neurobio 920 the genesis of the nervous system part 2 (proliferation neural tube neuroblasts filo
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the filopodia are constantly probing the environment for what?
the filopdia is probing for a substrate. Growth of the neurite occurs when a filopodium, instead of retracting, takes hold of the substrate and pulls the advancing growth cone foward
what is one important substrate that the filopodia probe for?
the extracellular matrix
what is fasciculation?
a mechanism that causes axons growing together to stick together.
what do pioneer axons do?
a common mode of pathway formation is the initial establishment of connections by pioneer axons
true or false? the trajectory of the axons is broken into small segments.
true. axaons conclude a segment when they arrive at an intermediate target. The interaction of the axon and the intermediate target throws a molecular switch that sends the axons onward to another intermediate target
what are guidance cues?
guidance cues are molecules in the environment of a neuronal cell that determines the direction and amount of growth of neurites. the guidance cues can be attractive or repulsive
how does the protein netrin act on the axons of dorsal horn neurons?
the gradient of netrin attracts axons of the dorsal horn neurons that will cross the midline to form the spinothalamic tract. These axons possess netrin receptors, and the binding of netrin to the receptor spurs growth toward the source of netrin
after axons of the dorsal horn neurons pass the midline, what happens
midline cells secrete a protein called slit, a chemorepellent that chases axons away.
what is the chemoaffinity hypothesis?
the idea that chemical markers on growing axons are matched with complementary chemical markers on their targets to establish precise connections
what is the tectum?
the tectum is the amphibian homologue of the mammalian superior colliculus
explain the experiment conducted by Roger Sperry on frogs
sperry cut the optic nerve of a frog and rotated it 180 degrees. Amphibians are able to regenerate their CNS axons, which is what happened to the frog. Despite the fact that the axons of the optic nerve were now scrambled from where they would occur naturally, the axons gew into the tectum to exactly the same sites that they occupied originally.
axons from a frog's nasal retina innervate which part of the tectum?
they innervate the neurons in the posterior part of the tectum
axons from a frog's temporal retina innervate which part of the tectum?
they grow into the anterior tectum and stop there
why do the axons from the frog's tempral retina stop at the anterior part of the tectum?
Experiments have shown that the cell membranes of anterior and posterior tectal neurons differentially express factors that allow the growth of nasal and temporal retinal axons.
what is the first step in synapse formation at a neuromuscular junction?
the fdirst step appears to be the induction of a cluster of postsynaptic receptors under the site of nerve-muscle contact. This clustering is triggered by an interaction between proteins secreted by the growth cone and the target membrane.
in the formation of a synapse in a neuromuscular junction, is it possible for the postsynaptic terminal to initiate an interaction with the presynaptic terminal?
yes. the induction of a presynaptic terminal also appears to involve proteins in the basal lamina. basal lamina factors provided by the target cell evidently can stimulate Ca entry into the growth cone, which triggers neurotransmitter release
how is the major reduction in neurons and glial cells mediated?
the widespread reduction reflects competition for tropic factors, life sustaining substances that are provided in limited quantities by the target cells.
during what period of life does the dramatic reduction in synapses occur?
what is synaptic capacity?
the number of synapses that a dendrite or soma can receive
true or false? initially, a muscle fiber receives synaptic input from several different motor neurons.
how is the polyneuronal innervation of each muscle fiber lost?
the process is regulated by electrical activity in the muscle. Silencing the activity of the muscle fiber leads to a retention of polyneuronal innervation, while stimulation of the muscle accelerates the elimination of all but one input
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