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 at different levels or layers. In this article we explain what it is and what the main characteristics of the cerebellar cortex are, and what type of functions it performs.
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 joins the brain) and is made up 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 the generation of movements. The cerebellum It is essential for precise and balanced muscle activity to occur, as well as for learning motor patterns and in muscle coordination.
At a structural level, 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 external part of gray matter, which we will talk about below.
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 granular layer (or granule cell layer), the piriform layer (or Purkinje cell layer), and the molecular layer. Let’s 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 that use glutamate) enter 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, medium in 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 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 piriformis layer
The piriform layer is made up of piriform or Purkinje cells, a very voluminous type of 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 cross the molecular layer to reach the surface of the cerebellar cortex.
The dendrites of Purkinje cells are highly developed and span the molecular layer. Its axon enters deep into the cortex and, unlike what happens with other types of cortical cells, ends up reaching the cerebellar nucleus or the lateral vestibular nucleus. Along its course, the axon gives rise to collateral branches destined mainly 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, parallel fibers and Bergmann fibers, as well as the radial processes of Golgi epithelial cells. The dendritic branches of Purkinje cells are the most extensive dendritic branches in 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.
In the molecular layer two different types of inhibitory GABAergic neurons can be found; Near the surface of the cerebellar cortex, stellate cells are located, small in size and whose axons project to the main trunk of origin of the dendritic tree of Purkinje cells.
Other cells called “basket cells” are located adjacent 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 cells and stellate cells receive information from parallel fibers.
Features
As we have explained previously, the most numerous neurons in the cerebellar cortex are the Purkinje cells, responsible for processing the information that comes from the cortex of the brain. These neurons activate as they detect and develop movements 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 learned responses. .
Furthermore, it has been proven 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 would have the capacity to release endocannabinoid substances that could reduce the potential of synapses (both inhibitory and excitatory).
