Textbook Notes (363,566)
Canada (158,433)
Psychology (2,948)
PSY100H1 (1,804)
Chapter 12

Chapter 12 - PSYB51

9 Pages
Unlock Document

University of Toronto St. George
Mathias Niemier

Chapter 12 – Touch  Touch: sensations caused by mechanical displacements of skin  Tactile: mechanical interactions  Kinesthesis: perception of position and movement of limbs in space  Proprioception: perception mediate by kinesthetic and vestibular receptors  Somatosensation: collective term for sensory signals from body  Pain serves as warning system that tells us when something might be internally wrong or when external stimulus may be dangerous  Temperature sensations enable to seek or create thermally safe environments  Mechanical sensations play important role in intimate sexual and productive activities and provide means of communicating thoughts and emotions  Touch important because use it to identify and manipulate objects that can’t be seen or heard  Eyes and ears an perceive signals from objects that are far from body, but must almost always be in direct contact with object to perceive it by touch Touch Physiology Sense Organ and Receptor Units for Touch  Human sense of touch is housed in largest and heaviest sense organs – skin – covers ~1.8 square meters and 4Kg  Epidermis: outer of 2 major layers of skin  Dermis: inner of 2 major layers of skin, consisting of nutritive and connective tissues, within which lie on mechanoreceptors  Receptors form basis for multiple “channels” specialized info-processing subsystems that each contribute to each overall sense of touch  Each type of receptor characterized by o 1. Type of stimulation to which receptor responds – receptors respond to different stimulus events o 2. Size of receptive field – receptors are activated when stimulation is applied to particular area of body, which constitutes receptors’ “receptive field”. Size of receptive field refers to extent of body area that elicits receptor response o 3. Rate of adaptation (fast vs. slow) – fast adapting (FA) receptor responds with bursts of action potentials, first when preferred stimulus is applied and again when it’s removed. Doesn’t respond during steady state between stimulus onset and offset. In contrast, slow adapting (SA) receptor remains active throughout period during which stimulus is in contact with receptive field Tactile Receptors  Mechanoreceptors: sensory receptors that are responsive to mechanical stimulation  All tactile nerve fibers fall into class called A-beta fibers, which have wide diameters that permit fast neural conduction  Tactile nerve fibers for various types are assumed to terminate in different expanded endings Response characteristics of 4 mechanoreceptor populations Size of receptive field Adaption Rate Small Large Slow SA I (Merkel) SA II (Ruffini) Fast FA I (Meissner) FA II (Pacinian)  Expanded endings of 4 different populations of tactile fibers in hairless skin of hand are named after anatomists who first described them o Meissner corpuscles: specialized nerve ending associated with fast adapting (FA I) fibers that have small receptive fields o Merkel cell neurite complexes: specialized nerve ending associated with slow adapting (SA I) fibers that have small receptive fields o Pacinian corpuscles: specialized nerve ending associated with fast adapting (FA II) fibers that have large receptive fields o Ruffini endings: specialized nerve ending associated with slow adapting (SA II) fibers that have large receptive fields  Endings of Meissner and Merkel receptors located at junction of epidermis and dermis whereas Pacinian and Ruffini receptors are embedded more deeply in dermis and underlying subcutaneous tissue  4 types of mechanoreceptors can be independently classified according to adaptation rates and sizes of receptive fields, as measured from activity of tactile nerve fibers o SA I fibers respond best to steady downward pressure, fine spatial details, and low frequency vibrations of less than about 5 Hz. When single SA I fiber stimulated, people report feeling “pressure” – fibers assumed to terminate Merkel cell neurite complexes o SA II fibers in skin respond to sustained downward pressure, and particularly to lateral skin stretch, which occurs when we grasp an object. SAII fibers help determine when your fingers are shaped properly for picking up cup. When single SA II fiber is stimulated, people don’t experience any tactile sensation at all o FA I fibers respond best to low frequency vibrations from 5-50 Hz. When lone FA I fiber stimulated, people report localized sensation that they describe as “wobble” or “flutter”. Fibers are assumed to terminate in Meissner corpuscles o FA II fibers respond best to high frequency vibrations from about 50-700 Hz. Vibrations occur whenever objects makes contact with skin. When single FA II fiber is stimulated, people report more diffuse sensation in skin that corresponds to “buzz”. Fibers shown to terminate Pacinian corpuscles Kinesthetic Receptors  Kinesthetic: perception involving sensory mechanoreceptors in muscles, tendons and joints  Muscle spindles: sensory receptor located in muscle that senses its tension  Receptors in tendons provide signals about tensions in muscles attached to tendons, and receptors directly in joints themselves come into play particularly when joint is bent to extreme angle  Importance of kinesthetic receptors is graphically illustrated by strange case of neurological patient named Ian Waterman o Lacking kinesthetic senses, Waterman is now completely dependent on vision to tell him about positions of limbs in space Thermoreceptors  Thermoreceptors: sensory receptors that signal info about changes in skin temperature  Warmth fibers: sensory nerve fiber that fires when skin temperature increases  Cold fiber: sensory nerve that fires when skin temperature decreases  Bodies constantly working to regulate internal temperature between 30-36 degrees  Thermoreceptors kick into gear when we make contact with object that is warmer or colder than skin Nociceptors  Nociceptors: sensory receptors that transmit info about noxious (painful) stimulation that causes damage or potential damage to skin. Can be divided into 2 types by nerve fibers, which no specialized endings have been found o A-delta fibers: intermediate-sized, myelinated sensory nerve fiber that transmits pain and temperature signals o C fiber: narrow diameter, unmyelinated sensory nerve fiber that transmits pain and temperature signals From Skin to Brain  Because receptors for sights, sounds, tastes, and smells located in skull, pathways that deliver info from receptors to brain are fairly short – touch travels as far as 2 meters to get from skin and muscles of fee to brain o To travel, info moves up through spinal cord  Right from start, see 2 major differences between visual and auditory pathways and pathways for touch o First, whereas there are only 2 optic nerves and 2 auditory nerves there are number of somatosensory nerve trunks, arising in hands, arms, feet, legs and other areas of skin o Second, axons in optic and auditory nerves go directly to brain, whereas axons in older nerve trunks, synapse first in spinal cord o Once in spinal cord, info proceeds upward toward brain via 2 major pathways  Spinothalamic pathway: route from spinal cord to brain that carries most of info about skin temperature. It’s slower of 2 and includes number of synapses within spinal cord, slowing conduction while providing mechanism for inhibiting pain perception when necessary  Dorsal column-medial lemniscal (DCML) pathway: route from spinal cord to brain that carries signals from skin, muscles, tendons and joints o Neurons in DCML pathway first synapse in cuneate and gracile nuclei, near base of brain and activity is passed on to neurons that snapse in ventral posterior nucleus of thalamus o From thalamus, much of touch info is carried up to cortex into somatosensory area 1 (S1) o Somatosensory area 1 (S1): primary receiving area for touch in cortex. Analogous to V1 in vision o Somatosensory area 2 (S2): secondary receiving area for touch in cortex. Lies in upper bank of lateral sulcus o Motor control movements of body parts, are located between somatosensory and motor control systems  Touch sensations that result from skin being stimulated are spatially represented in area S1 and somatotopically  Somatopic: spatially mapped in somatosensory cortex in correspondence to spatial events on skin. Somatotopy is analogous to topographic spatial representation of events on retina found in vision; adjacent areas on skin are ultimately connected to adjacent areas in brain  As result, somatosensory cortex is organized into spatial map of layout of skin – homunculus o Homunculus: map-like representation of regions of body in brain o Sensory homunculus is derived largely from work of Canadian neurosurgeon Wilder Penfield, who charted somatotopic map with aid of patients undergoing brain surgery to alleviate epilepsy o Correspondence between stimulation and sensation gave rise to map of body in brain o Like retinotopic map in area V1, Penfiel’s somatotopic map in S1 is distorted  Phantom limb: sensation perceived from physically amputated limb of body  Projections from S1 form basis for further analysis of objects and surfaces by cortex of brain  Results of studies suggest that analogously to vision, sense of touch may show division between what and where systems in higher cortical centers  Braille task activated only area S1 on left side of brain – but as days progressed, amount of activation in S1 declined while increasing in V1  Neural plasticity: ability of neural circuits to undergo changes in function or organization as result of previous activity  Downward pathways from brain alter sensations produced by stimulating periphery Pain  Multiple Levels of Pain o Substantia gelatinosa: jellylike region of interconnecting neurons in dorsal horn of spinal cord o Dorsal horn: region at rear of spinal cord that receives inputs from receptors in skin o Neurons receive info from brain as well as forming synapses with neurons that are conveying sensory info from nociceptors to brain o Gate control theory: description of system that transmits pain that incorporates modulating signals from brain. When gate neurons send excitatory signals, sensory info is allowed to go through, but inhibitory signals from gate neurons cancel transmission to brain – results are transmitted to somatosensory areas S1 and S2 o Signals arising at S1 and S2 don’t tell whole story as we’ve known that areas are responsible for sensory aspects of pain, but used new methods to identify other areas of brain that correspond to emotional aspects of painful experiences o Anterior cingulated cortex (ACC): region of brain associated with perceived unpleasantness of pain sensation. Concluded that ACC processes raw sensory data from S1 and S2 in such way as to produce emotional response o At high level, pain can produce what Price calls, “secondary pain effect” which reflects influence of cognition  Secondary affect is emotional response associated with long term suffering that occurs when painful events are imagined or remembered o Prefrontal cortex: region of brain concerned with cognition and executive control o Just as signals from brain can control pain perception, appear to come into play when we try to tickle ourselves o For itchiness, Schmelz suggests that pain and itch may be mediated by separate neural systems, researchers still understand relatively little about how itch works or how to relieve nasty sensations  Moderating Pain o Pain experiences are complex result of sensory signals interacting with many other factors that have moderating effects o Analgesia: decreasing pain sensation during conscious experience o At level of spinal cord, gate neurons that attenuate pain can be activated by “counter-irritation” or “diffuse noxious inhibitory control” o Appears that ascending signals from counterirritation reach brain stem and initiate new set of signals that are sent back down to pain blocking gate in spinal cord o Different , and certainly more pleasant, way of modulating pain is to use benign counterstimulation o Endogenous opiates: chemicals released by body that block release or uptake of neurotransmitters necessary to transmit pain sensations to brain o Externally produced substances such as morphine, heroin and codeine are similar in chemical structure to these endogenous opiates, and have similar analgesic effects o Acetaminophen and ibuprofen, alleviate pain at source by counteracting chemicals that would start nocieptors firing  Pain Sensitization o Nocieptors provide signal when there is impending or ongoing damage to body’s tissue – nocieptive pain o Once damage occurs, site can become more sensitive, triggering feeling of pain more readily than before o Hyperalgesia: increased or heightened response to normally painful stimulus
More Less

Related notes for PSY100H1

Log In


Don't have an account?

Join OneClass

Access over 10 million pages of study
documents for 1.3 million courses.

Sign up

Join to view


By registering, I agree to the Terms and Privacy Policies
Already have an account?
Just a few more details

So we can recommend you notes for your school.

Reset Password

Please enter below the email address you registered with and we will send you a link to reset your password.

Add your courses

Get notes from the top students in your class.