Physiolgy of Tactile Sensation- overview.txt

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Physiolgy of Tactile Sensation- overview.txt
2010-12-05 01:13:13

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  1. Physiolgy of Tactile Sensation: overview
    • tactile stimuli on the skin are transformed into the language of electrical activity
    • electrical signals follow the 3-neuron pathway to the cortex
    • electrical signals are modified along the pathway so that they represent features inherent in the stimulus (heavy vs. light; sharp, vs. dull; brief vs. continous)
    • Encoding: how a physical stimulus is transformed into the electrical activity of sensory receptors (1o neurons)
    • Processing: how neurons in the pathway reconstruct the shape of objects that touch the skin; involves integration of activity by synaptic connections at relays (nuclei) in pathway - divergence, convergence, inhibition
  2. First Order Neuron
    • terminals of DRG fibers in the skin are specialized to respond to touch (deformation)
    • most terminals have encapsulated endings - influence how mechanical energy is transformed into electrical energy
  3. Types of primary order receptors
    • Meissner's corpuscle: motion across skin; superficial, RAdaptation; small RF, Fine resolution <3mm
    • Merke disks: form, texture; superficial; SlowAdapting; RF=fingertip; fine resolution <3mm
    • Pacinian corpuscle: vibration; deep; Rapidly-Adapting; Large RF; coarse resolution >10mm
    • Ruffini ending: skin stretch; deep, SA; entire finger/hand = RF; Coarse >10mm
    • Hair follicle: hair bending; RA; Coarse resolution
  4. glamorous skin
    non-hairy skin contains the greatest variety and density of receptors and is therefore more sensitive compared to hairy skin
  5. Receptors transform touch into electrical energy
    • Mechanosensitive ion channels: in the nerve terminal open when the nerve membrane is physically deformed--> depolarization
    • Generator potential: is this initial depolarization, it is a GRADED potential
    • Depolarization spreads along nerve terminal AND, if reaches threshold, it produces action potentials at the first Node of Ranvier where Na+ channels are clustered
  6. Sensory receptors encode
    • modality
    • location
    • intensity
    • curation
  7. Modality
    • each receptor type responds to a specific type of stimulus
    • this info is preserved in the pathway to cortex
    • important principle of LABELED LINES: in sensory systems - specific axons carry specific modalities of sensory information
  8. Location on body
    each receptor has a specific RECEPTIVE FIELD: body location is preserved along the route to the cortex by the somatotopic map
  9. Intensity of stimulus
    encoded by ACTION POTENTIAL FREQUENCY: increasing stimulus intensity causes a larger generatory potential and a higher frequency of action potentials
  10. Duration of stimulus
    • the length of time a stimulus touches the skin is encoded iby the duration of electrical activity
    • Most receptors decrease their activity during a sustained stimulus: adaptation
    • Slowly Adapting (SA): receptors respond during the entire duration of contact with skin, providing static information
    • Rapidly Adapting (RA): receptors repined only briefly to sustained contact with skin and at onset and cessation of contact - provide dynamic (on/off) information
    • Adaptation determined by: the physical/mechanical properties of the capsule that surrounds the nerve terminal
  11. Receptive fields
    • define area of sensitivity on skin for each receptor; indicates where cells are analyzing stimuli
    • RFs of different receptors overlap on skin
    • RF size determined by: capsule properties/ size/skin depth and branching pattern on the nerve terminal
    • EACH nerve cell in the tactile pathway has an RF
  12. Superficial receptors RFs
    • Meissner's Merkel
    • smallest RFs 2-20mm for one DRG fiber
    • provide the finest spatial information
  13. Deepest receptors, RFs
    • Pacini, Ruffini
    • largest RFs 70-100mm in size
    • provide coarse spatial information
  14. Spatial Resolution
    • depends on size of RF and receptor density
    • measured by two point discrimination
    • reforest to our ability to detect fine stimuli on body
    • depends on cortical function - requires a perceptual comparison of tactile information
    • varies over the body
  15. Second and third order neurons
    • relay nuclei (nucleus gracilis/cuneatus and VPL) process tactile info
    • synaptic connections integrate the activity from presynaptic neurons through divergence, convergence, and inhibition
  16. Divergence
    • increases the number of active neurons
    • amplifies signal
    • ensures that the cortex is sufficiently excited by a stimulus
  17. Convergence
    • decreases neurons, decreases spatial resolution, maintains detectability
    • primary axons with neighboring RGs converge onto individual secondary neurons
    • RFs of secondary neurons are larger than those of primary neurons
    • conserves number of neurons, but lowers spatial resolution
    • Can increase detectability: by allowing several weak, subthreshold synaptic inputs to summate and evoke postsynaptic action potentials
  18. Divergence and Convergence
    increase excitatory activity in the pathway, ensuring that a stimulus on the skin causes a significant increase in activity at the cortex
  19. Inhibition
    • improves resolution
    • used in areas where spatial detail is important, e.g. fingers
    • Overlapping RFs normally would decrease spatial resolution, reducing out ability to discriminate fine tactile detail
    • primary axon inhibits activity of adjacent neurons
    • improves 2-point discrimination
  20. Divergence, convergence and inhibition
    • occur onto VPL neurons too
    • enable systems to reconstruct stimulus features faithfully
  21. Cortical neurons
    • in the postcentral gyrus continue to process tactile information in similar ways to previous levels
    • this results in perception - along with neighboring cortex
    • RFs become more complex as convergence onto cortical neurons combines primary RFs into more complex shapes --> this allow ID of object shape
    • Amount of cortex devoted to a region of the body: related to receptor density in that region
    • somatotopic map in cortex can be modified by experience (plasticity): use it or lose it principle - increased use of part of the body can expand the cortical area for that part
  22. how cortex organizes all the tactile information that arrives there?
    • Axons from VPL penetrate the cortex and travel VERTICALLY to terminate on neurons in layer 4: axons and dendrites from these neurons spread vertically to make synaptic connections with neurons in other layers
    • cortical column: this cylindrical population
    • Additional tertiary axons: that have overlapping RFs and come from the same type of receptor also synapse in this column
    • This type of processing indicates that the cortex analyzes information in a modular manner based on skin location and sensory modality
  23. Pattern of layered columns
    • is repeated in other functional regions of the cortex
    • is a fundamental feature by which the brain processes information for normal brain function
    • *** developmental problems that disrupt cortical layering have profound effects on brain connectivity and function leading to: seazures, mental retardation, and other deficits
  24. The Amazing Property of Perception
    • process by which we recognize, organize and make sense of sensations we receive from stimuli
    • occurs at the level of the cortex: after
    • 1) encoding of the stimulus
    • 2) progression/modification of that activity along the pathway
    • 3) cortical analysis first at the specific location where a sensory pathway "terminate" and then at neighboring cortical areas
  25. Requirements for Perception
    • encoding is NOT sufficient!!! ALSO requires attention to the stimulus - which is limited especially when multiple stimuli are receive simultaneously
    • Stimulation is NOT necessary for perception - can imagine and experiences can be generated by activity that begins in the cortex/