Slide 1Slide 2Punctate nature of cutaneous sensationInnervation-transductionShingles: disease of DRGWhat activates the virus?Slide 7TransductionAnatomical classification of first order fibersFiber typesInnervation densityLocalization and 2 point thresholdBasis of 2 point discriminationSomatosensory pathwaysOrganization of dorsal column systemSlide 16Slide 17Somatosensory cortexMap in somatosensory cortexReceptive fields in somatosensory cortexPlasticity in somatosensory cortexSOMATIC SENSATION1CUTANEOUS (SKIN) DEEPtouch pressurepain painwarm position coolSeparate and independent neural systems for each modality from periphery up to cortex.How are the cutaneous modalities distributed across the skin?Modalities of somatic sensation:qualitatively different sensations2Any point on skin gives rise to one and only one sensation, T, C, W, P.T C PW1 mmWhy is this true? There is a physiological/anatomical explanation: look at how the skin is innervated.Punctate nature of cutaneous sensation3Innervation-transductionDEFINITION: axons that innervate the body are the primary afferent fibers=first order fibers. Cell bodies are in the DRG (Dorsal Root Ganglion), axons branch: one end to periphery other into the spinal cord.Transduction is accomplished by channels at the fiber ending, no separate class of receptor cells. 4Shingles causes a painful rash with blisters. It is caused by the varicella virus, the same virus that causes chickenpox (it is a herpes virus, also called herpes zoster).The virus lies dormant in the DRG butmay reactivate as shingles. It travels down the sensory nerve fibers and affects the skin.Shingles: disease of DRG5Rash is seen in skin innervated by that nerve, sometimes with severe pain.Skin can remain painful and sensitive to touch for months or even years after the rash clears up “postherpetic neuralgia.”Conditions that weaken the body's immune system, such as aging, cancer, or certain drugs increase the chance that the virus will become active again.Not known why any particular DRG is affected nor what happens in the skin to cause the rash.Half of the nearly 1 million shingles cases in the United States each year occur in people aged 60 years and older.It is treated with antiviral drugs (there is a vaccine).What activates the virus?64 functional types1. Pain=nociceptors (noxious= damaging stimuli)2. Touch=mechanoreceptors3. Warm and 4. Cool=thermoreceptorsFunctional classification of primary afferent fibers innervating the skin7Fibers of different types are intermingled in the skin; sensation at each point determined by the kind of fiber that is innervating that point.warm fibercool fibertouch fiberpain fiberFibers for the different modalities differ at a microscopic level, channels specialized so that they open to different kinds of stimulus energy (depolarization, if stimulus intense enough, action potentials).Transduction83 anatomical types, fiber diameter and conduction velocity, correlate with function. myelinated: 1. A, 6-12 m, 30-70 m/secmechanoreceptors (touch, also pressure and position)some have “encapsulated endings”Ruffini endingsMerkel disksPacinian corpusclesKraus end bulbs Anatomical classification of first order fibers9Fiber types102. A,1-5 m, 12-30 m/secnociceptors (pain), thermoreceptors (warm, cool); all end as “free nerve endings.”unmyelinated:3. C, 0.4-1.2 m, .5-2 m/secnociceptors, thermoreceptors“free nerve endings”(assessing cv is important clinically in cases of nerve damage, “neuropathy”)fingertips, lipsbackInnervation density = how many axons/area of skin.Varies over the body surface, and correlates with somatosensory sensitivity.Innervation density11Localization –varies over body surface.2 point threshold also varies and is better in areas of higher innervation density.Receptive fields of axons or neurons in the somatosensory system: region of skin where appropriate stimulus affects the activity of axon or cell.Receptive field size of all cellsin the system varies over the bodysurface, smallest in regions of highest innervation density.Localization and 2 point threshold12High innervation density, smaller receptive fields, better 2 point discriminationLower innervation density, larger receptive fields, worse 2 point discrimination.Basis of 2 point discrimination13Somatosensory information from the skin and deep tissue travels to the brain in 2 anatomical systems.Different modalities are carried by the 2 systems. Dorsal Column Medial- Lemniscal Systemtouch, pressure, positioninput from A fibers, (skin and deep tissue)Anterolateral systempain, temperature, crude touchinput from A and C fibers, (again from skin and pain from deep tissue).Somatosensory pathways14There are orderly maps of the body at all levels, as in visual system (somatotopic organization). At the level of the spinal cord and medulla, maps are of the IPSILATERAL half of the body.At the level of thalamus and cortex, maps are of the CONTRALATERAL half of the body.Where do axons cross the midline?Organization of dorsal column system15Gracile and cuneate nuclei, medulla16Dorsal column-medial lemniscal systemSpinal cord: ascending tracts17Parietal lobe, postcentral gyrus, areas 3,1, 2 = S1 (somatosensory area 1)(numbered areas from anatomical studies)Occipital lobe174Somatosensory cortex18Sensory “homunculus” in somatosensory cortex, representation of the contralateral half of the body.Map is distorted with an expanded representation of areas with higher innervation density.coronal section through left somatosensory cortexMap in somatosensory cortex19Definition: area of body from which a single cell gets information, stimulus affects firing rate.SMALLER for areas with expanded representation and higher peripheral innervation density.LARGER for areas with smaller representations and lower peripheral innervation density.Receptive fields in cortex are larger than the receptive fields of primary afferent fibers as a consequence of convergence at every level of the system.Receptive fields in somatosensory cortex20“Plasticity”- changes in organization of the map of the body.How can this happen?1. denervation of periphery (e.g. amputation of finger).2. use, especially with learning and motivation, themap can reorganize.Plasticity in somatosensory