Cerebellar cortex: what it is, layers and functions

The cerebellum is an essential structure in the management and coordination of motor activities. As in the brain, there is a layer of gray matter that covers it, called the cerebellar cortex.

This cortex is made up of different types of neurons grouped into 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.

• Related article: "Human cerebellum: its parts and functions"

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 brainstem (where the spinal cord meets the brain) and is made up of two hemispheres or halves.

Receives information from sensory systems, the spinal cord, and other parts of the cerebral cortex, and projects it towards other structures involved in processes such as coordination, postural adaptation or the generation of movements. the cerebellum

it is essential for precise and balanced muscular activity to occur, as well as for learning motor patterns and in muscle coordination.

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

The cerebellar cortex: definition and structure

The cerebellar cortex is the part of gray matter that forms the covering of the cerebellum. This can be divided into two hemispheres (as occurs with the cortex of the brain), and between them is the vermis, which acts as a union 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 granule layer (or granule cell layer), the pyriform layer (or Purkinje cell layer), and the molecular layer. Let's see, in more detail, what each of them consists of.

granular layer

This inner layer contains multitudes 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 that use glutamate) enter into the constitution of the cerebellar glomeruli (synaptic arrays made up of mossy fibers and axons of cerebellar cells) golgi).

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 ends within the same granular layer or reaches the molecular layer; and unipolar brush cells, neurons located almost exclusively in the flocculonodular lobe, are made up of a single dendrite with endings similar to those of the bristles of a brush and receive a single synapse from a fiber muscoid.

• You may be interested in: "Types of neurons: characteristics and functions"

The piriform layer

The pyriform layer is made up of pyriform or Purkinje cells., a type of very large GABAergic neurons (with inhibitory effects). This entire layer is made up of a single row of Purkinje cells surrounded by a special type of glial cells: glial cells. Golgi epithelial cells, which have processes with a radial course that crosses the molecular layer to reach the surface of the cortex cerebellar.

The dendrites of the purkinje cells they are enormously developed and span the molecular layer. Its axon travels deep into the cortex and, unlike other types of cortical cells, ends up in the cerebellar nucleus or lateral vestibular nucleus. Throughout its course, the axon gives rise to collateral branches destined primarily for Golgi cells.

the molecular layer

The molecular layer is the outermost of all and It is almost completely occupied by the dendrites of the Purkinje cells., the parallel fibers and the Bergmann fibers, as well as the radial processes of the Golgi epithelial cells. The dendritic branches of Purkinje cells are the largest dendritic branches in the entire central nervous system; they are placed 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; small stellate cells are located near the surface of the cerebellar cortex and whose axons project to the main trunk of origin of the dendritic tree of the cells of Purkinje.

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

functions

As we have explained previously, the most numerous neurons in the cerebellar cortex are the Purkinje cells, in charge of processing the information that comes from the cerebral cortex. These neurons are activated as movements are detected and developed., and respond selectively to aspects such as muscle extension, flexion or contraction, or the position of the joints (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 responses learned.

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 the stimulus/response association and in the optimization of the execution of the motor response.

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

Bibliographic 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. SciAm 232:56
• Marr, D., & Thach, W. T. (1991). A theory of cerebellar cortex. In From the Retina to the Neocortex (pp. 11-50). Birkhauser Boston.