Betz cell: characteristics and functions of this type of neuron

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

In this brain region are found some of the largest cells in our nervous system, 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 are their main characteristics, where are they located, and in what pathological processes are they involved.

• Related article: "Types of neurons: characteristics and functions"

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 in the late 19th century. These pyramidal-like 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 movements muscular.

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 the soma of these cells are oriented along a vertical axis, which may contribute to columnar processing in the primary motor cortex. What's more, Betz cell somas 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 contact the lower motor neuron. Although Betz cells have an apical dendrite typical of pyramidal neurons, they have more dendritic axes primaries, and these do not leave the soma only at basal angles, but branch out from almost any point in a asymmetric.

The 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 the white matter. According to one study, Betz cells represent approximately 10% of the total population of pyramidal cells in the Vb layer 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, responsible for sending their long axons to the motor nuclei contralaterals of the cranial nerves and to lower motor neurons located in the ventral horn of the spinal cord spinal.

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

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 size of the cell bodies of these neurons continually decreases along a mediolateral gradient. This reduction in size appears to be related to motor somatotopia: the largest cells are found in the representation region of feet and legs, where efferent axons project further along the tract corticospinal.

It should be noted that the Betz cells are found in the motor cortex of all primates And, according to studies, the bodies of these neurons become proportionally larger with increases in body weight, brain weight, and encephalization. Furthermore, the phylogenetic variation in the volumetric scale of this type of neurons could be related to specific adaptations of each species.

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Neurodegenerative diseases

There appear to be only a few central nervous system pathologies 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 degenerative motor neuron diseases, such as Amyotrophic Lateral Sclerosis (THE A). This progressive disease is known to affect not only the motor system, but also various non-motor systems and subcortical areas, and it can occur sporadically or familially. The pathophysiological mechanism in ALS is loss of anterior horn cells and degeneration of the corticospinal tract with involvement of the upper motor neurons.

There are other neurodegenerative diseases within the spectrum of ALS, for example, the ALS-parkinsonism-dementia complex, 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 cellular cortical level, the degeneration of dendritic arborizations, changes in synapses and the loss of Betz cells in ALS and other diseases degenerative diseases involving the primary motor cortex suggest a participation of this neuronal subpopulation in the process of this type of disease neurological.

Normal brain aging

Ramón y Cajal was one of the first researchers to identify a difference in Betz cell morphology during lifespan between newborns and adults; the famous anatomist found that the basal dendrites of these neurons were longer in developed brains.

More recent studies have shown that in normally aging brains, Betz cells have reduced and swollen dendritic spines. These age-related changes have been considered a possible correlate of the slowdown in engine performance and agility, as well as increased stiffness over life, as the engine cells Betz are preferentially involved in stabilizing muscle tone.

Additionally, animal research has reported a decrease in the size of Betz cell somas in adult rhesus monkeys. normal, along with a progressive appearance 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 considering account for the fact that studies in this regard have only investigated the brains of elderly patients advanced. Still, 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 changes pathologies in large neurons are only seen in atypical cases with prominent motor symptoms or in cases of complex lateral sclerosis amyotrophic-parkinsonism-dementia.

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.