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A sensory neuron is an electrically excitable cell that processes information taken from one of the five senses: sight, hearing, taste, smell, and touch. The peripheral nervous system of the human body, which includes all the neurons outside brain and spinal cord, contains a substantial fraction of these neurons. The initial event that causes a neuron to activate is called a stimulus, and this can be anything from a breeze blowing across a person's arm to a door slamming. Sensory neurons on their own do not cause the human body to react to stimuli, and a reaction only occurs after the electrical impulses they carry have been processed.
The function and action of a sensory neuron is always the same, no matter what type of stimulus is being processed. Whether tasting a strawberry or touching something hot, the information from that experience is passed along the neuron as a series of impulses for processing that the neuron cannot do itself. The shape and size of the cells doesn't matter either, because the electrical signals can travel fast enough to the point where any differences in travel time are negligible.
Some chemical signaling also occurs while these neurons are active. Signals move from one neuron to another using small molecules called neurotransmitters, which excite neighboring neurons to continue carrying data. Any errors in this chemical signaling can cause problems in processing of sensory stimuli, and there are many disorders that have been either directly or indirectly linked to chemical signaling errors. Electrical signaling errors are also possible, and these can create particularly damaging effects, hampering not only the ability to prepare sensory information for processing but also the ability of the neuron to transmit that information.
Sensory neurons can be involved in both central nervous system processing and reflex loops. In central nervous system processing, electrical impulses travel to the spinal cord and then the brain, where a complex analysis of the stimulus is performed. This type of processing happens during complicated tasks, such as driving, talking, or solving a puzzle. Reflex loops, by contrast, route the neuron's signal through the spinal cord to a motor neuron, which generates an immediate reaction without a great deal of complicated processing. These loops often happen when a stimulus is unexpected, such as a flinch away after touching something hot.
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