Neural death: what is it and why is it produced?
All neurons in our body have a life cycle. They are formed, they live, they exercise their functions and finally they die and they are replaced. In fact, it is something that happens constantly in different systems of the organism.
However, the nervous system is a particular case in which, once in adulthood, they will hardly produce new neurons. And those that we already have will not live eternally: little by little and for different reasons, they will degenerate and die. It is because of that in this article we are going to talk about neuronal death and the two main processes so it occurs .
What is neuronal death?
The concept of neuronal death refers, as the name suggests, to the death of nerve cells known as neurons. This supposes a series of repercussions of great depth, like the fact that cell will no longer be able to exert its function of transmitting the information (with the consequent diminution of brain efficiency or even the loss of functions depending on the quantity, area and functions of dead cells).
However, it is not limited to this, and is that the death of a neuron can have an effect on neighboring cells: it supposes the existence of some remains that although they can usually be eliminated by the system, they can also reach stay in it and interfere with the normal functioning of the brain.
The process by which a neuron dies can vary greatly depending on its causes , as well as the results of said death. It is generally considered that there are two major types of neuronal death: that produced naturally by the cell itself or apoptosis and that produced by injury or necrosis.
Neuronal programmed death: apoptosis
In general, we tend to consider that the death of neurons is something negative, especially considering that once in adulthood practically no new neurons are produced (although some areas have been discovered in which there is neurogenesis). But neuronal death is not always negative, and is that in fact throughout our development there are even specific moments in which it is programmed. We are talking about apoptosis.
Apoptosis is itself the programmed death of the body's cells , which allows it to develop by getting rid of unnecessary material. It is a cell death that is beneficial (usually) to the body and serves to develop or to fight against possible damage and disease (diseased or harmful cells are removed). This process is characterized by requiring energy to be produced, not being able to be carried out in the absence of ATP (adenosine triphosphate, substance from which the cells obtain energy).
At brain level this happens especially at the time of neuronal or synaptic pruning, in which a high percentage of the neurons that have developed during our first years die to allow a more efficient organization of the system. Die neurons that do not establish strong enough synapses because they are not used regularly and those of more frequent use remain. This allows our maturation and increased efficiency in the use of mental resources and available energy. Another time that apoptosis also occurs is during aging, although in this case the consequences generate the progressive loss of faculties.
In the process of neuronal apoptosis the cell itself generates biochemical signals (either by positive induction in which the receptors of the membrane receptors bind to certain substances or by negative or mitochondrial induction in which the ability to suppress certain substances is lost. they would generate the activity of apoptotic enzymes) that cause them to condense and alter the cytoplasm, the cell membrane, the cell nucleus to collapse and fragment the DNA. Finally the microglial cells end up phagocytizing and eliminating the remains of the dead neurons, so that they do not generate an interference for the normative functioning of the brain.
A special type of apoptosis is called anoikis , in which the cell loses contact with the material of the extracellular matrix, which ends up causing its death by not being able to communicate.
Necrosis: death due to injury
But neuronal death does not occur only preprogrammed as a way to improve the efficiency of the system. They can also die due to external causes such as injuries, infections or poisoning . This type of cell death is what is known as necrosis.
Neural necrosis is that neuronal death caused by the influence of external factors, generally of a harmful nature. This neuronal death is mostly harmful to the subject. It does not require the use of energy, being a passive neuronal death. The neuron is unbalanced by the damage and loses control of its osmosis, breaking the cell membrane and releasing its contents.It is usual these remains produce an inflammatory reaction that can generate symptoms of diverse. On the contrary that happens in the apoptosis it is possible that the microglia does not get to phagocytize the dead cells correctly, remaining remains that can cause an interference in the normative operation. And although they are phagocytized over time, even if they are eliminated they tend to leave a scar of fibrous tissue that interferes with the neuronal circuit.
It is important to keep in mind that necrosis can also appear if a loss of ATP occurs in a process of apoptosis. Since the system needs energy to produce apoptosis, if it is left without neuronal death can not occur in a preprogrammed way so that although the neuron in question dies the process can not be completed, which will cause the death in question is necrotic .
Neural necrosis can occur due to multiple causes. It is common its appearance before processes such as hypoxia or anoxia , cerebrovascular accidents, traumatic brain injuries or infections. It is also well known neuronal death by excitotoxicity, in which neurons die due to the excessive influence of glutamate (the main exciter of brain activity), as occurs before some drug overdoses or drug intoxication.
The influence of neuronal death in dementia and neurological disorders
We can observe neuronal death in a large number of situations, not all of them clinical type. However, it is worth highlighting a recently discovered phenomenon in the relationship between dementia and neuronal death.
As we age our neurons do it with us, dying throughout our lives. The microglia is responsible for protecting the nervous system and phagocytosis the remains of dead neurons (through apoptotic processes), so that although faculties are lost, the brain usually remains healthy within the limits of normal aging.
However, recent research seems to indicate that in people with dementia, such as that of Alzheimer's disease, or with epilepsy, microglia do not exert their function to phagocytose dead cells, leaving remains that generate inflammation of the surrounding tissues. This means that even if brain mass is lost, there are still remains and scar tissue that, as they accumulate, increasingly harm the performance of the rest of the brain, facilitating, in turn, a greater neuronal death.
Although these are recent experiments that must be replicated to obtain more data and falsify the results, these data can make us understand better the process by which the nervous system deteriorates, so that we can establish better strategies and treatments to alleviate neuronal destruction. and perhaps, in the long run, to stop diseases that are still incurable.
Bibliographic references:
- Consentino, C. (1997). Apoptosis and Nervous System. Annals of the Faculty of Medicine, 58 (2). National University of San Marcos.
- Becerra, L.V .; Pepper, H.J. (2009). Neuronal apoptosis: the diversity of signals and cell types. Medical Colombia 40 (1): 125-133.Universidad del Valle. School of Health. Colombia.
- Abiega, O. et al. (2016). Neuronal hyperactivity disturbs ATP microgradients, impairs microglial motility, and reduces phagocytic receptor expression triggering apoptosis / microglial phagocytosis uncoupling. PLoS Biology.
