Neurons consist of the same components as other animal cells, but are specialised for processing information. This specialisation is evident morphologically in the fine processes, the dendrites and axon, by which neurons make contact with each other. Physiologically, the most important specialisation is the maintenance of a membrane potential, which varies in response to incoming signals.
The transmission of signals over long distances is accomplished by spikes, which are active changes in membrane potential and are conducted along an axon with a constant amplitude at a finite speed. Signal strength is coded as spike frequency, a digital process. Over short distances, neurons use graded potentials that vary in amplitude and duration. These graded potentials usually originate at postsynaptic sites. Their amplitude reduces and their waveform becomes smoothed with increasing distance from the point of origin. Signal strength is coded as the amplitude of change in membrane potential, an analogue process. These graded potentials can be summed together: like variations in potential reinforce each other, and unlike variations tend to cancel each other out. This simple electrical property is the basis of most of the integrative processing that occurs in neurons.
Thus, the nervous system uses analogue processes to combine information and digital processes to transmit it over long distances. These are basically simple processes but a great variety of operation is achieved by adjustment of the physical properties of each neuron and its connections with other neurons. Physical properties such as membrane resistance, which affect the space and time constants, determine how far signals that originate at separate synapses are able to travel and combine together. The input and output connections of each neuron, reflected in its specific branching pattern, control the flow of information and create functional circuits in the nervous system. By combining the basic components and processes of neurons together in different ways, evolution has generated an extremely efficient system for processing information, one that is well able to control an animal's behaviour.
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This ebook provides an introductory explanation of the workings of the human body, with an effort to draw connections between the body systems and explain their interdependencies. A framework for the book is homeostasis and how the body maintains balance within each system. This is intended as a first introduction to physiology for a college-level course.