Sensory Systems Lecture PDF

Sensory Systems J. Haenfler BIO225 1 Learning Objectives  To appreciate the general properties of sensory reception.  To explore the strategies used for encoding stimulus modality, location, intensity, & duration....

Sensory Systems J. Haenfler BIO225 1 Learning Objectives  To appreciate the general properties of sensory reception.  To explore the strategies used for encoding stimulus modality, location, intensity, & duration. 2 Sensory systems detect stimuli and send information to an integrating center 3 Sensory stimuli are detected in neurons or accessory cells 4 Sensory receptors convert incoming stimuli into changes in membrane potential 5  Sensory receptors can be classified by their stimulus modality  Chemoreceptors detect the presence of chemicals in the environment.  Mechanoreceptors detect pressure and movement, including proprioception.  Photoreceptors detect light.  Thermoreceptors detect temperature.  Electroreceptors detect electric fields.  Magnetoreceptors detect magnetic fields. 6 Stimulus Encoding  Sensory receptors convert information about the stimulus into action potentials  Encode four important features:  Stimulus modality  Stimulus location  Stimulus intensity  Stimulus duration 7  Stimulus Modality and Location  Receptor location encodes stimulus modality and location  Integrating center interprets modality and location  Modality Labeled Lines Discrete pathway from sensory cell to integrating center  Location Labeled lines 8 Sensory pathways can encode stimulus modality In general:  a particular afferent neuron is associated with one modality of receptor and stimulus (adequate stimulus)  each afferent neuron stimulates activity within a particular neural pathway  perception based on pathway activated, not the stimulus identity 9  Polymodal receptors have more than one adequate stimulus  Adequate stimulus  Preferred (most sensitive) stimulus modality  Polymodal receptors  Sensitive to more than one stimulus modality  For example, nociceptors detect various strong, potentially damaging stimuli ( 11  Polymodal receptors are sensitive to multiple sensory modalities Nociceptor (pain receptor) strategy: Sense organs in sharks are sensitive to electricity, touch, & temperature. Pattern of action potentials likely encodes Polymodal nociceptors transduce modality information from some thermal, mechanical, and chemical polymodal receptors. 12 cues into signals that are sensed as pain.  Receptive Field and Localization of Stimulus  Receptive fields encode stimulus location via labelled line  Receptive field: region of the sensory surface that causes a response when stimulated 13  Receptive fields vary in size Smaller receptive field allows more precise localization of acuity: ability to the stimulus (i.e., greater resolve fine detail spatial acuity) of a stimulus 15 Overlapping receptive fields can improve ability to localize the stimulus  Overlapping receptive fields can enhance localization, increase sensitivity, and improve contrast  However, they can lead to redundant information 18 Lateral inhibition further improves acuity  Lateral inhibition Signals from neurons at the center of the receptive field inhibit neurons in the surround Enhances contrast Decreases “noise” Improves edge detection/boundaries 19 Action potential frequency encodes signal intensity 20 Sensory receptors encode a limited range of stimulus intensities 21 Dynamic Range  Sensory neurons code stimulus intensity by changes in action potential frequency For example, strong stimuli  high frequency  Dynamic range Range of stimulus intensities over which a receptor exhibits an increased response Threshold of detection Weakest stimulus that produces a response in a receptor Saturation Top of the dynamic range (maximal response) 22 Dynamic range and discrimination 23 Dynamic Range vs. Discrimination  Trade-off between dynamic range and discrimination  Large dynamic range = poor sensory discrimination Large change in stimulus intensity causes a small change in AP frequency  Narrow dynamic range = good sensory discrimination Small change in stimulus intensity causes a large change in AP frequency 26 Sensory discrimination improves by distributing responses amongst the receptor population Range Fractionation 27 Logarithmic encoding of intensity allows for a compromise between dynamic range and discrimination. Light and Sound are encoded logarithmically by the nervous system. 28 Tonic receptors respond for the entire duration of a stimulus  Sensory Adaptation:  decreased response to stimulus as duration increases  Tonic receptors are typically slow adapting  For prolonged stimuli, sensory adaptation can continue until the stimulus is tuned-out 29 Phasic receptors encode changes in stimulus 30 Phasic receptors can encode stimulus changes in several ways 31 Learning Objectives  To describe the basic principles and varieties of chemosensation.  To understand the role of mechanoreception in touch and proprioception.  To examine the mechanisms involved in hearing and balance. 32 Chemoreceptors mediate detection of chemicals  Exteroceptors:  Olfaction (smell)  Gustatory (taste)  Nociception (pain)  Interoceptors:  Blood pH  Chemosensors in stomach  Internal nociception (pain) 33 Mammalian Olfactory System (smell)  Located in the roof of the nasal cavity  Mucus layer to moisten olfactory epithelium and dissolve odorants  Odorant binding proteins Allow lipophilic odorants to dissolve in mucus  Olfactory receptor cells are ciliated bipolar neurons 34  Odorant receptor neurons (ORNs) express G-protein coupled receptors Odorant receptor Odorant binding results proteins are located in ORN depolarization in the ciliated and action potential firing dendrites of bipolar odorant receptor neurons (ORNs) 35  Odorant Receptors  Each olfactory neuron expresses only one odorant receptor protein  Each receptor can recognize more than one odorant  Each odorant can stimulate more than one receptor  Odorant receptor is linked to G-protein Odorant binding causes formation of cAMP Opening of ion channels Depolarization  Odors are perceived by the brain using a combinatorial code Each perceivable odor activates a unique set of odorant receptors and olfactory neurons 36  Vertebrate gustatory receptors are taste buds  Taste buds are composed of Taste neuroepithelial taste receptor cells (TRCs) and accessory cells  Taste receptor cells are distributed across the tongue with regional differences in sensitivity to certain modalities (salty, sour, sweet, bitter, or umami)  Signals transmitted from the TRCs to primary gustatory afferent neurons 39 Gustatory signal transduction Book figures are outdated 41  Na+ Na+ H+ H+ Na+ H+ K+ 1) H+ from sour foods 1) Na+ from salty food enter channel activate pH-sensitive through sodium channels proton channels (Otop1) (ENaCs) and enter the cell 2) Lowered pH inhibits K+ 2) The resulting channels and depolarizes depolarization triggers the membrane, activating action potentials through voltage-gated Na+ voltage-gated sodium (VGNC) and Ca2+ channels (VGCC) channels 3) Increased intracellular 3) ATP released from the Ca2+ leads to taste receptor cell neurotransmitter release activates P2X receptors and activation of the GRN on the GRN 42  or Umami T1R1/T1R3 = voltage-gated Na+ channel 43  Mechanoreceptors couple mechanical stimuli to ion channels Epithelial Sodium Channel (ENaC) Transient Receptor Potential (TRP) Channel [Na+ Channel] [Non-Selective Cation Channel] 44  Variations on Mechanoreception in Vertebrates  Touch/Pressure  Proprioception  Equilibrium/Balance  Hearing  Baroreception (Blood Pressure Sensing) 45 Touch receptors are widely, but not evenly, dispersed in the skin Innervation Density (units/cm2) Slow-adapting (SA) Fast-adapting (FA)  The hands and face are the most densely innervated regions 46 Variety of touch receptors detect diverse mechanical stimuli  Large vs small receptive fields  Generator vs receptor potentials Fast vs slow sensory Generator Potential Generator Potential Receptor Potential Generator Potential  adaptation 48 Proprioceptors monitor the position of the body in space Spindle fibers and Golgi tendon organs detect the stretch of muscles Sensory (afferent) and tendons, respectively nerve fiber Sensory nerve endings Golgi tendon organ Sensory (afferent) Tendon nerve fiber Skeletal muscle fiber Muscle spindle Skeletal muscle fiber (a)Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display.(b) 49 Hair cells are mechanosensors for hearing and balance in vertebrates 50  Hair cells are bathed in “endolymph”, which contains high K+ relative to the cell Stereocilia Movement of the stereocilia opens the mechanically-gated cation channels (TMC1/2) High K+ Low K+ ***: a 2020 Neuron paper identified TMC1/2 as the pore-forming subunits of mechanosensitive ion channels in hair cells 51 Hair cells maintain transmitter release on to afferent neurons, even in the absence of deflection. 53 Stereocilila deflections modulate K+ conductance and transmitter release in hair cells 54 Stereocilila deflections modulate K+ conductance and transmitter release in hair cells 55 The mechanoreceptors for equilibrium (balance) are located in the inner ear Semicircular Utricle Vestibular apparatus canals Saccule Ossicles Cochlea Vestibulocochlear nerve Oval window Round window Tympanic membrane 56 Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display. Hair cells of the vestibular apparatus detect movements 57 semicircular canal Maculae of the utricle and saccule detect linear acceleration and tilting 58 Cristae of the semicircular canal detect angular acceleration Corrected Figure Excitation vs inhibition depends on the direction of movement and the location of the hair cells 59 Hearing involves the outer, middle, and inner ear Semicircular Utricle canals Saccule Ossicles Cochlea Vestibulocochlear nerve Pinna Oval window Round window Tympanic membrane Hearing 60 Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display.  Sound Transduction  Sound waves enter the ear canal and vibrate the tympanic membrane  Middle ear bones transmit vibration to the cochlea Oval window vibrates  Pressure waves in perilymph of cochlea cause basilar membrane and tectorial membrane to vibrate  Stereocilia on the hair cells bend Hair cells depolarize Inner hair cells release neurotransmitter (glutamate) Glutamate excites afferent neuron (Round window serves as a pressure valve) 61 Hair cells in the Organ of Corti detect basilar membrane movements Inner hair cells transduce basilar Outer membrane vibration The location along the cochlea Inner determines sound perception 62  Outer hair cells amplify sounds by “somatic electromotility” (aka dancing)  Change length in response to stimulation  Increase deflection of the basilar membrane  Amplify signals to inner hair cells 63  Learning Objectives  To explain the structure and function of photoreceptors  To describe the key features of the mammalian eye and vision 64 Photoreceptors convert light energy into changes in membrane potential 65 The photoreceptive opsins are seven-membrane- spanning GPCRs Opsins localize to membranes in the outer segments of vertebrate photoreceptors  Phototransduction occurs through chromophore isomerization 11-cis-Retinal 11-cis-Retinal All-trans-Retinal  Summary of light-induced events  Opsins covalently bind vitamin-A derived chromophores  Photons cause isomerization of the 11-cis-Retinal chromophore  Isomerization of the chromophore leads to changes in the opsin  Conformational change in opsin  Dissociation of chromophore from opsin (“bleaching”)  G-protein signaling events cause changes in membrane potential Two classes of photoreceptor are found throughout the animal kingdom * Rhabdomeric PRs were recently discovered in vertebrates too. These PRs help entrain our internal clocks to environmental light/dark cycles. Rhabdomeric photoreceptors signal through Gq Ciliary photoreceptors signal through Gi/transducin Cyclic nucleotide- gated (CNG) channel The mammalian eye allows formation of a bright, focused image 72  The mammalian eye allows formation of a bright, focused image  Muscle fibers of the iris change the pupil diameter and control the amount of light entering the eye  The cornea refracts light and focuses it onto the lens, which is responsible for fine-tuning the focus onto the retina  The lens changes shape depending on the distance of the light source in a process called accommodation  Light passes through the retina and excites the photoreceptors  The choroid is a pigmented layer that absorbs the light Muscle fibers of the iris change the pupil diameter in response to light 74 The choroids of nocturnal animals contain a reflective layer The tapetum lucidum reflects and amplifies dim light in nocturnal animals and makes their eyes “glow” 75 The cornea and lens are convex lenses that direct light onto the retina Most refraction occurs at the cornea. The lens is responsible for fine-tuning. Thus, most of the focusing is done by the cornea. 76  Accommodation refers to the ability of the eye to focus light from different distances During accommodation, the ciliary muscle contracts, the ligaments slack, and the lens bulges Distant object Nearby object Nearby object with = parallel rays = nonparallel rays accommodation 77 = short focal length = increased focal length = focal length restored Rods and cones are located at the back of the retina Direction of light Direction of signal 78 Mammals have two types of photoreceptor cells Rods Cones  Ciliary photoreceptors  Ciliary photoreceptors  Rod-shaped outer  Cone-shaped outer segment segment  Sensitive to very dim light  Sensitive to brighter light  Color detection 79 Many rods synapse on a single bipolar cell Multiple bipolar cells synapse onto a single ganglion cell convergence In rod pathways, each ganglion cell has a large receptive field 81 Ganglion cells in the fovea centralis receive input from a single bipolar cell, which receives input from a single cone 82 Smaller receptive fields allow for greater acuity 84  The fovea centralis is specialized to provide sharp, central vision  Important characteristics:  exclusively cones  devoid of capillaries  displaced obstructing cell layers OPTIC DISC (blind spot) FOVEA CENTRALIS 85 The absorbance spectra of human rods and cones are the basis for color discrimination 86 Retinal ganglion cells have complex receptive fields that enhance borders and contrast 87  Activated horizontal and/or amacrine cells inhibit neighboring cells  Horizontal cells function at the photoreceptor and bipolar cells, whereas amacrine cells function at the bipolar and ganglion cells 88  The Neural Circuitry of Retinal Processing: One Example 1). Light hyperpolarizes photoreceptor, reducing transmitter release Photoreceptor Inhibitory 2). A decrease in inhibitory Synapse signaling depolarizes the bipolar cell Bipolar Cell Excitatory 3). Depolarization increases Synapse excitation of ganglion cell. Ganglion Cell To CNS 90 There are many variations on the pattern of retinal connectivity 91