Nervous systems are programmed to ensure that behaviour changes in response to particular events during the normal life of an animal. Some changes are part of the developmental program of an animal, and others allow it to adapt its behaviour when its external environment alters. A good example of how a series of events is orchestrated at a particular time during development is provided by ecdysis in moths. The larva becomes fully committed to ecdysis by the positive feedback loop in which two different polypeptide hormones, ecdysis-triggering hormone and eclosion hormone, reinforce the release of each other. Pre-ecdysis is the first motor activity to be triggered because its pattern generator is very sensitive to ecdysis-trig-gering hormone and is switched on as soon as this hormone starts to be released. Eclosion hormone switches off the pre-ecdysis behaviour and, at the same time, switches on ecdysis. It mediates its effects by exciting cells that release crustacean cardioactive peptide, and these remain excited for some time after their initial stimulation by eclosion hormone, until ecdysis is complete.
Polypeptide hormones exert their effects on neurons by binding with protein receptors on the cell surface and triggering intracellular messenger pathways. Steroid hormones, on the other hand, operate directly in the cell nucleus, regulating gene expression. Before each moult in a moth, the level of the steroid hormone ecdysone rises. If this rise occurs when the titre of juvenile hormone is low, it can trigger widespread effects that play an important role in the reorganisation of the nervous system, sense organs and muscles that occurs during metamorphosis. The rise in ecdysone causes some neurons and muscles to die, and others, such as MNl, to grow new dendrites which enable them to participate in new circuits with other neurons.
Less persistent changes occur during associative learning, for example when a honey bee is conditioned to associate a particular odour with a sucrose reward. The trigger for the reinforcement of specific neuronal circuits is a particular pattern of activity, which happens when sensory signals about a specific stimulus, such as an odour, just precede signals about the sucrose reward. An identified neuron, VUMmxl, has been shown to carry information about the reward and to be able to reinforce the association between a specific odour and proboscis extension. This neuron branches to many brain regions and could reinforce a large number of different sensory stimuli, which is important so that the bee can make use of many types of sensory cues to increase its foraging efficiency. During conditioning, two areas where the action of VUMmxl is vital are the antennal lobes and the mushroom bodies. The large numbers of neurons in the mushroom bodies probably enable the bee to make novel association between many different sensory stimuli and particular actions. The wide range of possible associations that animals can make during learning provides a major challenge to neurobiologists.
The process of development of a male song bird's brain includes a program which ensures that it stores memories of the songs of adult tutors and uses the memories to mould its own, individual song pattern. There is no obvious immediate pressure for a young bird to learn songs, but learning is vital because young males that are prevented from using the songs of mature, singing adults as models for their own songs are unable to attract mates. Birds do not copy the songs of tutors exactly, and an intriguing problem is provided by the origin of the characters that distinguish the song of one individual from another.
A specific set of brain nuclei is involved in the development of song, although these nuclei play no known role in the control of mature song in the adult. During development, new neurons are added to some of these nuclei, and many neurons die in others. A number of loops and feedback pathways involving the nuclei probably enable comparisons between the sound of the bird's own song and the remembered songs of tutors. Increasing proficiency at singing is accompanied by an increase in the auditory responses of neurons in song nuclei until, in the adult, the neurons prefer recordings of the bird's own song to any other sound. The gradual change in the specificity of the auditory responses supports the idea that the development of the program for song occurs by instruction rather than by selection. Despite its complexity, bird song has become one of the most active areas of research in neuroethology. This is partly because it is intrinsically interesting, but also because the brains of animals that specialise in particular behaviours, such as bird song, almost invariably prove to be good sources of information about how the nerve cells dedicated to that behaviour work.
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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.