Single-celled Paramecium display avoidance behaviour, changing direction or speeding up appropriately in response to contact. This works through mechanical stimulation driving potassium or calcium concentrations, depolarization of the cell membrane, and changes in the beating of cilia; in many ways, each individual Paramecium resembles an animal neuron. This leads to a foray into the phylogenetics of ion channels and mechanoreceptors.
The barnacle shadow reflex is an example of a simple multicellular system. As well as a photosensitive system, it involves chemical signalling across synapses to link cells. And it has to cope with varying light intensities. Greenspan also touches on the broader evolution of photoreceptors.
Swimming in the jellyfish Aglantha requires different forms of coordination for ordinary slow swimming and faster "escape" swimming. These employ the same neurons, but with different sized action potentials. The swimming rhythm is maintained by a system of "pacemaker" neurons with regular depolarizations. The leech Hirudo medicinalis has a different body plan and mode of locomotion, with a neural system to match.
When Aplysia sea slugs are attacked, they close up their gill and siphon and emit ink. After activation, this alarm system becomes hypersensitized and responds faster; this modulation is controlled by serotonin release. Phylogenetic analysis of related species shows acquisition and loss of response to serotonin by tail neurons, while neuromodulators "appear to go back to the very origins of nervous systems". Noise and the large number of neurons involved make the system response highly variable.
The circadian rhythms of the fruit fly Drosophila are driven by three systems for detecting light, while its clock works through cycling of protein concentrations and gene expression. This is connected with seasonal changes and sleep, and with the broader evolution of circadian rhythms and their variation with latitude.
Flight is critical for fruit flies: "giant" fibers from the brain initiate the thoracic contractions that drive their wings, sensory organs called halteres provide feedback control to allow steady flight, and the visual system allows them to stay upright and follow a fixed course. Flies must also navigate, make decisions, and recognize novelty.
Drosophila have a complicated courtship system involving chemical and auditory signals in precise sequence. Greenspan touches on the neurobiology and genetics underlying sexual differences and the evolution of songs, pheromones, and courtship behaviours.
Examples of adaptation to the environment include the fine-tuning of mate discrimination and courtship depression in fruit flies, odor learning and navigation in the honey bee, simple place learning in cockroaches, and memory.
There's no room in so short a book to go into much depth on all these topics, and Greenspan offers just a paragraph on many of them. He avoids being blandly general, however, and those after more information are given good directions: there's a full bibliography at the end, broken down by chapter, but each chapter also has a much shorter "further reading" list. An Introduction to Nervous Systems would make a useful secondary text alongside a more standard introduction to neurobiology, but it is also perfectly pitched for generalists who want more than popular science.
July 2007
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