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Sensory Perception: A Summary AP Biology Spring 2011 Reception of stimulus energy Transduction of stimulus energy Brain response (sensation or perception) Stimulus: form of energy that activates receptor endings of a sensory neuron Sensations: conscious awareness of stimulus Perception: understanding of what a sensation means Mechanoreceptors: reseptors stimlate by physical stimuli, such as pressure, touch, stretch, sound, or motion Thermoreceptors: respond to either heat or cold and help maintain body temperature Chemoreceptors: detect chemical energy of substances dissolved in a fluid bathing them Gustatory (taste) receptors and olfactory (smell) receptors Osmoreceptors: detect changes in the solute levels of some body fluid Photoreceptors: detect differences in energy of visible light and UV light Pain receptors: respond to excess heat, pressure, or specific classes of chemicals released from damaged or inflamed tissues Many different groups All sensory receptors transduce stimulus energy into action potentials Brain assesses each stimuli by which nerve pathways are carrying action potentials, the frequency of action potentials traveling on each axon in the pathway, and the number of axons recruited by the stimulus Sensory Adaptation: a diminishing response to an ongoing stimulus Ex. The pressure exerted by a sock Example of how we make use of sensory pathways Figure 35.2, page 601 Somatic sensations: arise in the cerebrum’s outer layer of gray matter, the cerebral cortex. Some body parts have more sensory acuity (more neurons) Fingers, thumbs, lips Sensation of touch, pressure, cold, warmth, and pain Free Nerve Endings: unmyelinated or thinly myelinated, branched endings of sensory neurons in skin and internal tissues Thermoreceptos, mechanorecptors, and pain receptors Adapt slowly to stimualtion Different subpopulations respond to different stimuli Encapsulated receptors: detect somatic sesations, 4 types 1. Meissner’s Corpuscle Fingertips, lips, eyelids, nipples, genitals Adapts very slowly to low-frequency vibrations 2. Bulb of Krause Thermoreceptor, activated at 20 degrees C or lower, below 10 degrees C contributes to painful freezing sensations 3. Ruffini endings Detect steady touch and pressure, and temp.’s above 45 degrees C 4. Pacinian Corpuscle Sensitive to fine textures, occurs in dermis, near freely movable joints, and in some internal organs, sensitive to rapid pressure changes brought on by touch and vibrations Stretch receptors in muscle spindle fibers Increase firing rate as muscle stretches Signal brain about positions of the body’s limbs Pain: perception of a tissue injury Somatic pain: response to signals from pain receptors in skin, skeletal muscles, joints, tendons Superficial somatic pain arises at or near skin surface , sharp or pricking often does not last long Deep somatic pain arises deeper in skin or muscles or joints, diffuse and lasts longer Visceral pain: associated with internal organs Is response to high chemical stimulation, muscle spasm, muscle fatigue, excessive distension of gut, inadequate blood flow to organs, and other abnormal conditions Cells injured release chemicals that activate pain receptors Bradykinins: open floodgates for histamine, prostaglandins, and other participants in inflammatory response Signals from pain receptors enter spinal cord and cause release of a neuromodulator, substance P, which activates neurons that can signal the sensory cortex Assess the intensity and type of pain Different signals rouse body and mediate emotional responses Release natural opiates- endorphins and enkephalins Lower pain preception Influence pain tolerance: Emotional states, culture factors, possibly age (older people handle pain better) Hyperalgesia: intense or long lasting pain leads to this condition where pain is amplified Bad sunburn and hot shower Referred pain: perception of visceral sensations as somatic sensations Ex. Heart attack Makes mistake because of nervous system construction Sensory inputs of skin and certain internal organs enter same segments of spinal cord Skin encounters more pain than organs, so brain may interpret most sensory input as arriving from skin Phantom pain: amputees sense presence of a missing body part as if it were still there Severed sensory nerves continue to respond to the amputation Brain projects pain back to missing part, past the healed region Chemoreceptors become activated by binding molecules of a substance that is dissolved in the fluid bathing them Stimulus triggers signals that travel along nerves through thalamus, signals end in cerebral cortex Perception takes shape and fine tuning Also reaches limbic system, integrates emotional states and with stored memories Olfactory Receptors: fire off signals when exposed to water-soluble or volatile (easily vaporized) chemicals Receptor axons lead into one of two olfactory bulbs In these small brain structures, axons synapse with cells that sort out scent Then, information flows along olfactory tract to cerebrum, where further processed Use olfactory cues to navigate, find food, communicate Pheromones: signaling molecules secreted by one individual that change the social behavior of other individuals of its species Vomeronasal organ: reptiles, most mammals (not primates), cluster of sensory cells that detect pheromones Humans have reduced version Taste receptors: found on antennae, legs, tentacles, or inside mouth Chemoreceptors located in our surface of mouth, throat, and upper part of tongue (taste buds) 5 main sensations: sweet, sour, salty, bitter, umani (savory taste) Vestibular apparatus: one each ear, consist of two sacs (utricle and saccule) along with 3 semicircular canals Sacs and canals interconnect into a continuous, fluid- filled system where mechanoreceptors are stimulated when you move Organ of Dynamic Equilibrium: inside bulging semi-circular canal, gelatinous mass which hair cells project Any rotation of head displaces fluid Hydrostatic pressure exerted by moving fluid shifts gelatinous mass, which bends hair cells Bending causes signals to flow from sensory neurons to vestibular nerve, which carries signals about motion to brain Organ of Static Equilibrium: inside each utricle and saccule, send messages to brain about how head is oriented relative to ground Thick membrane rests on hair cells that project upward from floor of sac Membrane contains mass of crystals, weigh it down Head upright: weighted membrane presses down on hair cells, bends them slightly If posture changes or speed up or slow down movement, position of membrane above hair cells will shift Brain also evaluates input from receptors in skin, joints, tendons Integration of all information allows control of eye muscles that keep visual fields in focus even with movement Helps maintain awareness of body’s position and motion in space Vertigo: sensation that world is moving or spinning around Stroke, inner ear infection, loose particles in semicircular canals Arises from conflicting sensory inputs Motion Sickness: from mismatched signals Passengers in a vehicle, confusion of if in motion (outside vehicle) or stationary (inside vehicle) Amplitude: loudness (intensity), measure in decibels Frequency: number of wave cycles per second, more cycles per second higher the frequency and pitch Water readily transfers vibrations to body tissues Sound spreads out in air Outer ear: adapted for gathering sounds from air Pinna: folded flap of cartilage, sheathed in skin, projects from side of head Auditory canal leads from pinna to middle ear Middle ear: amplifies and transmits air waves to inner ear Eardrum: thin membrane, vibrates fast in response to pressure waves Behind drum is air-filled cavity and small bones: hammer, anvil, stirrup By interacting bones transmit force of sound waves from eardrum to a smaller surface (oval window) Oval window: elastic membrane in front of inner ear Inner ear: has vestibular apparatus, cochlea Cochlea: pea sized, fluid-filled structure, transduction of waves of sound into action potentials occurs here Sound waves make stirrup vibrate Middle ear bone pushes against oval window Transmits pressure waves to fluid in 2 of 3 cochlear ducts (scala vestibuli and scala tympani) Waves end at another membrane, round window (bows inward and outward in response) Third cochlear duct sorts out pressure waves Its basilar membrane wall, is stiff and narrow near oval window, then broadens and becomes more flexible deeper in coil High pitched sounds make stiff, narrow part of cochlear duct vibrate Low pitched sounds make flexible part vibrate Acoustical organ: attached to one surface of basil membrane Organ of Corti: has arrays of hair cells (acoustical receptor with modified cilia at one end) Cilia bend when pressure moves basilar membrane Mechanical energy transduced into action potentials that reach brain Damage of hairs results in hearing loss Vision: requires eyes and image perception in brain centers that can interpret patterns of visual stimulation Eyes: sensory organs that contain a tissue of many densely packed photoreceptors Photoreceptors: contain pigment molecules that can absorb photon energy, which can be converted to excitation energy in sensory neurons Ciliary photoreceptors: plasma membrane around the cilium develops into the photosensitive surface Cnidarians, some flatworms, vertebrates Rhabdomeric photoreceptors: photosensitive surface develops from microvilli around cilium Most flatworms, mollusks, annelids, arthropods, echinoderms Photoreceptors dispersed though integument (protective area) Ocellus: simplest eye, photoreceptors project into these pigmented sports or shallow cups of integument Can determine direction of light source Use light as cue for orientation, predators, bio-clocks Visual Field: part of outside world that eye sees Eye lens: helps image formation, transparent structure, bends all light rays from a given point in the visual field so they converge onto photoreceptors Cornea: helps sharpen images, transparent cover that directs light rays onto lens Simple arthropod eye: one lens for all photoreceptors Spiders Compound eye: many closely packed rhabdomeric units Some have thousands of units of a sort called ommatidia, inside each is a photoreceptor with rhabdomeric microvilli; visual pigment rhodopsin, embedded in membrane Visual mosaic: each unit samples small part of visual field Crustaceans and insects Camera eyes: most complex Light enters dark chamber through pupil An opening in ring of contractile tissue called iris Behind pupil, lens focus light on retina (tissue with many photoreceptors) Axons of sensory nerves converge to form optic tract One tract from each eye extends to brain Cephalopods: octopuses and squids Wall of eyeball (three layers) Sensory Tunic (inner layer) Retina: absorbs, transduces light energy Fovea: increases visual acuity Vascular Tunic (middle layer) Choroid: blood vessels nutritionally support wall cells, pigments prevent light scattering Ciliary Body: muscles control lens; shape; fine fibers hold lens upright Iris: adjusting iris controls incoming light Pupil: serves as enterance for light Start of optic Nerve: carries signals to brain Fibrous Tunic (outer layer) Sclera: protects eye ball (white of eye) Cornea: focuses light Interior of eyeball Lens Focuses light on photoreceptors Aqueous humor Transmits light, maintains pressure Vitreous body Transmits light, supports lens and eyeball Form an inverted, reversed image When ciliary muscle contracts, lens bulges, bending light rays from a close object so that they become focused on the retina When muscle relaxes, lens flattens, focusing light rays from a distant object on the retina Species that move about in night or dimly lit areas need to intercept as much of the available light as possible Large pupils let in more light Large irises can be dilated more to let in more light In between retina and choroid is pigmented epithelium Anchored to epithelium is rod and cone cells Ciliary photoreceptors Rod and cone cells transduce photon energy into action potentials, messages by which brain cells monitor the visual field Rod Cells: detect very dim light, basis for coarse perception of movement across visual field Most abundant outside fovea, small circular region near centre of retina Has rhodopsin pigments Membrane folding and high density of pigments increases odds of intercepting photons Cone cells: detect bright light, basis of sharp vision and colour perception Sense of colour and daytime vision starts when red, green, and blue cone cells, each with different kind of visual pigment, absorb photons Fovea has greatest density of cone cells Basis of greatest visual acuity (most precise discrimination between any 2 points in visual field) Distinct types of sensory neurons lie above rods and cones These neurons receive, process, and start to integrate signals that arise from transduction of photon energy Input from about 125 million rods and cones converge on the retinal neurons known as bipolar cells Humans have 11 types of bipolar neurons- 10 for cones, and 1 for rods Sort out objects that are lighter than darker ones in visual field’s background Information also flows laterally among amacrine cells and horizontal cells Messages converge on 1 million ganglion cells These are output neurons; axons are start of an optic nerve that carries action potentials to the brain Before a transduced signal leaves the retina, neurons start integrating and processing it Humans have 2 optic nerves, 1 from retina in each eye Each optic nerve delivers signals concerning a stimulus from the left visual field to the right cerebral hemisphere And from right visual field to left hemisphere Optic nerve axons end in layered brain region- lateral geniculate nucleus Each layer has map corresponding to receptive fields Deals with one kind of visual stimulus: form, movement, depth, colour texture After early processing signals reach different parts of visual cortex Integration organizes action potentials and produces visual sensations Colour blindness: Cone cells fail to develop Focusing problems: Astigmatism: unevenly curved corneas, cannot properly bend all incoming light rays to same focal point Myopia (nearsightedness): horizontal axis of eyeball is longer than vertical axis, or ciliary muscle responsible for adjusting lens contracts too strongly Outcome: images of distant get focused in front of retina instead of on it Hyperopia (farsightedness): eyeballs vertical axis is longer than horizontal Outcome: light rays from closeobjects get focused behind retina