Brain Structure and Function

Every vertebrate brain is hierarchically organized into forebrain, midbrain or mesencephalon, and hindbrain. The forebrain can be further divided into telencepha-lon and diencephalon, and hindbrain can be divided into rhombencephalon and myelencephalon. Brain tissue in all animals consists of neurons as information processing units and glia and other cells that are, in effect, supporting tissue. Neurons are often specialized with respect to neurotransmitters, shape, and size. Sizes, for example, range from the granule cells of the cerebellum (soma less than 10-mm diameter) to the giant Mauthner cells (soma about 100-mm diameter) that mediate startle responses in fish and in amphibian tadpoles. The full size of a nerve cell includes the arborization of axon and dendrites, which may account for 95% or more of the volume of a neuron and which varies enormously in pattern both within a brain and between species.

Underlying this diversity, there is surprising uniformity about principles of nerve action in the transmission of information, which makes it possible to use almost any neuron from any species as a model for neuronal action. There is, furthermore, a uniformity at the level of networks of cells in vertebrate brains, evident even in the neocortex in mammals, which encourages one to emphasize information-processing capacity for the brain as a whole as well as in its specialized component systems, such as those for color vision, binocular vision, sound localization, and olfaction.

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