Chapter 14: The Cutaneous Senses PDF

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CHAPTER 14: THE CUTANEOUS SENSES I. Cutaneous Senses: Perception of touch and pain from stimulation of the skin. A. touch, vibration, tickle, and pain. II.

Overview of Cutaneous System A. The Skin: Heaviest organ in the body i. Protects the organism by keeping damaging areas from penetrating the body ii. Layers- Epidermis and Dermis 1. Epidermis is the outer layer of the skin, which is made up of dead skin cells 2. Dermis is below the epidermis and contains mechanoreceptors that respond to stimuli such as pressure, stretching and vibration.

B. Mechanoreceptors: Four types, different shapes and functions i. Two located close to surface of the skin 1. Merkel receptor fires continuously while stimulus is present a. Responsible for sensing fine details 2. Meissner corpuscle fires only when a stimulus is first applied and when it is removed a. Responsible for controlling hand grip.

ii.

Two located deeper in the skin 1. Ruffini Cylinder fires continuously to stimulation a. Associated with perceiving stretching of the skin

2. Pacinian Corpuscle fires only when a stimulus is first applied and when it is removed a. Associated with sensing rapid vibrations and fine texture

C. Pathways from Skin to Cortex: i. Nerve fibers travel in bundles (peripheral nerves) to the spinal cord. ii. Two major pathways in the spinal cord 1. Medial lemniscal (touch and limb position)- consists of large fibers 2. Spionthalamic (pain and temperature) – consists of smaller fibers a. These cross over to the opposite side of the body and synapse in the thalamus D. The Somatosensory Cortex: i. Signals travel from the thalamus to the somatosensory receiving area (S1) and the secondary receiving area (S2) in the parietal lobe 1. The somatosensory cortex is organizing into maps that correspond to locations on the body. 2. The larger the body part the higher the tactile acuity represented by largers areas on the cortex

E. The Plasticity of Cortical Body Maps: i. Experience-dependent plasticity; Increase S1 areas in monkeys with experience; Cortical area larger for body part used in playing instrument

III.

Perceiving Details A. Method: Measuring Tactile Acuity (the ability to detect details on the skin) i. Two points thresholds: minimum separation needed between two points to perceive them as two units ii. Grating acuity: placing a grooved stimulus on the skin and asking the participant to indicate the orientation of the grating iii. Raised pattern identification: using such patterns to determine the smallest size that can be identified.

B. Receptor Mechanisms for Tactile Acuity: i. There is a high density of Merkel receptors in the fingertips 1. Densely packed similar to cones in the fovea 2. Better acuity is associated with less spacing between Merkel receptors (the more dense it is) ii. Both two-point thresholds and grating acuity studies show these results. C. Cortical Mechanisms for Tactile Acuity: i. Body areas with high acuity have larger areas of cortical tissue devoted to them 1. Parallels the “magnification factor” feen in the visual cortex for the cones in the fovea 2. Areas with higher acuity also have smaller receptive fields on the skin a. Receptive fields of monkey cortical neurons i. Two points on the arm overlap on cortex…two points on the fingers have different spots on cortex

IV.

Perceiving Vibration & Texture A. Vibration: i. Pacinian corpuscle is the mechanoreceptor primarily responsible for perceiving vibration 1. Nerve fibers associated with PCs respond best to high rates of vibration 2. Fibers w/o the PC only respond to continuous pressure

B. Surface Texture: Duplex theory of texture perception i. Kats (1925) proposed that perception of texture depends on two cues 1. Spatial Cues (size, shape, distance) a. Supported by past research 2. temporal cues (rate of vibrations), Movement needed to perceive roughness of fine surface a. Hollins and Reisner show support for temporal cues

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In order to detect differences between fie textures, participants needed to move their fingers across the surface. ii. the visual perception of texture is influenced by illumination V.

Perceiving Objects A. Humans use active rather than passive touch to interact with the environment i. Haptic Perception- is the active exploration of 3-D objects with the hand ii. Uses three distinct systems 1. Sensory 2. Motor 3. Cognitive iii. Psychophysical research shows that people can identify objects haptically in one to two seconds iv. Klatzky et al. have shown that people use exploratory procedures (Eps) 1. Lateral motion 2. Contour following 3. Pressure 4. Enclosure

B. The Physiology of Tactile Object Perception: i. Firing pattern of mechanoreceptors signals shape such as the curvature of an object. ii. Neurons become more specialized higher in pathway, 1. Monkey’s thalamus shows cells that responds to centersurround receptive fields 2. Somatosensory cortex shows cells that respond maximally to orientations and direction of movement. iii. Visual attention affects neural firing in haptic task VI.

PainA. Pain- is a multimodal phenomenon containing sensory component and an affective or emotional component i. Three types 1. Inflammatory pain: caused by damage to tissues and joints or by tumor cells 2. Neuropathic pain: caused by damage to the central nervous system a. Stroke, repetitive movements

3. Nociceptive pain: signals impending damage to the skin a. Response to heat, chemicals, severe pressure and cold b. Threshold of receptor response must be balanced to warn of damage, but not be affected by normal activity. c. Direct Pathway Model- Created by activation of nociceptors in the skin that respond to different types of stimulation. Signals are send directly to brain.

VII.

The Gate Control Model by Melzack & Wall (1965) A. Early model stated nociceptors are stimulated and send signals to the brain B. Problems with the direct pathway model i. Pain can be affected by a person’s mental state ii. Pain can occur when there is no stimulation of the skin 1. Without any transmission from receptor to brain…Phantom limb iii. Pain can be affected by a person’s attention C. Pain must not originate in the skin but in the brain D. Gate control: pain signals enter spinal cord from the body and transmitted to the brain E. Additional pathways that influence the signals from spinal cord to the brain, can ope or close a gate in the spinal cord which determine the strength of the signal leaving the spinal cord. i. Three pathways 1. Nociceptors 2. Mechanoreceptors 3. Central control

VIII.

Top-Down Processes: A. Expectation: when surgical patients are told what to expect, they request less pain medication and leave the hospital earlier i. Placebos can also be effective in reducing pain B. Shifting attention: Virtual reality technology has been used to keep patients’ attention on other stimuli than the pain inducing stimulation C. Content of emotional distraction: participants could keep their hands in cold water longer when pictures they were shown were positive i. Looking a picture ii. Listening to music (Roy, 08’) 1. Rate intensity and pleasantness under three conditions (silence, unpleasant and pleasant music) 2. Unpleasant music did not affect pain but pleasant music decreased both the intensity and unpleasantness of pain.

IX.

The Brain and Pain: A. Areas in the brain responsible for pain i. Subcortical areas including the hypothalamus, amygdala and thalamus ii. Cortical areas including S1 in the somatosensory cortex, the insula and the anterior cingulate and prefrontal cortex 1. All called the “Pain Matrix”

B. Hoffauer et al. i. Participants were presented with potentially painful stimuli and asked 1. To rate subjective pain intensity 2. To rate the unpleasantness of the pain ii. Brain activity measured while they placed their hands in hot water iii. Hypnosis was used to increase or decrease the sensory and affective components. iv. Results: 1. Suggestions to change the subjective intensity led to changes in both ratings and in activity in S1 2. Suggestions to change the unpleasantness of pain did not affect the subject’s ratings, but did change ratings of unpleasantness and in activity in ACC. C. Opioids and pain i. Brain tissue releases neurotransmitters called endorphins ii. Evidence shows that endorphins reduce pain 1. Injecting naloxone blocks the receptor sites causing more pain 2. Naloxone also decreases the effectiveness of placebos 3. People whose brains release more endorphins can withstand higher pain levels. X.

Observing Pain in Others A. Singer et al. (2004) demonstrated the connection between brain response to pain and empathy i. Romantically couples; women’s brain activity measured by fMRI 1. Receive shocks or watch partner receive shocks ii. Higher empath scores showed higher activation of their ACC iii. Activation in (b) is related to emathy for the other person

B. Klimecki et al. (2014): empath training i. Those in empathy training group showed more empathy to people suffering due to injury and greater activation of ACC. 1. Showed videos depicting others suffering XI.

Something to Consider: Social Pain and Physical Pain. A. Eisenberger et al. (2015): does social rejection hurt? i. Cyberball experiment 1. Excluded in second half of the game, ball is not thrown to their avatar ii. Dorsal anterior cingulate cortex is activated by feelings of social exclusion B. Physical-social pain overlap hypothesis- pain resulting from negative social experience is processed by some of the same neural circuitry that processes physical pain....