Chapter 4: Brain Development Flashcards
proliferation 117
Proliferation is the production of new cells. Early in development, the cells lining the ventricles of the brain divide.
stem cells 117
Some cells remain where they are as stem cells, continuing to divide. Others become primitive neurons and glia that migrate to other locations.
myelination 118
A later and slower stage of neuronal development is
myelination, the process by which glia produce the insulating fatty sheaths that accelerate transmission in many vertebrate axons. Myelin forms first in the spinal cord and then in the hindbrain, midbrain, and forebrain. Unlike the rapid proliferation and migration of neurons, myelination continues gradually through adolescence and early adulthood, and perhaps beyond.
differentiates 118
At first, a primitive neuron looks like any other cell. Gradually, the neuron differentiates, forming its axon and dendrites. The axon grows first. In many cases, a migrating neuron tows its growing axon along like a tail, allowing its tip to remain at or near its target. In other cases, the axon needs to grow toward its target, finding its way through a jungle of other cells and
fibers. After the migrating neuron reaches its destination, its dendrites begin to form.
migrate 118
Early in development, the primitive neurons begin to
migrate (move). Some migrate faster than others, and a few of the slowest don’t reach their destinations until adulthood. Some neurons move radially from the inside of the brain to the outside, some move tangentially along the surface of the brain, and some move tangentially and then radially. Chemicals known as immunoglobulins and chemokines guide neuron migration. A deficit in these chemicals leads to impaired
migration, decreased brain size, decreased axon growth, and mental retardation.
synaptogenesis 118
The final stage is synaptogenesis, or the formation of synapses. Although this process begins before birth, it continues throughout life, as neurons form new synapses and discard old ones. However, the process generally slows in older people, as does the formation of new dendritic branches.
nerve growth factor (NGF) 122
Initially, the sympathetic nervous system forms far more
neurons than it needs. When one of its neurons forms a synapse onto a muscle, that muscle delivers a protein called nerve growth factor (NGF) that promotes the survival and growth of the axon (Levi-Montalcini, 1987). An axon that does not receive NGF degenerates, and its cell body dies. That is, each neuron starts life with a “suicide program”.
apoptosis 122
If its axon does not make contact with an appropriate postsynaptic cell by a certain age, the neuron kills itself through a process called apoptosis, a programmed mechanism of cell death.
neurotrophin 123
Nerve growth factor is a neurotrophin, meaning a chemical that promotes the survival and activity of neurons. the nervous system responds to brain-derived
neurotrophic factor (BDNF) and several other neurotrophins. Neurotrophins are essential for growth of axons and dendrites, formation of new synapses,
and learning.
fetal alcohol syndrome 123
The infant brain is highly vulnerable to damage by alcohol. Children of mothers who drink heavily during pregnancy are born with fetal alcohol syndrome, a condition marked by hyperactivity, impulsiveness, difficulty maintaining attention, varying degrees of mental retardation, motor problems, heart defects, and facial abnormalities. Drinking during pregnancy leads to thinning of the cerebral cortex that persists to adulthood. More drinking causes greater deficits, but even moderate drinking produces a measurable effect.Exposure to alcohol damages the brain in several ways. At the earliest stage of pregnancy, it interferes with neuron proliferation. A little later, it impairs neuron migration and differentiation. Still later, it impairs synaptic transmission. Alcohol kills neurons partly by apoptosis. To prevent apoptosis, a brain neuron must receive input from incoming axons. Alcohol inhibits receptors for glutamate, the brain’s main excitatory
transmitter, and enhances receptors for GABA, the main inhibitory transmitter. Because of the decrease in net excitation many neurons undergo apoptosis.
focal hand dystonia 130
One or more fingers may go into constant contraction.
This condition, known as “musician’s cramp” or more formally as focal hand dystonia, can be a career ender for a musician.
Neural Darwinism
These results suggest a general principle, called
neural Darwinism. In the development of the nervous system, we start with more neurons and synapses
than we can keep. Synapses form with approximate accuracy, and then a selection process keeps some and rejects others.
