The cerebellum is an essential structure in the management and coordination of motor activities. Just like 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.
What is the cerebellum?
The cerebellum is one of the brain structures with the highest neuronal density and plays a key role in the integration of sensory and motor pathways. It is located behind the top of the brainstem (Where the spinal cord joins the brain) and is made up of two hemispheres or halves.
It receives information from 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 movement. the cerebellum it is essential for precise and balanced muscle activity, as well as for learning motor patterns and in muscle coordination.
Structurally, the cerebellum can be divided into two parts: internal white matter, made up of three gray matter nuclei in each hemisphere that make up the intracerebellar nuclei; and the cerebellar cortex, the outer part of gray matter and which we’ll talk about next.
The cerebellar cortex: definition and structure
The cerebellar cortex is the part of gray matter that forms the cerebellar cover. This can be divided into two hemispheres (as with the cortex of the brain), and between them is the vermis, which connects and connects the two parts. The architecture of this crust is uniform in all its parts, except for the abnormal distribution of the so-called “unipolar brush cells”..
From the inside out, the cerebellar cortex consists of the granular layer (or granular 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 splits 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 arrangements formed by the mossy fibers and axons of Golgi cells).
In the granular layer, there are three other types of neurons: Golgi cells, medium neurons with dendrites that connect to parallel fibers; Lugaro cells, of medium size, their axon ends in the same granular layer or reaches the molecular layer; and unipolar brush cells, neurons located almost exclusively in the flocculonodular lobe, are formed from a single dendrite with terminations similar to those of brush hairs and receive a single synapse from a muscoid fiber.
The pear-shaped layer
The piriformis layer consists of piriformis or Purkinje cells, A type of very large gabaergic neurons (with inhibitory effects). This entire layer consists 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. .
Purkinje cell dendrites are enormously developed and encompass the molecular layer. Its axon is introduced deep into the cortex and, unlike other types of cortical cells, eventually reaches the cerebellar nucleus or the lateral vestibular nucleus. Throughout its course, the axon gives rise to collateral branches intended mainly for Golgi cells.
The molecular layer
The molecular layer is the outermost of all and it is occupied, almost entirely, by the dendrites of the Purkinje cells, Parallel fibers and Bergmann fibers, as well as radial processes of Golgi epithelial cells. The dendritic branches of Purkinje cells are the most extensive dendritic branches of the entire central nervous system; they are placed perpendicular to the parallel fibers, with which they connect at many 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 small stellate cells, and their axons project onto the original main trunk of the dendritic tree of Purkinje cells.
Other cells called “basket cells” are located next to the piriformis layer and are larger than stellate cells, with axons that branch out repeatedly and wrap around cell bodies Purkinje cells. The basket and the stellate cells receive information from the parallel fibers.
As we explained above, the most numerous neurons in the cerebellar cortex are the Purkinje cells, which are responsible for processing information from the cortex of the brain. These neurons are activated when they sense and develop movements, And selectively respond 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, so far, the results conclude that the absence of a cerebellar cortex would not affect this learning of motor sequences, but the execution of responses. learned.
In addition, it was found that the cerebellum also plays an important role in developing goal-oriented behaviors, Without knowing to what extent it contributes to a change in the stimulus / response association and to the optimization of the performance of the motor response.
Finally, it should be noted that in recent research, it has been suggested that Purkinje neurons in the cerebellum have the ability to release endocannabinoid substances that may decrease the potential of synapses (both inhibitory and excitatory).
- Galea, JM, Vázquez, A., Pasricha, N., Orban de Xivry, JJ and Celnik, P. (2010). Dissociate the roles of the cerebellum and the motor cortex during adaptive learning: the motor cortex retains what the cerebellum is learning. Cerebral cortex, 21 (8), 1761-1770.
- Linas, R. (1975) The cortex of the cerebellum. Sci Am 232: 56
- Marr, D. and Thach, WT (1991). A theory of the cerebellar cortex. A From the retina to the neocortex (p. 11-50). Birkhäuser Boston.