Mathilde Rahmann

Chemical Aspects of Neuronal Information Transmission in Synapses

The totality of mechanisms involved in the process of information transmission is immensely complex, comprised as it is of biochemical, biophysical, and ultrastructural elements. More-over, these are not identical for all types of nerve terminals. Indeed, quite heterogeneous varieties have become apparent.

Behavioral-Physiological Basis of Memory

It is essential to be entirely familiar with the behavioral spectrum of an organism in order to understand the phenomenon of memory. The determination as to whether an organism has or has not formed memory can be established only on the basis of lasting changes in behavior in response to specific environmental stimuli.

Zelluläre Grundlagen des Gedächtnisses

Zweifelsfrei stellt das Nervensystem hoher entwickelter Tiere, speziell das der Wirbeltiere, die komplexeste Organisationsstufe der lebenden Materie dar. Hinsichtlich seines zellularen Aufbaus ist es im wesentlichen aus nur zwei verschiedenen Zelltypen, den Nervenzellen oder Neuronen (WALDEYER, 1891) sowie den Gliazellen (Neuroglia) zusammengesetzt. Auf die Neuronen entfallen dabei die spezifischen Sonderleistungen des Nervengewebes, namlich die Aufnahme, Verarbeitung und Weiterleitung von Informationen von Zelle zu Zelle und vor allem die Informationsspeicherung und damit die Deponierung von Gedachtnisinhalten.

The Cellular Basis of Memory

Without question, the nervous system of the more highly developed animals, especially of the vertebrates, represents the most complex level of organization of all living matter. In essence, it is comprised of only two different types of cells, nerve cells (neurons; Waldeyer, 1891) and glial cells (neuroglia). The neurons perform the specific, specialized activity of the nerve tissue, i.e., reception, processing, and transmission of information from cell to cell and, most importantly, the storage of information, thereby serving as the repository of memory content. The functions of the neuroglia, in contrast to those of the neurons, cannot be defined so readily. Indeed, the glial cells insulate, protect, and support the nerve cells from external, mechanical influences, but the neuroglia also perform metabolic tasks in the sense of metabolic symbiosis with the nerve cells.

Functional Morphology of the Nervous System in Vertebrates

The preceding chapter dealt with cellular, molecular, and morphogenetic considerations as they apply to the formation of neuronal structures. Several molecular aspects of neuronal differentiation were addressed, as well. This background will serve as a foundation for the discussion of functional morphology. Functional morphology in vertebrates is of particular interest to us in that it serves as the basis for our own higher associative brain and memory performance. A brief discussion of the invertebrate nervous system is presented in Chapt. 4.

Evolution and Architecture of the Nervous System in Invertebrates

The formation of nerve cells, i.e., elements whosefunction within an organism is to conductimpulses, was immensely important for higherdevelopment (anagenesis) in animals during their phylogeny. It was, indeed, the development of nerve cells that enabled animals to develop their own system of communication which, in turn,gave the entire organism the capacity to act andreact. Higher development in the organisms of multicellular animals (metazoa) was coupled withever-advancing development and refinement of the nerve cells into nervous systems. The more differentiated the composition and functional modality of the nervous system of a species was,the more complex and adaptive did its performance become. That meant that it had all the more positive advantages of selection.

Principles of Circuitry in Neurobiological Information Processing

Nerve cells never occur in isolation within anorganism. In addition to their close association with glial cells (see Chap. 1.2), they are always connected to other neurons and receptor or effector cells in the region of their nerve fiberendings via the formation of synaptic contacts.The integration of nervous systems into highly complex functional systems, i.e., into information and control centers, increases proportionately to the animal’s higher developmental phylogeneticstanding.

Modulation of Neuronal Information Transmission

In light of the myriad details presented thus far relative to the molecular structure of the synaptic membrane, the vesicles, the synaptic transmitters (neurotransmitters, neuropeptides), and their neuroreceptors, one might well conclude that the relatively strict sequencing of events in the transmission process of electrical information signals from one neuron to the next requires that the component processes be modulated in order to attain transmission. Of course, a neuronal impulse triggers the release of transmitter substances. Nonetheless, the quantity of the released substance as well as the strength, duration, and modality of the postsynaptic effects (inhibitory or excitatory) must be coordinated precisely.

Development of the Nervous System in Vertebrates

Having introduced the essential building blocks of the nervous system in the previous chapter and having discussed their morphology, a closer look will be directed in this chapter toward the development and differentiation of neuronal systems. In particular, the cell-specific and molecular fundamentals of neuronal differentiation, i.e., the morphogenesis of larger neuronal assemblies in the central nervous system in vertebrates, will be examined in greater detail. An understanding of the processes upon which development is based leads to an understanding of the way in which the nervous system functions. These processes also constitute the basis for higher associative brain functions, in particular, the formation of the more complex functions of learning and memory.

Neurobiological Models of Memory

Discussions concerning the essentials of information processing were presented in preceding chapters. Cellular, morphogenetic, and functional principles were addressed concerning the ways in which organisms receive, conduct, transmit, and store information. The remarkable plasticity (i.e., adaptability) of neuronal processes, especially relative to information transmission and synaptic events, bears reemphasizing. In light of these discussions, it is appropriate now to reflect upon the neuronal basis of long-term memory and examine the existing memory hypotheses in terms of soundness, efficiency, and effectiveness.