Redrawn from

Redrawn from Warwick R, Williams PL: Grays Anatomy, th Br. ed. London: Longman Group Ltd, Figure Limbic system, showing the key position of the hypothalamus. forebrain bundle, which extends from the septal and orbitofrontal regions of the cerebral cortex downward through the middle of the hypothalamus to the brain stem reticular formation. This bundle carries fibers in both directions, forming a trunk line communication system. A second route of communication is through short pathways among the reticular formation of the brain stem, thalamus, hypothalamus, and most other contiguous areas of the basal brain. Hypothalamus, a Major Control Headquarters for the Limbic System The hypothalamus, despite its very small size of only a few cubic centimeters, has twoway communicating pathways with all levels of the limbic system. In turn, it and its closely allied structures send output signals in three directions: backward and downward to the brain stem, mainly into the reticular areas of the mesencephalon, pons , and medulla and from these areas into the peripheral nerves of the autonomic nervous system; upward toward many higher areas of the diencephalon and cerebrum, especially to the anterior thalamus and limbic portions of the cerebral cortex; and into the hypothalamic infundibulum to control or partially control most of the secretory functions of both the posterior and the anterior pituitary glands . Thus, the hypothalamus, which represents less than per cent of the brain mass, is one of the most important of the control pathways of the limbic system.

A major

A major part of the limbic system is the hypothalamus, with its related structures. In addition to their roles in behavioral control, these areas control many internal conditions of the body, such as body temperature, osmolality of the body fluids, and the drives to eat and drink and to control body weight. These internal functions are collectively called vegetative functions of the brain, and their control is closely related to behavior. Functional Anatomy of the Limbic System; Key Position of the Hypothalamus Figure shows the anatomical structures of the limbic system, demonstrating that they are an interconnected complex of basal brain elements. Located in the middle of all these is the extremely small hypothalamus, which from a physiologic point of view is one of the central elements of the limbic system . Figure illustrates schematically this key position of the hypothalamus in the limbic system and shows surrounding it other subcortical structures of the limbic system, including the septum, the paraolfactory area, the anterior nucleus of the thalamus, portions of the basal ganglia, the hippocampus, and the amygdala. And surrounding the subcortical limbic areas is the limbic cortex, composed of a ring of cerebral cortex in each side of the brain beginning in the orbitofrontal area on the ventral surface of the frontal lobes, extending upward into the subcallosal gyrus, then over the top of the corpus callosum onto the medial aspect of the cerebral hemisphere in the cingulate gyrus, and finally passing behind the corpus callosum and downward onto the ventromedial surface of the temporal lobe to the parahippocampal gyrus and uncus.

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.