The How of Tactile Sensation http://neuroscience.uth.tmc.edu/s2/chapter02.html Chris Cohan, Ph.D. Dept. of Pathology/Anat Sci University at Buffalo
Objectives 1. Understand how sensory stimuli are encoded by receptors. 2. Understand the concepts of divergence, convergence, and inhibtion and how they contribute to processing sensory signals. 3. Understand how cortical columns are formed. 4. Understand the concept of cortical plasticity and how it relates to cortical sensory maps. 5. Understand the concept of perception and how it depends on attention.
The 3-neuron pathway defines the route tactile information takes from skin to brain But our sensory experience of FEELING is determined by the properties of nerve cells and the connections they make.
Overview How are tactile stimuli transformed into electrical activity Electrical signals follow the 3-neuron pathway, but what happens to the signals along the way. Electrical signals are modified along pathway to represent the unique qualities of stimuli: 1. Encoding - convert mechanical to electrical signal (1 neuron) 2. Processing - how activity of many neurons reconstruct stimulus features (divergence, convergence, inhibition)
Tactile Sensory Receptor - Encoding Tactile sensory receptor = 1 neuron Entire body surface is covered with tactile sensory receptors. Specialized structures that encapsulate the ends of dendrites in the skin. Physical properties of capsule determine how dendrite responds to touch. 2mm x 1mm
First Order Neuron - Encoding Specialized Endings: meissner corpuscle merkel disk pacinian corpuscle ruffini ending hair follicle naked nerve ending muscle spindle, golgi tendon organ Specialized epidermal cells surround dendrite. merkel cells
First Order Neuron - Encoding Locations: skin, viscera, muscle, joints SKIN: vary in density vary in depth hairy vs non-hairy Light Touch - tests superficial receptors by touching skin lightly with cotton
Receptor Differences Only about 5 different types of receptors, but quality of our feeling is vast!! Receptors are stimulated in different combinations during touch to provide a wide range of feeling.
Transformation to Electrical Signals generator/ Touch = deformation Mechanosensitive ion channels in dendritedepolarization. Graded potential = Generator Potential vary in size - encodes magnitude and duration of stimulus Depolarization spreads to first node of Ranvier where it will generate action potentials if large enough.
Encoding Sensory stimuli are brief; they come and go quickly; they change rapidly. The nervous system must capture (encode) them quickly, accurately, and allow us to distinguish occurrences. Encoding does NOT ensure perception! The process of attention is crucial for perception.
Transformation to Electrical Signals Attributes of encoding: Modality- stimulus vs receptor Location- receptive field- global/local Intensity- action potential frequency Duration- duration of activity slowly adapting receptors rapidly adapting receptors
Modality Modality Law of Specific Energies: each type of receptor is activated only by a specific type of energy. depends on receptor structure, physiology each tactile receptor responds to deformation, but in different ways (pressure, stretch, vibration, joint position).
Location - Receptive Field The area on the body where a stimulus affects the activity of a neuron. Its size is influenced by: type of capsule depth in skin amount of branching Size affects resolution
Spatial Resolution of Skin What s the smallest object you can feel? depends on receptor density and RF size 2-point discrimination - a measure of spatial resolution. High density Small RF Low density Large RF
Intensity The intensity of a stimulus is encoded by frequency of action potentials Bigger stim > larger GP > higher freq of action potentials
Duration Response determined by physical properties of the capsule and membrane properties of dendrite. Slowly adapting active during the entire duration of a stimulus rapidly adapting active only during stimulus onset and offset
After stimulus is encoded by receptor, electrical activity is integrated and processed at synaptic relays that continue to reveal features of the stimulus.
Changes at 2 and 3 Neurons Electrical activity is processed by synaptic interactions in Nucleus gracilis/cuneatus/vpl. The effects: increase activity to cortex decreases number of neurons while maintaining touch sensitivity/detectability HOW? 1. Divergence 2. Convergence 3. Inhibition
Changes at 2 and 3 Neurons 1. Divergence- increases the number of active neurons each 1 axon contacts many 2 neurons. 2. Convergence- conserves neurons, ß resolution axons with neighboring RFs converge onto 2 neurons RFs of 2, 3 neurons increase in size conserves neurons, but decreases spatial resolution z Used in locations where spatial resolution less important - back
Changes at 2 and 3 Neurons 3. Inhibition- increases resolution where detail is needed used in fingers where spatial detail is important. inhibitory interneurons in relay nuclei (2, 3 neurons) increases contrast (sharpens edges) between neighboring RFs improves 2-pt discrimination. RF 1 - - 2 inhib
Processing (synaptic interactions) along pathway causes RFs to become larger and more complex BUT modality and location information are preserved along axonal routes
Cortical Neurons More processing - RFs more complex as individual 1 RFs combine in cortex respond to shape, orientation, motion Size of cortical area related to receptor density
Cortical Organization Incoming 3 axons to cortex travel vertically (perpendicular to surface) pia 0.5mm synapse in layer 4, but connections continue to spread vertically - column Indicates cortex analyzes tactile info in a modular manner
Cortical Plasticity Can somatotopic organization of postcentral gyrus be altered? Cortical map is changeable based on experience! Evidence: amputation, anesthetics, and training
Plasticity If amputation causes loss of sensation from body area, corresponding cortical area has no activity initially. Eventually, silent area responds to stimulation from adjacent intact body area. Functional cortical areas expand into nearby unused cortical areas. Lose digits 1, 3-5 Digit 2 expands into area Increased use of body area causes its cortical area to expand into nearby areas as it competes for cortical landscape.
Plasticity People who play stringed instruments: fingers of left hand move over strings continually fingers of right hand receive much less stimulation of individual fingers. functional imaging shows amount of cortex devoted to fingers of left hand is greatly enlarged compared to that for right hand.
Plasticity Cortical map reorganizes by expansion of functional areas. Expansion of one area requires retraction of another! Our brains continuously change structurally and functionally. Experience/use is the key to change. Brain function is shaped by what we do rather than what genetic information can specify. Recovery from brain injury is possible because of the brain s ability to functionally reorganize its connections.
Perception How we recognize and make sense of stimuli. It involves encoding, transmission along pathway, cortical analysis. Encoding does NOT ensure perception! The process of attention is crucial for perception. Attentional resources are limited. Stimulation is not necessary for perception! Cortex can generate its own activity normally or abnormally. How do we differentiate external stimuli from internal activity? do you ever hear a song in your head do you ever have an image stuck in your head people with schizophrenia cannot differentiate internal stimuli from external stimuli