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VT 106 Comparative Anatomy and Physiology Sense Organs SENSE ORGANS – organs that detect conscious sensations (esthesia) Sensory Receptors – specialized cells or dendrites that detect stimuli in the internal or external environment thermoreceptors – dendrites that detect temperature changes mechanoreceptors – dendrites or cells that detect mechanical distortions of cell membranes caused by touch or pressure, stretch or bend, vibrations photoreceptors – cells that detect electromagnetic stimuli (light) chemoreceptors – dendrites or cells that detect concentration of specific chemicals pain receptors (nociceptors) – dendrites stimulated mainly by chemicals released by tissue damage or inflammation may also be stimulated by intense mechanical or thermal stimuli anesthesia – loss of conscious sensation general anesthesia – complete loss of sensation and consciousness blocks stimulation of cerebral cortex local (regional) anesthesia – loss of sensation in a localized area blocks conduction of impulses by sensory nerves GENERAL SENSES – distributed throughout skin, connective tissues, and organs of the body pain sensations pain receptors (nociceptors) superficial pain – skin and subcutaneous tissues deep pain – muscles, joints, bones visceral pain – internal organs (no pain receptors in brain) analgesics – drugs that decrease pain perception temperature sensations thermoreceptors that detect heat and cold superficial thermoreceptors – found in skin consciously perceived – mainly cause behavioral changes central thermoreceptors – hypothalamus detect core temperature (temperature of blood) control reflexes for sweating, shivering, piloerection (hair elevation), and thyroid function 1 proprioception mechanoreceptors in muscles, tendons, and joints that monitor location of body parts monitored by cerebellum to coordinate movements muscle spindle – detects stretching of skeletal muscle brain regulates sensitivity to control muscle tone tactile sensations mechanoreceptors in skin and connective tissues that detect touch and pressure Meissner's corpuscles – encapsulated dendrites in dermal papillae of skin detect fine touch hair root plexuses – dendrites wrapped around hair follicles detect movements of hairs Pacininan corpuscles – encapsulated dendrites found in many tissues detect deep pressure visceral sensations – related to organ functions chemoreceptors – detect levels of chemicals (eg. nutrients, water) mechanoreceptors – detect stretch of organs (intense stimulation causes pain) stimulate conscious sensations such as hunger, thirst, nausea SPECIAL SENSES – sensory receptors in complex sensory organs OLFACTION – sense of smell chemoreceptors in nasal cavity Olfactory Epithelium – sense organ for smell 3 types of cells: 1) olfactory receptors – first-order neurons bipolar neurons with 1 knob-shaped dendrite with olfactory cilia cilia have receptors for odorant molecules olfactory nerve – bundled axons extending through cribriform plate 2) supporting cells – columnar epithelium support and protect olfactory receptors 3) basal stem cells – divide and differentiate to produce new receptors olfactory glands – in underlying connective tissue secrete mucus on surface – dissolves odorant molecules Vomeronasal Organ – specialized olfactory organ within incisive bones of some species, may communicate with nasal cavity and/or mouth detects pheromones (odorant molecules used for communication purposes) 2 Physiology of Olfaction odorants dissolve in mucus odorants bind to receptors on olfactory cilia, causing stimulation threshold stimulation produces an action potential, which propagates along the olfactory nerve, through cribriform plate (olfactory threshold is low – as little as 4 odorant molecules) olfactory nerves (cranial nerve I) synapse in olfactory bulbs with second-order neurons olfactory tract – second-order neurons running to olfactory cortex (temporal lobe) GUSTATION – sense of taste chemoreceptors on papillae of tongue and in mouth and pharynx Primary Tastes: sour, sweet, salty, bitter umami – meaty or savory (watery) taste is augmented by olfaction and tactile sensations Taste Buds – sense organs for taste on papillae of tongue 2 types of cells: gustatory cells – sensory receptors taste hairs – long microvilli with receptors for certain molecules taste pore – opening in papilla; tastant molecules pass through to bind to taste hairs supporting cells – support and protect gustatory cells differentiate to form new gustatory cells Physiology of Gustation tastants dissolve in saliva tastants bind to and stimulate taste hairs threshold stimulation triggers action potential in first-order neuron first-order neurons run via cranial nerves VII, IX, X to brainstem second-order neurons – brainstem to thalamus third-order neurons – thalamus to gustatory cortex (parietal lobe) VISION – sense of sight photoreceptors in eyes Accessory Structures of the Eye eyelids (palpebrae) – skin flaps that protect eye nictitating membrane – third eyelid; movement is passive tarsal (Meibomian) glands – modified sebaceous glands of eyelids oily secretions coat margin of eyelid prevents eyelids sticking together and tears spilling out of eye 3 conjunctiva – protective mucous membrane on surface of eye and inner eyelids; mucus contains lysozyme – bacteriocide bulbar conjunctiva – lines eye palpebral conjunctiva – lines inner eyelid conjunctival sac – space between; site for giving eye medications lacrimal apparatus – moistens and protects eye lacrimal glands – produce tears (watery secretion) lacrimal puncta and lacrimal sac – drain tears from eye nasolacrimal duct – drains tears into nasal cavity extrinsic eye muscles – move eyeball innervated by cranial nerves III, IV, VI dorsal rectus – upward rotation ventral rectus – downward rotation lateral rectus – lateral rotation medial rectus – medial rotation dorsal and ventral obliques – roll or stabilize eyeball retractor bulbi – retracts eye into orbit ANATOMY OF THE EYE 3 layers of eye: 1) Fibrous Layer – outer layer cornea – anterior transparent portion, avascular 3 layers of cornea: outer – stratified squamous epithelium middle – orderly layers of collagen inner - simple squamous epithelium sclera – “white” of eye dense fibrous connective tissue gives rigid shape to eyeball attachment site for extrinsic muscles 2) Vascular Layer (uvea) – middle layer choroid – posterior portion highly vascular - provides nutrients to retina pigmented melanin – absorbs stray light tapetum lucidum – reflective region in some species aids vision in low light conditions iris – anterior colored portion regulates light entering eye pupil – opening in iris where light enters eye; shape varies circular smooth muscle – decreases pupil diameter parasympathetic response (ANS) radial smooth muscle – increases pupil diameter sympathetic response (ANS) 4 ciliary body - between choroid and iris secretes aqueous humor ciliary muscle – focuses lens suspensory ligaments – attach lens to ciliary body 3) Nervous Layer (retina) – inner layer on posterior eyeball location of visual receptors ophthalmoscope – instrument for viewing retina optic disc – optic nerve (cranial nerve II) and blood vessels pass through eye wall blind spot – no visual receptors 2 layers of retina: 1) pigmented layer – outer layer simple cuboidal epithelial cells containing melanin 2) neural layer – inner layer contains photoreceptors and neurons which process visual impulses before passing them to the brain 3 cell layers of neural layer: 1) photoreceptor layer – outermost layer rods – very sensitive to low light (night vision) black and white vision only many rods in nocturnal animals cones – sensitive to different light wavelengths(colors) need more light for stimulation give sharper vision in color central fovea – depression in retina with high concentration of photoreceptors area of sharpest vision, only found in some species (eg. primates, birds) 2) bipolar cell layer – middle layer adjusts contrast of visual image 3) ganglion cell layer – inner layer cell bodies of first-order neurons (CN II) axons cross retinal surface to optic disc Lens – transparent, avascular structure composed of layers of transparent proteins (crystallins) normally round – elasticity allows it to change shape and recoil cataract – opacity of lens which obscures vision Lens Divides Eye Into 2 Compartments: 1) aqueous compartment – anterior to lens anterior chamber – anterior to iris posterior chamber – posterior to iris 5 aqueous humor – watery fluid filling aqueous compartment produced by ciliary body capillaries filtering blood circulates to anterior chamber returns to blood at the canal of Schlemm found at limbus –junction of cornea and sclera 2) vitreous compartment – posterior to lens vitreous humor – jelly-like substance filling compartment holds retina in place intraocular pressure – pressure in eye regulated by aqueous humor glaucoma – eye disease characterized by increased intraocular pressure usually due to decreased drainage of aqueous humor PHYSIOLOGY OF VISION Photoreceptors – (rods and cones) photopigments – organic molecules that absorb light retinal – vitamin A derivative that absorbs light activates a chain of reactions that stimulates receptor cell opsins (4 types) – molecules that promote absorption of different wavelengths of light rhodopsin (in rods) – absorbs any visible light cones – different opsins absorb different wavelengths of light stimulation of photoreceptors releases neurotransmitters bipolar cell layer integrates photoreceptor signals ganglion cells (first-order neurons) integrate bipolar layer signals action potentials are triggered and conducted to the optic disc optic nerves travel to optic chiasm optic chiasm – optic nerves converge and half of axons cross over optic tracts – first-order axons run to thalamus second-order neurons – thalamus to visual cortex (occipital lobe) cerebral hemispheres process information from opposite visual field binocular vision – depth perception due to overlap of visual fields of right and left eyes; placement of eyes varies depending on lifestyle predators – eyes placed forward prey – eyes placed laterally 6 Image Formation Refraction – bending of light rays as they pass through substances of differing densities cornea – accounts for most refraction lens – refraction varies due to changing shape of lens produces inverted image on retina (upside-down and left-right reversal); brain interprets correct orientation Accommodation – focusing light rays on retina by changing curvature of the lens = changing refraction ciliary muscle – circular muscle contraction reduces tension on lens = lens more round relaxation increases tension on lens = flattens lens distant vision (20 ft or more) – parallel light rays focus on retina with relaxed ciliary muscle = flattened lens (no work required) near vision (less than 20 ft) – divergent light rays require more refraction to focus image on retina contracted ciliary muscle = rounder lens (work required) HEARING AND EQUILIBRIUM mechanoreceptors in inner ear ANATOMY OF THE EAR – 3 main regions 1) External ear – collects and funnels sound waves into ear pinna – funnel of elastic cartilage covered by skin skeletal muscles can direct pinna towards sounds external auditory canal – leads from pinna to eardrum L-shaped in most animals vertical canal horizontal canal passes into temporal bone ceruminous glands and hairs protect ear from foreign material otitis externa – excess secretions, hair, and narrowed canals can trap moisture and promote growth of bacteria and yeast tympanic membrane (eardrum) – between external and middle ear outer layer – epidermis middle layer – connective tissue with collagen & elastic fibers inner layer – simple cuboidal epithelium 7 2) Middle ear – air-filled cavity in temporal bone; lined by cuboidal epithelium auditory ossicles – 3 smallest bones in body suspended across middle ear by ligaments malleus (hammer) – attached to inner tympanic membrane articulates with incus incus (anvil) – middle bone; articulates with malleus and stapes stapes (stirrup) – articulates with incus periosteum attaches to oval window (opening to inner ear) 2 muscles protect ear from loud noises by limiting vibrations tensor tympani muscle – attached to malleus stapedius muscle – attached to stapes round window – membrane-covered opening to inner ear Eustachian (auditory) tube – connects middle ear to nasopharynx opens to equalize air pressure in middle ear otitis media – middle ear infection bacteria from pharynx can enter through auditory tube guttural pouch – sac-like extension of the auditory tube in horses 3) Inner ear – location of organs for hearing and equilibrium bony labyrinth – series of cavities in temporal bone that enclose organs of hearing and equilibrium perilymph – fluid similar to CSF within bony labyrinth 3 regions of bony labyrinth: 1) vestibule – central portion saccule and utricle – 2 membranous sacs which function in static equilibrium; filled with endolymph (fluid) 2) 3 semicircular canals – at 90 degrees to each other semicircular ducts – 3 membranous tubes which function in dynamic equilibrium; filled with endolymph 3) cochlea – snail shell-shaped structure cochelar duct – coiled, membranous duct which functions in hearing; filled with endolymph organ of Corti – sensory organ for hearing hair cells – mechanoreceptors; hairs which produce graded potentials when bent supporting cells – support and protect hair cells tectorial membrane – stiff membrane in center of cochlear duct cochlear duct divides cochlea into 2 channels vestibular duct – begins at oval window tympanic duct – ends at round window 8 PHYSIOLOGY OF HEARING sound waves – alternating ripples of high and low pressure air originating from a vibrating object frequency – number of waves/second (Hz) high frequency = high pitch Pathway for Hearing: sound waves funneled into external ear and strike tympanic membrane malleus transmits vibrations to incus, incus to stapes, and stapes to oval window vibrations at oval window membrane produce pressure waves in perilymph of cochlea perilymph pressure waves travel from vestibular duct to tympanic duct to round window (round window prevents echo) pressure waves produce vibrations of cochlear duct hair cells bend against tectorial membrane each region of the cochlear duct vibrates at a different frequency brain interprets as pitch stimulated hair cells release of neurotransmitter neurotransmitter triggers action potential in first-order neurons of cochlear branch of c.n.VIII c.n. VIII runs to brainstem second-order neurons – brainstem to thalamus third-order neurons – thalamus to auditory cortex (temporal lobe) EQUILIBRIUM – sense of balance vestibular apparatus – organs of equilibrium (saccule, utricle, semicircular ducts) equilibrium is also affected by visual and proprioceptive sensations Static Equilibrium – detects body position relative to gravity saccule and utricle (otolithic organs) macula – thickened region in wall of each organ hair cells – mechanoreceptors otolithic membrane – gel-like layer resting on hair cells otoliths – calcium carbonate crystals in gel as head tilts, otoliths (due to gravity) pull otolithic membrane downhill, bending sensory hairs stimulated hair cells release neurotransmitter neurotransmitter triggers action potential in first-order neurons of vestibular branch of c.n.VIII 9 Dynamic Equilibrium – detects head movements semicircular ducts – lie at right angles to each other in 3 planes (sagittal, dorsal, and transverse) ampulla - dilated portion at base of each duct crista – elevation with hair cells cupula – gelatinous material coating hair cells as head moves, ducts and their structures move, but endolymph & cupula inside lag (due to inertia) and bend hair cells bent hair cells release neurotransmitter triggers action potentials in vestibular branch of c.n.VIII vestibular branch of c.n. VIII runs to brain stem second-order neurons run to thalamus and cerebellum complex integration pathways monitor position and movements and use input to coordinate movements of eyes, head & neck, and adjust muscle tone to maintain balance AVIAN SENSES VISION – highly developed sense in birds diurnal (active in day) birds have color vision and may detect UV light eyes are large – sclera is supported by sclerotic ring shape of eye is variable – round, flattened, tubular nictitating membrane has voluntary muscle – birds can use it to protect eye iris contains voluntary muscle – birds can regulate amount of light entering eye retina has more photoreceptors and fewer blood vessels than mammals pecten – vascular structure on retina commonly hemorrhages due to head trauma HEARING no pinna external auditory canal is covered by auricular feathers columella – single bone in middle ear nocturnal birds (eg. owls) have extremely sensitive hearing ears are large with more hairs cells ear placement is asymmetrical to help localize sounds TOUCH feather follicles are supplied with sensitive touch receptors many birds have touch sensitive bills and tongues used to locate food taste and smell are usually not well-developed in birds main sense used for finding food in most species is vision 10