Neurobiology in Statu Nascendi

Abstract

This chapter provides an introduction to neurobiology. Much less is known about the way the billions of neurons acquire those intrinsic characteristics that define their later functional idioms. Some seem to be preprogrammed at an early stage, others seem to gain specifications secondarily from their surroundings or contacts, and others remain consistently pluripotent. Conclusive evidence is that early preprogramming of neurons is a fact that is manifested by Mauthner cells of amphibian larvae, a pair of single neurons in the medulla, distinctive by their giant sizes and destined for some sensory function in aquatic life. Studies show that a small fragment of future medulla of an egg in neurulation, explanted into a neutral medium, will still give rise to a singular giant Mauthner cell amidst a mass of conventionally small-sized neurons. Aside from its gigantism, this peculiar cell also seems pre-endowed with the potential for its specific future reactivity. According to Weiss and Rossetti, the larva of Xenopus grows conspicuously for about a month and so do its two Mauthner cells. Thyroid secretion then supervenes and transforms the larva into the metamorphosed frog, which continues to grow. While the other brain cells likewise keep enlarging, the two Mauthner cells shrink down to indistinctiveness. No external intervention, such as amputating the tail, transecting the long Mauthner fibers, etc., has any effect.