Physiolgy of Tactile Sensation- overview.txt

• 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

• 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

• 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

non-hairy skin contains the greatest variety and density of receptors and is therefore more sensitive compared to hairy skin

• 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

• modality
• location
• intensity
• curation

• 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

each receptor has a specific RECEPTIVE FIELD: body location is preserved along the route to the cortex by the somatotopic map

encoded by ACTION POTENTIAL FREQUENCY: increasing stimulus intensity causes a larger generatory potential and a higher frequency of action potentials

• 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

• 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

• Meissner's Merkel
• smallest RFs 2-20mm for one DRG fiber
• provide the finest spatial information

• Pacini, Ruffini
• largest RFs 70-100mm in size
• provide coarse spatial information

• 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

• relay nuclei (nucleus gracilis/cuneatus and VPL) process tactile info
• synaptic connections integrate the activity from presynaptic neurons through divergence, convergence, and inhibition

• increases the number of active neurons
• amplifies signal
• ensures that the cortex is sufficiently excited by a stimulus

• 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

increase excitatory activity in the pathway, ensuring that a stimulus on the skin causes a significant increase in activity at the cortex

• 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

• occur onto VPL neurons too
• enable systems to reconstruct stimulus features faithfully

• 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

• 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

• 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

• 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

• 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/