9.4

• Synaptogenesis depends on the presence of glial cells, particularly astrocytes and microglia.
• True or False: . Retinal ganglion cells maintained in culture formed seven times more synapses when astrocytes were present. Moreover, synapses formed in the presence of astrocytes were quickly lost when the astrocytes were removed.

  True
• True or False: Neuron death should not happen and it harms neurodevelopment.

  False
• True or False: In neurodevelopment, many more neurons are produced than required.

  True
• Passive cell death is called necrosis.
• Active cell death is called apoptosis.
• True or False: Necrosis is safer than apoptosis.

  False, Necrotic cells break apart and spill their contents into the surrounding extracellular fluid, and the consequence is potentially harmful inflammation. In apoptotic cell death, the internal structures of a cell are cleaved apart and packaged in membranes before the cell breaks apart. These membrane packages contain molecules that attract microglia who engulf and consume them.
• If genetic programs for apoptotic cell death are blocked, the consequence can be cancer.
• If the programs are inappropriately activated, the consequence can be neurodegenerative disease.
• Triggers of Apoptosis in developing Neurons:

  Genetically programmed for an early death and failure to obtain the life-preserving chemicals that are supplied by their targets
• The most prominent class of life-preserving chemicals is the neurotrophins.
• Nerve growth factor (NGF) was the first neurotrophin to be isolated and the second was brain-derived neurotrophic factor
• The neurotrophins promote the growth and survival of neurons, function as axon guidance molecules, and stimulate synaptogenesis.
• During the period of cell death, neurons that have established incorrect connections are particularly likely to die. As they die, the space they leave vacant on postsynaptic membranes is filled by the sprouting axon terminals of surviving neurons. Thus, cell death results in a massive rearrangement of synaptic connections. This is what we call the __________ ________.

  Synapse Rearrangement
• This phase tends to focus the output of each neuron on a smaller number of postsynaptic cells, thus increasing the selectivity of transmission.

  Synapse Rearrangement
• True or False: The human brain develops more slowly than those of other species, not achieving full maturity until late adolescence or early adulthood.

  True
• The last part of the human brain to reach maturity.

  Prefrontal Cortex
• True or False: There is a general increase in synaptogenesis in the human cortex shortly after birth, but there are differences among the cortical regions.

  True
• Myelination increases the speed of axonal conduction.
• True or False: The myelination of various areas of the human brain during development roughly parallels their functional development.

  True