Somatosensory System: Touch and Pain PDF

Somatosensory system: touch and pain Somatosensory system: three subsystems Exteroceptive cutaneous system (outside body) Mechanical stimulation (touch) Thermal stimulation (temperature) Nociceptive stimuli (surface pain) Proprioceptive system (body position) Interoceptiv...

Somatosensory system: touch and pain Somatosensory system: three subsystems Exteroceptive cutaneous system (outside body) Mechanical stimulation (touch) Thermal stimulation (temperature) Nociceptive stimuli (surface pain) Proprioceptive system (body position) Interoceptive system (within body)  Learning Objectives Name the cutaneous receptors and explain the functional significance of fast versus slow receptor adaptation. Describe the two major somatosensory pathways. Describe the cortical somatosensory areas and their somatotopic layout. Name the areas of association cortex that somatosensory signals are sent to and describe the functional properties of one of those areas. Cutaneous Receptors Types of receptors: Ruffini corpuscles and Merkel receptors (slow-adapting, skin indentation and stretch) Free nerve endings (pain and temperature) Pacinian corpuscles (deep fast- adapting, skin displacement) Figure 7.9 Cutaneous Receptors Cutaneous perception: All cutaneous receptors respond to stimuli with ion flow across neuronal membrane Stereognosis (identification of objects by touch) use both fast and slow-adapting receptors. Tactile sensations are not specific to individual receptors but are produced by multiple receptor mechanisms. means by combined receptors Two Major Somatosensory Pathways 1) Dorsal-column medial-lemniscus system Carries information about touch and proprioception movemnet of the body 2) Anterolateral system Mediates pain and temperature 1. Dorsal-Column-Medial-Lemniscus Carries information about touch and proprioception to the cortex Axons of receptors enter spinal cord and ascend to hindbrain, thalamus. Axons of the ventral posterior nucleus (thalamus) ascend to the somatosensory cortex Figure 7.10  2. Anterolateral System Mediates pain and temperature Axons synapse as soon as they enter the spinal cord Axons travel up three paths/tracts all reaching the thalamus. Information is then distributed to somatosensory cortex Figure 7.11 Cortical Areas of Somatosensation Projections of somatosensory information from thalamus Primary somatosensory cortex Secondary somatosensory cortex Primary somatosensory cortex Organized somatotopically: according to a map of the body surface Somatosensory homunculus means little man the name of the map is in the cortext Primary and Secondary Somatosensory Cortex Figure 7.12 The locations of human somatosensory cortex The conventional portrayal of the somatosensory homunculus. Cortical Areas of Somatosensation Damage to primary somatosensory cortex Minor effects on tactual sensitivity Due to numerous parallel pathways Somatosensory System and Association Cortex Somatosensory signals are projected to: Posterior parietal cortex – multisensory integration Secondary somatosensory cortex – perception of shapes of objects s for the shape Bimodal neurons of posterior parietal cortex Respond to somatosensory and visual stimuli Receptive fields closely associated What about the female homunculus? https://www.the-scientist.com/news-opinion/her-munculus-situates- female-genital-sensation-in-the-cortex-69555  Learning Objectives Describe the two major types of somatosensory agnosia. Describe the rubber-hand illusion and its neural mechanisms. Explain why the perception of pain is said to be paradoxical. Somatosensory Agnosias Astereognosia Loss of ability to recognize objects by touch Damage to the parietal cortex Asomatognosia Loss of ability to recognize parts of one’s own body Damage to the temporal and parietal lobes Rubber-Hand Illusion Procedure Hide healthy volunteer’s hand while stroking both the hidden hand and rubber hand The rubber hand is placed in clear sight Volunteers quickly perceive rubber hand as their own https://www.youtube.com/watch?v=DphlhmtGRqI Rubber-Hand Illusion Procedure Hide healthy volunteer’s hand while stroking both the hidden hand and rubber hand The rubber hand is placed in clear sight Volunteers quickly perceive rubber hand as their own Neural mechanism https://www.youtube.com/watch?v=DphlhmtGRqI Possibly mediated by bimodal neurons Posterior parietal and frontal cortices Perception of Pain is Paradoxical Paradox = Contradiction Pain is adaptive: important for survival Cases of congenital pain insensitivity There is a genetic component (mutation in a gene that influences the synthesis of sodium channels) Perception of Pain is Paradoxical Paradox = Contradiction Pain is adaptive: important for survival Cases of congenital pain insensitivity There is a genetic component (mutation in a gene that influences the synthesis of sodium channels) Lack of clear cortical representation of pain SI and SII are both activated by pain Removal of SI and SII does not change perception of pain Perception of Pain is Paradoxical Pain is modulated by cognition and emotion Ability of emotional and cognitive factors to block pain Evidence of a circuit descending from the brain that can block pain signals coming from the spinal cord. Study Aid : fill in the table Sensory System Stimulus Receptor Type Key Pathway Vision Visible light Photoreceptors: Optic nerve to LGN, rods and cones to primary visual cortex Audition Somatosensation Perception and Attention How do we construct a percept of the world using all our senses?  Learning Objectives Use examples to illustrate the role of experience in perception. Explain perceptual decision making, using some examples of phantom percepts to illustrate. Describe the two characteristics of selective attention and explain what is meant by exogenous versus endogenous attention. Describe the phenomenon of change blindness. Role of Prior Experience in Perception Prior knowledge has a major influence on how we perceive the world Role of Prior Experience in Perception Prior knowledge has a major influence on how we perceive the world Perceptual Decision Making Mental models of the world based on predictable and recurring sensory events Perceptual decision making consumes a large proportion of the energy used by the brain Perceptual decision making at work Bistable figures Phantom percepts Bistable Figures The vase-face illusion Phantom percepts In the absence of sensory input, we still perceive. Phantom limb syndrome Charles Bonnet syndrome (hallucinations) Phantom sounds (tinnitus) Selective Attention Improves the perception of the stimuli that are its focus, interferes with the perception of the stimuli that are not its focus. May be focused on internal cognitive processes Endogenous attention Believed to be top-down (from higher to lower levels) May be focused on external events Exogenous attention Believed to be bottom-up (from lower to higher levels) Cocktail-party phenomenon Selective Attention Improves the perception of the stimuli that are its focus, interferes with the perception of the stimuli that are not its focus. May be focused on internal cognitive processes Endogenous attention Believed to be top-down (from higher to lower levels) May be focused on external events Exogenous attention Believed to be bottom-up (from lower to higher levels) Cocktail-party phenomenon Selective Attention Improves the perception of the stimuli that are its focus, interferes with the perception of the stimuli that are not its focus. May be focused on internal cognitive processes Endogenous attention Believed to be top-down (from higher to lower levels) May be focused on external events Exogenous attention Believed to be bottom-up (from lower to higher levels) Cocktail-party phenomenon Change Blindness Example of selective attention Demonstrated by showing people two photographs identical in every aspect but one If allowed a brief delay, people have difficulty seeing change Change Blindness Figure 7.22 Change Blindness Figure 7.22 Change Blindness Figure 7.22