摘要:

Compared to other vertebrates, even including lampreys and hagfishes in some respects, salamanders exhibit a relatively simple organization of brain and sense organs which is illustrated here using the visual system as an example. The greatest simplicity is found in the bolitoglossine salamanders, yet all bolitoglossines possess highly projectile tongues and rely on vision for survival; furthermore, some species are agile and acrobatic. The unusual features of the visual system of salamanders include small numbers of large neurons, a low degree of morphological differentiation among neurons, a small proportion of myelinated axons in the optic nerve, and an optic tectum consisting essentially of a periventricular cellular layer and a superficial fiber layer. Similar features are found throughout the central nervous system of salamanders and in the lateral line, auditory and olfactory systems as well. Phylogenetic analysis shows that the most parsimonious interpretation of these data is that the simple organization of the brain and sense organs of salamanders was derived secondarily from a more complex ancestral state. We hypothesize that increased genome size has led to simplification of the nervous system in salamanders. Increased genome size appears to have had profound effects on neural development in salamanders, leading to paedomorphosis, the retention of juvenile or even embryonic characteristics into adulthood. In particular, large genome size is associated with large cell size and reduced rates of cell proliferation, migration and differentiation. Secondary simplification has constrained the function of the salamanders'' visual system, primarily by increasing cell size and decreasing cell numbers. However, it also has provided an opportunity for the evolution of compensating mechanisms, which have helped to restore or even enhance visual function. Most apparent among the compensatory mechanisms of bolitoglossine salamanders is the presence of well developed ipsilateral retinotectal projections, which apparently enhance depth perception. It is difficult to explain the unusual history of the nervous system in salamanders solely in terms of natural selection and adaptation. Increasing genome size through selfish replication appears to have played a major role in the evolution of salamander brains by imposing functional constraints as well as creating opportunities for overcoming them.

ISSN:0006-8977    

DOI:10.1159/000113321

出版商:S. Karger AG    

年代:1997

数据来源: Karger

ISSN:0006-8977    

DOI:10.1159/000113322

出版商:S. Karger AG    

年代:1997

数据来源: Karger

摘要:

This review summarizes our current understanding of the neural circuit underlying the larval proleg withdrawal reflex (PWR) of Manduca sexta and describes how PWR function changes in two contexts: metamorphosis and learning. The first form of PWR plasticity occurs during the larval-pupal transformation, when the reflex is lost. One mechanism that contributes to this loss is the weakening of monosynaptic excitatory connection from proleg sensory neurons to proleg retractor motor neurons. This change is associated with the hormonally-mediated regression of proleg motor neuron dendrites, which may break synapse contacts between the sensory and motor neurons. After pupation, some of the proleg motor neurons die in a segment-specific pattern that persists even after individual motor neurons are isolated from the nervous system and exposed to hormones in vitro. The second form of PWR plasticity involves short-term, activity-dependent changes in neural function during the larval stage. The nicotinic cholinergic connections from proleg sensory neurons to motor neurons exhibit several forms of plasticity including facilitation, depression, post-tetanic potentiation and two types of muscarinic modulation. Larval PWR behavior exhibits two simple forms of learning - habituation and dishabituation - which involve alterations in the central PWR circuit. These studies of a simple circuit illustrate neural mechanisms by which behaviors undergo both short- and long-term modifications.

ISSN:0006-8977    

DOI:10.1159/000113356

出版商:S. Karger AG    

年代:1997

数据来源: Karger

ISSN:0006-8977    

DOI:10.1159/000113360

出版商:S. Karger AG    

年代:1997

数据来源: Karger

ISSN:0006-8977    

DOI:10.1159/000113361

出版商:S. Karger AG    

年代:1997

数据来源: Karger

ISSN:0006-8977    

DOI:10.1159/000113362

出版商:S. Karger AG    

年代:1997

数据来源: Karger