Sensory neurons are neurons that are activated by sensory input (vision, touch, hearing, etc.), and send projections into the central nervous system that convey sensory information to the brain or spinal cord. Unlike neurons of the central nervous system, whose inputs come from other neurons, sensory neurons are activated by physical modalities such as light, sound, temperature, chemical stimulation, etc.
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 (electrical impulses) 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 stimulus it will be sensitive to. For example, neurons containing mechanoreceptors are sensitive to tactile stimuli, while olfactory receptors make a cell sensitive to odors.