Sensory neurons (neurones) are nerve cells within the nervous system responsible for converting external stimuli from the organism's environment into internal electrical motor reflex loops and several forms of involuntary behavior, including pain avoidance. In humans, such reflex circuits are commonly located in the spinal cord.
In complex organisms, sensory neurons relay their information to the central nervous system or in less complex organisms, such as the hydra, directly to motor neurons and sensory neurons also transmit information to the brain, where it can be further processed and acted upon. For example, olfactory sensory neurons make synapses with neurons of the olfactory bulb, where the sense of olfaction (smell) is processed.
At the molecular level, sensory receptors located on the cell membrane of sensory neurons are responsible for the conversion of stimuli into electrical impulses. The type of receptor employed by a given sensory neuron determines the type of stimuli it will be sensitive to. For example, neurons containing mechanoreceptors are sensitive to tactile stimuli, while olfactory receptors make a cell sensitive to odors.
Peripheral nerve fibers can be classified based on axonal conduction velocity, mylenation, fiber size etc. For example, there are slow-conducting unmyelinated C fibers and faster-conducting myelinated Aδ fibers.