The human central nervous system begins to form when the embryo is about ___ ____ old.
The dorsal surface thickens and then long thin lips rise, curl, and merge, forming a ____ ____ that surrounds a fluid filled cavity.
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.
Neuroscientists distinguish these processes in the ____ of ____: preparation, migration, differentiation, myelination, and synaptogenesis.
development of neurons
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.
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.
immunoglobulins and chemokines
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.
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.
The final stage is ____, or the formation of synapses.
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.
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.
few or no
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.
Growing axons find their way close to the right locations by following ____. Then they array themselves over a targeted area by following chemical gradients.
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.
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.
According to Weiss the muscles were like ____ tuned to different stations: each muscle received many signals but responded to only one.
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.
Roger ____, a former student of Weiss, performed a classic experiment that showed how sensory axons find their way to their correct targets.
In Sperrys experiment with newts each axon regenerated to the same place where it had originally been, presumably by following a ____ ____.
Axons find their correct target with remarkable ____.
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.
Eventually, axons sort themselves over the surface of their target area by following a ____ (concentrations) of chemicals.
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 ____ ____.
Instead, each axon forms synapses onto ____ cells in approximately the correct location, and each target cell receives synapses from ____ axons.
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.
strengthens : eliminates
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.
keeps : rejects
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 ____?
Levi-Montalcini eventually discovered that the muscles do not determine how many axons ____; they determine how many ____.
form : survive
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.
nerve growth factor
And axon that does not receive NGF ____, and its cell body dies.