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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
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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
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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
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glamorous skin
non-hairy skin contains the greatest variety and density of receptors and is therefore more sensitive compared to hairy skin
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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
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Sensory receptors encode
- modality
- location
- intensity
- curation
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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
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Location on body
each receptor has a specific RECEPTIVE FIELD: body location is preserved along the route to the cortex by the somatotopic map
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Intensity of stimulus
encoded by ACTION POTENTIAL FREQUENCY: increasing stimulus intensity causes a larger generatory potential and a higher frequency of action potentials
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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
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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
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Superficial receptors RFs
- Meissner's Merkel
- smallest RFs 2-20mm for one DRG fiber
- provide the finest spatial information
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Deepest receptors, RFs
- Pacini, Ruffini
- largest RFs 70-100mm in size
- provide coarse spatial information
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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
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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
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Divergence
- increases the number of active neurons
- amplifies signal
- ensures that the cortex is sufficiently excited by a stimulus
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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
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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
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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
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Divergence, convergence and inhibition
- occur onto VPL neurons too
- enable systems to reconstruct stimulus features faithfully
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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
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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
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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
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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
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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/
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