Psychophysical laws

Legge di Weber: ΔS=K*S

Legge di Fechner: I=K*log(S/S0) 450 Part V / Sensory receptors

Vision Smell

Touch Thermal

Hearing Balance

Figure 21–1 The major sensory modalities in are (touch, , 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 organs. Each class of receptor cell transforms are mediated by receptors in the eye, nose, one type of energy into electrical signals that are mouth, skin, and inner , respectively. The other somatosen- encoded as trains of action potentials. The principal receptor sory modalities—thermal senses, pain, and proprioception— cells include photoreceptors (vision), (smell, are mediated by receptors distributed throughout the body. taste, and pain), thermal receptors, and

the complex forms that are the basis of . 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 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 , All tissues except central (thermal, mechanical, mechanoreceptor, and nervous system and chemical stimuli) 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 ) 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 C Types of inhibition in relay nuclei

Stimulus

Somatosensory cortex Skin

Receptors Output to

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 primary somatic 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 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 of excitability

Aδ fiber C fiber stimulation stimulation Glutamate Substance P

AMPA AMPA NMDA AMPA NMDA NMDA NK1 Ion channels

Ca2+ Ca2+

Fast membrane Long-lasting depolarization depolarization (transient) (cumulative)

2+ Figure 24–10 Mechanisms for enhanced excitability of which relieves the Mg block of the N-methyl-D-aspartate dorsal horn neurons. (NMDA)-type receptors. Activation of the postsynaptic NMDA- A. Typical responses of a dorsal horn neuron in the rat to electri- type receptors and neurokinin-1 (NK1) antagonist receptors by cal stimuli delivered transcutaneously at a frequency of 1 Hz. C fbers generates a long-lasting cumulative depolarization. The 2+ With repetitive stimulation the long-latency component evoked cytosolic Ca concentration in the dorsal horn neuron increases 2+ by a C fber increases gradually, whereas the short-latency because of Ca entry through the NMDA-type and AMPA-type 2+ 2+ component evoked by an A fber remains constant. channels and voltage-sensitive Ca channels. The elevated Ca and activation of NK1 receptors through second-messenger B. The dorsal horn neuron receives monosynaptic input from systems enhances the performance of the NMDA-type recep- mechanoreceptors (A fbers) and polysynaptic input from 2+ tors. Activation of NK1 receptors, cumulative depolarization, (C fbers). Elevation of Ca in the presynaptic 2+ elevated cytosolic Ca , and other factors regulate the behavior terminal leads to increased release of glutamate and sub- of ion channels responsible for action potentials, resulting in stance P. Activation of the postsynaptic -amino-3-hydroxy-5- α enhanced excitability. methylisoxazole-4-propionate (AMPA)-type glutamate receptors by A fbers causes a fast transient membrane depolarization,

and terminates in both the reticular formation and the I and V. Information transmitted along this tract is thalamus (Figure 24–12). The axons of spinoreticular thought to contribute to the affective component of tract neurons do not cross the midline. pain. This tract projects in the anterolateral quadrant The spinomesencephalic (or spinoparabrachial) tract of the spinal cord to the mesencephalic reticular for- contains the axons of projection neurons in laminae mation and periaqueductal gray matter (Figure 24–12). Descending systems that modulate the transmission of ascending pain signals Descending systems that modulate the transmission of ascending pain signals Descending systems that modulate the transmission of ascending pain signals Referred Pain