The cerebellum is an essential structure in the management and coordination of motor activities. As in the brain, there is a layer of grey substance that covers the brain, called the cerebellar cortex .
This cortex is composed of different types of neurons grouped in different levels or layers. In this article we explain what it is and what are the main characteristics of the cerebellar cortex, and what kind of functions it performs.
What is the cerebellum?
The cerebellum is one of the brain structures with the highest neuronal density and plays a fundamental role in the integration of sensory and motor pathways. It is located behind the upper part of the brain stem (where the spinal cord meets the brain) and consists of two hemispheres or halves.
It receives information from the sensory systems, the spinal cord and other parts of the cerebral cortex, and projects it to other structures involved in processes such as coordination, postural adaptation or movement generation. The cerebellum is essential for precise and balanced muscle activity, as well as for learning motor patterns and in muscle coordination.
At the structural level, the cerebellum can be divided into two parts: the inner white substance, composed of three nuclei of grey substance in each hemisphere that constitute the intracerebellar nuclei; and the cerebellar cortex, the outer part of grey substance and which we will discuss next.
The Cerebellar Cortex: Definition and Structure
The cerebellar cortex is the part of gray substance that forms the covering of the cerebellum. It can be divided into two hemispheres (as with the cortex of the brain), and between them is the vermis, which joins and connects the two parts. The architecture of this cortex is uniform in all its parts, except for the anomalous distribution of the so-called “unipolar brush cells” .
From the inside out, the cerebellum’s cortex comprises the granular layer (or granular cell layer), the pyriform layer (or Purkinje cell layer), and the molecular layer. Let’s take a closer look at what each of these consists of.
The granular layer
This inner layer contains a multitude of granular cerebellar cells, the smallest neurons in the entire brain . They have several short dendrites and a long axon that reaches the molecular layer, where it divides into a “T” shape to form the parallel fibers. The dendrites of the granules (excitatory neurons that use glutamate) enter the constitution of the cerebellar glomeruli (synaptic arrangements formed by mossy fibers and axons of Golgi cells).
In the granular layer there are three other types of neurons: Golgi cells, medium sized neurons with dendrites that connect to parallel fibers; Lugaro cells, medium sized, their axon ends within the same granular layer or reaches the molecular layer; and unipolar brush cells, neurons located almost exclusively in the flocculonodular lobe, are formed by a single dendrite with brush bristle-like ends and receive a single synapse of a muscoid fiber.
The pyriform layer
The piriform layer is made up of piriform or Purkinje cells , a type of gabaergic neurons (with inhibitory effects) that are very bulky. This entire layer is made up of a single row of Purkinje cells surrounded by a special type of glial cell: Golgi epithelial cells, which have processes with a radial course that crosses the molecular layer to reach the surface of the cerebellar cortex.
The dendrites in Purkinje cells are highly developed and cover the molecular layer. Their axon goes deep into the cortex and, unlike other types of cortical cells, ends up in the cerebellar or lateral vestibular nucleus. Along its course, the axon gives rise to collateral branches destined mainly to Golgi cells.
The molecular layer
The molecular layer is the outermost of all and is occupied almost entirely by the dendrites of the Purkinje cells , the parallel fibres and Bergmann’s fibres, as well as the radial processes of the Golgi epithelial cells. The dendritic branches of the Purkinje cells are the most extensive dendritic branches of the entire central nervous system; they are placed at right angles to the parallel fibers, with which they make connections at the level of numerous synaptic spines present at their distal end.
Two different types of gabaergic inhibitory neurons can be found in the molecular layer; near the surface of the cerebellar cortex are located the small stellate cells whose axons project to the main stem of origin of the dendritic Purkinje cell tree.
Other cells called “basket cells” are located next to the pyriform layer and are larger than the stellate cells, with axons that branch out repeatedly and wrap around the cell bodies of the Purkinje cells. Both the basket cells and the stellate cells receive information from the parallel fibres.
As explained above, the most numerous neurons in the cerebellar cortex are the Purkinje cells, which are in charge of processing the information coming from the cortex of the brain. These neurons are activated as they detect and develop movements , and respond selectively to aspects such as muscle extension, flexion or contraction, or the position of the joints (fundamental for coordination and balance).
In recent years, the relationship between the cerebellum and motor learning has been investigated and, for the time being, the results conclude that the absence of the cerebellum cortex would not affect this learning of motor sequences, but it would affect the execution of learned responses.
Furthermore, the cerebellum has also been found to play an important role in the acquisition of targeted behaviours , without it being clear to what extent it contributes to a change in the stimulus-response association and in the optimisation of motor response performance.
Finally, it should be noted that recent research has suggested that Purkinje neurons in the cerebellum would have the ability to release endocannabinoid substances that could impair the potential of synapses (both inhibitory and excitatory).
- Galea, J. M., Vazquez, A., Pasricha, N., Orban de Xivry, J. J., & Celnik, P. (2010). Dissociating the roles of the cerebellum and motor cortex during adaptive learning: the motor cortex retains what the cerebellum learns. Cerebral cortex, 21(8), 1761-1770.
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- Marr, D., & Thach, W. T. (1991). A theory of cerebellar cortex. In From the Retina to the Neocortex (pp. 11-50). Birkhäuser Boston.