Showing papers in "International Review of Neurobiology in 1971"

The Axon Reaction: A Review of the Principal Features of Perikaryal Responses to Axon Injury

Abstract: Publisher Summary This chapter provides an overview of the principal features of perikaryal responses to axon injury. The neuron is an unusual cell. Its axon terminals may be situated at what in cellular terms is an enormous distance from the cell body (perikaryon); the volume of the latter may be but a small fraction of the total cellular volume. Yet the neuronal processes are maintained and their substance is constantly renewed from the perikaryon. The separation of an axon from its cell body results (in vertebrates) in the degeneration of the separated portion and is followed by a series of morphological changes in the perikarya. The most conspicuous of these is the disintegration, redistribution, and apparent disappearance from the cell body of cytoplasmic basophil material. Changes in axotomized neurons are generally assessed by comparison with the corresponding contralateral neurons of the experimental animal.

Central Cholinergic Mechanism and Behavior

Abstract: Publisher Summary This chapter discusses central cholinergic mechanism functions and behaviors. It presents data that establishes acetylcholine (ACh) as a central neurotransmitter and its role in the elicitation and maintenance of behavior. The chapter also highlights the role of the central adrenergic and serotonergic mechanisms to demonstrate and emphasize that a balance between the involved systems, rather than an individual mechanism, is essential for proper maintenance of a behavior. A large volume of pharmacological, chemical, and physical evidence indicates that ACh is present in the nervous system. Usually, bioassay methods are employed in the determination of ACh concentration in the brain. In the brain, the concentration of ACh is highest in the brain stem and caudate nucleus, lowest in the cerebellum, and intermediate in the cerebral cortex, pons, and medulla. Much more ACh is present in the gray matter than in the white matter, because of its localization in the synaptic region. ACh is synthesized from choline and acetyl-coenzyme A with the help of the enzyme, ChA. The chapter explains the multitransmitter control of central functions and central cholinergic modulation of behavior.

CO2 fixation in the nervous tissue.

Abstract: Publisher Summary This chapter explains CO 2 fixation in the nervous tissue. The primary CO 2 fixation reaction occurs at the oxaloacetate level. Both pyruvate carboxylase and phosphoenolpyruvate carboxykinase reactions lead to the formation of oxaloacetate, while the reaction of malic enzyme synthesizes malate. In the peripheral nerves, oxaloacetate is found to be the primary product. After CO 2 is fixed into oxaloacetate, it is transferred to all other tricarboxylic acid intermediates. The importance of CO 2 fixation at the oxaloacetate level is reflected by the 10% contribution of CO 2 fixation pathway versus the acetyl-CoA pathway in pyruvate utilization. This implies a heavy loss of carbons that are fed into the tricarboxylic acid. Glucose and pyruvate are generally considered to serve as an energy producer instead of providing carbons for other intermediates. In view of the extremely fast glutamine synthesis in the brain and the fact that glutamine is a readily diffusible compound across neuronal membrane, it is conceivable that CO 2 fixation replenishes the loss of glutamine from neuronal tissue to the blood or the cerebrospinal fluid. The chapter also discusses the functional aspects of CO 2 fixation.

The Pharmacology of Thalamic and Geniculate Neurons

Abstract: Publisher Summary This chapter describes the pharmacology of thalamic and geniculate neurons. It discusses the progress that has been made in identifying transmitter agents in brain. The substances discussed are acetylcholine (ACh), monoamines, including dopamine, noradrenaline, and 5-hydroxytryptamine, and the neutral and acidic amino acids, such as γ-aminobutyric acid (GABA) and glutamic acid. The problem of transmitter identification should be approached with caution. The complex organization of the central nervous system with a close proximity of many neurons, glial cells, and capillaries makes it difficult to study the actions of putative transmitters on single neurons in isolation. The complexity of the synaptic junction itself is such that several possible mechanisms of drug action must be considered before the actions of a particular substance upon a neuron can be evaluated. A series of criteria have been developed from the classical studies on the peripheral nervous system, which has established acetylcholine as a transmitter at neuromuscular and ganglionic synapses. These criteria constitute a useful guide in any assessment of the possibility that a substance is a transmitter at a particular synaptic junction.

The Chemical Anatomy of Synaptic Mechanisms: Receptors

Abstract: Publisher Summary This chapter presents an overview of chemical anatomy of synaptic mechanisms. The chemical nature of the receptor sites for transmitters is one of the most interesting problems of molecular biology. Certain portions of the membrane of neurons and muscle cells are specialized so that they can bind transmitters, with the result that the ionic permeability of the cell membrane becomes altered. If the transmission of sodium ions is enhanced, the membrane becomes depolarized; if potassium or chloride ions are involved, the membrane becomes hyperpolarized and inhibition results. The chapter explains the possible molecular complexes involved in receptors. One difficulty in constructing hypotheses about the possible molecular nature of the receptor knows where to start. One way is to itemize all the possibilities and then develop the most promising of these. The problem can be approached by asking what type of electrostatic bond proteins can get involved in.

Reflections on the Role of Receptor Systems for Taste and Smell

Abstract: Publisher Summary The role of chemoreceptors in the life of animals and in the evolution of their nervous systems poses problems of the greatest complexity but with fascinating implications. The tactile receptors, for all their complexity in detail, are relatively simple in principle. This chapter describes the differences in structure of taste and odor receptors. These receptors have not evolved independently but as the afferent arms of complete reaction systems. In air-breathing animals, taste became concentrated in the oral cavity. Olfaction, on the other hand, as soon as it can be distinguished from taste, is concentrated at the cranial end of the animal. Odorants are detected only in solution on the olfactory membrane. The detailed structure of taste bud cells indicates that the receptor cells are more slender and contain chromatic irregular nuclei, while the larger supporting cell has a less chromatic nucleus. In addition, the olfactory mechanism is much more complex. Organization of the olfactory nerves also is puzzling. Immediately beneath the epithelium, the fibers join into small fascicles held together by a common sheath cell. These fascicles interweave and exchange fiber. The evolutionary significance of olfaction is highlighted in the chapter.