#14113 - Sterogenesis - Neuroscience 1

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 MECHANORECEPTORS- responsible 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

-Mechansim

-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+ channels

-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

-Force applied by an object boundary displaces the skin asymmetrically-enhances responses from receptive fileds located along edges of an object

Ruffini endings

-Capsulated dendritic endings-innervate slowly adating axons

-Deep below epidermis-located along the palmar folds or at finger joints

-Provide information about shape of large objects grasped with entire hand- all five fingers press an object against the palms

Adaptability

-All receptors display adaptability

-If they are continuously stimulated they stop firing- the faster they adapt the faster they stop firing. Mechanism prevents permanent stimulation elicing a response- when wearing clotes we don’t notice that we are wearing them

Receptive field: The area over which a stimulus elicits a response

-This is linked to the depth of the receptor below the epidermis- the deeper a receptor is the wider an area it will respond to

-More superficial receptors- merkel discs and Meissner corpuscles are better at sensing fine touch but over a small area.

-Tactile acuity- ability to resolve spatial details is highest on fingertips-receptive field are smallest- used in braille reading-each braille dot stimulates different slowly adapting fibres-specific combinations of SA1 fibres fire synchronously signal spatial arrangement of Braille dots

Dorsal column medial lemniscus pathway

tabes dorsalis, where syphilitic infection causes degeneration of the myelin sheath of neurons in the dorsal columns of the spinal cord. This leads to deficits in touch but little loss of other sensation (e.g. pain temperature).

-Route from afferent neurons from receptors in skin to the brain- dorsal column medial leminiscus pathway

-Afferent neurons have cell body in dorsal root ganglia – enter spinal cord through dorsal root

-From sacrum to the lower thoracic level- nerves form the gracile fascicle medially

-Upper thoracic and cervical nerves form the cuneate fascicle laterally

-These two bundles form the dorsal column and terminate by synapsing with neurons in the gracile and cuneate nuclei in the medulla oblongata- the lower part of the brainstem

-Dorsal coloumn nuclei- single impulses evoked in peripheral afferent information is faithfully reproduced in Dorsal coloumn nuclei

-Secondary neurons from these 2...