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Sterogenesis Notes

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Even blindfolded, most people can easily rummage through a bag full of diverse objects to retrieve, for example, a bunch of keys. To do this, they must be able to almost 'see with their fingers'. Describe neural structures and mechanisms that contribute to this remarkable ability Introduction Stereognosis: ability to recognise objects based on the way they feel when touched and held
-To do this-we need to respond to a wide variety of somatosensory stimuli and associate them with previous experiences of objects to deduce what the objects are Touch
-Touch is defined as direct contact between 2 physical bodies
-can be active-you move your hand oor some other part of the body against another surface-used when hand manipulates objects
- passive when seomeone or something else touches you-used for naming objects or describing sensations
-Passive touch-important clinically to measure sensory loss-measurements of detection thresholdsd, vibtration sense, two point or texture discrimination
-Active and passive modes of tactile stimulation excite the same population of receptors in the skin and evoke similar responses in afferent fibres Receptors
-Object contacts the hand, skin conforms to its contours-forms a mirror image of the object surface
-Resultant displacement and indentation of the skin stretches the tissue stimulates sensory endings of mechanoreceptors-provide information to brain about position of stimulus, shae, surface texture
-Psychophysical studies show that tactile perception from the hand is highly integrative and rapid-without visual input we can effortlessly recognise and manipulate infinite number of objects with our hands- Study done by Klatzky showed that humans can recognise 100 common objects with accuracy of 96% , with mean response time of less than 5secodns. Subjects used perceptual dimensions- texture, size, shape
-These studies show that accurate object recognition is multidimensional and inputs from different afferent types are integrated in the cortex
-4 main types of mechano-receptors in the skin that respond to touch
-Receptors are innervated by either slowly adapting or rapidly adapting axons
-Slowly adapting fibres respond to steady skin indentation with sustained discharge
-Radpily adapting fibres stop firing as soon as the indentation is stationary-sensation of motion across the skin is fired by these fibres
-Slowly adapting fibres: Detect object pressure and form

-Pancinian corpuscle - RAPIDLY ADAPTING TYPE 2 MECHANORECEPTORS-high frequency vibration
-Meissner corpuscles- RAPIDLY ADPTING MECHANORECEPTORS TYPE 1- low frequency vibrations and motion across the skin
-Merkel cells and endings- SLOWLY ADAPTING TYPE 1 MECHANORECEPTORSresponsible for perception of 2D form and texture
-Ruffini endings= SLOWLY ADAPTING TYPE 2 MECHANORECEPTORS-afferents for skin stretch- afferents that encode information about the hand conformation
-Recognition of objects- tight integration of inputs from cutaneous afferents with inputs from proprioceptive afferents from muscles, joints, skin. The inputs from cutaneous slowly adaptive 1 and rapidly adapting systems provide information about local form, texture and motion at each loctation where skin contacts the object. Multiple points of contact- produce multiple independent views of the object. Global properties are determined by matching a composite of these three dimensional views with previously stored representations of objects
-In older study- Davidson showed that accuracy of humans to identify objects increases rapidly as the number of fingers used to contact the object increases
-Other receptors- thermoreceptors and nociceptors- add to experience and help in recognition of an object
-Multiple receptors are advantageous
-Offers redundancy- if there is a mutation that causes a certain type of receptor to be dysfunctional the others offer a backup and individual still have touch sensation
-Also offers a wider range and finer control of sensation Pancinian corpuscle
-Subcutaneous tissue- pancinain corpuscles receives single rapidly adapting axons
-Function- ampifiy high frequency vibration
-Unmyelinated nerve endings of neurons - wrapped in multiple layers/ lamellae
-Lamellae are compsed of fibrous connective and Schwann cells
-The nerve ending itself is in a fluid filled capsule- this protects nerve ending from all stimuli but mechanical deformation
-Deformation causes lamella, capsule and membrane of the ending to be deformed- forms a generator potential (definition = first depolarisaiton of variable amplitude that will cause an action potential if it exceeds threshold. 1960s experiments of Loewenstein-remove the capsule it's still able to transduce)
-Pancinain corpuscles are most sensitive to deep pressures Experiments on Pancian corpuscles- relatively easy to isolate and test properties. We can control deformation by Piezo electric crystal- crystal changes shape in proportion to the voltage applied to it- this is then used to measure change in membrane permeability

-Also need tetrodotoxin to prevent action potentials by blocking VGNaC- this is because it would interfere with changes in permeability from the stimulus
-Studies found stimuli caused a depolarisation which can be explained by increased permeability to Na+
-Receptor Adaptation:
-Adaptation to static loading- shown in experiments in which pancinain corpuscle is compressed to a fixed level and this level is maintained for some time- resulting generator potential declines then to zero within few millisecond
-Readjustments in the structure of the receptors: Pancinian corpuscles is a viscoelastic structure-distorting forces transmitted by the viscous compoenent-within microseconds, fluid redistributes
-Electrical type of accommodation: progressive inactivation of the Na+
-Rapidly adapting receptors important- able to detect change in stimulus Adaptivity is due to the lamella

-When stimulus is applied the firing rate of neurons increases- it is the combination of such signals from different receptors that is integrated in the cerebral cortex as touching and feeling an object Meissner corpuscles
-Globular fluid filled structure that encloses flattened, lamellar cells-lamellae are coupled mechanically to the edge of the papillary ridge by collagen fibres-this results in fine mechanical sensitivity to frictional forces as hand is moved across surfaces
-Innervated by rapidly adjusting neurons-each axon innervates 10 to 20 Mesissner corpuscles-integrates information from several adjacent papillary ridges
-Each corpuscle receives 5 to 6 axons
-Function-detect events that produce low frequency, low amplitude skin motion- hand motion over surface of objects, detection of low frequency vibration
-Sensitive to lighter and softer touch as they don't have a developed lamella
-Found beneath the epidermis and coupled to the papillary ridge
-Not enclosed in a capsule- clear cells in the stratum basale of the epidermis and are associated with enlarged nerve endings Merkel cells
-Small epithelial cells that surround terminal branches of a slowly adapting axon
-Merkel cell encloses a semirigid structure-transmits compressive strain to sensory nerve
-Indentation foce of a flat/ genly curved suface is distributed symmetrically within the central contact zone

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