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03 Sensibilita Somatica Psychophysical laws Legge di Weber: ΔS=K*S Legge di Fechner: I=K*log(S/S0) 450 Part V / Perception Sensory receptors Vision Smell Taste Touch Thermal senses Pain Hearing Balance Proprioception Figure 21–1 The major sensory modalities in humans are (touch, hearing, balance, and proprioception). The classic fve mediated by distinct classes of receptor neurons located in senses—vision, smell, taste, touch, and hearing—and the specifc sense organs. Each class of receptor cell transforms sense of balance are mediated by receptors in the eye, nose, one type of stimulus energy into electrical signals that are mouth, skin, and inner ear, respectively. The other somatosen- encoded as trains of action potentials. The principal receptor sory modalities—thermal senses, pain, and proprioception— cells include photoreceptors (vision), chemoreceptors (smell, are mediated by receptors distributed throughout the body. taste, and pain), thermal receptors, and mechanoreceptors the complex forms that are the basis of cognition. Sen- specialized receptors. The sensory information is sory pathways are also recursive. The higher centers in transmitted to the central nervous system by trains of the brain modify and structure the incoming fow of action potentials that represent particular aspects of sensory signals by feeding information back to earlier the stimulus. The question that has intrigued philoso- stages of processing; thus percepts are shaped by inter- phers and scientists alike is whether experienced sen- nal as well as environmental factors. sations accurately refect the stimuli that produce them In each sensory modality a specifc type of stim- or whether our knowledge of the world is inherently ulus energy is transformed into electrical signals by subjective and imprecise. Sensory460 receptors Part V / Perception Table 21-1 Classifcation of Sensory Receptors Sensory system Modality Stimulus Receptor class Receptor cells Visual Vision Light (photons) Photoreceptor Rods and cones Auditory Hearing Sound (pressure Mechanoreceptor Hair cells in cochlea waves) Vestibular Head motion Gravity, acceleration, Mechanoreceptor Hair cells in vestibular and head motion labyrinths Somatosensory Cranial and dorsal root gan- glion cells with receptors in: Touch Skin deformation and Mechanoreceptor Skin motion Proprioception Muscle length, muscle Mechanoreceptor Muscle spindles and joint force, and joint angle capsules Pain Noxious stimuli Thermoreceptor, All tissues except central (thermal, mechanical, mechanoreceptor, and nervous system and chemical stimuli) chemoreceptor Itch Histamine Chemoreceptor Skin Visceral (not Wide range (thermal, Thermoreceptor, Gastrointestinal tract, uri- painful) mechanical, and mechanoreceptor, and nary bladder, and lungs chemical stimuli) chemoreceptor Gustatory Taste Chemicals Chemoreceptor Taste buds Olfactory Smell Odorants Chemoreceptor Olfactory sensory neurons Multiple Subclasses of Sensory Receptors Are based on physiological experiments (the white and Found in Each Sense Organ black curves in Figure 21–7A). The tuning curve shows the range of sensitivity of the receptor, including Sensory receptors are found in specialized epithe- its threshold, the minimum stimulus intensity at which lia called sense organs, principally the eye, ear, nose, the receptor is activated. For example, blue cones tongue, and skin. The arrangement of receptors in an in the retina are most sensitive to light of 437 nm; for organized structure allows further specialization of that reason, they are also termed S or short-wavelength function within each sensory system. receptors. Green cones, termed M receptors for their Each major sensory system has several constitu- sensitivity to middle wavelengths, respond best to ent qualities or submodalities. For example, taste can be 533 nm; red cones, the L or long-wavelength recep- sweet, sour, salty, or bitter; objects that we see differ in tors, respond most vigorously to 564 nm wavelengths. color; and touch has qualities of temperature, texture, The blue, green, and red cones respond to other and rigidity. Submodalities exist because each class of wavelengths of light but these responses are weaker receptors contains a variety of specialized receptors (see Chapter 26). that respond to limited ranges of stimulus energies. The graded sensitivity of photoreceptors means The receptor behaves as a flter for a narrow range that each rod and cone responds to a wide spectrum or bandwidth of energy. For example, an individual of colors yet signals a specifc wavelength by the photoreceptor is not sensitive to all wavelengths of amplitude of the evoked receptor potential. However, light but only to a small part of the spectrum. We say because the tuning curve is symmetric around the that a receptor is tuned to an optimal or best stimu- best frequency, wavelengths of greater or lesser lus, the unique stimulus that activates the receptor at values may evoke identical responses. For example, low energy and evokes the strongest response. As a red cones respond equally well to light of 520 and result, we can plot a tuning curve for each receptor 600 nm. How does the brain interpret these signals? Spatial resolution Somatosensory afferents convey information from the skin surface to central circuits The skin harbors a variety of morphologically distinct mechanoreceptors Transduction in a mechanosensory afferent (a Pacinian corpuscle) Receptive fields and two-point discrimination threshold (Part 1) Receptive fields and two-point discrimination threshold Slowly and rapidly adapting mechanoreceptors respond differently to a stimulus Proprioceptors provide information about the position of body parts Schematic representation of the main mechanosensory pathways Schematic representation of the main mechanosensory pathways Chapter 21 / Sensory Coding 467 Lateral inhibition A Neural circuits for sensory processing C Types of inhibition in relay nuclei Stimulus Somatosensory cortex Skin Receptors Output to thalamus Neurons in relay Feedback nucleus Feed-forward Descending B Spatial distribution of excitation and inhibition To spinal cord Stimulus Receptors Relay neurons Frequency Figure 21–11 Neural networks in relay nuclei integrate sen- C. Inhibitory interneurons in a relay nucleus are activated by sory information from multiple receptors. three distinct excitatory pathways. Feed-forward inhibition is A. Sensory information is transmitted in the central nervous produced by the afferent f bers of receptors that terminate on system through hierarchical processing networks. A stimulus to the inhibitory interneurons. Feedback inhibition is produced the skin is registered by a large group of postsynaptic neurons by recurrent collateral axons of neurons in the output pathway in relay nuclei in the brain stem and thalamus, but most from the nucleus. The interneurons in turn inhibit nearby output strongly by neurons in the center of the array (red neuron). neurons, creating sharply def ned zones of excitatory and inhibi- The receptive f eld of an individual relay neuron is larger than tory activity in the nucleus. In this way the most active relay that of any of the presynaptic sensory neurons because of neurons reduce the output of adjacent, less active neurons, the convergent connections. (Adapted, with permission, from permitting a winner-take-all strategy that ensures that only one Dudel 1983.) of two or more competing responses is expressed. Inhibi- tory interneurons are also activated by neurons in other brain B. Inhibition (gray areas) mediated by local interneurons con- regions such as the cerebral cortex. The descending pathways f nes excitation (orange area) to the central zone where stimu- allow cortical neurons to control the relay of sensory informa- lation is strongest, enhancing the contrast between strongly tion centrally, providing a mechanism by which attention can and weakly stimulated relay neurons. select sensory inputs. his or her attention is engaged, previous experience of modify the incoming sensory information. For these that stimulus, and recent activation of the pathway by reasons, neural responses to sensory stimulation or similar stimuli. Similarly, behavioral conditions dur- during motor behaviors are usually illustrated both ing stimulus presentation, subjective intentions, motor by raster plots that depict the trial-to-trial variability plans that may evoke feedback responses, or intrinsic of fring (see Figure 21–15) and by histograms that oscillations of the neuron’s membrane potential can all average neural activity across trials. Somatic sensory portions of the thalamus and their cortical targets in postcentral gyrus Somatotopic order in the human primary somatic sensory cortex Connections within the somatosensory cortex establish functional hierarchies Neurons in the primary somatosensory cortex form functionally distinct columns Functional expansion of a cortical representation by a repetitive behavioral task Properties of SII neurons Thermoception Experimental demonstration that nociception involves specialized neurons Pain can be separated into first (sharp) and second (duller, burning) pain The anterolateral system The anterolateral system sends information to different parts of the brainstem/forebrain Inflammatory response to tissue damage 542 Part V / Perception Enhanced excitability of dorsal horn neurons A Repetitive stimulation of C and A fibers 80 Aδ fiber 60 Response to C fiber B 40 20 Response to Aδ fiber C fiber Number of spikes 0 036912 15 Stimulus number B Enhancement
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