell has excitatory inputs from many parallel fibres.

Cerebellar modules are unidirectional , i.e. they carry infomation in one direction through the module, without internal feedback; this arrangement is known as a feedforward system. The information processing is therefore rapid.

So cells of the deep cerebellar nuclei receive excitatory inputs from mossy and climbing fibres, but inhibitory inputs from the Purkinje cells.

Many neuroscientists believe that the cerebellum is involved in motor learning. It has been suggested that the climbing fibres, with their powerful inputs to Purkinje cells, are important in selecting when the appropriate combinations of inputs (from parallel fibres) characteristic of a particular movement occurs.