Betz cell: characteristics and functions of this type of neuron.

Betz cells are among the largest types of motor neurons in the human body.

Our brain is responsible for planning, coordinating and executing the movements necessary to carry out everyday activities, and it does this mainly through the primary motor area.

In this brain region are some of the largest cells of our nervous system, the Betz cells; a type of giant pyramidal neuron that is responsible for transmitting motor commands through nerve impulses that travel from the neocortex to the spinal cord.

In this article we explain what Betz cells are, what their main characteristics are, where they are located, and what they do.What are their main characteristics, where they are located, and in what pathological processes they are involved.

Betz cells: definition and characteristics

Betz cells are some of the largest motor neurons in the human nervous system, named after the Ukrainian scientist Vladimir A. Betz, who described this type of nerve cell in the late 19th century.They are named after the Ukrainian scientist Vladimir A. Betz, who described this type of nerve cell in the late 19th century. These pyramidal-type cells are gigantic in size (compared to most neurons) and are located in the gray matter of the primary motor cortex, a brain region responsible, along with other adjacent areas, for planning and executing Muscle movements.

Betz neurons are characterized by large somas and extensive 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 contribute to columnar processing in the primary motor cortex. In addition, the somas of Betz cells have a heterogeneous shape, including pyramidal cell bodies.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 contact the lower motor neuron. Although Betz cells have an apical dendrite typical of pyramidal neurons, they possess more primary dendritic axons, and these do not leave the soma only at basal angles, but branch from almost any point asymmetrically.

The perisomatic and basal dendrites of Betz neurons project into all cortical layers, but most of their horizontal projections populate layers V and VI.some of which reach the white matter. According to one study, Betz cells represent approximately 10% of the total pyramidal cell population in layer Vb of the human primary motor cortex.

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, which are 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 horn of the spinal cord.

The axons of Betz neurons are part of the corticospinal tract.and although these nerve cells do not compose the complete motor output of the cortex, they are responsible for providing a clear marker for the primary motor cortex (Brodmann's area 4). This region of the brain contains a topographical map of the muscles of our body, in which the head is represented laterally, the leg medially and the other parts in intermediate positions.

Betz cells are found solitary or in small groups of three to four neurons, especially in the dorsal part of the primary motor cortex. The size of the cell bodies of these neurons decreases continuously along a mediolateral gradient. This reduction in size appears to be related to motor somatotopy: the largest cells are found in the feet and leg representational region, where the efferent axons project farthest along the corticospinal tract.

It should be noted that Betz cells cells are found in the motor cortex of all primates. and, according to the studies, the somas of these neurons become proportionally larger with increases in body weight, brain weight and encephalization. In addition, phylogenetic variation in the volumetric scale of these neurons may be related to species-specific adaptations.

Neurodegenerative diseases

Apparently, there are only a few pathologies of the central nervous system that involve Betz cells. These are, generally, neurodegenerative diseases that more or less specifically affect the primary motor cortex and its projections..

The extent to which Betz cells are affected in motor neuron degenerative 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 may occur sporadically or familially. The pathophysiological mechanism in ALS is loss of anterior horn cells and degeneration of the corticospinal tract with involvement of upper motor neurons.

There are other neurodegenerative diseases included in the ALS spectrum, e.g., ALS-parkinsonism-dementia complex, a disorder involving the motor pathways of the brain.a disorder involving the cortical motor pathways and primary lateral sclerosis involving only the upper motor neurons with a total loss of Betz cells.

At the cortical cellular 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 an involvement of this neuronal subpopulation in this type of neurological disease process.

Normal brain aging

Ramón y Cajal was one of the first researchers to identify a difference in the morphology of Betz cells during the lifespan between newborns and adults; the celebrated anatomist discovered that the basal dendrites of this type of neurons were longer in developed brains..

More recent studies have shown that in normal aging brains, Betz cells have reduced and swollen dendritic spines. These age-related changes have been considered a possible correlate of slowing motor performance and agility, as well as increased stiffness during life, since Betz cells are preferentially involved in the tone of stabilizing muscles..

In addition, animal research has reported a decrease in the size of Betz cell somas in normal adult rhesus monkeys, along with a progressive appearance of highly specific age-related inclusion bodies (abnormal subcellular structures). However, these data contradict previous observations of Betz cell swelling during aging in humans.

The fact that Betz cells may be affected during aging is important in view of the fact that studies in this regard have only investigated brains of elderly patients. All in all, it should be noted that the primary motor cortex is generally spared from Alzheimer's disease, at least until the very late stages of dementia, and pathological changes in large neurons are only observed in atypical cases with prominent motor symptoms or in cases of the amyotrophic lateral sclerosis-parkinsonism-dementia complex.

Bibliographic references:

• Eisen, A., & Weber, M. (2001). The 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. Comptes Rendus Biologies, 329(5-6), 354-362.
• Sasaki, S., & 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.