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Sympathetic afferent fibers originate from a visceral organ, course in the thoracolumbar rami communicantes, have cell bodies located in dorsal root ganglia, and terminate in the gray matter of the spinal cord. Sympathetic afferent fibers from the heart transmit information about noxious stimuli associated with myocardial ischemia, i. e. angina pectoris. Previous reviews have described the characteristics of cardiovascular sympathetic afferent fibers (Bishop et al. 1983; Malliani 1982). This review summarizes that work and focuses on the neural mechanisms underlying the complexities of angina pectoris. In order to understand anginal pain, cells forming the classical pain pathway, the spinothalamic tract (STn, were chosen for study. These cells were chosen to address questions about anginal pain because they transmit nociceptive informa of pain. Antidromic tion to brain regions that are involved in the perception activation of STT cells provided a means of identifying cells involved with trans mission of nociceptive information in anesthetized animals. Other ascending pathways may also transmit nociceptive information, but many studies show that the STT plays an important role. Visceral pain is commonly referred to overlying somatic structures. The pain of angina pectoris can be sensed over a wide area of the thorax: in the retrosternal, precordial anterior thoracic, and anterior cervical regions of the chest; in the left or sometimes even the right shoulder, arm, wrist, or hand; or in the jaw and teeth (Harrison and Reeves 1968).
I fancy that many of you, like myself, have woken up in the night with a "sleeping" arm or leg. It is a very peculiar feeling to have that arm or leg, cold and lifeless, hanging there at your side as if it were something which does not belong to you. In such situations you recover some of the motor functions before the sensory functions, which en ables you to move the limb like a pendulum. For a few sec onds the arm functions as an artificial limb - a prosthesis without sensors. In general we are not aware of the importance of our sensory organs until we lose them. You do not feel the pressure of your clothes on the skin or the ring on your finger. In the nineteenth century such phenomena generally named adaptation, were studied to a great extent, partic ularly in vision, as well as in the so-called lower senses. The question whether sensory adaptation was due to changes in the peripheral sensory receptors or in the central nervous structure remained in general open until the 1920s. Then the development of the electronic arsenal gave us the means to attack the problem by direct observations of the electrical events in the peripheral as well as the central nervous system. But even today there are still some blank areas in our knowledge of adaptation.
Stability of the internal environment in which neuronal elements are situated is unquestionably an important prerequisite for the effective transmission of information in the nervous system. During the past decade our knowledge on the microenvironment of nerve cells has expanded. The conception that the microenvironment of neurones comprises a fluid with a relatively simple and stable composition is no longer accepted; the microenvironment is now envisaged as a dynamic structure whose composition, shape, and volume changes, thereby significantly influencing neuronal function and the trans mission of information in the nervous system. The modern conception of the neuronal microenvironment is based on the results of research over the last 20 years. The extracellular space (ECS) is comprehended not only as a relatively stable microenvironment containing neurones and glial cells (Bernard 1878), but also as a channel for communica tion between them. The close proximity of the neuronal elements in the CNS and the narrowness of the intercellular spaces provides a basis not only for interaction between the elements themselves, but also between the elements and their microenvironment. Substances which can cross the cell membranes can easily find their way through the microenvironment to adjacent cellular elements. In this way the microenvironment can assure non-synaptic com munication between the relevant neurones. Signalization can be coded by modulation of the chemical composition of the ECS in the vicinity of the cell membrane and does not require classic connection by axones, dendrites, and synapses.
The study of the functional organization of the first synapse of the centripetal visual pathway at the outer plexiform layer level (OPL) ought to be made through the application of combined histological, electrophysiological, and neurochemical techniques. A large amount of new evidence has been accumu lated in the past 20 years on the structure of the retina and on the electrical responses of retinal cells to light stimulus. Also, recently, many substances considered as neurotransmitters in the brain have been found in the retina. The goal of the study of retinal function is to integrate the data obtained by structural and electrophysiological techniques and to identify and determine the role played by neurotransmitters or neuromodulators in the function of the retina. In this study it is important to realize the morphological and biochemical diversi ty displayed by the visual cells in the vertebrate retina which, according to Cresci telli (1972), has been produced "through the interaction of natural selection with diversity in the photic environment." The evidence obtained shows that bipolar and especially horizontal cells, closely related to visual cells, display morphologi cal and probably biochemical differences among classes, genus, and even species according to the photic environment. These differences give peculiarities to the organization of the OPL, which must be taken into account when studying a par ticular retina with electrophysiological or neurochemical techniques.