Electromagnetic receptors are sensory cells that respond to electrical, magnetic, and light stimuli. Of these, light-detecting photoreceptors are the most common and diverse. Most animals sense light in their environment. Photoreceptors have been common since the first ones evolved in cnidarians (jellyfish) and ctenophores (comb jellies) more than 500 million years ago.

Light-detecting photoreceptors are the sensory receptors in eyes (Fig. 36.2c). Photoreceptors respond to individual photons of light energy. In most invertebrates, light causes Na+ channels to open, depolarizing the cell. By contrast, vertebrate photoreceptors close Na+ channels in response to light, causing the cell to become hyperpolarized. Most receptors excite neurons that they synapse with, but vertebrate photoreceptors are unusual in that they can either excite or inhibit neurons in the eye. Photoreception in the vertebrate eye is discussed in section 36.4.

Some fish, such as catfish, contain specialized electroreceptors arranged in a lateral line along their bodies. Electroreceptors enable these fish to detect weak electrical signals emitted by all organisms. They likely evolved as an adaptation for locating prey or potential predators in poorly lit habitats where vision was less useful. Some specialized “weakly electric” fish actually generate an electromagnetic field by emitting pulses from an electric organ located in the tail. Disturbances in the field detected by electroreceptors of the lateral line system signal the location of nearby prey. These fish also inhabit rivers with poor visibility. The bill of the duckbilled platypus also contains electroreceptors that locate prey in dimly lit water.

Finally, specialized receptors with dendritic branches in the skin can respond to heat and cold (thermoreceptors) or to pain (nociceptors). Thermoreceptors help to control an animal’s metabolism, and they also regulate body temperature by controlling patterns of blood flow that in turn alter rates of heat gain and loss (Chapter 40). As a result, they help to maintain homeostasis. Pain receptors send action potentials to the brain or spinal cord when exposed to excessive heat, force, or chemical damage. A quick withdrawal from the painful stimulus is the typical response.