Chapter 5 - Chapter 5 The brain and control of food intake...

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Chapter 5: The brain and control of food intake We already noted that the human brain is an extraordinary energy drain – some 20% of the energy we consume goes directly to fueling the brain. You’d like to think this was a useful investment, right? From a “selfish brain” perspective, one of the critical things that the brain does is to ensure that our physiology and behavior are adequate to keep itself in business. We will consider three domains of executive control that the brain may direct. These domains do not work independently; indeed, their seamless gestalt or whole is what must produce feeding behavior in all its complex manifestations. We will name the three domains homeostasis, reward, and foraging . The first two of these were introduced in previous treatments of multiple brain systems (eg Berthoud 2002); we have added a third because the first two do not capture the decision making aspect. 5.1. Overview of neurons and brain architecture We recognize that some readers may come to this book with little formal background in neurobiology, so the purpose of this section is to present a concise and selected overview of how the brain is organized. The human brain contains about 100 billion (10 11 ) neurons with about 10 14 synapses between them. Neurons are information- carrying and integrating cells in the brain. They connect with other neurons via synapses, and from the foregoing approximate numbers, it is evident that on average each neuron has a thousand synapses. Most synapses are unidirectional: signals flow from a presynaptic neuron to a postsynaptic neuron and most synapses are chemical: they use neurotransmitters as the signal. Specific transmitters are released from terminals of the presynaptic neuron and activate corresponding receptors on the postsynaptic cell. One of the goals of behavioral neurobiology is to understand which transmitters and synapses are critically involved in specific behaviors, such as feeding.
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Many different transmitters have been so implicated, and some of these are shown in Table…. . A cartoon of a typical neuron is shown in Figure … It consists of the cell body which contains the nucleus and is filled with a fluid, cytoplasm. Branching off of the cell body is one axon and (typically) many dendrites. The dendrites are highly branched, like a tree, and contain receptors to receive information from other cells. The length of the axon depends on the function of the cell, and may be microscopically short or several cm long. At the end, an axon typically branches into several terminal zones or buttons. Each of these buttons forms a synapse with another cell, often on the dendrites of that next cell. As we noted before, a typical neuron may have a thousand or more receptor sites on its dendrites, and a smaller number on the soma. These receptors are the input devices for the cell: they receive (chemical) signals from other cells. At any given instant in time, a cell will be bombarded with many inputs, and the next instant with a
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Chapter 5 - Chapter 5 The brain and control of food intake...

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