Sensory Receptors – Part II

Sensory Receptors – Part II • Based on type of stimuli the receptors can detect (stimulus modality) • Chemoreceptors – chemicals, e.g., smell and taste • Mechanoreceptors – pressure and movement, e.g., touch, hearing, balance, blood pressure • Photoreceptors – light, e.g., vision; detect photons • Electroreceptors – electrical fields • Magnetoreceptors – magnetic fields • Thermoreceptors - temperature

Mechanoreceptors • Transform mechanical stimuli into electrical signals • All organisms and cells can sense and respond to mechanical stimuli • Two main types • ENaC – epithelial sodium channels • TRP – transient receptor potential

Touch and Pressure • Three classes • Baroreceptors – interoceptors that detect pressure changes • Tactile receptors – exteroceptors that detect touch, pressure, and vibration on the body surface • Proprioceptors – monitor the position of the body

Insects • Two types of mechanoreceptors

Type 1 – External Surface • Two common types of sensilla • Trichoid – hairlike • Campaniform – bell-shaped Figure 7.13

Type 1 – Internal Surface • Scolopidia – bipolar neuron and complex accessory cell (scolopale) • Can be isolated or grouped to form chordotonal organs • Most function in proprioception • Can be modified into tympanal organs for sound detection Figure 7.14

Vertebrate Tactile Receptors • Widely dispersed • Function as isolated sensory cells • Free nerves endings or enclosed in accessory structures (e.g., Pacinian corpuscle) Figure 7.15

Proprioceptors • Monitor the position of the body • Three major groups • Muscle spindles – located on the surface of the muscle and monitor muscle length • Golgi tendon organs – located at the junction between skeletal muscles and tendons and monitor tendon tension • Joint capsule receptors – located in the capsules that enclose joints and detect pressure, tension, and movement in the joint

Equilibrium and Hearing • Utilize mechanoreceptors • Equilibrium or balance – detecting position of the body relative to gravity • Hearing – detecting and interpreting sound waves • Vertebrates: ear is responsible for both equilibrium and hearing • Invertebrates: organs for equilibrium are different from organs of hearing (e.g., tympanal organs)

Statocysts • Organ of equilibrium in invertebrates • Hollow, fluid filled cavities lined with mechanosensory neurons • Contain statoliths – dense particles of calcium carbonate Figure 7.16a

Hair cells • Mechanoreceptor cells used for hearing and balance in vertebrates • Modified epithelial cells • Have extensive extracellular structures and cilia that extend from the apical end

Signal Transduction in Hair Cells • Can detect movement and direction

Fish • Use hair cells in ears for hearing and for detecting body position and orientation • Have neuromasts that detect water movement • Neuromast – hair cell and accessory cupula • Lateral line system – array of neuromasts within pits or tubes running along the side of the body

Vertebrate Ears • Function in both equilibrium and hearing

Equilibrium • Vestibular apparatus detects movements • Vestibular apparatus – three semi-circular canals with enlarged region at one end (ampulla) and two sacklike swellings (utricle and saccule) • All regions contain hair cells

Vestibular Apparatus • Utricle and saccule contain mineralized otoliths suspended in a macula covering >100,000 hair cells • Ampullae lack otoliths and contain cristae (hair cells located in a cupula)

Maculae Detect Linear Acceleration and Tilting Figure 7.23

Cristae Detect Angular Acceleration Figure 7.24

Sound Detection • Inner ear detects sound • In fish, incoming sound waves cause otoliths to move which bend cilia of hair cells • Some fish use the swim bladder to amplify sounds Figure 7.25

Terrestrial Vertebrates • Hearing involves the inner, middle, and outer ears • Problem: sound transfers poorly between air and the fluid-filled inner ear • Solution: amply 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 Figure 7.26a

Mammalian Inner Ear • Specialized for sound detection • Cochlea is coiled in mammals • Perilymph – fills vestibular and tympanic ducts and is similar to extracellular fluids • Endolymph – fills cochlea duct and is high in K+ and low in Na+ • Organ of Corti contains hair cells and sits on basilar membrane • Two types of hair cells • Inner hair cells detect sound • Outer hair cells amplify sounds Figure 7.26b

Sound Transduction • Steps • Incoming sound • Oval window vibrates • Waves in perilymph of vestibular duct • Basilar membrane vibrates • Stereocilia on the inner hair cells bend • Depolarization • Release of neurotransmitter (glutamate) • Excite sensory neuron • Round window serves as a pressure valve

Sound Encoding • Basilar membrane is stiff and narrow at the proximal end and flexible and wide at distal end • Frequency • High  stiff end vibrates • Low  flexible end vibrates

Amplification • Loudness • Loud sounds   movement of basilar membrane   depolarization of inner hair cells   AP frequency • Outer hair cells • Change shape in response to sound instead of releasing neurotransmitter • Change in shape causes basilar membrane to move more and causes a larger stimulus to the inner hair cells • Amplifies sound

Sound Location • Brain uses information on time lags and differences in sound intensity • Sound to right ear first  sound located to the right • Sound louder in right ear  sound located to the right

Photoreception • Ability to detect a small proportion of the electromagnetic spectrum from ultraviolet to near infrared • Concentration on this range or wavelengths supports idea that animals evolved in water Figure 7.27

Photoreceptors • Organs range from single light-sensitive cells to complex, image forming eyes • Two major types • Ciliary photoreceptors – have single, highly folded cilium; folds form disks that contain photopigments • Rhabdomeric photoreceptors – apical surface is covered with multiple outfoldings called microvillar projections • Photopigments - molecules that absorb energy from photons

Vertebrate Photoreceptors • All are ciliary photoreceptors • Two types • Rods • Cones Figure 7.29

Characteristics of Rods and Cones • Nocturnal animals have relatively more rods

Photopigments • Photopigments have two covalently bonded parts • Chromophore – pigment that is a derivative of vitamin A, e.g., retinal • Opsin – G-protein-coupled receptors • Steps in photoreception • Chromophore absorbs energy from photon • Chromophore changes shape • Photoreceptor protein changes shape • Signal transduction cascade • Change in membrane potential • Bleaching – process where activated retinal no longer bonds to opsin, thereby activating opsin

Phototransduction • Transduction cascades differ in rhabdomeric and ciliary photoreceptors

The Eye • Eyespots are single cells or regions of a cell that contain photosensitive pigment, e.g., protist Euglena • Eyes are complex organs Figure 7.33

Flat-sheet Eyes • Provide some sense of light direction and intensity • Most often seen in larval forms or as accessory eyes in adults Figure 7.33a

Cup-shaped Eyes • Retinal sheet is folded to form a narrow aperture • Better discrimination of light direction and intensity • Seen in the Nautilus

Vesicular Eyes • Use a lens in the aperture to improve clarity and intensity • Lens refracts light and focuses it onto a single point on the retina • Present in most vertebrates Figure 7.33c

Convex Eye • Photoreceptors radiate outward forming a convex retina • Present in annelids, molluscs, and arthropods (eeeeeeeeeek)

Compound Eyes • Most complex convex eyes found in arthropods • Composed of ommatidia • Form images in two ways • Apposition compound eyes – ommatidium operate independently; afferent neurons make interconnection to generate an image • Superposition compound eyes – ommatidium work together to form an image on the retina

The Vertebrate Eye • Forms bright, focused images • Parts • Sclera – white of the eye • Cornea – transparent layer • Choroid – pigmented layer • Tapetum – layer in the choroid of nocturnal animals that reflects light Figure 7.35

The Vertebrate Eye, Cont. • Parts • Iris – two layers of pigmented smooth muscle • Pupil – opening in iris • Lens – focuses image • Ciliary body – muscles for changing lens shape • Aqueous humor – fluid in the anterior chamber • Vitreous humor – gelatinous mass in the posterior chamber Figure 7.35

Image Formation • Refraction – bending light rays • Both the cornea and the lens act as converting lens to focus light on the retina • In terrestrial vertebrates, most of the refraction occurs between the air and the cornea Figure 7.36a

Image Accommodation • Accommodation - incoming light rays must converge on the retina to produce a clear image • Focal point – point at which light waves converge • Focal distance – distance from a lens to its focal point • Distant object: light rays are parallel when entering the lens • Close object: light rays are not parallel when entering the lens and must be refracted more • Light rays are focused on the retina by changing the shape of the lens

The Retina • Arranged into several layers • Rods and cones are are at the back and their tips face backwards • Axons of ganglion cells join together to form the optic nerve • Optic nerve exits the retina at the optic disk (“blind spot”) Figure 7.37a

The Fovea • Small depression in the center of the retina where overlying bipolar and ganglion cells are pushed to the side • Contains only cones • Provides the sharpest images Figure 7.37a

Signal Processing in the Retina • Rods and cones form different images • Rods • Principle of convergence – as many as 100 rods synapse with a single bipolar cell  many bipolar cells synapse with a ganglion cell • Large visual field • Fuzzy image • Cones • One cone synapses with one bipolar cell which connects to one ganglion cell • Small visual field • High resolution image

Signal Processing in the Retina, Cont. • Complex “on” and “off” regions of the receptive fields of ganglion cells improve their ability to detect contrasts between light and dark Figure 7.39

The Brain Processes the Visual Signal • Optic nerves  optic chiasm  optic tract  lateral geniculate nucleus  visual cortex Figure 7.41

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) Figure 7.42

Thermoreception • Central thermoreceptors – located in the hypothalamus and monitor internal temperature • Peripheral thermoreceptors – monitor environmental temperature • Warm-sensitive • Cold-sensitive • Thermal nociceptors – detect painfully hot stimuli • ThermoTRPs – TRP ion channel thermoreceptor proteins

Specialized Thermoreception • Specialized organs for detecting heat radiating objects at a distance • Pit organs – pit found between the eye and the nostril of pit vipers • Can detect 0.003°C changes (0.5°C for humans)

Magnetoreception • Ability to detect magnetic fields • e.g., migratory birds, homing salmon • Neurons in the olfactory epithelium of rainbow trout contain particles that resemble magnetite

Sensory Receptors – Part II