Cerebellar cortex: what is it, its layers and functions?

What is the cerebellar cortex and what are the neural layers that compose it?

The cerebellum is an essential structure in the management and coordination of motor activities. As in the cerebrum, there is a layer of gray matter a layer of gray matter that covers it, 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 encephalic 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 joins 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 muscle coordination. and muscle coordination.

Structurally, the cerebellum can be divided into two parts: the inner white matter, composed of three gray matter nuclei in each hemisphere that constitute the intracerebellar nuclei; and the cerebellar cortex, the outer gray matter part, which will be discussed below.

The cerebellar cortex: definition and structure

The cerebellar cortex is the part of gray matter that forms the covering of the cerebellum. It can be divided into two hemispheres (as is the case with the cortex of the brain), and between them is the vermis, which acts as a junction and connects both 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 cerebellar cortex comprises the granular layer (or granular cell layer), the pyriform layer (or Purkinje cell layer) and the molecular layer. Let us see, in more detail, what each of them consists of.

The granular layer

This inner layer contains a multitude of cerebellar granule 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 in a "T" shape to form parallel fibers. The dendrites of the granules (excitatory neurons using glutamate) enter into the constitution of the cerebellar glomeruli (synaptic arrays 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 with parallel fibers; Lugaro cells, of medium size, their axon terminates within the granular layer itself or reaches the molecular layer; and unipolar brush cells, neurons located almost exclusively in the flocculonodular lobe, are formed by a single dendrite with endings similar to those of the bristles of a brush and receive a single synapse from a muscoid fiber.

The piriform layer

The piriform layer is made up of piriform or Purkinje cells.The Golgi epithelial cells, a type of very voluminous GABAergic neurons (with inhibitory effects). This entire layer is composed of a single row of Purkinje cells surrounded by a special type of glial cells: 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 of the Purkinje cells are enormously developed and span the molecular layer. Their axon enters deep into the cortex and, unlike other cortical cell types, eventually reaches the cerebellar nucleus or the lateral vestibular nucleus. Along its course, the axon gives rise to collateral branches destined mainly for the Golgi cells.

The molecular layer

The molecular layer is the outermost of all and is almost entirely is occupied, almost entirely, by the dendrites of the Purkinje cells.The dendritic branches of the Purkinje cells, the parallel fibers and Bergmann fibers, as well as the radial processes of the Golgi epithelial cells, are the most extensive dendritic ramifications of the entire central nervous system. The dendritic branches of the Purkinje cells are the most extensive dendritic ramifications of the entire central nervous system; they are positioned at right angles to the parallel fibers, with which they make a connection at the level of numerous synaptic spines present at their distal end.

Two different types of inhibitory GABAergic neurons can be found in the molecular layer; near the surface of the cerebellar cortex are located the stellate cells, of small size and whose axons project to the main trunk of origin of the dendritic tree of the Purkinje cells.

Other cells called "basket cells" are located in the vicinity of the piriform layer and are larger than the stellate cells, with axons that branch out to the main stem of the Purkinje cell dendritic tree. axons that branch repeatedly and wrap around the cell bodies of the stellate cells. of the Purkinje cells. Both basket and stellate cells receive information from parallel fibers.

Functions

As explained above, the most numerous neurons in the cerebellar cortex are the Purkinje cells, which are responsible for processing information coming from the cortex of the brain. These neurons are activated as movements are detected and develop, and they respond selectively to the movements of the cerebellar cortex.They respond selectively to aspects such as muscle extension, flexion or contraction, or joint position (essential for coordination and balance).

In recent years, the relationship between the cerebellum and motor learning has been investigated and, for the moment, the results conclude that the absence of the cerebellar cortex would not affect this learning of motor sequences, but it would affect the execution of the learned responses.

In addition, it has been shown that the cerebellum also plays an important role in the acquisition of goal-directed behaviors, without it being clear to what extent it contributes to a change in stimulus/response association and in the optimization of motor response execution.It is unclear to what extent it contributes to a change in stimulus/response association and optimization of motor response execution.

Finally, it should be noted that recent research has suggested that Purkinje neurons in the cerebellum would have the capacity to release endocannabinoid substances that could impair the potential of synapses (both inhibitory and excitatory).

Bibliographical references:

• 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.
• Linas, R. (1975) The cortex of the cerebellum. Sci Am 232:56
• Marr, D., & Thach, W. T. (1991). A theory of cerebellar cortex. In From the Retina to the Neocortex (pp. 11-50). Birkhäuser Boston.