The cerebellar cortex consists of three layers. The innermost layer, the granule cell layer, is composed of 5 × 1010 small, densely packed granule cells. The intermediary layer, the Purkinje cell layer, is only one cell thick. The molecular layer is made up of granule cell axons and Purkinje cell dendrites, as well as a few other cell types. The Purkinje cell layer separates the granule and molecular layers.
Granule cells are the body’s tiniest cells. Granule cells, which make up the vast bulk of the cerebellum’s neurons, are extremely small, tightly packed neurons. Cerebellar granule cells create more than half of all neurons in the brain. Mossy fibre input is received by these cells, which then sends it to Purkinje cells.
The Purkinje system is made up of cells called Purkinje cells. Purkinje cells are one of the most visible cell types in the mammalian brain. Its apical dendrites have a large fan of finely branched processes. Surprisingly, when viewed from the side, this dendritic tree appears to be almost two-dimensional. In addition, all Purkinje cells are in a straight line. This arrangement has important functional ramifications, as we’ll see shortly.
Cells of various sorts In addition to the major cell types, the cerebellar cortex features a variety of interneuron types, including the Golgi cell, basket cell, and stellate cell (granule cells and Purkinje cells).
Connectivity. The cerebellar cortex has a consistent connection pattern that spans throughout the entire organisation. Cerebellar input is divided into two categories.
Mossy fibres are excitatory projections in the cerebellar cortex from the pontine nuclei, spinal cord, brainstem reticular formation, and vestibular nuclei to the cerebellar nuclei and granule cells. Mossy fibres get their name from the tufted appearance of their synaptic connections with granule cells.
The mossy fiber-granule cell interaction is substantially different since each mossy fibre innervates hundreds of granule cells. The granule cells’ axons travel to the cortical surface. Each axon bifurcates in the molecular layer, sending collaterals in opposite directions. Parallel fibres go parallel to the folds of the cerebellar cortex, where they connect to Purkinje cells to form excitatory synapses. Purkinje cell dendrites have perpendicular two-dimensional arbours to the parallel fibres. Purkinje cells and parallel fibres are arranged in a way that resembles the flow of telephone lines between poles.
Due to the great degree of divergence of the mossy fiber-granule cell synapses, thousands of mossy fibres can impact (disynaptically) the firing of one Purkinje cell.
The inferior olive sends excitatory projections to the cerebellar nuclei and Purkinje cells, which are known as climbing fibres. Climbing fibres get their name from their axons, which wrap around the Purkinje cell’s dendrites like a climbing vine. A single ascending fibre provides a single, extraordinarily strong input to each Purkinje cell. Unlike mossy and parallel fibres, ascending fibres only make 300 connections with each Purkinje cell on average.
As a result, the ascending fibre provides Purkinje cells with a restricted but extremely powerful excitatory input.
The Purkinje cell is the cerebellar cortex’s only source of output. Purkinje cells have inhibitory connections to the cerebellum’s nuclei, which is crucial. Parallel-fiber inputs account for over 80% of the spikes generated by the Purkinje cell. The Purkinje cell fires at a high resting rate (70 spikes/sec) in response to these inputs, tonically suppressing its cerebellar nucleus targets. Climbing fibre inputs are more strong but occur less frequently (1 spike/sec), therefore they only have a little impact on the total firing rate of the Purkinje cell. Purkinje cell spikes in climbing fibres, on the other hand, are calcium-spikes, allowing the climbing fibres to activate a variety of calcium-dependent Purkinje cell changes.