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Sensory Receptors 8th ed 50.1 to 50.4 7th ed 49.1 to 49.4 Physiological basis for all animal activity: processing sensory information and generating motor output in response to that information This is a continuous cycle Sensory information can be from external or internal environment. Sensory pathway: Sensory reception Transduction Transmission Perception Amplification and adaptation Sensory pathway: Sensory reception Transduction Transmission Perception Amplification and adaptation Sensory reception 1st step of sensory pathway Sensory receptors: Specialized neurons or epithelial cells; Single cells or a collection of cells in organs Very sensitive Sensory pathway: Sensory reception Transduction Transmission Perception Amplification and adaptation Sensory transduction: Conversion of physical, chemical and other stimuli to change in membrane potential Receptor potential: change in membrane potential itself Sensory pathway: Sensory reception Transduction Transmission Perception Amplification and adaptation Transmission: Sensory information is transmitted through the nervous system as nerve impulses or action potential to the Central Nervous System (CNS). Some axons can extend directly into the CNS and some form synapses with dendrites of other neurons Sensory neurons spontaneously generate action potential without stimulus at a low rate Magnitude of receptor potential controls the rate at which action potentials are generated (larger receptor potential results in more frequent action potentials) Weak muscle stretch Muscle Stretch receptor Membrane potential (mV) Dendrites Strong muscle stretch –50 Receptor potential –50 –70 –70 Action potentials 0 0 –70 –70 Axon 0 1 2 3 4 5 6 7 Time (sec) 0 1 2 3 4 5 6 7 Time (sec) Sensory pathway: Sensory reception Transduction Transmission Perception Amplification and adaptation Perception: Action potential reach the brain via sensory neurons, generating perception of a stimulus All action potentials have the same property, what makes the perceptions different are the part of the brain they link to. Sensory pathway: Sensory reception Transduction Transmission Perception Amplification and adaptation Amplification and adaptation: Strengthening of stimulus energy during transduction (involves second messengers) Continued adaptation: decrease in responsiveness upon prolonged stimulation Types of sensory receptors: Mechanoreceptors Chemoreceptors Electromagnetic receptors Thermoreceptors Pain receptors Mechanoreceptors: sense physical deformation caused by pressure, touch, stretch, motion, sound Stretch receptors are mechanoreceptors are dendrites that spiral around small skeletal muscle fibers Weak muscle stretch Muscle Dendrites Stretch receptor Membrane potential (mV) Strong muscle stretch –50 Receptor potential –50 –70 –70 Action potentials 0 0 –70 –70 Axon 0 1 2 3 4 5 6 7 Time (sec) Crayfish stretch receptors have dendrites embedded in abdominal muscles. When the abdomen bends, muscles and dendrites stretch, producing a receptor potential in the stretch receptor. The receptor potential triggers action potentials 0 1 2 3 4 5 6 7 Time (sec) in the axon of the stretch receptor. A stronger stretch produces a larger receptor potential and higher frequency of action potentials. Light touch Touch receptors (light and deep touch) are embedded in connective tissue Strong pressure Epidermis Dermis Hypodermis Nerve Connective tissue Hair Chemoreceptors: General receptors: respond to total solute concentrations Specific receptors: respond to concentrations of specific molecules Electromagnetic receptors: Detect various forms of electromagnetic energy like visible light, electricity, magnetism Snakes can have very sensitive infrared receptors – detect body heat of prey Animals can use earth’s magnetic field lines to orient themselves during migration (magnetite in body) – orientation mechanism Eye Infrared receptor This rattlesnake and other pit vipers have a pair of infrared receptors, one between each eye and nostril. The organs are sensitive enough to detect the infrared radiation emitted by a warm mouse a meter away. Some migrating animals, such as these beluga whales, apparently sense Earth’s magnetic field and use the information, along with other cues, for orientation. Heat Cold Thermoreceptors: Detect heat and cold Located in skin and anterior hypothalamus Mammals have many thermoreceptors each for a specific temperature range Nerve Connective tissue Hair Capsaicin triggers the same thermoreceptors as high temperature Menthol triggers the same receptors as cold (<28oC) Pain Pain receptors (nociceptors): Stimulated by things that are harmful – high temperature, high pressure, noxious chemicals, inflammations Defensive function Nerve Connective tissue Hair Sensing gravity and sound in invertebrates: Hair of different stiffness and length vibrate at different frequencies and pick up sound waves and vibrations Statocyts: organ with ciliated receptor cells surrounding a chamber containing statoliths in invertebrates – sense gravity Ciliated receptor cells Cilia Statolith Sensory nerve fibers Tympanic membrane 1 mm Tympanic membrane stretched over their “ear” help sense vibrations Sensing gravity and sound in humans: Human ear: sensory organ for hearing and equilibrium Our organ for hearing “hair cells” are mechanoreceptors because they respond to vibrations Moving air pressure is converted to fluid pressure Ear structure: Outer ear: pinna, auditory canal, tympanic membrane (separates outer and middle ear) Middle ear: three small bones malleus (hammer), incus (anvil) and stapes (stirrup) transmit vibrations from tympanic membrane to the oval window. Eustachian tube connects middle ear to the pharynx and equalizes pressure Inner ear: consists of fluid filled chambers including semicircular canals (equilibrium) and cochlea (hearing) Moving air travels through the air canal and causes the tympanic membrane to vibrate Three bones of the middle ear transmit the vibrations to the oval window, a membrane on the cochlear surface That causes pressure waves in the fluid inside the cochlea The cochlea: Upper vestibular canal and inner tympanic canal filled with perilymph; middle cochlear duct filled with endolymph Organ of Corti: Floor of the cochlear duct is the basilar membrane Organ of corti is located on the basilar membrane, with hair cells which has hair projecting into the cochlear duct. Many of the hairs are attached to the overhanging tectorial membrane. Sound waves cause the basilar membrane to vibrate. This results in displacement and bending of the hair cells within the bundle. This activates the mechanoreceptors, changes the hair cell membrane potential (sensory transduction) which generates action potential in the sensory neuron. Cochlea Stapes Vestibular canal Oval window Perilymph Apex Base Round window Tympanic canal Axons of sensory neurons Basilar membrane Equilibrium: In the vestibule behind the oval window are urticle, saccule and three semicircular canals Three semicircular canals arranged in three spatial planes detect angular movements of the head The hair cells form a cluster. They have a gelatinous capula. Fluid in the semicircular canals pushes against the capula deflecting the hairs, stimulates the neurons Semicircular canals Ampulla Flow of endolymph Flow of endolymph Vestibular nerve Cupula Hairs Hair cell Vestibule Nerve fibers Utricle Saccule Body movement Utricle (oriented horizontally), saccule (oriented vertically) tell the brian which way is up and the position of the body and acceleration Sheet of hair cells project into a gelatinous capula embedded with otoliths (ear stones). Movement of the head causes otoliths to in different directions against the hair protruding from the hair cells. This movement is detected by the sensory neurons Dizziness: false sensation of angular motion http://www.dizziness-andbalance.com/disorders/bppv/otoliths.html Hearing and equilibrium in fish: Vibrations in the water – conducted by skeleton to inner ear canals, move otoliths which stimulate hair cells Swim bladder: air filled, responds to sound Lateral line sense organ Lateral line sense organ: Water flows through the system Bends hair cells; generates receptor potential Nerve carries action potential to the brain Helps them sense water currents, moving objects, low frequency sounds Lateral line Lateral line canal Scale Neuromast Epidermis Segmental muscles of body wall Opening of lateral line canal Lateral nerve Cupula Sensory hairs Supporting cell Nerve fiber Hair cell Vision Light Photoreceptors Planarians: Ocelli or eye spots in the head region Light stimulates photoreceptors Brain compares rate of action potential coming form the two ocelli Photoreceptor Brain directs the body to turn until sensation form both ocelli are equal and minimal Visual pigment Animal can move to shade, under a rock away Ocellus from predators Light shining from the front is detected Nerve to brain Screening pigment Light shining from behind is blocked by the screening pigment Compound eyes: Very good at detecting movement Very good at detecting flickering light (6 times faster than human eye) Some bees can see in the ultraviolet range of light Several thousand omatidia (facets) in every eye Cornea and crystalline cone form the lens which focuses light on the rhabdom Light stimulates the photoreceptors to generate receptor potential which generates action potential Cornea Crystalline Lens cone Rhabdom Axons Photoreceptor Ommatidium Vertebrate eye Single lens system (very different from invertebrate single eyes) Sclera Eyeball or globe consists of Choroid Iris Sclera: tough white outer Cornea connective tissue Cornea: clear part of sclera in the front of the eye – lets light into the eye, acts as a fixed lens Choroid: pigmented inner layer: forms iris (doughnut shaped) – can change size to regulate the amount of light Pupil coming in Optic nerve Central artery and vein of the retina Optic disk (blind spot) Retina: innermost layer with neurons and photoreceptors Aqueous humor: fluid that fills the anterior cavity (blockage of ducts increases pressure and causes glaucoma) Vitreous humor: jellylike, fills the posterior chamber Retina Fovea (center of visual field) Aqueous humor Vitreous humor Ciliary body Suspensory ligament Lens Lens: clear disk of protein Humans and other mammals Front view of lens and ciliary muscle Choroid Lens (rounder) Retina spherical ( ciliary muscles contract, sensory ligaments relax – near objects) Ciliary muscle Near vision (accommodation) flatter (ciliary muscles relax, edge of choroid moves away from lens, suspensory ligaments contract and pull the lens – distant objects) Suspensory ligaments Lens (flatter) Distance vision Fishes, squids and octopuses focus by moving lens forward and backward Photoreceptors Rods: sensitive to light, do not distinguish colors Cones: detect color, not very sensitive to light Nocturnal animals have a higher proportion of rods Rods and cones have stacks of disks rhodopsin (retinal - vitamin A derivative + opsin) in the membrane get activated and cause sensory transduction Rod Outer segment Disks Inside of disk Cell body Synaptic terminal Cytosol Retinal Rhodopsin Opsin Information from the each eye is carried by the optic nerve (each with about a million axons) Optic nerves meet cross at the optic chiasm Information from right visual field of both eyes goes to the left side of the brain Information from the left visual field of both eyes goes to the right side of the brain Optic nerve Synapse with interneurons which take the information to the primary visual cortex Lateral geniculate nucleus Primary visual cortex Left visual field Left eye Right visual field Right eye Optic chiasm Perception of gustation (taste) and olfaction (smell) Insects In insects taste sensation is located within sensory hairs called sensilla (on feet and mouthparts) Olfactory odorants are detected by olfactory hairs Sensillum Mammals In mammals specialized epithelial cells form taste buds Tastants detect five perceptions of taste: sweet, sour, salty, bitter and savory (MSG) Chemoreceptors generate receptor potential by triggering a chain of reactions involving different proteins for different tastes in the receptor cells Taste pore Taste bud Tongue Sugar molecule Sensory receptor cells Sensory neuron Sensory neurons lining the nasal cavity and extending into the mucus layer get stimulated by odorants. The stimulus is transmitted directly to the olfactory bulb of the brain Brain Olfactory bulb Nasal cavity Bone Odorant Epithelial cell Odorant receptors Chemoreceptor Plasma membrane Odorant Cilia Mucus