Sensation and Perception
Touch and Pain
Learning Objectives
- Explain the receptors that respond to touch
- Explain the importance of pain and give examples of how expectations and context affect pain and touch experiences.
Touch, Thermoception, and Noiception
The skin can convey many sensations, such as the biting cold of a wind, the comfortable pressure of a hand holding yours, or the irritating itch from a woolen scarf. The different types of information activate specific receptors that convert the stimulation of the skin to electrical nerve impulses, a process called transduction. There are three main groups of receptors in our skin: mechanoreceptors, responding to mechanical stimuli, such as stroking, stretching, or vibration of the skin; thermoreceptors, responding to cold or hot temperatures; and chemoreceptors, responding to certain types of chemicals either applied externally or released within the skin (such as histamine from an inflammation). For an overview of the different receptor types and their properties, see Table 1. The experience of pain usually starts with activation of nociceptors—receptors that fire specifically to potentially tissue-damaging stimuli. Most of the nociceptors are subtypes of either chemoreceptors or mechanoreceptors. When tissue is damaged or inflamed, certain chemical substances are released from the cells, and these substances activate the chemosensitive nociceptors. Mechanoreceptive nociceptors have a high threshold for activation—they respond to mechanical stimulation that is so intense it might damage the tissue. Sensory information collected from the receptors and free nerve endings travels up the spinal cord and is transmitted to regions of the medulla, thalamus, and ultimately to somatosensory cortex, which is located in the postcentral gyrus of the parietal lobe.
| Identity of receptor | Size of receptor* | Type of skin where found | Speed of adaptation* | Adequate stimulus* |
| Merkel’s disks | small, sharp borders | glabrous* | slow | pressure |
| Meissner’s corpusles | small, sharp borders | glabrous | rapid | indentation |
| Ruffini corpuscles | large, diffuse borders | hairy + glabrous | slow | stretching |
| Pacinian corpuscles | large, diffuse borders | hairy + glabrous | rapid | vibration |
| *Terms:
Adequate stimulus-the type of stimulus that the receptor is specialized to receive and respond to. Glabrous skin-the hairless skin found on our palms and the soles of our feet. This skin has a higher density of receptors of a more complex range, which reflects the fact that we use these areas of our body to actively explore our surroundings and to discriminate tactile properties of objects we’re interacting with. Low-threshold mechanoreceptors-mechanoreceptors that respond to stimulus that is so light it doesn’t threaten to damage the tissue around it. high-threshold mechanoreceptors respond to stimulation of higher intensity, and are a type of nociceptor. Receptive field-the space of skin or ti