Holosteans and Teleosts

Recent studies on neural pathways in a broad spectrum of vertebrates suggest that, in addition to migration and an increase in the number of certain select neurons, a significant aspect of neural evolution is a “parcellation” (segregation-isolation) process that involves the loss of selected connections by the new aggregates. A similar process occurs during ontogenetic development. These findings suggest that in many neuronal systems axons do not invade unknown territories during evolutionary or ontogenetic development but follow in their ancestors' paths to their ancestral targets; if the connection is later lost, it reflects the specialization of the circuitry.The pattern of interspecific variability suggests (1) that overlap of circuits is a more common feature in primitive (generalized) than in specialized brain organizations and (2) that most projections, such as the retinal, thalamotelencephalic, corticotectal, and tectal efferent ones, were bilateral in the primitive condition. Specialization of these systems in some vertebrate groups has involved the selective loss of connections, resulting in greater isolation of functions. The parcellation process may also play an important role in cell diversification.The parcellation process as described here is thought to be one of several underlying mechanisms of evolutionary and ontogenetic differentiation.

Evolution and ontogeny of neural circuits

The ultimate aim in the study of the nervous system is the explanation of behavior: the demonstration of how behavioral responses are produced as a function of nerve cell activity. Even for small bits of neural tissue and restricted aspects of behavior, the number of nerve cells involved is so large and their connections are so complex that large numbers of hypotheses can be imagined. As a result, in the history of beavioral studies, a great deal of “neurologizing” has occurred. Broad speculation about the overall “organization of the brain” and the manner in which it controls behavior has been entertained because, in most cases, the number of facts available to rule out hypotheses has been small. Thus, no comprehensive hypothesis really could be proved. It has seemed necessary to pick a situation where it would be fruitful to gather a great number of behavioral, neuroanatomical, and neurophysiological facts and thus to narrow down the allowable hypotheses to a small number. If enough relevant facts can be brought to bear, and hypotheses can be conclusively ruled out, then a strong inference (Platt, 1964) of the correct hypothesis will finally be possible. In turn, principles can be stated clearly and can be tested.

Neural Mechanisms of Female Reproductive Behavior

Folowing unilateral HRP injections in the optic tectum of Ran pipiens, HRP - positive cells were seen in three pretectal nuclei: bilaterally in the dorsal posterior nucleus; in the dorsal half of the ipsilateral posterior nucleus; and ipsilaterally in the large-celled pretectal nucleus. HRP -positive cells were also seen ipsilaterally in the anterodorsal, posterodorsal and posteroventral tegmental fields, the nucleus isthmi, and the dorsal gray columns of the cervical spinal cord; bilaterally in the suprapeduncular nucleus, a paramedian cell group dorsal to the interpeduncular nucleus; and in the deep layers of the contralateral tectum. In addition, evidence for a bilateral ventral preopto-tectal projection was seen in half the experimental animals. No tectal afferents from telencephalic or rostral thalamic areas were seen. Both the ascending and descending tectal efferent fibers were also filled with reaction product. The pale reaction indicative of terminating tectal efferents was seen in the dorsal pretectum, partially overlapping the lateral nucleus and uncinate neuropil; in the core of nucleus isthmi; and in the superior olive.

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Afferents to the optic tectum of the leopard frog: an HRP study.

From the dawn of comparative neuroanatomy the central nervous system of the lamprey has attracted the attention of a great number of workers. Owing t o its alleged primitiveness and simplicity’ it was expected that analysis of the brain and spinal cord of these forms would yield the “prototype”2 i.e., the natural starting point for the study of the development and differentation of the vertebrate neuraxis. Hence, many so-called evolutionary trends, like for example the progressive migration, differentiation, segregation, and specialization of neurons and neuron groups, were considered as having their point of departure in the brain of the lamprey or in lamprey-like brains. Unfortunately, many of these trends have been described in a straightforwardly phylogenetic setting without any clear justification for doing so. In fact, all of these trends are naturally based on the study of a single or a few related features and should be considered on that account as typologic rather than phylogenetic. Lampreys have a long, independent phylogenetic history behind them and n o recent group of vertebrates, let alone the entire vertebrate kingdom, can be said t o have arisen from them. However, it cannot be denied that the lampreys retain more features of the presumed ancestral craniates than d o any other members of the group,3 and therefore deserve especial attention. The first part of the present paper will survey the structural organization of the brain and spinal cord of the lamprey. This survey is based on a study of the literature and o n our own Nisslstained and silver-impregnated material of Lampetra fluviatilis. In the second part of this paper the central nervous system of the lamprey will be placed in a comparative perspective. Some of the “trends” referred to above will be review ed. It should be noted that our present-day knowledge of the central nervous system of the lamprey is based on the study of many different species. It stands t o reason that these species differ sufficiently t o be distinguished by the taxonomist. However, neuroanatomically there are no appreciable differences among them; hence, I feel justified in using in the present treatise the term “lamprey” without any further specification.*

The brain of the lamprey in a comparative perspective.

Lesions were made at varying levels in the spinal cords of a number of dogfish, rudd, and frogs. After a suitable survival period the resulting fibre degeneration was demonstrated by means of a silver impregnation technique. The findings have provided experimental confirmation of a profuse spinal projection to the reticular formation of the hindbrain in all three classes examined. These fibres were found to project to a constant sector of the reticular formation which closely corresponded not only between individual specimens of the same species, but between the three species, and which was situated ventrolaterally in the medulla oblongata. The density of this dengeneration was always greatest in the caudal brainstem and became progressively less at more rostral levels. All spinal projections to the reticular formation exhibited some degree of bilaterality, including those in specimens with lesions of hemisections or less. A spinotectal pathway was also demonstrated in certain specimens of dogfish. No involvement of cranial nerve nuclei was seen. These findings are discussed.

A comparative study of spinal projections to the brain (except cerebellum) in three classes of poikilothermic vertebrates.

Spinocerebellar systems have been investigated in three classes of poikilothermic vertebrates. Lesions were made in the spinal cord of a number of dogfish, rudd and frogs which were selected as representatives of their classes. After a postoperative survival period the resulting fibre degeneration was demonstrated by means of a suitable silver impregnation technique. The findings have provided experimental confirmation of a spinocerebellar pathway in each species examined. In the dogfish these fibres form part of a mixed spinal lemniscus, which ascends through the brainstem, and leave it to enter the corpus cerebelli, through its peduncles. In the rudd two separate systems were found, one of which corresponds to that seen in the dogfish. The other forms a separate compact fasciculus which lies more dorsally in the brainstem throughout its course. This one can be followed into the cerebellum through the peduncle. Within the corpus cerebelli it was not possible to distinguish between the two systems. In the frog also two separate systems were seen. These retained their individual identity from the spinal cord to their entrance into the cerebellar peduncle beyond which they appeared to merge. These findings have been compared with the results of other authors, and an attempt has been made to show how and why such systems have evolved.

T. H. Hayle

Papers

A comparative study of spinal projections to the brain (except cerebellum) in three classes of poikilothermic vertebrates.

A comparative study of spinocerebellar systems in three classes of poikilothermic vertebrates