During embryonic development, regulated gene expression and signal transduction establish the overall structure of the NS. Two processes then dominate the remaining development of the NS.
-What is the first?
o Neurons compete for growth-supporting factors, which are produced in limited quantities by tissues that direct neuron growth. Cells that don't reach the proper locations fail to receive such factors and undergo programmed cell death. The competition is so severe tht half of the neurons formed in the embryo are eliminated. The net effect is the preferential survival of neurons that are loacated properly within the nervous system.
1- competition among neurons for survival.
What is the second?
- A developng neuron forms numerous synapses, more than are required for its prroper function. The activity of that neuron then stabilizes some synapses and destabilizes others. By the end of __, neurons on average have lost more than half of their initial synapses, leaving behind the connections that survive into adulthood.
True or False:
Together, neuron and synapse eliminateion set up the network of cells and connections within the NS required throughout life.
Although the basic architecture of the CNS is established during embryonic development, it can change after birth. This capacity for the NS to be remodeled, espeically in response to its own activity, is called __.
Much of the reshaping of the NS occurs at __. When activity at a __ correlates with that of other __, changes may occur that reinforce that synaptic connection. Conversely, when the activity of a __ fails to correlate with that of other __, the synaptic connection sometimes becomes weaker.
synapses x all
__ and __ of the NS occurs in many contexts. For example, these processes are necessary steps in how we develop the ability to sense our surroundings. They are also critical to the NS's limited ability to recover from injury or disease.
Remodeling and refining
Though we may not be aware of it, we are constantly checking what is happening against what just happened a few moments ago. We hold info for a time in __ locations and then release it if it becomes irrelevant. If we wish to retain knowledge of a name, phone # or other fact, the mechanisms of __ are activated. If we later need to recall the name or #, we fetch it from __ and return it to __.
long -term memory
Both types of memory involve the storage of info in the __. In __, this info is accessed via temporary links or assoiations formed in the __. Whem memories are made long-term, the links in the __ are replaced by more permanent connections within the __ itself.
The __ is thus essential for acquiring new long-term memories, but not for maintaining them. For this reason, people who suffer damage to the __ are to some extent trapped in the past: They cannot form any new lasting memories but can freely recall events from before their injury.
What evolutionary advantage might be offered by organizing short-term and long-term memoies differently?
- Current thinking is that the delay in forming connection sin the __ allows long-term memories to be integrated gradually into the existing store of knowledge and experience, providing a basis for more meaningful assocations. Consistent with this idea, the transfer of information from short term to long-term meory is enhaced by the association of new data with data perviously learned and stored in long-term memory.
__, like walking, tying shoes or writing are usually learned by repetition.
- Learning skills and procedures appears to involve cellular mechanisms very similar to those responsible for brain growth and development. In such cases, neurons actually make new connections. In contrast, memorizing phone #s, facts and places- which can be very rapid and may require only one exposure to the relevant item- may rely mainly on change sin the strength of existing neuronal connections.
Reserachers have identified several processes that can alter a synaptic connection, making the flow of communication either more efficient or less efficient.
- __ is a lasting increase in the strength of synaptic transmissin.
long-term potentiation (LTP)
__, which was fist characterized in tissue slices from the hippocampus, involves a __ neuron tha releases the excitatory neurotransmitter glutamate. For __ to occur, there must be a brief high-frequency series of action potentials in this __ neuron. In addition, these action potentials must arrive at thes ynaptic terminal at the same time that the __ cell receives a depolarizing stimulus.
__ involves two types of glutamate receptors, each named for a moleucle- __ or __ - taht artificially activates tht particular receptor. The set of receptors present on the __ membranes changes in response to an active synapse and a depolarizing stimulus.
The result is __- a stable increase in the size of the postsynapic potentials at the synapse. Because __ can lasts for days or weeks in dissected tissue, it is thought to represent one of the fundamental processes by which memories are stored and learning takes place.