The somatosensory system, also known as somatic senses, touch or tactile perception, is a complex sensory system. It is considered one of the five traditional senses. It is made up of a number of different receptors, including thermoreceptors, photoreceptors, mechanoreceptors and chemoreceptors. It also comprises essential processing centres, or sensory modalities, such as proprioception, mechanoreception (touch), thermoception (temperature), and nociception (pain). The sensory receptors cover the skin and epithelial tissues, skeletal muscles, bones and joints, internal organs, and the cardiovascular system.