There are three main kinds of neurons. Sensory neurons are coupled to receptors specialised to detect and respond to different attributes of the internal and external environment. The receptors sensitive to changes in light, sound, mechanical and chemical stimuli subserve the sensory modalities of vision, hearing, touch, smell and taste. When mechanical, thermal or chemical stimuli to the skin
exceed a certain intensity, they can cause tissue damage and a special set of receptors called nociceptors are activated; these give rise both to protective reflexes and to the sensation of pain (see chapter 5 on Touch and Pain). Motor neurons, which control the activity of muscles, are responsible for all forms of behaviour including speech. Interposed between sensory and motor neurons are
Interneurones. These are by far the most numerous (in the human brain). Interneurons mediate simple reflexes as well as being responsible for the highest functions of the brain. Glial cells, long thought to have a purely supporting function to the neurons, are now known to make an important contribution to the development of the nervous system and to its function in the adult brain. While much more numerous, they do not transmit information in the way that neurons do.