Slide 1Slide 2Slide 3Slide 4Slide 5Slide 6Slide 7Slide 8Slide 9Slide 10Slide 11Slide 12Slide 13Slide 14Slide 15Slide 16Slide 17Slide 18Slide 19Slide 20Slide 21Slide 22Slide 23Slide 24Slide 25Slide 26Slide 27Somatosensory SystemTouch - mechanoreceptionLimb posture movements and forces – proprioception(Haptics – object recognition through touch + proprioception, usually with the hand)Readings: Squires et al Ch 25Supplementary reading: Kandel et al Chs 21-23http://www.youtube.com/watch?v=FKxyJfE831QSA1RAPCSA1120-50 corpuscles/afferentTransmitted vibrationtexturesSlipping object/large rf’sHigh densityForm/brailleTexture-hardnessroughness??Stretch? + Hair follicle afferentsMost SensitivetodeformationReaction to slippage: 65 msec - 45 msec is peripheral nerve and motor response time.Applied forces in response to shape of grasped object - 100 msec.Mechanoreceptors Differ in Morphology and Skin LocationSA1RASAIImerkelpacinianmeissnerruffiniDorsal root ganglion cells serve a dual roleof transduction and information transmission.Multiple fibers from the axon branch toform a large receptive field.Receptive fields in the skin overlap.Large variation in two-point threshold across the body surfaceThe Spatial Resolution Varies Because the Density of Mechanoreceptors VariesRA mechanoreceptor responds to sinusoidal mechanical stimuli with a single action potential for each cycle.The lowest stimulus intensity that evokes one action potential per cycle of the sinusoidal stimulus is called the receptor's “tuning threshold.”25 Hz vibratory stimulusTuning thresholdsfor vibrationMerkel: 5-15 HzMeissner: 20-50 HzPacinian: 60-400 HzRA’s – lower thresholdsFigure 22-7 The shape and size of objects touching the hand are encoded by populations of Merkel disk receptors.A. The area of contact on the skin determines the total number of stimulated Merkel disk receptors in the population. The pink region on the fingertip shows the spread of excitation when probes of different diameters are pressed upon the skin with constant force. The intensity of color is proportional to the firing rates of the stimulated receptors. 1. A small-diameter, sharp probe activates a small population of Merkel receptors. However, the active receptors fire intensely be cause all of the force is concentrated at the small probe tip. 2. An intermediate-size probe excites more receptors but the peak firing rate in the population is reduced. The probe does not feel as sharp as the small-diameter probe. 3. A gently rounded, large-diameter probe stimulates a large population of receptors spread across the width of the finger. These receptors fire at low rates because the force is spread over a larger area of skin. (Adapted from Goodwin et al. 1995.)B. The firing rate of individual Merkel disk receptors signals the probe diameter. These recordings of action potentials fired by a Merkel disk receptor illustrate the responses evoked when probes of decreasing size are pressed on the center of the receptive field. All of the probes evoke a stro ng initial response as contact is made with the skin. The firing rate of the neuron during steady pressure is proportional to the curvature of each probe. The weakest responses are evoked by flat surfaces and gently rounded (large diameter) probes. The firing rate increases as the probe diameter becomes smaller. (Adapted from Srinivasan and LaMotte 1991 .)Figure 22-8 The firing patterns of mechanoreceptors in the superficial layers of the skin encode the texture of objects rubbed a cross the skin.A. 1. The nerve responses to textures are measured with the hand immobilized. The receptive field of a single receptor on a monkey's finger is stimulated with an embossed array of raised dots on a rotating drum. The pattern moves horizontally over the receptive field as the drum rotates. T he experimenter thus controls the speed of movement and the location of the dot pattern in the receptive field. The pattern is moved laterally on successive rotat ions to allow the dots to cross the medial, central, and lateral portions of the receptive field on successive rotations. The composite response of an individual ne rve fiber to successive views of the raised dots simulates the distribution of active and inactive nerve fibers in the population. 2. Sequential action potentials discharged by individual receptors during each revolution of the drum are represented in spatial event plots in which each action potential is a small dot, and each horiz ontal row of dots represents a scan with the pattern shifted laterally on the finger.B. Spatial event plots of three types of mechanoreceptors to dot patterns with different spacing. Slowly adapting Merkel disk rece ptors and rapidly adapting Meissner's corpuscles differentiate between dots and blank space when the spacing of the dots exceeds the receptive field diamet er. A receptor fires bursts of action potentials for each dot, spaced by silent intervals. As the dots are brought closer together, the resolution of individual dots blurs. Pacinian corpuscles do not distinguish texture patterns because their receptive fields are larger than the dot spacing. (Reproduced from Connor et al. 1990 .)The spatial resolution of detail within a pattern depends on the total area of skin innervated by each sensory nerve (see Figure 21-6). The Merkel disk receptors provide the sharpest resolution of spatial pattern, as each receptor axon monitors a single dot. Meissner's corpuscles also reso lve individual dots but the image of the pattern that they provide is not as sharp because they have slightly larger receptive fields. Pacinian corpuscles do not signal changes in surface contour because their large receptive fields encompass several dots in the textured surface. I nstead they fire continuously, measuring the speed at wh ich the hand moves across the surface. The P.441activity of Pacinian corpuscles provides timing information that allows the brain to convert the number of bursts per second fir ed by Meissner's corpuscles and Merkel disk receptors into spatial information about the number of dots per centimeter on the textured surface.The pure sensory experiences evoked by the stimuli used in the neurological examination—a light tap, pressure from a pin, or a s inusoidal vibratory stimulus—are quite different from the tactile sensations evoked by the complex natural stimuli that we usually encounter. Natural stimuli rar ely activate a single type of
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