Our brain is responsible for planning, coordinating, and executing the movements necessary to perform daily activities, and it does so primarily through the primary motor area.
In this region of the brain are some of the largest cells in our nervous system, the Betz cells; a type of giant pyramidal neuron that is responsible for transmitting motor commands by nerve impulses that travel from the neocortex to the spinal cord.
In this article we tell you what Betz cells are, What are their main characteristics, where are they located and in what pathological processes are they involved.
Betz cells: definition and characteristics
Betz cells are some of the largest motor neurons in the human nervous system, And are named after the Ukrainian scientist Vladimir A. Betz, who described this type of nerve cell at the end of the 19th century. These pyramid-like cells are giant in size (compared to most neurons) and are located in the gray matter of the primary motor cortex, a region of the brain responsible, along with other adjacent areas, for planning and execution. muscle movements.
Betz neurons are characterized by large somas and large basilar dendrites. These dendrites are significantly larger than those of other superficial and deep pyramidal neurons. The apical dendrites and soma of these cells are oriented along a vertical axis, which may aid columnar processing in the primary motor cortex. Outraged, the sums of Betz cells have a heterogeneous shape, Including pyramidal, triangular, and spindle-shaped cell bodies.
These motor neurons send their axons through the corticospinal tract to the anterior horn of the spinal cord, where they come into contact with the lower motor neuron. Although Betz cells have apical dendritis typical of pyramidal neurons, they have more primary dendritic axes, and these do not leave the soma only at basal angles, but branch out from almost any point in a way asymmetric.
Perisomatic and basal dendrites of Betz neurons project into all cortical layers, but most of its horizontal projections populate layers V and VI, Some of which reach white matter. Betz cells make up about 10% of the total pyramidal cell population in the Vb layer of the human primary motor cortex, according to one study.
The primary motor cortex
Betz cells are located in layer V of the primary motor cortex. This layer contains this type of giant pyramidal neurons, responsible for sending their long axons to the contralateral motor nuclei of the cranial nerves and to the lower motor neurons located in the ventral shaft of the spinal cord.
Axons of Betz neurons are part of the corticospinal tractAnd although these nerve cells do not constitute the complete motor output of the cortex, they are responsible for providing a clear marker for the primary motor cortex (Brodmann zone 4). This region of the brain contains a topographic map of the muscles of our body, in which the head is shown laterally, the leg medially and the rest of the parts in intermediate positions.
Betz cells are found singly or in small groups of three to four neurons, especially in the dorsal part of the primary motor cortex. The cell body size of these neurons continually decreases along a mediolateral gradient. This reduction in size appears to be related to motor somatotopy: larger cells are located in the region representing the feet and legs, where the efferent axons project further along the corticospinal tract.
It should be noted that Betz cells they are found in the motor cortex of all primates and, according to studies, the sums of these neurons become proportionately larger with increasing body weight, brain weight and encephalization. In addition, the phylogenetic variation of the volumetric scale of this type of neuron could be linked to species-specific adaptations.
Apparently there are only a few central nervous system pathologies involving Betz cells. These are usually neurodegenerative diseases that more or less specifically affect the primary motor cortex and its projections.
The extent to which Betz cells are affected in degenerative motor neuron diseases, such as amyotrophic lateral sclerosis (ALS), is still unknown. This progressive disease is known to affect not only the motor system, but also various non-motor systems and subcortical areas, and can occur sporadically or in the family. The pathophysiological mechanism of ALS is the loss of previous stem cells and degeneration of the corticospinal tract with involvement of the higher motor neurons.
There are other neurodegenerative diseases included in the spectrum of ALS, for example the ALS-parkinsonism-dementia complex, A disorder involving the motor cortical pathways and primary lateral sclerosis involving only the upper motor neurons with complete loss of Betz cells.
At the cellular cortical level, degeneration of dendritic arborizations, changes in synapses and loss of Betz cells in ALS and other degenerative diseases involving the primary motor cortex suggest the involvement of this neuronal subpopulation in the process of this type of neurological disease.
Normal aging of the brain
Ramón i Cajal was one of the first researchers to identify a difference in the morphology of Betz cells during the useful life between infants and adults; the famous anatomist to find that the basal dendrites of this type of neuron were longer in developed brains.
In more recent studies, it has been observed that in brains with normal aging, Betz cells have reduced and swollen dendritic spines. These age-related changes were considered to be a possible correlate of deceleration in motor performance and agility, as well as increased stiffness over the course of life, such as Betz cells they are preferably involved in the tone of the stabilizing muscles.
In addition, animal research has reported a decrease in the size of Betz cell sums in normal adult rhesus monkeys, as well as a gradual onset of highly specific inclusion bodies (abnormal subcellular structures) related to age. However, these data contradict previous observations of Betz cell inflammation during aging in humans.
The fact that Betz cells can be affected during aging is important since studies in this regard have only investigated the brains of elderly patients. However, it should be noted that the primary motor cortex is usually saved from Alzheimer’s disease, at least until the very advanced stages of dementia, and pathological changes in large neurons are observed only in atypical cases with significant motor symptoms or in case of amyotrophic laterality. sclerosis-parkinsonism-dementia complex.
- Eisen, A. and Weber, M. (2001). Motor cortex and amyotrophic lateral sclerosis. Muscle & Nerve: Official Journal of the American Association of Electrodiagnostic Medicine, 24 (4), 564-573.
- Jeannerod, M. (2006). The origin of voluntary action. History of a physiological concept. Accounts of Biology, 329 (5-6), 354-362.
- Sasaki, S. and Iwata, M. (2001). Ultrastructural study of Betz cells in the primary motor cortex of the human brain. The Journal of Anatomy, 199 (6), 699-708.