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.

They also

They also seem to project into the limbic system ones current status in relation to both surroundings and thoughts. On the basis of this information, the amygdala is believed to make the persons behavioral response appropriate for each occasion. Function of the Limbic Cortex The most poorly understood portion of the limbic system is the ring of cerebral cortex called the limbic cortex that surrounds the subcortical limbic structures. This cortex functions as a transitional zone through which signals are transmitted from the remainder of the brain cortex into the limbic system and also in the opposite direction. Therefore, the limbic cortex in effect functions as a cerebral association area for control of behavior. Stimulation of the different regions of the limbic cortex has failed to give any real idea of their functions. However, as is true of so many other portions of the limbic system, essentially all behavioral patterns can be elicited by stimulation of specific portions of the limbic cortex. Likewise, ablation of some limbic cortical areas can cause persistent changes in an animals behavior, as follows. Ablation of the Anterior Temporal Cortex. When the anterior temporal cortex is ablated bilaterally, the amygdalas are almost invariably damaged as well. This was discussed earlier in this chapter; it was pointed out that the KluverBucy syndrome occurs. The animal especially develops consummatory behavior: it investigates any and all objects, has intense sex drives toward inappropriate animals or even inanimate objects, and loses all fearand thus develops tameness as well.

These people

These people can recall most previously learned memories satisfactorily. However, they often can learn essentially no new information that is based on verbal symbolism. In fact, they often cannot even learn the names of people with whom they come in contact every day. Yet they can remember for a moment or so what transpires during the course of their activities. Thus, they are capable of shortterm memory for seconds up to a minute or two, although their ability to establish memories lasting longer than a few minutes is either completely or almost completely abolished. This is the phenomenon called anterograde amnesia that was discussed in Chapter Theoretical Function of the Hippocampus in Learning. The hippocampus originated as part of the olfactory cortex. In many lower animals, this cortex plays essential roles in determining whether the animal will eat a particular food, whether the smell of a particular object suggests danger, or whether the odor is sexually inviting, thus making decisions that are of lifeordeath importance.Very early in evolutionary development of the brain, the hippocampus presumably became a critical decisionmaking neuronal mechanism, determining the importance of the incoming sensory signals. Once this critical decisionmaking capability had been established, presumably the remainder of the brain also began to call on the hippocampus for decision making. Therefore, if the hippocampus signals that a neuronal input is important, the information is likely to be committed to memory.

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.