The same basic processes that have been illustrated in a simple mechano-receptor underlie the operation of more complicated sense organs. There is no sense organ more appropriate to the study of animal behaviour than one of the arrays of photoreceptors, commonly called eyes. The surface of the Earth is continuously bathed in light, and the speed with which it travels means that light can be a source of almost instantaneous information about events that are occurring at a distance. Hence, it is not surprising that eyes have evolved in many different animal groups, and that vision plays a crucial role in guiding behaviour patterns in many species.
Two fundamental factors influence the design of any sense organ. First, there are the physical constraints imposed by the particular modality involved. In the case of eyes, the physical properties of light set the limits to performance. Features such as the overall size of the eye, the design of lenses, and the arrangement of light-sensing elements will partly be determined by the physical properties of light, just as in any other optical instrument. The second factor that influences the design of a sense organ is the need to gather information that is relevant to the animal's particular way of life. Thus, animals that are active at night time will have eyes of a construction different from those of animals that are usually only active in daylight. The design of an animal's sense organs must form an integrated part of its overall adaptive strategy.
Eyes reach their highest level of complexity amongst the vertebrates and arthropods, especially insects, as well as in cephalopod molluscs. The eyes of insects are quite different in their design from those of vertebrates. Nevertheless, both types of eye have to deal with the same problems in converting patterns of change in light intensity into signals that are useful for guiding behaviour. The same physical constraints influence the design of the optics of both types of eye, and there are many similarities in the way the two types of eyes operate. In photoreceptors, the same basic sensory processes that have been described in a mechanoreceptor occur. Energy from the environment is focused onto a photoreceptor by specialised accessory structures (lenses) and it is then transduced into a receptor potential, which typically adapts fairly rapidly. Most photoreceptors do not have long axons, and the receptor potential itself regulates the transmission of signals across synapses to the next layer of cells, the second-order neurons. Here, the compound eyes of insects are used to illustrate the principal features of eyes and the properties of the receptor potential.
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This ebook provides an introductory explanation of the workings of the human body, with an effort to draw connections between the body systems and explain their interdependencies. A framework for the book is homeostasis and how the body maintains balance within each system. This is intended as a first introduction to physiology for a college-level course.