![]() Similarly, any one neuron can contact up to 10,000 postsynaptic cells. Consequently, the potential complexity of the networks is vast. Figure 1 (click on "Neuron Receiving Synaptic Input") shows an example of three presynaptic neurons contacting the one tan-colored postsynaptic neuron, but it has been estimated that one neuron can receive contacts from up to 10,000 other cells. One neuron can receive contacts from many different neurons. The tan-colored terminal to the left is consequently referred to as the presynaptic neuron. The aqua-colored neuron in Figure 1 (click on "Neuron Connected to a Postsynaptic Neuron") is referred to as the postsynaptic neuron. The synapse is the terminal region of the axon and it is here where one neuron forms a connection with another and conveys information through the process of synaptic transmission. Axons can be rather long extending up to a meter or so in some human sensory and motor nerve cells. The axon is a key component of nerve cells over which information is transmitted from one part of the neuron (e.g., the cell body) to the terminal regions of the neuron. The cell body or soma contains the nucleus and the other organelles necessary for cellular function. Dendrites are the region where one neuron receives connections from other neurons. Neurons are different from most other cells in the body in that they are polarized and have distinct morphological regions, each with specific functions. ![]() The 100 billion neurons in the brain share a number of common features (Figure 1). These nanocircuits constitute the underlying biochemical machinery for mediating key neuronal properties such as learning and memory and the genesis of neuronal rhythmicity. Networks are also prevalent within neurons. So, multiple levels of networks are ubiquitous in the nervous system. ![]() Macrocircuits mediate higher brain functions such as object recognition and cognition. More complex networks ( macrocircuits) consist of multiple imbedded microcircuits. Just a few interconnected neurons (a microcircuit) can perform sophisticated tasks such as mediate reflexes, process sensory information, generate locomotion and mediate learning and memory. Synaptic transmission comes in two basic flavors: excitation and inhibition. To understand neural networks, it is necessary to understand the ways in which one neuron communicates with another through synaptic connections and the process called synaptic transmission. What makes the nervous system such a fantastic device and distinguishes the brain from other organs of the body is not that it has 100 billion neurons, but that nerve cells are capable of communicating with each other in such a highly structured manner as to form neuronal networks. This chapter will begin with a discussion of the neuron, the elementary node or element of the brain, and then move to a discussion of the ways in which individual neurons communicate with each other. Fortunately, much is known about the properties of individual neurons and simple neuronal networks, and aspects of complex neuronal networks are beginning to be unraveled. Its phenomenal features would not be possible without the hundreds of billions of neurons that make it up, and, importantly, the connections between those neurons. The three pounds of jelly-like material found within our skulls is the most complex machine on Earth and perhaps the universe.
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