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The Somatosensory System

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The Somatosensory System The Somatosensory General Topics System Chapter 3 in text • Touch • Thermal sensations • Proprioception & Kinesthesis • Pain 1 2 Touch Teaches Us About Touch: Subtopics Other Senses • The stimuli of touch • Multimodality • Anatomy of the skin • Transduction • Types of mechanoreceptors • Parallel vs. Serial processing • Brain areas processing touch sensations • Receptive fields • Tactile acuity and vibration sensitivity • Temporal and spatial resolution • Haptic object perception 3 4 The Many Stimuli of Touch Pressure Somatosensory neurones respond to • 2 physical changes in skin, mainly involving: • Measured in Pascals (N/m ) or, if area is kept constant (and gravity), can be measured in • Pressure (stretch & compression) grams (g) • Vibration (movement) • Pressure thresholds vary depending on many Temperature (cold or heat) factors, such speed and depth/distance of • skin deformation, body location, area of skin • These combine to elicit higher-order affected, etc. sensations such wetness or smoothness 5 6 Vibration Temperature • Variations in skin pressure across time Measured in degrees Celsius (C°) • Frequency of vibration is measured in • Hertz (Hz) = cycles/second • Decreases or increases from physiological • Amplitude of vibration measured in zero (≈32 C°) are detected respectively by meters (usually µm) cold and warmth thermoreceptors. The basis for the sense of movement • Threshold for detection depends on rate • 2 across the skin, which is important in (e.g.) of change (C°/s) and area of skin (cm ) tool use affected, among other things. 7 8 Pain Anatomy of Skin • Heaviest and largest sense organ by far • Excessive intensities of pressure, vibration, • Two main layers: and temperature (as well as pH) produce • Epidermis: Outer layer, made up mainly of painful sensations. keratinized skin cells • The pain threshold varies according to • Dermis: Below the epidermis. Contains three many factors. categories of mechanoreceptors Pain is in general a very complex and • Two types of skin: Hairy and hairless (no, shaving • doesn’t change the type), with different elastic subtle topic. properties. 9 10 Skin & (some) Mechanoreceptors Mechanoreceptors • Sensory neurones. Signal pressure on skin • Respond to mechanical deformation of the receptor or associated structure • Three main categories: • Encapsulated • Accessory-structure-associated • Free nerve endings 11 12 Encapsulated Mechanoreceptors • Meissner corpuscle • Ruffini corpuscle • Pacinian corpuscle • Each of these is a neurone with a specialized ending growing out of its dendrite(s) 13 14 Mechanoreceptors w/ Free Nerve Endings Accessory Structures • Simply neurones with extensions* that • Merkel Disc: Dendrites from mechanoreceptor terminate close to skin’s surface surround a separate skin cell called a Merkel cell • No corpuscle or accessory structure • Root Hair Plexus: Dendrites surround follicle • Some are mechanoreceptors, but: • Some respond to heat and/or cold • These neurones lack a specialized ending, instead (thermoreceptors) relying on contact with a separate structure. (we will see this idea again in proprioception) • Others signal various sorts of pain (nociceptors) * FNE “extensions” are not (strictly speaking) dendrites, for reasons we’ll get into later... 15 16 Questions? From Skin to Spine • When appropriately stimulated, mechanoreceptors produce action potentials • These are transmitted to the spine by dorsal root ganglion neurones, a type of bipolar What are the three main types of • neurone mechanoreceptors? • What are the two main layers of skin? 17 18 DRG Neurones DRG Neurones • Come in two basic types: A (myelinated) and C (unmyelinated) The A type is further broken down into • Another way to classify • DRG neurones is by • α (alpha): Big diameter, very fast transmission (≈100 m/s). how fast they adapt to Connect to proprioceptors (about which, more later). stimuli. Two basic types: β (beta): Medium diameter and transmission speed (≈50 m/s), • Fast adapting (FA): carry info from all mechanoreceptors but free nerve endings • Respond at onset and • δ (delta): Small diameter, slow transmission (≈20 m/s). Carry offset of stimulus signals from some free nerve endings (e.g. cold thermoreceptors) • Slow adapting (SA): Respond • C fibres: Smallest and slowest (≈.5 m/s, so up to 4s from toe to continuously to brain), 100 times slower than Aα. Carry pain and warmth signals. stimulus 19 20 Fundamental Concept: DRG Neurones Receptive Field • FA and SA further break down based on depth in skin: • The area of a sense organ affecting the firing of a given • FA-1 & SA-1 are found near surface. Have small receptive fields neurone • FA-II & SA-II are found deeper. Have large receptive fields • Applies to all sense organs: Skin, retina, inner ear, etc. • Determined by measuring neurone firing via microelectrode and then hunting around on the surface for areas that affect firing rate. • Can have excitatory and inhibitory components, often in “centre/surround” organization. & Stretch • Note that RFs overlap one another and that any given receptor is usually part of multiple higher-level RFs FA-II & c 21 22 Fundamental Concepts: Centre-surround RF Temporal & Spatial Resolution • Area of skin or retina that affects firing of a given neurone. • Spatial Resolution: How many receptors • Varies in size and has either an are there across an area of the sense excitatory centre/inhibitory surround organ? Determines (in part) how precisely inhibitory centre/excitatory surround one can know where a stimulus happened. – • Temporal Resolution: How often does a given receptor respond to stimuli? – + + – + Determines how precisely one can know + + + – – + when a stimulus happened. – – – – + + 23 24 RF Size & Spatial RF Size & Spatial Resolution Resolution • The size of the RF Stimuli determines the upper limit of spatial resolution. Receptive • Think about pixel size and Fields resolution on a monitor Mechano- • RF size does not (directly) receptors affect temporal resolution. 25 26 Temporal vs. Spatial Resolution & The Mechanoreceptors Temporal vs. Spatial Resolution Spatial Temporal RF size Adaptation Resolution Resolution Merkel Small High Slow Low • Why not just Meissner? Resolution isn’t the whole story. Sensitivity is often traded Meissner Small High Fast High off with resolution. • Also, some receptors are specialized for Ruffini Large Low Slow Low particular important types of stimuli (e.g., Ruffini endings detect skin stretch) Pacinian Large Low Fast High Free nerve Varies Varies Varies Varies endings 27 28 Review: Anatomical Directions Review: Anatomical Directions in the Brain & Spine 29 30 Review: Anatomical Directions in the Brain & Spine Questions? • What is a receptive field? • In what direction are one’s arms relative to one’s sternum (two correct answers). • What is the relationship between RF size and spatial resolution? 31 32 From Skin to Brain: Dorsal Root Ganglion Parallel Pathways • Dorsal Column-Medial • Anterolateral pathway lemniscal pathway • large fibres (Aα & Aβ) • small fibres (Aδ & C) • cross over the medial • cross over in the spine lemniscus of brainstem • carry kinesthetic and • carries temperature tactile information and pain information • fast conduction • slow conduction • evolutionarily newer • evolutionarily older 33 34 DRG Neurones (Aα & Aβ) Anterolateral (Aδ & C) 35 36 Fundamental Concept: Fundamental Concepts of Serial & Parallel Processing Sensory System Organization • Serial processing: Neurones connect to one another in • Contralateral Processing: Sensations from left side of sequence. E.g., DRG neurones connect to spinal ones, body cross over to right side of brain, and vice versa which connect to ones in thalamus, which connect to S-I, etc. • However, signals are then communicated to ipsilateral side, so that the whole brain is (normally) • Parallel processing: Several streams or channels of involved. neurones, each dealing with different aspects of perception, bring information to the brain simultaneously. • Topographic Organization: Neurones from adjacent The channels tend to be only semi-independent, however. parts of sensory organs synapse with adjacent neurones in brain modules (somatotopic, retinotopic, • This kind of arrangement is found in all sensory systems. etc.) 37 38 Somatotopic Organization • Throughout the somatosensory system, adjacent neurones/fibres carry signals from adjacent parts of the body • It didn’t have to be this way (information doesn’t care where it is located) but evolutionarily this is the way it turned out • The size of brain areas is related to number of receptors in an area, not the size of the area 39 40 Subcortical Regions in Somatosensory Cortex Touch • Spinal neurones from the medial lemniscal pathway (touch), terminate in the ventral posterior nucleus (VPN) of the thalamus • All senses have thalamic relay nuclei except smell • Anterolateral pathway (pain, temperature) neurones terminate in several subcortical areas, which in turn send projections to many parts of the brain • This diffuse connectivity reflects the importance of pain signals 41 42 Fundamental Concept: Subareas of Area S-1 Integration • Although sensory signals are initially processed in separate parallel streams, they eventually rejoin Presumably they interact to perform more Tactile • complex, higher-order sensory analyses Tactile & Proprioceptive • Example: Subarea 2 of S-I integrates Tactile proprioceptive and tactile stimuli. • Integration is seen in all sensory systems. Proprioceptive 43 44 Organization of Neurones Questions? within Somatosensory
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