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The Somatosensory System

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The Somatosensory General Topics System Chapter 3 in text • Touch • Thermal sensations • Proprioception & Kinesthesis • Pain 1 2 1 2 Touch: Subtopics The Many Stimuli of Touch The stimuli of touch • Somatosensory neurones respond to • physical changes in skin, mainly involving: • Anatomy of the skin • Pressure (stretch & compression) • Types of mechanoreceptors • Vibration (movement) • Brain areas processing touch sensations • Temperature (cold or heat) • Tactile acuity and vibration sensitivity • These combine to elicit higher-order • Haptic object perception sensations such wetness or smoothness 3 4 Pressure Vibration • Measured in Pascals (N/m2) or, if area is kept constant (and gravity), can be measured in grams (g) • Variations in skin pressure across time • Pressure thresholds vary depending on many factors, such • Frequency of vibration is measured in Hertz speed and depth/distance of skin deformation, body (Hz) = cycles/second location, area of skin affected, etc. • Amplitude of vibration measured in meters An Esthesiometer (usually µm) • The basis for the sense of movement across the skin, which is important in (e.g.) tool use 5 6 Temperature Pain • Measured in degrees Celsius (C°) • Excessive intensities of pressure, vibration, and temperature (as well as pH) produce Decreases or increases from physiological • painful sensations. zero (≈32 C°) are detected respectively by cold and warmth thermoreceptors. • The pain threshold varies according to many factors. • Threshold for detection depends on rate of change (C°/s) and area of skin (cm2) • Pain is in general a very complex and subtle affected, among other things. topic. 7 8 Skin & (some) Mechanoreceptors Anatomy of Skin • Heaviest and largest sense organ by far • Two main layers: • Epidermis: Outer layer, made up mainly of keratinized skin cells • Dermis: Below the epidermis. Contains three categories of mechanoreceptors • Two types of skin: Hairy and hairless (no, shaving doesn’t change the type), with different elastic properties. 9 10 9 10 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 w/ Mechanoreceptors Accessory Structures Meissner corpuscle • Merkel Disc: Dendrites from mechanoreceptor • surround a separate skin cell called a Merkel cell • Ruffini corpuscle • Root Hair Plexus: Dendrites surround follicle • Pacinian corpuscle • These neurones lack a specialized ending, instead • Each of these is a neurone with a specialized relying on contact with a separate structure. ending growing out of its dendrite(s) (we will see this idea again in proprioception) 13 14 13 14 Free Nerve Endings Questions? • Simply neurones with extensions* that terminate close to skin’s surface • No corpuscle or accessory structure Some are mechanoreceptors, but: • What are the three main types of • mechanoreceptors? Some respond to heat & cold • What are the two main layers of skin? (thermoreceptors) • • Others signal various sorts of pain (nociceptors) * FNE “extensions” are not (strictly speaking) dendrites, for reasons we’ll get into later... 15 16 From Skin to Spine DRG Neurones • When appropriately stimulated, • Come in two basic types: A (myelinated) and C (unmyelinated) mechanoreceptors produce action potentials • The A type is further broken down into • These are transmitted to the spine by dorsal • α (alpha): Big diameter, very fast transmission. Connect to root ganglion neurones, a type of bipolar neurone proprioceptors (about which, more later). • β (beta): Medium diameter and transmission speed, carry info from all mechanoreceptors but free nerve endings • δ (delta): Small diameter, slow transmission. Carry signals from some free nerve endings (e.g. cold thermoreceptors) • C fibres: Smallest and slowest (up to 4s from toe to brain), 100 times slower than Aα. Carry pain and warmth signals. 17 18 DRG Neurones DRG Neurones FA and SA further break down based on depth in skin: • Another way to classify • DRG neurones is by how • FA-1 & SA-1 are found near surface. Have small receptive fields fast they adapt to stimuli. Two basic types: • FA-II & SA-II are found deeper. Have large receptive fields • Fast adapting (FA): Respond at onset and offset of stimulus • Slow adapting (SA): Respond continuously & Stretch to stimulus FA-II 19 20 Fundamental Concept: Centre-surround RF Receptive Field • Area of skin or retina that affects firing of a • The area of a sense organ affecting the firing of a given given neurone. neurone • Applies to all sense organs: Skin, retina, inner ear, etc. • Has either an ! excitatory centre/inhibitory surround • Determined by measuring neurone firing via microelectrode and then hunting around on the surface for areas that affect ! inhibitory centre/excitatory surround firing rate. – • Can have excitatory and inhibitory components, often in “centre/surround” organization. – + + – + • Note that RFs overlap one another and that a given receptor + is usually part of multiple higher-level RFs + + – – + – – – – + + 21 22 Fundamental Concepts: Temporal & Spatial Resolution RF Size & Resolution Spatial Resolution: How many receptors are • • The size of the RF there across an area of the sense organ? determines the upper limit Determines how precisely one can know of spatial resolution. where a stimulus happened. • Think about pixel size and • Temporal Resolution: How often does a resolution on a monitor given receptor respond to stimuli? • RF size does not (directly) Determines how precisely one can know affect temporal resolution. when a stimulus happened. 23 24 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 off Meissner Small High Fast High 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 25 26 Review: Anatomical Directions Review: Anatomical Directions in the Brain & Spine 27 28 Review: Anatomical Directions in the Brain & Spine Questions? • What is a receptive field? • In what direction are ones arms relative to one’s sternum (two possible right answers). • What is the relationship between RF size and spatial resolution? 29 30 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 31 31 32 DRG Neurones (Aα & Aβ) (Aδ & C) 34 33 34 Fundamental Concept: Fundamental Concepts of Serial & Parallel Processing Sensory System Organization • Contralateral Processing: Sensations from left side of • Serial processing: Neurones connect to one another in body cross over to right side of brain, and vice versa sequence. E.g., DRG neurones connect to spinal ones, 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) involved. • Parallel processing: Several streams or channels of neurones, each dealing with different aspects of perception, • Topographic Organization: Neurones from adjacent bring information to the brain simultaneously. The channels parts of sensory organs synapse with adjacent neurones tend to be only semi-independent, however. in brain modules (somatotopic, retinotopic, etc.) • This kind of arrangement is found in all sensory systems. • However, the size of areas of brain modules do not correspond directly to sizes of areas of sense organs 35 36 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 37 38 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 area, which in turn send projections to many parts of the brain • This diffuse connectivity reflects the importance of pain signals 39 40 Summary of Connections in Subareas of Area S-1 Somatosensory System (peripheral) Skin DRG Spine Subcortical Cortical Aβ dorsal Internal Mechanoreceptors column capsule VPN of Free Nerve Endings medial S1 Aδ Thalamus lemniscal Various Proprioceptors patway Tactile Aα Nociceptors Medulla Tactile & C antero- Proprioceptive Heat receptors Tactile C lateral Midbrain Cold receptors pathway Aδ Proprioceptive 41 42 Summary of Connections in Fundamental Concept: Somatosensory System (cortical) Integration • Although sensory signals are initially processed in Mechanoreceptors S1 3b S1 1 PPC separate parallel streams, they eventually rejoin Free Nerve Endings
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