Introduction to Physiological Principles Physiology

Sensory Systems Sensory Receptors Range from single cells to complex sense organs Types: chemoreceptors, mechanoreceptors, photoreceptors, electroreceptors, magnetoreceptors, thermoreceptors All transduce incoming stimuli into changes in membrane potential Figure 6.1 Sensory reception: Reception Transduction Amplification Transmission Perception Graded Potentials Generator potential Sensory receptor is the primary afferent neuron ?Vm spreads along membrane Receptor potential Sensory receptor is separate from the afferent neuron ?Vm triggers release of neurotransmitter Classification of Sensory Receptors Based on stimulus location: Telereceptors – detect distant stimuli, e.g., vision and hearing Exteroceptors – detect stimuli on the outside of the body, e.g., pressure and temperature Interoceptors – detect stimuli inside the body, e.g., blood pressure and blood oxygen Tells little or nothing about how receptors work In School: Problems? Classification of Sensory Receptors Based on type of stimuli the receptors can detect (stimulus modality) Chemoreceptors – chemicals, e.g., smell, taste Mechanoreceptors – pressure and movement, e.g., touch, balance, bp, sound Photoreceptors – light, e.g., vision Electroreceptors – electrical fields Magnetoreceptors – magnetic fields Thermoreceptors – temperature Sensitivity to Multiple Modalities Adequate stimulus Preferred (most sensitive) stimulus modality Many receptors can be excited by other stimuli, if sufficiently strong E.g. pressure on eyelid ? perceive light Polymodal receptors Sensitive to more than one stimulus modality For example, nociceptors; polymodal receptors for multiple types of pain Ampullae of Lorenzini Stimulus Encoding All stimuli are ultimately converted into action potentials in the primary afferent neurons How can organisms differentiate among stimuli or detect the strength of the signal? Sensory receptors must encode four types of information: Stimulus modality Stimulus location Stimulus intensity Stimulus duration Receptor Location Receptor location encodes both stimulus modality and location Theory of labeled lines – discrete pathway from the sensory cell to the integrating center Polymodal receptors are exceptions Encode modality via temporal patterns of APs Receptive Field Region of the sensory surface that generates a response when stimulated Smaller receptive field ? more precise location of the stimulus ? greater acuity Maximally sensitive at the center of the receptive field Higher density of receptors How then does an afferent neuron distinguish between a strong stimulus at edge of field and a weak stimulus at center? Using more than one sensory receptor cell Lateral inhibition – signals from neurons at the center of the stimulated area inhibit neurons on the periphery Receptive Field and Location of Stimulus Figure 7.3 Stimulus Intensity - Dynamic Range How can a sensory receptor code for a wide range of stimulus intensity with a small range of AP frequencies? Sensory neurons code stimulus intensity by changes in action potential frequency E.g 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 50% of the time Saturation Top of the dynamic range (maximal response) Figure 7.4a Stimulus Intensity - Range Fractionation Figure 7.4b-c Logarithmic coding by single neurons Trade-off between Dynamic range and discrimination Large Dynamic range Large change in stimulus causes a small change in AP frequency Poor sensory discrimination Narrow dynamic range Small change in stimulus causes a large change in AP frequency Good sensory discrimination Range fractionation – groups of receptors work together to increase dynamic range without decreasing sensory discrimination Stimulus Duration - Tonic and Phasic Receptors Two classes of receptors encode stimulus duration: Phasic Produce APs at the beginning or end of the stimulus Encode change in stimulus, but not stimulus duration Tonic Produce APs as long as the stimulus continues Encode duration of stimulus Receptor adaptation – AP frequency decreases if stimulus intensity is maintained at the same level Chemoreception Transform chemical stimuli into electrical signals Olfaction (smell) Detection of chemicals in air Gustation (taste) Detection of dissolved chemicals emitted from food Olfaction and gustation are distinguished by structural criteria Performed by different sense organs Different signal transduction mechanisms Processed and perceived in different integrating centers The Olfactory System Evolved independently in vertebrates and insects Vertebrate can distinguish thousands of odorants Located in the roof of the nasal cavity Receptor cells are ciliated bipolar neurons Odorant receptor proteins are located in the cilia Mucus layer to moisten olfactory epithelium Odorant binding proteins Allow lipophilic odorants to dissolve in mucus Signal transduction in olfactory receptor cell Figure 7.7 Each olfactory neuron expresses only one odorant receptor protein There are 1000s of different receptor proteins Receptors recognize more than one odorant Odorants can stimulate more than one receptor The Gustatory System Taste receptor cells differ from odorant receptor cells Epithelial cells release neurotransmitter Express more than one kind of taste receptor protein A single taste neuron may synapse with more than one taste receptor cell Diverse signal transduction mechanisms Five classes of tastants: Salty Sweet Umami Bitter Sour Taste Buds in Vertebrates Group of taste receptor cells Clustered in groups On tongue, soft palate, larynx, and esophagus On external surface of the body in some fish Figure 6.10 Tastants Taste Receptor Transduction Pathways Figure 6.11a-b (Salamanders) Taste Receptor Transduction Pathways Figure 6.11c-d or umami? Taste in Invertebrates Located on sensilla inside (pharynx) and outside (proboscis) the mouth along the wing margin at the end of the legs female vaginal plates Receptors Bipolar sensory neurons G-protein coupled Express only a single receptor protein Differences suggest independent evolution In fact invert. taste sensilla are more similar to vertebrate olfactory neurons Mechanoreception Transform mechanical stimuli into electrical signals All organisms (and most cells) sense and respond to mechanical stimuli Two main types of mechanoreceptor proteins: ENaC Epithelial sodium channels TRP channels Transient receptor potential channels Channels are linked to extracellular matrix Mechanical stimuli alter permeability Touch and Pressure Three classes: Baroreceptors – interoceptors that detect pressure changes; vertebrate only Tactile receptors – exteroceptors that detect touch, pressure, and vibration on the body surface Proprioceptors – monitor the position of the body Vertebrate Tactile Receptors Widely dispersed in skin Not grouped into complex organs Function as isolated sensory cells Free nerves endings Slow adapting tonic Small receptive field Enclosed in accessory structures (e.g., Pacinian corpuscle) Figure 6.15 Vertebrate Proprioceptors Monitor the position of the body Three major groups: Muscle spindles Located in skeletal muscles Monitor muscle length Golgi tendon organs Located in tendons Monitor tendon tension Joint capsule receptors Located in capsules that enclose joints Monitor pressure, tension, and movement Typically do not adapt Insect Tactile Receptors Two common types of sensilla: Trichoid – hairlike Touch, but also olfaction, gustation Stretch-sensitive TRP ion channel Campaniform – bell-shaped Proprioception Figure 6.13 Insect Proprioceptors Scolopidia – bipolar neuron and complex accessory cells (scolopale) Can be isolated or grouped to form chordotonal organs Most function in proprioception Can be modified into tympanal organs for sound detection Figure 6.14 Retrieved from http://natashamhatre.blogspot.com/2006/08/insect-ears.html 26/02/08 Equilibrium and Hearing Utilize mechanoreceptors Equilibrium (“balance”) Detect position of the body relative to gravity Hearing Detect and interpret sound waves Vertebrates Ear is responsible for equilibrium and hearing Invertebrates Organs for equilibrium are different from organs of hearing Statocysts Figure 6.16a Organ of equilibrium in invertebrates Hollow, fluid filled cavities lined with mechanosensory neurons Statocysts contain statoliths Dense particles of calcium carbonate Movement of statoliths stimulate mechanoreceptors Vertebrate Hair Cells Figure 6.17 Mechanoreceptor for hearing and balance Modified epithelial cells Cilia on apical surface Kinocilium (a true cilium) Stereocilia (microvilli) Tips of stereocilia are connected by proteins (tip links) Mechanosensitive ion channels in stereocilia Movement of stereocilia ? change in permeability Change in membrane potential Change in release of neurotransmitter from hair cell Signal Transduction in Hair Cells Can detect movement, direction, and duration Figure 6.18 Retrieved from http://oto2.wustl.edu/cochlea/intro3.htm, feb 6, 2009. Vertebrate Ears Figure 6.20 Function in both equilibrium and hearing Outer ear Not in all vertebrates Middle ear Not in all vertebrates Interconnected bones in air-filled cavity Inner ear Present in all vertebrates Series of fluid-filled membranous sacs and canals Contains mechanoreceptors (hair cells) Equilibrium – Inner ear Figure 6.21 Vestibular apparatus detects movements Three semi-circular canals with enlarged region at one end (ampulla) Two sacklike swellings (utricle and saccule) Lagena Extension of saccule Extended in birds and mammals into a cochlear duct or cochlea for hearing Hair cells present in vestibular apparatus and lagena (cochlea) Vestibular Apparatus Macula Present in utricle and saccule Mineralized otoliths suspended in a gelatinous matrix Stereocilia of hair cells embedded in matrix >100,000 hair cells Detect linear acceleration and tilting of head Cristae Present in ampullae of semicircular canals Gelatinous matrix (cupula) lacks otoliths Stereocilia of hair cells embedded in matrix Detect angular acceleration (turning) of head Mechanoreceptors of the inner ear Maculae Detect Linear Acceleration and Tilting Figure 6.23 Saccule is oriented vertically Cristae Detect Angular Acceleration Figure 6.24 Important in Vestibular Ocular Reflex Terrestrial Vertebrates Hearing involves the inner, middle, and outer ears Problem: sound transfers poorly between air and the fluid-filled inner ear Solution: amplify sound Pinna acts as a funnel to collect more sound Middle ear bones increase the amplitude of vibrations from the tympanic membrane to the oval window by lever action Tympanic membrane SA > oval window SA Figure 6.26a Hearing Mammalian Inner Ear Figure 6.26b Specialized for sound detection Coiled Cochlea in mammals Perilymph Fills vestibular and tympanic ducts Similar to extracellular fluids (high Na+ and low K+) Endolymph Fills cochlear duct/semicircular canals/utricle/saccule Different from extracellular fluid (high K+ and low Na+) Organ of Corti Organ of sound transduction Hair cells on basilar membrane Inner and outer rows of hair cells Inner hair cells detect sound Outer hair cells amplify sounds Stereocilia embedded in tectorial membrane in cochlear duct Sound Transduction Figure 6.26b Steps: Sound waves vibrate tympanic membrane Middle ear bones transmit vibration to oval window Oval window vibrates Pressure waves in perilymph of vestibular duct Basilar membrane vibrates Stereocilia on the inner hair cells bend Opening stretch-sensitive TRP ion channels Hair cells depolarize (K+ in) Hair cells release neurotransmitter (glutamate) Excites sensory neuron Round window serves as a pressure valve Cochlea essentially transduces pressure waves into electrical signals Encoding sound frequencies Basilar Membrane Oscillations Frequency Detection Basilar membrane is stiff and narrow at the proximal end and flexible and wide at distal end High frequency sound vibrates stiff end Low frequency sound vibrates flexible end Specific regions of brain respond to specific frequencies Place coding Amplitude, Amplification, and Location ohc Brain uses time lags and differences in sound intensity to detect location of sound Sound in right ear first Sound located to the right Sound louder in right ear Sound located to the right Rotation of head helps localize sound Amplitude Detection Loud sounds cause larger movement of basilar membrane than quiet sounds ?depolarization of inner hair cells ?AP frequency Outer hair cells amplify quiet sounds Change shape in response to sound Do not release neurotransmitter Change in shape increases movement of basilar membrane Increased stimulus to inner hair cells Photoreception Figure 6.27 Ability to detect visible light A small proportion of the electromagnetic spectrum from ultraviolet to near infrared Ability to detect this range of wavelengths supports idea that animals evolved in water Visible light travels well in water; other wavelengths do not Photoreceptors Range from single light-sensitive cells to complex, image-forming eyes Two major types of photoreceptor cells: Ciliary photoreceptors Have a single, highly folded cilium Folds form disks that contain photopigments Rhabdomeric photoreceptors Apical surface covered with multiple outfoldings called microvillar projections Microvillar projections contain photopigments Photopigments Molecules that absorb energy from photons Vertebrate Photoreceptors Figure 6.29 Vertebrates have ciliary photoreceptors Rods Cones Both have inner and outer segments Inner and outer segments connected by a cilium Outer segment contains photopigments Inner segment forms synapses with other cells Characteristics of Rods and Cones Table 6.2 Diverse shapes of rods and cones among vertebrates Shape does not determine properties of photoreceptor Properties of photoreceptor depend on its photopigment Photopigments Photopigments have two covalently bonded parts Chromophore Derivative of vitamin A (e.g. Retinal) Contains carbon-carbon double bonds Absorption of light converts bond from cis to trans Opsin G-protein-coupled receptor protein Opsin structure determines photopigment characteristics For example, wavelength of light absorbed Phototransduction Steps in photoreception Chromophore absorbs energy from photon Chromophore changes shape Double bond isomerizes from cis to trans Activated chromophore dissociates from opsin “Bleaching” Activated opsin activates G-protein Formation of second messenger Ion channels open or close Change in membrane potential The Eye Eyespots Cells or regions of a cell that contain photosensitive pigment For example, protist Euglena Eyes are complex organs Detect direction of light Light-dark contrast Some can form an image Figure 6.33a Types of Eyes Flat sheet eyes Some sense of light direction and intensity Often in larval forms or as accessory eyes in adults Cup-shaped eyes (e.g., Nautilus) Retinal sheet is folded to form a narrow aperture Discrimination of light direction and intensity Light-dark contrast Image formation Poor resolution Vesicular and Convex Eyes Figure 6.33c,d Vesicular Eyes (present in most vertebrates) Lens in the aperture improves clarity and intensity Lens refracts light and focuses it onto a single point on the retina Image formation Good resolution Convex Eye (annelids, molluscs, arthropods) Photoreceptors radiate outward Convex retina Compound Eyes of Arthropods Composed of ommatidia (photoreceptor) Each ommatidium has its own lens Images formed in two ways Apposition compound eyes Ommatidia operate independently Each one detects only part of the image Afferent neurons interconnect to form an image Superposition compound eyes Ommatidia work together to form image Resolving power can be increased by: Reducing size of ommatidium Limited by wave properties of light Increasing the number of ommatidia Limited by size of eye Compound Eyes Figure 6.34 lens Rhabdomeric Photoreceptive cells The Vertebrate Vesicular Eye Forms bright, focused images Parts Sclera – white of the eye Cornea – transparent layer on anterior Retina – lines inside of choroid Choroid – pigmented layer behind retina Blood vessels Tapetum – layer in the choroid of nocturnal animals that reflects light Figure 6.35 The Vertebrate Eye, Cont. Figure 6.35 Iris Two layers of pigmented smooth muscle Pupil Opening in iris allows light into eye Lens Focuses image on retina Ciliary body Muscles that change lens shape Aqueous humor Fluid in the anterior chamber Vitreous humor Gelatinous mass in the posterior chamber Image Formation Refraction – bending light rays Both the cornea and the lens are convex and act as converging lenses to focus light on the retina In terrestrial vertebrates, most of the refraction occurs between the air and the cornea Lens does fine tuning Figure 6.36a Image Accommodation Incoming light rays must converge on the retina to produce a clear image Accommodation ensures this – vergence, pupil size, and lens shape Focal point – point at which light waves converge Focal length – distance from a lens to its focal point Distant objects Light rays are parallel when entering the lens Ciliary muscles relax Lens is pulled and becomes thinner Little refraction of light by lens Close objects Light rays are not parallel when entering the lens Ciliary muscles contract Lens becomes thicker More refraction of light by lens The Retina - Phototransduction Figure 6.37a Arranged into several layers Rods and cones are in the retina and their outer segments face backwards Other cells are in front of rods and cones Bipolar cells, ganglion cells, horizontal cells, amacrine cells Axons of ganglion cells join together to form the optic nerve Optic nerve exits the retina at the optic disk (“blind spot”) Direction of Retinal visual processing The Fovea Figure 7.37a Retrieved from http://thalamus.wustl.edu/course/eyeret.html, 01/02/2006 Region in center of retina Overlying bipolar and ganglion cells are pushed to the side Contains only cones Color vision Provides the sharpest images Image is focused on the fovea Signal Processing in the Retina Rods and cones form different images Rods Principle of convergence – many rods synapse with a single bipolar cell ? many bipolar cells synapse with a ganglion cell Large visual field Fuzzy image Cones in Fovea One cone synapses with one bipolar cell which connects to one ganglion cell Small visual field High resolution image Figure 6.39 Signal Processing in the Retina Complex “on” and “off” regions of the receptive fields of ganglion cells improve their ability to detect contrasts between light and dark Signal Processing in the Retina “On” and “off” regions of the receptive field of ganglion cells improve contrast of light and dark “Center-surround” organization of receptive field “On-center” ganglion cells Stimulated by light in center of receptive field Inhibited by light in periphery of receptive field “Off-center” ganglion cells Stimulated by dark in center of receptive field Inhibited by dark in periphery of receptive field Photoreceptors in center and periphery inhibit each other by lateral inhibition Hermann Grid Consider an “ON” center receptive field The Brain Processes the Visual Signal Action potentials from retina travel to brain Optic nerves ? optic chiasm ? optic tract ? lateral geniculate nucleus ? visual cortex Binocular vision Eyes have overlapping visual fields Binocular zone Combine and compare information from each eye to form a three-dimensional image Depth perception If poor binocular vision – most of sensory information crosses over Color Vision Detecting different wavelengths of light Requires multiple types of photoreceptors with different maximal sensitivities Humans: three (trichromatic) Most mammals: two (dichromatic) Some bird, reptiles and fish: three, four, or five (pentachromatic) Retina and brain compare output from each type of receptor and infer the color Figure 7.42 Retrieved from http://webexhibits.org/causesofcolor/17.html, 01/02/06 How do we see yellow? S – Short Wavelength – Blue M – Medium Wavelength – Green L – Long Wavelength - Red S M L ? ? ? ? ? ? ? ? ? S M L ? ? ? ? ? ? ? ? ? Blue Green Red Yellow Purple White Ratios of stimulation are coded and transmitted in separate parallel pathways to the brain. Vision test Test for Colorblindness On-line Color Deficiencies Thermo- and Magnetoreception Thermo: Central - in hypothalamus and monitor internal temperature Peripheral – monitor environmental temperature Warm-, and cold-sensitive Thermal nociceptors – detect painfully hot stimuli ThermoTRPs – TRP ion channel thermoreceptor proteins Pit organs of vipers can detect temperature changes of 0.003 ?C Magneto: Ability to detect magnetic fields e.g., migratory birds, homing salmon