5.1 Development Of The Brain

Question 0

The dorsal surface thickens and then long thin lips rise, curl, and merge, forming a ____ ____ that surrounds a fluid filled cavity.

Answer 0

neural tube

Question 1

The human central nervous system begins to form when the embryo is about ___ ____ old.

Answer 1

two weeks

Question 2

As the neural tube sinks under the surface of the skin, the ____ end enlarges and differentiates into the hindbrain, midbrain, and forebrain. The rest becomes the spinal-cord.

Answer 2

forward

Question 3

Neuroscientists distinguish these processes in the ____ of ____: preparation, migration, differentiation, myelination, and synaptogenesis.

Answer 3

development of neurons

Question 4

After cells have differentiated as neurons or glia, they ____ (move). Some neurons migrate much faster than others, and a few of the slowest don’t reach their final destinations until adulthood.

Answer 4

migrate

Question 5

Chemicals known as ____ and ____ guide neuron migration. A deficit in these chemicals leads to impaired migration, decreased brain size, decreased axon growth, and mental retardation.

Answer 5

immunoglobulins and chemokines

Question 6

At first, primitive neurons looks like any other cell. Gradually, the neuron ____, forming it’s axon and dendrites. The axon grows first. After the migrating neuron reaches its destination, its dendrites begin to form.

Answer 6

differentiates

Question 7

A later and slower stage of development is ____, the process by which glia produce the insulating fatty sheaths that accelerate transmission in many vertebrate axons. Myelination continues gradually for decades.

Answer 7

myelination

Question 8

The final stage is ____, or the formation of synapses.

Answer 8

synaptogenesis

Question 9

Possible formation of new neurons in the mature primate cerebral cortex has been controversial. The best evidence against their formation came from a clever study using a ____ ____ of carbon, 14C.

Answer 9

radioactive isotope

Question 10

When researchers examined neurons in the cerebral cortex, they found a 14C concentration corresponding to the year of the persons birth. These results indicate that the mammalian cerebral cortex forms ___ or __ new neurons after birth, at least under normal circumstances.

Answer 10

few or no

Question 11

However, the situation may be different after ____ ____. After damage to the sensory axons from a monkey’s hand, the cerebral cortex on the contralateral side gradually reorganises, and during this process new neurons do form, as confirmed by chemicals that specifically label newly formed neurons.

Answer 11

brain damage

Question 12

Growing axons find their way close to the right locations by following ____. Then they array themselves over a targeted area by following chemical gradients.

Answer 12

chemicals

Question 13

In 1924 Paul Weiss, conducted an experiment in which he grafted an extra leg to a ____ and then waited for axons to grow into it. After the axons reached the muscles, the extra leg moved in synchrony with the normal leg next to it.

Answer 13

salamander

Question 14

Weiss dismissed the idea that each axon found its way to exactly the correct muscle in the extra limb. He suggested instead that the nerves attached to muscles at ___ and then sent a variety of messages, each one tuned to a different muscle.

Answer 14

random

Question 15

According to Weiss the muscles were like ____ tuned to different stations: each muscle received many signals but responded to only one.

Answer 15

radios

Question 16

Later evidence supported the interpretation that Weiss rejected: the salamanders extra leg moved in synchrony with its neighbour because each axon found ____ the correct muscle.

Answer 16

exactly

Question 17

Roger ____, a former student of Weiss, performed a classic experiment that showed how sensory axons find their way to their correct targets.

Answer 17

Sperry

Question 18

In Sperrys experiment with newts each axon regenerated to the same place where it had originally been, presumably by following a ____ ____.

Answer 18

chemical trail

Question 19

Axons find their correct target with remarkable ____.

Answer 19

precision

Question 20

A growing axon follows a path of ____ ____, attracted by some chemicals and repelled by others, in a process that steers the axon in the correct direction.

Answer 20

cell-surface molecules

Question 21

Eventually, axons sort themselves over the surface of their target area by following a ____ (concentrations) of chemicals.

Answer 21

gradient

Question 22

When exons initially reach their targets, chemical gradients steer them to approximately their correct location, but it would be hard to imagine that they achieve ____ ____.

Answer 22

perfect accuracy

Question 23

Instead, each axon forms synapses onto ____ cells in approximately the correct location, and each target cell receives synapses from ____ axons.

Answer 23

many

Question 24

Over time, each postsynaptic cell ____ some synapses – presumably the most appropriate ones – and ____ others. This adjustment depends on the pattern of input from incoming axons.

Answer 24

strengthens : eliminates

Question 25

In the development of the nervous system, we start with more neurons and synapses then we can keep. Synapses form with only approximate accuracy, and then a selection process ____ some and ____ others.

Answer 25

keeps : rejects

Question 26

Getting the right number of neurons for each area of the nervous system is more complicated than it might seem. Consider, the sympathetic nervous system sends axons to muscles and glands. Each ganglion has enough axons to supply the muscles and glands in its area, with no axons left over. How does the match come out so ____?

Answer 26

exactly

Question 27

Levi-Montalcini eventually discovered that the muscles do not determine how many axons ____; they determine how many ____.

Answer 27

form : survive

Question 28

Initially, the sympathetic nervous system forms far more neurons that it needs. When one of those neurons forms a synapse onto a muscle, that muscle delivers a protein called ____ ____ ____ (NGF) that promotes the survival and growth of the axon.

Answer 28

nerve growth factor

Question 29

And axon that does not receive NGF ____, and its cell body dies.

Answer 29

degenerates