Neurotrophins are a family of neurotrophic factors including nerve growth factor (NGF) and brain-derived nerve growth factor (BDNF). One key characteristic of neurotrophins, and neurotrophic factors in general, is that synaptic targets release just enough of these chemicals to maintain the amount of innervation they need. If there are too many neurons synapsed to the target structure, some neurons won't receive enough neurotrophins to survive and will die out, and vice versa.

Neurotrophins

How does a cell "decide" to enter apoptosis by expressing death genes and activating the apoptotic cascade? A cell's competition for resources with other cells is an important apoptosis determinant. These limited resources may include the following: 

-Synaptic targets: There are only a limited number of synaptic targets (e.g., muscles). 

-Neurotrophic factors: Neurotrophic factors are chemicals produced by synaptic targets that enables the neuron to survive. There are a limited number of neurotrophic factors.

-Experience: Experience may either continuously stimulate a synapse to strengthen it, or it may never use the synapse and discard of it. This is because synapses are energetically costly to maintain throughout the life span.

Brandeis University

Apoptosis is a term used to describe programmed cell death. Cell death is an important aspect of development in both animals and plants, and apoptosis is a selective and organized way to kill cells that was likely established long ago in the course of evolution. 

Apoptosis

Breedlove, S. & Watson, N. (2017). Behavioral neuroscience, eighth edition. Oxford University Press.

Behavioral Neuroscience 8th Edition

A death gene is only expressed when a cell has "decided" to undergo apoptosis. The expression of death genes has important functions in apoptotic cascades. 

Death Genes

There are many different types of apoptotic cascades involving death genes. Our understanding of apoptosis is outlined by the following: 

-An influx of calcium ions (Ca+2) from the exterior of the cell, as well as interior stores of calcium, enters the mitochondria.

-The mitochondria releases a pro-apoptotic protein called Diablo into the cell. This may be blocked by the Bcl-2 anti-apoptotic proteins. 

-Diablo binds to and inhibits a family of proteins known as the inhibitors of apoptosis proteins (IAPs) that normally inhibit proteins called caspases. Caspases are types of enzymes that degrade proteins and nuclear DNA.

-A cascade of caspase enzymes are activated, leading to destruction of proteins and nuclear DNA needed for cell survival. 

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