The most

The most important of the physical structural changes that occur are the following: Increase in vesicle release sites for secretion of transmitter substance. Increase in number of transmitter vesicles released. Increase in number of presynaptic terminals. Changes in structures of the dendritic spines that permit transmission of stronger signals. Thus, in several different ways, the structural capability of synapses to transmit signals appears to increase during establishment of true longterm memory traces. Number of Neurons and Their Connectivities Often Change Significantly During Learning During the first few weeks, months, and perhaps even year or so of life, many parts of the brain produce a great excess of neurons, and the neurons send out numerous axon branches to make connections with other neurons. If the new axons fail to connect with appropriate subsequent neurons, muscle cells , or gland cells, the new axons themselves will dissolute within a few weeks. Thus, the number of neuronal connections is determined by specific nerve growth factors released retrogradely from the stimulated cells. Furthermore, when insufficient connectivity occurs, the entire neuron that is sending out the axon branches might eventually disappear. Therefore, soon after birth, there is a principle of use it or lose it that governs the final number of neurons and their connectivities in respective parts of the human nervous system. This is a type of learning. For example, if one eye of a newborn animal is covered for many weeks after birth, neurons in alternate stripes of the cerebral visual cortexneurons normally connected to the covered eyewill degenerate, and the covered eye will remain either partially or totally blind for the remainder of life.

It is

It is a type of negative memory that causes the neuronal circuit to lose its response to repeated events that are insignificant. Conversely, if a noxious stimulus excites the facilitator terminal at the same time that the sensory terminal is stimulated, then instead of the transmitted signal into the postsynaptic neuron becoming progressively weaker, the ease of transmission becomes stronger and stronger; and it will remain strong for minutes, hours, days, or, with more intense training, up to about weeks even without further stimulation of the facilitator terminal. Thus, the noxious stimulus causes the memory pathway through the sensory terminal to become facilitated for days or weeks thereafter. It is especially interesting that even after habituation has occurred, this pathway can be converted back to a facilitated pathway with only a few noxious stimuli. Molecular Mechanism of Intermediate Memory Mechanism for Habituation. At the molecular level, the habituation effect in the sensory terminal results from progressive closure of calcium channels through the terminal membrane, though the cause of this calcium channel closure is not fully known. Nevertheless, much smaller than normal amounts of calcium ions can diffuse into the habituated terminal, and much less sensory terminal transmitter is therefore released because calcium entry is the principal stimulus for transmitter release as was discussed in Chapter Chapter Cerebral Cortex, Intellectual Functions of the Brain, Learning and Memory Mechanism for Facilitation.