Chapter 14: The Cutaneous Senses

Chapter 14: The Cutaneous Senses

Somatosensory System • There are three parts • Cutaneous senses - perception of touch and pain from stimulation of the skin • Proprioception - ability to sense position of the body and limbs • Kinesthesis - ability to sense movement of body and limbs

Mechanoreceptors • Two types located close to surface of the skin • Merkel receptor fires continuously while stimulus is present. • Responsible for sensing fine details • Meissner corpuscle fires only when a stimulus is first applied and when it is removed. • Responsible for controlling hand-grip

Figure 14.1 A cross section of glabrous (without hairs or projections) skin, showing the layers of the skin and the structure, firing properties and perceptions associated with the Merkel receptor and Meissner corpuscle - two mechanoreceptors that are near the surface of the skin.

Mechanoreceptors - continued • Two types located deeper in the skin • Ruffini cylinder fires continuously to stimulation • Associated with perceiving stretching of the skin • Pacinian corpuscle fires only when a stimulus is first applied and when it is removed. • Associated with sensing rapid vibrations and fine texture

Figure 14.2 A cross section of glabrous skin, showing the structure, firing properties and perceptions associated with the Ruffini cylinder and the Pacinian corpuscle - two mechanoreceptors that are deeper in the skin.

Pathways from Skin to Cortex • Nerve fibers travel in bundles (peripheral nerves) to the spinal cord. • Two major pathways in the spinal cord • 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.

Maps of the Body on the Cortex • Body map (homunculus) on the cortex in S1 and S2 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.

Figure 14.4 (a) The sensory homunculus on the somatosensory cortex. Parts of the body with the highest tactile acuity are represented by larger areas on the cortex. (b) The somatosensory cortex in the parietal lobe. The primary somatosensory area, S1 (light shading), receives inputs from the ventrolateral nucleus of the thalamus. The secondary somatosensory area, S2 (dark shading), is partially hidden behind the temporal lobe. (Adapted from penfield & Rasmussen, 1950).

Figure 14.5 (a) Each numbered zone represents the area in the somatosensory cortex that represents one of the monkey’s five fingers. The shaded area on the zone for finger 2 is the part of the cortex that represents the small area on the tip of the finger shown in (b). (c) The shaded region shows how the area representing the fingertip increased in size after this area was heavily stimulated over a 2-month period. (From Merzenich et al., 1988)

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 • Raised pattern identification - using such patterns to determine the smallest size that can be identified

Figure 14.7 Methods for determining tactile acuity (a) two-point threshold; (b) grating acuity.

Receptor Mechanisms for Tactile Acuity • There is a high density of Merkel receptors 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.

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.

Figure 14.10 Two-point thresholds for males. Two-point thresholds for females follow the same pattern. (From S. Weinstein, 1968.)

Figure 14.11 Receptive fields of monkey cortical neurons that fire (a) when the fingers are stimulated; (b) when the hand is stimulated; and (c) when the arm is stimulated. (d) Stimulation of two nearby points on the finger causes separated activation on the finger area of the cortex, but stimulation of two nearby points on the arm causes overlapping activation in the arm area of the cortex. (From Kandel & Jessell, 1991 (a-c).

Perceiving Vibration • Pacinian corpuscle (PC) is primarily responsible for sensing vibration. • Nerve fibers associated with PCs respond best to high rates of vibration. • The structure of the PC is responsible for the response to vibration - fibers without the PC only respond to continuous pressure.

Figure 14.12 (a) When a vibrating pressure stimulus is applied to the Pacinian corpuscle, it transmits these pressure vibrations to the nerve fiber. (b) When a continuous pressure stimulus is applied to the Pacinian corpuscle, it does not transmit the continuous pressure to the fiber. (c) Lowenstein determined how the fiber fired to stimulation of the corpuscle (at A), and to direct stimulation of the fiber (at B) (Adapted from Lowenstein, 1960)

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 14.13 (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.

Figure 14.16 (a) Response of 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 Goodwin, 1998)

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.

Figure 14.18 Receptive fields of neurons in the monkey’s somatosensory cortex. (a) This neuron responds best when a horizontally oriented edge is presented to the monkey’s hand. (b) This neuron responds best when a stimulus moves across the fingertip from right to left. (From Hyvarinin & Poranen, 1978)

Figure 14.19 The response of a neuron in a monkey’s parietal cortex that fires when the monkey grasps a ruler but that does not fire when the monkey grasps a cylinder. The monkey grasps the objects at time = 0. (From Sakata & Iwamura, 1978)

Figure 14.20 Firing rate of a neuron in area S1 of a monkey’s cortex to a letter being rolled across the fingertips. The neuron responds only when the monkey is paying attention to the tactile stimulus. (From Hsiao, O’Shaughnessy, & Johnson, 1993)

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 or by tumor cells • 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

Figure 14.21 Nociceptive pain is created by activation of nociceptors in the skin that respond to different types of stimulation. Signals from the nociceptors are transmitted to the spinal cord and then from the dorsal root of the spinal cord in pathways that lead to the brain.

Direct Pathway Model of Pain Perception • Early model that stated nociceptors are stimulated and send signals to the brain • Problems with this model • Pain can be affected by a person’s mental state. • Pain can occur when there is no stimulation of the skin. • Pain can be affected by a person’s attention.

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.

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 • Placebos can also be effective in reducing pain. • 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 • Experiment by Derbyshire to investigate hypnotically induced pain. • Participants had a thermal stimulator attached the to palm of their hand.

Experiment by Derbyshire et al. - continued • Three conditions • Physically induced pain • Hypnotically induced pain • Control group that imagined painful stimulation • Both subjective reports and fMRI scans showed that hypnosis did produce pain perception.

Figure 14.24 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.

Opioids and Pain • Brain tissue releases neurotransmitters called endorphins. • Evidence shows that endorphins reduce pain. • Injecting naloxone blocks the receptor sites causing more pain. • Naloxone also decreases the effectiveness of placebos. • People whose brains release more endorphins can withstand higher pain levels.

Figure 14.28 (a) Naloxone reduces the effect of heroin by occupying a receptor site normally stimulated by heroin. (b) Stimulating sites in the brain that cause the release of endorphins can reduce the pain by stimulating opiate receptor sites. (c) Naloxone decreases the pain reduction caused by endorphins, by keeping the endorphins from reaching the receptor sites.

Pain in Social Situations • Experiment by Eisenberger et al. • Participants watched a computer game. • Then, they were asked to play with two other “players” who did not exist but were part of the program. • The “players” excluded the participant. • fMRI data showed increased activity in the anterior cingulate cortex and participants reported feeling ignored and distressed.

Pain in Social Situations - continued • Experiment by Singer et al. • Romantically involved couples participated. • The woman’s brain activity was measured by fMRI. • The woman either received shocks or she watched while her partner received shocks. • Similar brain areas were activated in both conditions.

Chapter 14: The Cutaneous Senses