Chap 6- the developing brain

Three main stages of fetal brain development

Induction, proliferation, migration

After \____ week's gestation, the human brain begins to form. The first stage is the neural tube. By \__________, the individual sections of the brain can be recognized. In \____________, the ridges of the brain can be observed.

3
4 weeks
6 months

During the very early stages of embryonic development, three layers emerge: \______________________. These layers give rise to the rich variety of tissue types that make up the human body. The explanation for this diversity lies in \______________________________.

ectoderm, mesoderm, endoderm
signals produced by surrounding tissues

Signals from the \______________ trigger some ectoderm cells to become \_______________, a process called \_______________. Subsequent signaling interactions refine the nerve tissue into the basic categories of \_________________ or \________________, and then into subclasses of each cell type

mesoderm
nerve tissue, neural induction
neurons, glia

The fate of a developing cell is largely determined by its proximity to various sources of \_________________________.

signaling molecules

The concentration of each type of signaling molecule decreases farther from its source, creating \_____________ throughout the brain.

gradients

A particular signaling molecule, called sonic hedgehog, is secreted from __________________ tissue lying beneath the developing ___. As a result of exposure to this signal, adjacent nerve cells are converted into a specialized class of _______*. Cells that are farther away are exposed to lower concentrations of sonic hedgehog, so they become _*_______________ that control the movement of muscles. An even lower concentration promotes the formation of _________, which don't relay messages to muscles but to other neurons. Interestingly, the mechanism of this molecular signaling is very similar in species as diverse as ___ and humans.

mesodermal, spinal cord
glia
motor neurons
interneurons
flies

In the brain, neurons arise from a fairly small pool of neural \_________ and \________________ cells, special cells that can divide and become a variety of mature cell types. Before achieving their mature cell fate, this pool of cells undergoes a series of divisions — increasing the number of cells that will ultimately form the brain. Early divisions are \______________ , but as these divisions progress, the cells begin to divide \___________________, giving rise to only one daughter cell that keeps proliferating and a second that progresses towards its ultimate cell fate as a neural or glial cell

stem, progenitor
symmetric, asymmetrically

After \________________ is complete, only a few neural stem and progenitor cells remain within the brain, and neurogenesis in adulthood is limited to a few regions of the brain, such as those involved with \_________________memory.

proliferation

Scientists have proposed that protein defects causing a premature switch from symmetric to asymmetric divisions may be a cause of \_____________________. This disorder, characterized by a \_____________________________, is associated with serious neurological disabilities and sometimes death in \_________. Similarly, excessive proliferation of brain cells can lead to a disorder called \___________________ — a brain that is abnormally large and heavy — which is also associated with a variety of neurodevelopmental complications.

microcephaly
severe reduction in brain size
infancy
megalencephaly

After \______________________ and \__________________ occur, new neurons journey from the \___________ surface of the embryonic brain, where they formed, to their long-term locations in the brain. This process is called \__________________

neural induction, proliferation
inner
migration

During migration, the \________________ starts to thicken and build up along the \______________ of the embryo. As the cells continue to divide, a flat \__________________ grows, followed by the formation of parallel \________________, somewhat resembling the creases in a paper airplane, that rise along \___________________ of the midline.

ectoderm, midline
neural plate
ridges
either side

The ridges around the midline extend from the "\_____________ end", where the future \_____________ will form, along the \_______________ of the embryo where the future \__________________ will develop. Within a few days, the ridges \____________________________ and fuse into a hollow \________________________

head, brain
length, spinal cord
fold toward each other, neural tube

The head end of the neural tube thickens into three bulges that form the \______________________________. Later in the process, at week \___ in humans, the first signs of the \__________ and the brain's \___________________ appear. As new neurons are produced, they move from the neural tube's \_________________ zone, which lies along the \_____________ surface of the tube, toward the border of the \_____________ zone, or \__________ surface. After neurons stop dividing, they form an \____________________ zone where they gradually accumulate as the brain develops.

hindbrain, midbrain, forebrain
7
eyes, hemispheres
ventricular, inner
marginal, outer
intermediate

The most common guidance mechanism, accounting for about \____ percent of migration in humans, is the \________________, which project radially from the \_______________________ to the \__________. Neurons use these glia as scaffolding, inching along \__________________ until they reach their final destinations.

radial glia
intermediate zone, cortex
glial projections

The process of radial migration occurs in an "inside-out" manner; that is, the cells that arrive the earliest (the \___________ ones) form the \_______________ layer of the cortex, whereas the late-arriving (\______________) neurons form the \______________ layer. Through a different mechanism, other neurons migrate sideways, or \_________________ (rather than radially), moving \____________ to the brain's surface and \___________ the radial cortical columns.

oldest, deepest
youngest, outermost
tangentially
parallel, across

Migration is a finely tuned process that can be influenced by many factors. \__________________,\______________, or \______________ can prevent proper migration, resulting in misplacement of cells, which can lead to \____________________ or \_____________. Furthermore, mutations in the genes that regulate migration have been shown to cause rare \_____________ forms of intellectual disability and epilepsy in humans.

Exposure to alcohol, cocaine, or radiation
intellectual disability, epilepsy
genetic

After neurons reach their final locations, they begin making the \________________ that will determine how particular functions such as vision or hearing can occur. Induction, proliferation, and migration occur internally during \_________ development, but the next phases of brain development depend increasingly on \___________________. After birth, factors such as watching a mobile spin, listening to a voice, and even proper nutrition influence the connections formed by neurons.

connections
fetal
external experience

Neurons become interconnected through \__________________ . Axons produce and transmit signals to other neurons, and dendrites receive signals from the axons that contact them. To reach their targets, axons can span distances many times the size of their cell body, many crossing to the \______________ side of the brain. The longest human axons are in the \________________, extending from the \_________________________ all the way to \______________________.

dendrites and axons
opposite
periphery, lower spinal cord, muscles in the toes

A developing axon grows by the extension of its \_____________________, an enlargement at the \________ of the axon that actively explores the environment to seek out its precise destination. A growth cone is guided to that final destination by \___________________ in its environment. Some of these molecules stud the surfaces of cells, while others are secreted into areas near the growth cone. \________________ on the growth cone enable its responses to these environmental cues.

growth cone, tip
molecular cues
receptors

Binding of environmental molecules tells the growth cone whether to move forward, stop, recoil, or change direction. \________________ cues lay a path growth cones follow, while \____________ molecules funnel growth cones through precise corridors. Signaling molecules include families of proteins with names such as \_____________, \________________ and \_____________.

Attractive, repellent
netrin, semaphorin, ephrin

Most axon growth signaling proteins are common to many organisms — worms, insects, and mammals including humans. Each family of proteins is smaller in flies or worms than in mice or people, but its functions are very similar. As a result, simpler animals are highly useful experimental models for gaining knowledge that \________________________________.

directly applies to humans

\_________ was first discovered in a \_________, where it was found to guide neurons around the worm's "\_______________." Later, vertebrate netrins were found to guide axons around the \______________ spinal cord.

Netrin, worm, nerve ring
Mammalian

Once axons reach their targets, a specialized connection called a \___________ begins to form

synapse

\______________ are also actively involved in initiating contact with axons, and both sides produce \____________ that span the space between them and anchor the \_______________ together.

Dendrites, proteins, synapse

Once initial contact is established, a synapse continues to \_____________. On the presynaptic side, the tiny \_______________ that contacts the dendrite becomes specialized for releasing \______________, stocking itself with \__________________, and \____________ that enable them to be held in place and then released. On the dendritic — or postsynaptic — side, \_____________________ begin to dot the membrane. Both processes ensure that a synapse can transmit signals quickly and effectively.

differentiate
axon terminal
neurotransmitters, neurotransmitter packets
proteins
neurotransmitter receptors

New evidence has implicated a third important player in the proper formation of a synapse. \_________________ are a type of glial cell in the brain previously thought to simply provide scaffolding and passive support to neurons. They are now known to exert their own influence on synaptic development and function.

Astrocytes

Many synapses in the brain are contacted by astrocytes, and studies in rodents have found that a single astrocyte can contact \________________ of synapses across \____________ neurons. Some neurons form only a few synapses when developing in a culture dish from which \_________________________, and recent research has discovered that molecules secreted by astrocytes regulate aspects of synaptic development.

thousands, multiple
astrocytes are absent

Scientists are learning that molecules from multiple sources work together to promote proper synapse formation. It is now thought that defects in such molecules could contribute to disorders such as \_____________. In addition, the loss of certain other molecules might underlie the degradation of synapses that occurs during \___________.

autism
aging

An array of signals determines which type of neurotransmitter a neuron will use to communicate. For some cells, such as \____________ neurons, the type of neurotransmitter is fixed (\__________________), but for other neurons, it is not.

motor, acetylcholine

Scientists have found that when certain immature neurons are maintained in a culture dish with no other cell types, they produce the neurotransmitter \_______________. In contrast, when the same neurons are cultured with specific cells, such as \___________ tissue, they produce the neurotransmitter \________________.

norepinephrine
cardiac, acetylcholine

Just as genetic and environmental signals can modulate the development of specialized cells, a similar process leads to production of \_____________ \__________________. Many researchers believe that the signal to engage the gene, and therefore the final determination of the chemical messenger a neuron will produce, is influenced by factors that come from the \____________ of the synapse itself.

specific neurotransmitters
location

\_________________, the fatty wrapping of axons by extensions of \_____, increases — by as much as \_______ times — the speed at which \____________________________________.

Myelination, glia
100
signals travel along axons

The increase in signal speed along an axon is a function of \______________________________, with somewhat regularly spaced gaps called \__________________ interrupting the sheath. The alternating pattern of \_____________ and \________ allows electrical signals to move down an axon faster, jumping from one node to the next. This phenomenon, called \__________________ \________________ is responsible for more rapid transmission of electrical signals.

how the sheath is wrapped
nodes of Ranvier
insulation, nodes
saltatory conduction

True or False: Myelination occurs until around age 60

False. Formation of myelin occurs throughout the lifespan

After its initial growth, the neural network is \__________________, creating a more efficient system. Only about \_________ the neurons generated during development survive to function in an adult. Entire populations of neurons are removed through \___________________.

pared back
half
apoptosis

Apoptosis is activated if a neuron fails to receive enough life-sustaining chemical signals called \_______________, which are produced in limited quantities by target tissues. Each type of trophic factor supports the survival of a \__________________________. For example, \_____________________ is important for the survival of \____________ neurons.

trophic factors
distinct group of neurons
nerve growth factor, sensory

It has recently become clear that apoptosis is maintained into adulthood but constantly \___________________. Based on this, researchers have found that injuries and some neurodegenerative diseases kill neurons not by directly inflicting damage but by \______________________. This discovery has led to new avenues for therapy.

held in check
activating apoptosis

Just as too many brain cells develop early on, these cells initially form an excessive number of \_________________. In primates, for example, neural projections from the \__________ to the brain initially overlap; then, in some portions of the brain, they sort into separate territories devoted to one eye or the other. Furthermore, connections between neurons in a young primate's cerebral cortex are more \______________ and \___________ as concentrated as in an adult primate.

connections
two eyes
numerous, twice

The \_____________ of these excess connections is heavily dependent on the \___________________ of each connection. Connections that are active and generating electrical currents \___________, while those with relatively little activity are \__________.

pruning
relative activity
survive, lost

\________________ and other glia also play an important role in pruning. For example, astrocytes are known to aid the formation of \_______ -specific connections by engulfing and eliminating unnecessary synapses. Thus, at least to some extent, the circuits of the adult brain are formed by \__________________________________________

Astrocytes
eye
pruning away incorrect connections to leave only the correct ones

New neurons are born throughout life in a specific region of the brain's hippocampus called the \__________________. This region is involved in pattern separation.

dentate gyrus

pattern separation

The ability to discriminate between very similar memories

A human brain contains \________________s of synapses

trillions