As would

As would be expected, these three systems have different effects on levels of excitability in different parts of the brain. The norepinephrine system spreads to virtually every area of the brain, whereas the serotonin and dopamine systems are directed much more to specific brain regions the dopamine system mainly into the basal ganglial regions and the serotonin system more into the midline structures. Neurohormonal Systems in the Human Brain . Figure shows the brain stem areas in the human brain for activating four neurohormonal systems, the same three discussed for the rat and one other, the acetylcholine system. Some of the specific functions of these are as follows: The locus ceruleus and the norepinephrine system. The locus ceruleus is a small area located bilaterally and posteriorly at the juncture between the pons and mesencephalon. Nerve fibers from this area spread throughout the brain, the same as shown for the rat in the top frame of Figure , and they secrete norepinephrine. The norepinephrine generally excites the brain to increased activity. However, it has inhibitory effects in a few brain areas because of inhibitory receptors at certain neuronal synapses. In Chapter , we will see that this system probably plays an important role in causing dreaming, thus leading to a type of sleep called rapid eye movement sleep REM sleep. The substantia nigra and the dopamine system. The substantia nigra is discussed in Chapter in Multiple centers in the brain stem, the neurons of which secrete different transmitter substances specified in parentheses.

Studies by

Studies by Byrne and colleagues, also in the snail Aplysia, have suggested still another mechanism of synaptic memory. Their studies have shown that stimuli from separate sources acting on a single neuron, under appropriate conditions, can cause longterm changes in membrane properties of the postsynaptic neuron instead of in the presynaptic neuronal membrane, but leading to essentially the same memory effects. LongTerm Memory There is no obvious demarcation between the more prolonged types of intermediate longterm memory and true longterm memory. The distinction is one of degree. However, longterm memory is generally believed to result from actual structural changes, instead of only chemical changes, at the synapses, and these enhance or suppress signal conduction. Again, let us recall experiments in primitive animals where the nervous systems are much easier to study that have aided immensely in understanding possible mechanisms of longterm memory. Structural Changes Occur in Synapses During the Development of LongTerm Memory Electron microscopic pictures taken from invertebrate animals have demonstrated multiple physical structural changes in many synapses during development of longterm memory traces. The structural changes will not occur if a drug is given that blocks DNA stimulation of protein replication in the presynaptic neuron; nor will the permanent memory trace develop. Therefore, it appears that development of true longterm memory depends on physically restructuring the synapses themselves in a way that changes their sensitivity for transmitting nervous signals.

Then the

Then the monkey is taught to recognize different objects with its right eye while its left eye is covered. Next, the right eye is covered and the monkey is tested to determine whether its left eye can recognize the same objects. The answer to this is that the left eye cannot recognize the objects. However, on repeating the same experiment in another monkey with the optic chiasm split but the corpus callosum intact, it is found invariably that recognition in one hemisphere of the brain creates recognition in the opposite hemisphere. Thus, one of the functions of the corpus callosum and the anterior commissure is to make information stored in the cortex of one hemisphere available to corresponding cortical areas of the opposite hemisphere. Important examples of such cooperation between the two hemispheres are the following. Cutting the corpus callosum blocks transfer of information from Wernickes area of the dominant hemisphere to the motor cortex on the opposite side of the brain. Therefore, the intellectual functions of Wernickes area, located in the left hemisphere , lose control over the right motor cortex that initiates voluntary motor functions of the left hand and arm, even though the usual subconscious movements of the left hand and arm are normal. Cutting the corpus callosum prevents transfer of somatic and visual information from the right hemisphere into Wernickes area in the left dominant hemisphere. Therefore, somatic and visual information from the left side of the body frequently fails to reach this general interpretative area of the brain and therefore cannot be used for decision making.

An animal

An animal builds up strong memory traces for sensations that are either rewarding or punishing but, conversely, develops complete habituation to indifferent sensory stimuli. It is evident that the reward and punishment centers of the limbic system have much to do with selecting the information that we learn, usually throwing away more than per cent of it and selecting less than per cent for retention. Specific Functions of Other Parts of the Limbic System Functions of the Hippocampus The hippocampus is the elongated portion of the cerebral cortex that folds inward to form the ventral surface of much of the inside of the lateral ventricle. One end of the hippocampus abuts the amygdaloid nuclei, and along its lateral border it fuses with the parahippocampal gyrus, which is the cerebral cortex on the ventromedial outside surface of the temporal lobe. The hippocampus and its adjacent temporal and parietal lobe structures, all together called the hippocampal formation has numerous but mainly indirect connections with many portions of the cerebral cortex as well as with the basal structures of the limbic systemthe amygdala, the hypothalamus, the septum, and the mamillary bodies. Almost any type of sensory experience causes activation of at least some part of the hippocampus, and the hippocampus in turn distributes many outgoing signals to the anterior thalamus, hypothalamus, and other parts of the limbic system, especially through the fornix, a major communicating pathway.