Neurophysiology - Discuss the roles of development learning and evolution in the construction of the nervous system

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Chris Holland        Jesus College


Discuss the roles of development learning and evolution in the construction of the nervous system

The nervous system is responsible for the initiation, propagation and co-ordination of animal behaviour. How it is constructed and what factors are involved encompasses many fields of biology, from ethology and neurophysics to evolution. In this essay I will describe the roles of development learning and evolution in the construction of the nervous system and give experimental evidence that backs up these theories.


Evolution deals with the origins of the nervous system, where it comes from determines how it will be constructed. This will have direct consequences on the relative fitness of an individual as the layout of the nervous system relates to how the animal behaves. Phylogeny is very important therefore to analyse how changes in the nervous system relate to the evolution of behaviour. The only realistic way of studying the evolution of nervous systems, particularly the events, which lead to, their current day form, is through comparative biology. By comparing closely related species in similar niches, the difference in their behaviour must have a genetic/nervous system origin. A good example of how behaviour can be genetic in origin and show that nervous systems can evolve to create different behavioural responses is found in deer. The white tailed deer odocoileus virginianus and the mule deer O. hemionus use different gaits when alarmed. The white deer gallop and the mule deer stott. This alone doesn’t confer that the difference is due to their nervous systems but the genetic origin for the behaviour is inferred when a cross between the two species results in a hybrid that bounds when alarmed.

In order to attain quantitative data the use of complex nervous systems, such as mammals, is unfeasible. A simpler nervous system is better suited and comparisons can then be extrapolated for the more complex animals. Within the invertebrates the model organism is, as ever, Drosophila. Since its genome has been sequenced and the relatively short generation time it plays a key role in the study of all type of nervous system construction. Zebra fish have been termed “flies with backbones” and are perfect for the study of nervous system development in vertebrates. However these relatively simple organisms are still too complex to study fully and so scientists tend to use a part of a nervous system for detailed analysis. The Crustacean Stomatogastric Ganglion STG, which comprises of only 30 ganglions, is most popular for several reasons, mainly because it has been preserved for about 350 million years and is seen across many taxa. This allows for comparison on a smaller scale and although the overall synaptic circuitry is similar there are differences in the relative strength of connections and the amount of electrical coupling across the taxa.

The reason why the STG is seen across so many taxa is because on the whole the nervous system is a very evolutionary conserved organ. This reflects its importance to an animal. As it is so conserved certain inferences can be made regarding the evolution of the nervous system. The first is that the neural networks must be pretty similar across species meaning that the nervous system is more of a generalist than a specialist. Therefore only small changes to the nervous system are needed in order to produce markedly different behaviours. It is these behaviours that are then subsequently acted upon by natural selection and contribute to the nervous system layout in the next generation.

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Once the genetic instructions that determine the constitution of the nervous system have been selected the next step in the construction of the nervous system is the subsequent application of that code, the development. The nervous system develops during embryogenesis and continues in some form or another throughout the animals’ life, but that latter stages of this development I shall relate to the learning part of this essay. From before we have learnt that the basic mechanisms for constructing a nervous system are highly conserved during evolution. There is a set of general tools that are used ...

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