Sensory neurons are  nerve cells that transmit sensory information (sight, sound, feeling, etc.). They are activated by sensory input, and send projections to other elements of the nervous system, ultimately conveying sensory information to the brain or spinal cord. In complex organisms, when stimulation of a peripheral sensory neuron (a first-order sensory neuron) receptor exceeds a set level of intensity, an electrical impulse travels down the nerve fiber to the central nervous system, where it may activate a motor neuron or another sensory neuron (a second- or third-order neuron), or both. In less complex organisms, such as the hydra, sensory neurons transmit data to motor neurons or ganglia. Different types of receptor respond to different kinds of stimulus.