1. Chapter 14: The Cutaneous Senses

2. Overview of Questions
Are there specialized receptors in the skin for sensing different tactile qualities?
What is the most sensitive part of the body?
Is it possible to reduce pain with your thoughts?
Do all people experience pain in the same way?

3. Cutaneous System
Skin - heaviest organ in the body
Protects the organism by keeping damaging agents from penetrating the body
Epidermis is the outer layer of the skin, which is made up of dead skin cells
Dermis is below the epidermis and contains mechanoreceptors that respond to stimuli such as pressure, stretching, and vibration

4. Somatosensory Systems
We will focus on the cutaneous senses (touch, pain)
We will only briefly discuss
Proprioception: Location of body parts and joint positions
Kinesthesis: Position and movement of the limbs

5. Mechanoreceptors

6. Hairy Skin (most of body)
Follicle receptor
Principle mechanoreceptor
Triggered by distortion of follicle

7. Mechanoreceptors
Merkel receptor - disk-shaped receptor located near the border between the epidermis and dermis
Meissner corpuscle - stack of flattened disks in the dermis just below epidermis
Ruffini cylinder - branched fibers inside a cylindrical capsule
Pacinian corpuscle - onion-like capsule located deep in the skin

8. Mechanoreceptors - continued
Properties of mechanoreceptor nerve fibers
Temporal properties - adaptation:
Slowly adapting fibers (SA) found in Merkel and Ruffini receptors - fire continuously as long as pressure is applied
Rapidly adapting fibers (RA) found in Meissner receptor and Pacinian corpuscle - fire at onset and offset of stimulation

10. Properties of Mechanoreceptor Nerve Fibers
Spatial properties - detail resolution
SA1 fibers (Merkel receptor) respond to patterns of grooves
RA2 fibers (Pacinian corpuscle) do not respond to the details of these stimuli
Frequency response
Ranges from .3 Hz for SA1 fibers 500 Hz for RA2 fibers

11. Figure (a) The firing of Merkel receptors/SA1 fiber signals the grooved stimulus pattern, but (b) the firing of the Pacinian corpuscle/RA2 fiber does not. Results such as these indicate that the Merkel receptor/SA1 fiber signals details

12. Table 14.1 Properties of Four Types of Mechanoreceptors
The instructor can finish up this section by using the table on this slide to talk about the types of stimuli the cells respond to.
Table Properties of Four Types of Mechanoreceptors

13. Pathways from Skin to Cortex
Nerve fibers travel in bundles (peripheral nerves) to the spinal cord
Two major pathways in the spinal cord:
Dorsal Column/Medial lemniscal pathway consists of large fibers that carry proprioceptive and touch information
Spinothalamic pathway consists of smaller fibers that carry temperature and pain information
These cross over to the opposite side of the body and synapse in the thalamus

14. Both Pathways Reach Contralateral Cortex
DC-ML Fibers
Ascend ipsilateral SC
Secondary fibers cross in medulla and project to thalamus
ST Fibers
Cross in SC
Ascend contralateral SC and reach thalamus directly

16. Maps of the Body on the Cortex
Signals travel from the thalamus to the somatosensory receiving area (S1) and the secondary receiving area (S2) in the parietal lobe
Body map (homunculus) on the cortex shows more cortical space allocated to parts of the body that are responsible for detail
Plasticity in neural functioning leads to multiple homunculi and changes in how cortical cells are allocated to body parts

18. Plasticity in the Cortex
Stimulation (or use) can change the homonculus (a little).
The experiment in this slide can be used to explain research that has shown that experience has an impact on the amount of cortical tissue devoted to specific parts of the body.

19. Perceiving Details
Measuring tactile acuity
Two-point threshold - minimum separation needed between two points to perceive them as two units
Grating acuity - placing a grooved stimulus on the skin and asking the participant to indicate the orientation of the grating

20. Figure 14.9 Methods for determining tactile acuity (a) two-point threshold; (b) grating acuity.

21. Receptor Mechanisms for Tactile Acuity
There is a high density of Merkel receptor/SA1 fibers in the fingertips
Merkel receptors are densely packed on the fingertips - similar to cones in the fovea
Both two-point thresholds and grating acuity studies show these results

22. Correlation between density of Merkel receptors/SA1 fiber density and tactile acuity.

23. Cortical Mechanisms for Tactile Acuity
Body areas with high acuity have larger areas of cortical tissue devoted to them
This parallels the “magnification factor” seen in the visual cortex for the cones in the fovea
Areas with higher acuity also have smaller receptive fields on the skin

26. Perceiving Vibration
Pacinian corpuscle is primarily responsible for sensing vibration
RA2 fibers associated with them respond best to high rates of vibration
The structure of the corpuscle is responsible for the response to vibration - RA2 fibers without the corpuscle only respond when pressure is applied and removed

27. Figure 14. 13 A Pacinian corpuscle
Figure A Pacinian corpuscle. (From “Biological Transducers,” by W. R. Lowenstein, 1960, p Copyright © 1960 by Scientific American, Inc. All rights reserved.)

28. Perceiving Texture
Katz (1925) proposed that perception of texture depends on two cues:
Spatial cues are determined by the size, shape, and distribution of surface elements
Temporal cues are determined by the rate of vibration as skin is moved across finely textured surfaces
Two receptors may be responsible for this process - called the duplex theory of texture perception

29. Perceiving Texture - continued
Past research showed support for the role of spatial cues
Recent research by Hollins and Reisner shows support for the role of temporal cues
In order to detect differences between fine textures, participants needed to move their fingers across the surface

30. Figure (a) Participants in Hollins and Reisner’s (2000) experiment perceived the roughness of two fine surfaces to be essentially the same when felt with stationary fingers, but (b) could perceive the difference between the two surfaces when they were allowed to move their fingers.

31. Perceiving Objects
Humans use active rather than passive touch to interact with the environment
Haptic perception is the active exploration of 3-D objects with the hand
It uses three distinct systems:
Sensory system
Motor system
Cognitive system

32. Perceiving Objects - continued
Psychophysical research shows that people can identify objects haptically in 1 to 2 sec
Klatzky et al. have shown that people use exploratory procedures (EPs)
Lateral motion
Pressure
Enclosure
Contour following

33. Figure Some of the exploratory procedures (EPs) observed by Lederman and Klatzky as participants identified objects. (From “Hand Movements: A Window into Haptic Object Recognition,” by S. J. Lederman and R. L. Klatzky, 1987, Cognitive Psychology, 19, , figure 1. Academic Press, Inc.)

34. The Physiology of Tactile Object Perception
The firing pattern of groups of mechanoreceptors signals shape, such as the curvature of an object
Neurons further upstream become more specialized
Monkey’s thalamus shows cells that respond to center-surround receptive fields
Somatosensory cortex shows cells that respond maximally to orientations and direction of movement

35. Figure (a) Response of SA1 fibers in the fingertips to touching a high-curvature stimulus. The height of the profile indicates the firing rate at different places across the fingertip. (b) The profile of firing to touching a stimulus with more gentle curvature. (From A. W. Goodwin (1998). Extracting the shape of an object from the responses of the peripheral nerve fibers. In J. W. Morely (Ed.), Neural aspects of tactile sensation. Elsevier Science, pp , Fig. 12a, b. Used with permission.)

36. Figure An excitatory-center, inhibitory-surround receptive field of a neuron in a monkey’s thalamus.

37. The Physiology of Tactile Object Perception - continued
Monkey’s somatosensory cortex also shows neurons that respond best to:
Grasping specific objects
Paying attention to the task
Neurons may respond to stimulation of the receptors, but attending to the task increases the response

38. Edge and motion detectors in the somatosensory cortex - Note similarities to V1

39. Cells in the parietal cortex can fire when a monkey grasps a ruler not when the monkey grasps a cylinder.

40. Pain Perception
Pain is a multimodal phenomenon containing a sensory component and an affective or emotional component
Three types of pain:
Nociceptive - signals impending damage to the skin
Types of nociceptors respond to heat, chemicals, severe pressure, and cold
Threshold of eliciting receptor response must be balanced to warn of damage but not be affected by normal activity

41. Types of Pain
Inflammatory pain - caused by damage to tissues and joints that releases chemicals that activate nociceptors
Neuropathic pain - caused by damage to the central nervous system, such as:
Brain damage caused by stroke
Repetitive movements which cause conditions like carpal tunnel syndrome

42. Origins of pain signals
Signals from nociceptors travel up the spinothalamic pathway and activate:
Subcortical areas including the hypothalamus, limbic system, and the thalamus
Cortical areas including S1 and S2 in the somatosensory cortex, the insula, and the anterior cingulate cortex
These cortical areas taken together are called the pain matrix

44. Experiment by Hoffauer et al.
Participants were presented with potentially painful stimuli and asked:
To rate subjective pain intensity
To rate the unpleasantness of the pain
Brain activity was measured while they placed their hands into hot water
Hypnosis was used to increase or decrease the sensory and affective components

45. Experiment by Hoffauer et al. - continued
Results showed that:
Suggestions to change the subjective intensity led to changes in those ratings and in S1
Suggestions to change the unpleasantness of pain did not affect the subjective ratings but did change:
Ratings of unpleasantness
Activation in the anterior cingulate cortex

46. Conclusion: Evaluation of subjective intensity comes before evaluation of unpleasantness.

47. Cognitive and Experiential Aspects of Pain
Expectation - when surgical patients are told what to expect, they request less pain medication and leave the hospital earlier. (When you know pain is on the way, it helps to understand its causes)
Shifting attention - virtual reality technology has been used to keep patients’ attention on other stimuli than the pain-inducing stimulation

48. Cognitive and Experiential Aspects of Pain - continued
Content of emotional distraction - participants could keep their hands in cold water longer when pictures they were shown were positive
Individual differences - some people report higher levels of pain than others in response to the same stimulus
This could be due to experience or to physiological differences

49. How long can you stand cold water?
The results of deWied and Verbaten’s (2001) experiment showing that participants kept their hands in cold water longer when looking at positive pictures than when looking at neutral or negative pictures.

50. Gate Control Model of Pain Perception
The “gate” consists of substantia gelatinosa cells in the spinal cord (SG- and SG+)
Input into the gate comes from:
Large diameter (L) fibers - information from tactile stimuli
Small diameter (S) fibers - information from nociceptors
Central control - information from cognitive factors from the cortex

51. Gate Control Model of Pain Perception - continued
Pain does not occur when the gate is closed by stimulation into the SG- from central control or L-fibers into the T-cell
Pain does occur from stimulation from the S-fibers into the SG+ into the T-cell
Actual mechanism is more complex than this model suggests

52. Closing the Gate

53. Opioids and Pain
For centuries, humans have known and experimented with the effects of heroine, morphine.
Why would a chemical made by a flower have such dramatic effects on animals?

54. Evidence shows that endorphins reduce pain
Opioids and Pain
Brain tissue releases neurotransmitters called endorphins (endogenous morphine)
Evidence shows that endorphins reduce pain
Injecting naloxone blocks the receptor sites causing more pain (also can be used to treat heroin addiction)
Naloxone also decreases the effectiveness of placebos!
Tolerance of pain may be related to how fast and in what quantities endorphins are released
Probably varies from person to person
The instructor can include material about the discovery of endorphins and how this relates to opiates.

55. Play Through it?
Endorphins are released during times of stress or exercise. Athletes, soldiers, trauma victims, may be “protected” by endorphins for a period of time while they seek or wait for help.
…a merciful provision by our benevolent creator for lessening the pain of death. - Livingstone, 1857