Chapter 14: The Cutaneous Senses

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?

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

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

Mechanoreceptors

Hairy Skin (most of body)• Follicle

receptor– Principle mechanoreceptor

– Triggered by distortion of follicle

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

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

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

Figure 14.4 (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

Table 14.1 Properties of Four Types of Mechanoreceptors

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

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

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

Plasticity in the CortexStimulation (or use) can change the homonculus (a little).

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

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

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

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

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

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

Figure 14.13 A Pacinian corpuscle. (From “Biological Transducers,” by W. R. Lowenstein, 1960, p. 103. Copyright © 1960 by Scientific American, Inc. All rights reserved.)

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

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

Figure 14.14 (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.

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:

1. Sensory system

2. Motor system

3. Cognitive system

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

Figure 14.16 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, 342-368, figure 1. Academic Press, Inc.)

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

Figure 14.17 (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. 55-87, Fig. 12a, b. Used with permission.)

Figure 14.18 An excitatory-center, inhibitory-surround receptive field of a neuron in a monkey’s thalamus.

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

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

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

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

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

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

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

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

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

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

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

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.

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

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

Closing the Gate

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?

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

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