Cellular Organization

The fully grown brain weighs only about 3 pounds. The brain can be divided into two major components: the structures that perform the mental computations and the supportive structures. About 100 years ago Santiago Ramón y Cajal revealed that the basic units of the computational portion of the brain are individual living cells called neurons (see Figure 2.1). The human brain has about 10 million neurons. Most of the neurons in the cerebral cortex have a cell body that gives off branches similar to those of a tree. These branches of the neuron are called dendrites. In some neurons these dendritic branches are given off on one side of the neuron's body. On the other side of the body, the neuron has a single long branch called an axon (see Figure 2.1). Some dendrites in the cortex of the brain gather information from nerves that bring sensory information to the brain, and other dendrites gather information from other neurons in the brain that are close neighbors. Some axons leave the brain and are important in motor control. Other axons connect with other neurons. Thus, the neurons in the brain have a rich network of interconnections. There are more than a trillion of such connections. Neurons are not physically connected together but, when activated, they can communicate with each other by giving off chemicals. The areas where one chemical is given off by an axon or a dendrite and excites or inhibits another adjacent neuron is called a synapse (see Figure 3.1). The chemicals given off at synapses are called neurotransmitters, and depending on the type of neurotransmitter that is given off, neurons can either activate (e.g., glutamate) or inhibit (e.g., gamma-aminobu-tyric acid or GABA) the adjacent neurons that receive this chemical information.

Most of the brain's neurons are found in the cerebral cortex; that is, the mantle that covers the brain. The cerebral cortex is about 3 millimeters thick, and if one looks at the cerebral cortex in the microscope, one can easily recognize that it has six layers. At the turn of the 20th century, Korbinian Brodmann examined these six layers and noticed that in different parts of the brain one or more of these layers was

Synaptic Clef!-

Synaptic Clef!-

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Postsynaptic Membrane

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Figure 3.1. Diagram of a neuronal synapse.

either thicker or thinner than the same layer in a different part of the brain. The microscopic differences he found in different anatomic regions of the brain was related to how dense the neurons are in each layer and to how the neurons in these areas are connected to other parts of the brain. Some neurons bring sensory information into the brain (afferent) and others send messages out of the brain (efferent), like those that control movements. Most neurons in the brain, however, communicate with each other. Thus, neurons in the brain gather information from the outside world and from one's body. They also send information to and receive information from other neurons, and it is the strength of connections between these neurons that stores knowledge.

Brain Games

Brain Games

A Fantastic Treasury of Mind Bending Puzzles, Games, and Experiments for All the Family. If you are one of those people who takes great pleasure in playing games, and also happens to be extremely competitive, you know how frustrating it can be to fail at solving a game or puzzle.

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