Mapping the human somatosensory cortex the sensory homunculus

Mapping the human somatosensory cortex – the sensory homunculus – perception of touch, temperature, pain, proprioception, kinesthetics, haptics, sexual sensation, tickle, & itch The model represents what the body would look like (in this case a male body) if it grew in proportion to the area of the cortex that was dedicated to its sensory perception

Big Picture of the Somatosensory Systems: types of “sensing” • • Cutaneous – related to skin Tactile – related to touch Haptic – related to search Sensations: – Discriminative touch (haptic search) – Touch related to the skin – Pressure and vibration

Big Picture of the Somatosensory Systems: types of “sensing” • Kinesthesis: – Sensation “below the surface” – The sense of position or location in space of the limbs and body parts – The sense of movement of the limbs and body parts – Muscle stretch, joint movement and tendon tension • Proprioception: – All of the above – But, integrated with information from the vestibular system (Balance)

Big Picture of the Somatosensory Systems: types of “sensing” • Thermoreception & pain: – Temperature – Pain – Tickle and itch

Cutaneous sensation – stimulation of receptors in the skin: The largest and the heaviest organ of the body Protection from bacteria & disease Maintains the integrity of our body (contains our fluids) Informs us about our immediate surroundings (touch) Cross-section of skin

“Mechanoreceptors” process different types of tactile, proprioceptive and kinethetic stimulation * Located in the Epidermis & Dermis * Mechanical (physical) change of the outer cells activate internal nerve endings (transduction from mechanical to neural impulse) Cross-section of skin

Slow Adapting (SA) Rapid Adapting (RA) Firing Stimulus on Merkel receptor Ruffini cylinder off on off Meissner corpuscle Pacinian corpuscle

• Mechanoreceptors – Merkel receptor: located between Epidermis & Dermis • Sensitive to ongoing pressure on the skin, particularly where stimulation is “tonic” (ongoing, maintained) such as in the fingertips; used in processing pressure. • Action potential caused by deformation of myelinated nerve ending • Slow Adapting (SA 1) with small receptive field; fires in “continuous stimulation”

Copyright © 2002 Wadsworth Group. Wadsworth is an imprint of the Wadsworth Group, a division of Thomson Learning Fine detail of touch is processed by Merkel receptors (slow adapting)

• Mechanoreceptors – Meissner corpuscle: tactile corpuscle located in Dermis just below the Epidermis • Sensitive to light touch, “flutter” (vibration) or tapping on the skin: fingertips, palms, soles, lips, tongue, face, nipples, genitalia • Action potential caused by deformation and reformation of unmyelinated nerve ending • Rapid Adapting (RA 1) with small receptive field; fires in “onoff” bursts

• Mechanoreceptors – Ruffini Cylinder: located in Dermis • Sensitive to stretching of the skin or movements of the joints • Action potential caused by deformation of the nerve ending • Slow Adapting (SA 2) with large receptive field; fires in continuous stimulation

• Mechanoreceptors – Pacinian corpuscle: located deeper in the dermis, near joints, on internal organs • Sensitive to deep & diffuse pressure & vibrations • Action potential caused by deformation and reformation of unmyelinated nerve ending that are in the middle of the corpuscle (outer layered mechanism)

Pacinian corpuscle • Rapid Adapting (RA 2) with large receptive field; fires in “on-off” bursts • “Graded response” – small or greater deformations associated with smaller/greater response • The larger the deformation/reformation the higher the firing frequency of the neuron

Surface receptor – small receptive field RA Meissner: fine vibration – recognize texture Hair: responds to a localized “flutter” SA Merkel: sensing fine details of a surface or edge Deep receptor – large receptive field Pacinian: a diffuse vibration (holding a power saw) or deep pressure (when firmly grabbed and released) Ruffini: a skin stretch (sensing the position of your fingers)

Thermoreceptors & Temperature • Receptors – Warm fiber – Cold fiber • Rate of neural firing determines the degree of increased or decreased temperature • Firing rate continues as long as the stimulus is present

Cold fibers fire “best” to 30 -deg (range of 20 -to-45 -deg C) Impulses per second 15 10 Warm fibers fire “best” to 44 -deg (range of 30 -to-48 -deg C) (remember: body temperature is 37 -deg C) Warm fibers Cold fibers 5 20 25 30 35 40 Temperature (deg-C) 45 50

Tactile Acuity: Merkel receptors account for much of the information that is disproportionately represented in the cortex Two-point threshold test: what is the smallest distance between two different points on the body that can be discriminated as different? Compare your: Finger tips, Lips & Side of the hip

Size of the “receptive field” correlates with the cortical magnification factor

Size of the receptive field determines fineness of acuity (i) Large fields more insensitive: near touch more likely to activate same receptive field; (ii) Small fields more sensitive: near touch more likely to activate different receptive fields neural firing from 1 receptive field no response neural firing from both receptive field

Notice that the size of the receptive field correlates with the receptors depth in the skin Fast Adapting Slow Adapting

Haptic Perception: Three-dimensional exploration of objects with the hands • Haptic Perception: information-knowledge acquired from sensations in skin, muscles, tendons & joints involving “active exploration” • Active v. passive touch: whose doing the touching? You on the world or the world on you. – Passive touch: “I feel a pricking sensation on my skin” – Active search: “I feel a pointed object”

Human Haptics: perception of 3 -D objects with the hands Haptic feedback is comprised of both tactile (touch) and force (pressure) that is always sensed when physically interacting with the hands Lateral motion Pressure Enclosure Static contact Unsupported Holding Contour Following Texture Hardness Global shape Temperature Weight Precision shape

Factors contributing to specificity of haptic information processing • Type of stimulation and receptors’ specialization for processing that information – “Object Affordance” – what an object “tells you” • Size of the receptive field (small field more sensitive) Examples of Exploratory Procedures (EPs) of active touch (Lederman & Klatzky, 1987) – usually only use 1 or 2 methods to explore an object.

Object recognition, mechanoreceptors and cortical organization How long would it take you to “recognize” these objects in your hands?

Factors contributing to specificity of haptic information processing • Cortical magnification factor (the homunculus) – Approximately 10 different “maps” as a function of different types of information – Form v. texture v. direction of movement, etc. represented by different homunculi – Cortical cells are similar in action to visual simple and complex cells (showing sensitivity to edges, ends of stimuli, etc. )

Factors contributing to specificity of haptic information processing • Neurons organized with a Center-Surround onset-offset organization (also occurs in many areas not involved with haptics) – Throughout the thalamus and somatosensory cortex

Object recognition, mechanoreceptors and cortical organization • Many sources of information are coming from various receptors in time-space synchrony • Texture, hardness, orientation signaled by different receptors & receptive fields, differing CNS pathways, differing mid-brain & cortical receptive areas, etc. in simultaneous orchestration • Typically, most people can recognize most objects in a matter of one or two seconds

Tracts from receptors through the Central Nervous System (CNS) • Lemniscal System (dorsal-column mediallemniscal pathway) – Phasic sensations (recurring cycle; vibrations) – Sensations of movement against the skin – Fine positional and pressure sensations • Proprioception – Touch sensations • High degree of localization and specificity of stimuli • Fine graduations in intensity of stimuli

lemniscal system pathway • Begins with somatosensory axons entering the spinal cord via the dorsal root • Ascends in the dorsal columns ipsilaterally • First synapse point for this pathway is in the dorsal column nuclei located in the medulla • Axons of neurons originating in the dorsal column nuclei decussate (cross over) to contralateral side • Axon then ascends via the Medial Lemniscus to the contralateral ventral posterior thalamic nucleus (VPN) • The majority of VPN neurons project to the primary somatosensory cortex (S 1) • Remaining neurons project to the secondary somatosensory cortex (S 2) of the posterior parietal lobe

Summary: lemniscal system pathway (in your book: dorsal-column-medial-lemniscal (DCML) pathway) • Secondary somatosensory cortex (S 2) • Primary somatosensory cortex (S 1) • Ventral posterior thalamic nucleus (VPN) • Dorsal column nuclei in medulla • Dorsal Root ganglion of spinal cord

The somatosensory map on the cortex – the “homunculus”

The somatosensory map on the cortex – the sensory “homunculus”

Tracts from receptors through the Central Nervous System (CNS) • Spinothalamic System – Thermal sensations: cold and warm – Pain sensations – Crude (diffuse) pressure and touch sensations – Tickle and itch sensations – Sexual sensations

Spinothalamic System Pathway • Somatosensory axons entering the spinal cord via the dorsal root and synapsing upon entry • The majority of these 2 ndorder axons decussate, and ascend to the brain via the anterolateral portion of the spinal cord white matter • This ascending system is composed of three separate tracts: – spinothalamic tract – spinoreticular tract – spinotectal tract

Spinothalamic System Pathway • The spinothalamic tract projects to the ventral posterior nucleus of the thalamus – This tract is involved in the perception of touch, temperature, and sharp pain • The spinoreticular tract projects to the brain stem reticular formation on its way to the parafasicular nucleus and intralaminar nucleus of the thalamus – This pathway seems to be selectively involved in the perception of deep, chronic pain • The spinotectal tract projects to the tectum of midbrain – This tract is likely involved in some aspect of pain perception (not yet understood) • The tracts of the Spinothalamic System project to both the primary and secondary somatosensory cortex, and to more posterior locations within the parietal lobe

Object recognition, mechanoreceptors and cortical organization • Many sources of information are coming from various receptors in time-space synchrony • Texture, hardness, orientation signaled by different receptors & receptive fields, differing CNS pathways, differing mid-brain & cortical receptive areas, etc. in simultaneous orchestration • Typically, most people can recognize most objects in a matter of one or two seconds

Perception of pain • Pain defined (International Association for the Study of Pain): – “Pain is an unpleasant sensory and emotional experience associated with actual or potential tissue damage, or described in terms of such damage (Merskey, 1991)” • Cannot be fully explained by physical dimensions alone (sensory & emotional) – Sensory experience: throbbing, hot, sharp, dull – Emotional experience: annoying, torturing, excruciating

Perception of pain • Proof of the multimodal nature of pain? Clinicians’ treatment of pain symptoms – Cutting nerve fibers in pathways from receptors to higher cortical levels do NOT eliminate pain perception – Best evidence: phantom limb findings • Cognitive factors in pain perception – Expectations – preparation helps – Shifting attention – distraction from pain – Content of emotional distraction – thinking “peaceful thoughts” – Individual differences in pain perception – differences in tolerance and experience

Perception of pain: Melzack & Wall’s “Gate Control Theory” • Pain perception can be affected by psychological dimensions (i. e. , expectations) and competing stimulation of pain pathways (i. e. , simultaneous stimulation of cutaneous receptors) • Two types of fibers in the Dorsal Root Ganglion of the spinal cord – Short fibers carry pain information – Long fibers carry tactile/non-pain information – Activation of long fibers competes with activity of short fibers (hence, diminishing perception of pain) • Pain perception pathways far more complex than first described by Melzack & Wall, but central thesis remains basic: competing gating mechanisms