Gustation & Eye Structure PDF

Gustation Sense of taste Detection of chemicals by chemoreceptors within the oral cavity Allows us to sample the contents of the food and drink that we consume Aided by our olfactory sense Organization of gustatory...

Gustation Sense of taste Detection of chemicals by chemoreceptors within the oral cavity Allows us to sample the contents of the food and drink that we consume Aided by our olfactory sense Organization of gustatory system Papillae possess taste buds Found on the anterior surface of the tongue Found on the soft palate Taste buds house the gustatory cells Onion-shaped structure containing a variety of gustatory cells and support cells Gustatory cells are the taste receptor cells Papillae of the tongue Foliate Leaf-like Lateral boundary Not well developed in humans Functioning mostly in infancy and early childhood Filiform Thread Short and spiky Anterior 2/3rd of tongue Do not house taste buds Detect texture and manipulate food Papillae of the tongue Circumvallate Surrounding About 10 to 12 creating a boundary between the anterior and posterior aspects Form an inverted “V” Each papillae is surround by a deep, narrow depression Greatest concentration of taste buds Fungiform Mushroom-shaped About 300 over the whole tongue Primarily anterior margin of the tongue Contain a few taste buds Taste buds Taste pores Open to the oral cavity Dissolved taste-producing chemicals (tastants) come into contact with the variety of gustatory cells Structure Gustatory cells are packaged in taste buds Basal cells: stem cells Transitional cells: support cells Nerve fibers found between cells Life span of a taste receptor cell ~10 days Gustatory cells Specialized neuroepithelium Dendritic ending is formed by the gustatory microvilli (taste hairs) Extends through the taste pore Contacts the saliva and the environment of the oral cavity Tastants interact with receptors on the microvillus Four types of cells Type I Respond to Na+ ions (salt) Type II Transduce sweet, umami and bitter Use GPCRs to detect tastants Type III Respond to sour stimuli Type IV Serve as the stem cell Gustatory pathway Gustatory cells contact sensory neurons of the facial nerve (CN VII) and glossopharyngeal nerve (CN IX) CN VII innervates anterior 2/3rd of tongue CN IX innervates posterior 1/3rd of tongue CN X innervates the epiglottis and lower pharynx Axons project to Medulla Oblongata to 2° neurons projecting to thalamus Tertiary neurons to cortical gustatory area Other neurons project from thalamus to hypothalamus and limbic system Add dimensions to taste (pleasantness, etc.) Process behavioral aspects associated with taste and smell Five primary tastes Salty Stimulated by chemical salts. Direct entry of Na+ through specialized Na+ channels, called ENaC channels Found on Type I gustatory cells Sour Caused by free H+. H+ binds to and blocks K+ channels in the receptor membrane reducing passive movement of K+ Found on Type III gustatory cells Type I (salty) & type III (sour) cells Five primary tastes Sweet Specific configuration of glucose. Activates a GPCR to cause depolarization through second messenger cascade Found on Type II gustatory cell Umami Triggered by amino acids, esp. glutamate. Activates a GPCR to cause depolarization through second messenger cascade Found on Type II gustatory cell Five primary tastes Bitter Chemically diverse group including alkaloids Activates a GPCR to cause depolarization through second messenger cascade Found on Type II gustatory cell These cells contain ~50-100 differing bitter taste receptors to respond to different bitter flavors Type II cells Receptor identification has been fairly recent Do not memorize anything on this slide Other taste receptors? Taste perception is also influenced by other information from other receptors, especially olfactory receptors Carbon Dioxide receptor Car4 Fatty receptors ?? The taste bud tongue map myth 1901 D. Hänig publishes paper Contains data on taste sensitivity in different regions of the tongue. The data are later misinterpreted, giving rise to the myth of the ‘tongue map’. Eye Structure Almost spherical organ Receded into the orbit of the skull Cushioned on anterior and lateral sides by orbital fat Three principal layers (outside to inside) Fibrous tunic Includes Sclera and cornea Vascular tunic Includes Iris, Ciliary body, and Choroid Nervous tunic Includes retina and optic nerve Visible light A small portion of the total electromagnetic spectrum Form of electromagnetic radiation composed of photons that travel in waves Wavelength Distance between two wave peaks Represents energy carried by the photon Amplitude Height between the trough and peak Represents the intensity Protective Structures of the Eye Eyebrows Thick row of short hairs on supraorbital ridge Prevent sweat from dripping into open eyes Eyelashes Extend from the margin of the eyelids Prevent large foreign objects from contacting anterior surface of the eye Eyelids, aka palpebrae Superior and inferior movable anterior coverings of the eye Consist of fibrous core, muscles, glands, conjunctiva, and skin Tarsal glands (aka Melbonian glands) produce sebum to prevent tear overflow from eye and superior/inferior adherence Space between the two eyelids is the palpebral fissure Lacrimal caruncle found medially contain ciliary glands that produce the gritty particulate found on waking Conjunctiva Specialized stratified squamous epithelium Forms a continuous lining over the external, anterior surface of the eye and the internal eyelid Two parts Ocular conjunctiva Palpebral conjunctiva Fold between two is called conjunctival fornix Contains numerous goblet cells that lubricate and moisten the eye Highly vascularized to supply nutrients to the sclera Highly innervated Does not cover the cornea Lacrimal Apparatus Produces, collects, and drains tears from the eye Lacrimal fluids (aka tears) Lubricate the anterior surface of the eye know!! Cleanses and moistens eye surface Antibacterial (contains lysozyme) Lacrimal gland located in superolateral depression of each orbit Continually produces tears Blinking washes the fluid towards the lacrimal caruncle Tears pass through lacrimal puncta into the lacrimal canaliculi Into the lacrimal sac and down through the lacrimal duct into the nasal cavity Fibrous (external) Tunic Sclera Posterior structure “white” of the eye Continuous with the dura mater that surrounds optic nerve Functions: Provides eye shape Protects the eye Attachment for extrinsic eye muscles Cornea Anterior structure Convex, transparent structure Composed of epithelium Avascular thus supplied nutrients from lacrimal gland (anteriorly) and aqueous humor (posterior) Functions: Allows for the passage of light into the interior of the eye Refracts (bends) light as it enters Extrinsic eye muscles Allow eye movement Keep eyes stable in the orbital Attached to the fibrous tunic Vascular Tunic Highly vascularized Contains intrinsic muscles of the eye Three parts Choroid Posterior vascular region Provides nutrients to the retina Numerous melanocytes to absorb extraneous light Ciliary Body Anterior to choroid Ciliary muscles Smooth muscle attached to suspensory ligaments attached to the lens Contraction/relaxation alters the shape of the lens Ciliary processes Produces aqueous humor Iris Anterior structure Colored portion of the eye Two groups of smooth muscle responsible for adjusting the amount of light entering into the eye Iris Thin pigmented smooth muscle Controls amount of light entering the eye Pupil Opening in the center of the iris through which light passes Two smooth muscles Sphincter pupillae Circular Contractile Parasympathetic innervation Dilator pupillae Radial Dilating Sympathetic innervation Retina Innermost tunic Two layers Pigmented layer Adjacent to the choroid Provides Vitamin A for the neural layer Absorbs light passing through the neural layer Neural layer Innermost layer of eye Contains photoreceptor cells and associated neurons Transduces light energy into nerve signals Cells of the neural layer Photoreceptor cells Outermost layer of cells Rods & Cones Convert light energy into a nerve signal Generate graded potentials Bipolar cells Lie between photoreceptor cells & ganglion cells Fewer cells than photoreceptor cells Converge visual input onto the ganglion cells Generate graded potentials Ganglion cells Innermost layer of neurons Axons send signals to CNS via optic nerve Generate action potentials Others Horizontal cells Integrate stimuli between photoreceptor and bipolar cells Generate graded potentials Amacrine cells Integrate visual information between bipolar and ganglion cells Generate action potentials Other cells Photosensitive ganglionic cells Found in the neural layer Light sensitive Possess melanopsin Function Help to reset the internal circadian clock by sending information to the suprachiasmatic nucleus in the hypothalamus Help to regulate pupil size by sending information about light hitting the retina to the olivary pretectal nucleus in the midbrain Help to regulate the release of melatonin by the pineal gland Retinal pigment epithelial cells Found in the pigmented layer Contain melanin to trap light Function Capturing unused photons to prevent reflection Store and recycle vitamin A Components of the retina Distribution of rods and cones is not uniform Optic disc Contains no photoreceptors Point where the axons of ganglion cells converge to exit the retina and form the optic nerve Aka “blind spot” Medially located Macula lutea Lies directly in line with the pupil, lateral to the optic disc Point where light entering the eye is directed Fovea centralis Depressed pit High concentration of cones, lacks rods Area of sharpest vision Peripheral retina Contains mostly rods Mostly effictive in low light The lens Transparent structure that focuses light onto the retina Shape determines degree of refraction Modified by the suspensory ligaments and ciliary muscles When ciliary muscles are relaxed Suspensory ligaments are pulled taut, lens becomes flat Distant objects are viewed Default position When ciliary muscles are contracted Suspensory ligaments are loose, lens becomes rounded Near objects are viewed Process of changing shape of lens from flattened to round is called accommodation Refraction Different mediums affect the speed at which light waves travels Transition from one medium to another causes the light to bend (refract) Lens shape directs the direction light bends Concave Thin in the middle Cause light to bend away from a focal point Convex Thick in the middle Cause light to bend toward a focal point Cavities of the Eye Posterior Cavity Posterior to lens and anterior to retina Filled with vitreous humor Gelatinous fluid Functions Anterior cavity Anterior chamber Maintains the shape of the eye Holds retina against the choroid Iris Transmits Light Posterior chamber Anterior Cavity Lens Anterior to the lens and posterior to the cornea Filled with aqueous humor Posterior cavity Filtrate from the blood plasma Produced by the ciliary bodies Provides nutrients to the cells of the lens and cornea Subdivided into two chambers Posterior chamber lies between lens and iris Anterior chamber lies between iris and cornea Focusing light on the retina Bending of light (refraction) to focus on the light sensitive cells of the retina Every structure (and liquid) that light passes through affects the course of light Cornea is initial structure Modification of lens shape (accomodation) helps to focus near or far light to retina Focal point of collected light is the fovea centralis High density of cones Rods and Cones Rods One type Outer segment is filled with membrane bound discs Concentrated towards the periphery ~100 x 106/retina Responsible for vision in dim light High sensitivity, low acuity Can not distinguish colors Exhibit a high degree of convergence on ganglion cells Cones Three types Each type is maximally sensitive to different wavelengths of light Gives Rise to color vision Outer segment consists of folds of the plasma membrane Concentrated in the fovea centralis ~3-10 x 106/retina Responsible for vision in bright light Low sensitivity, high acuity Exhibit a low degree of convergence on ganglion cells Photopigment Light absorbing photoreceptor responsible for initiating light energy transduction Found in the membrane of the membrane-bound disc Two parts Opsin G-protein coupled receptor Several types each stimulated over a range of light wavelengths Rhodopsin found in rods, range ~400 – 600 nm, λmax ~500 nm Photopsins found in cones Blue (S) cones, range ~380 – 550 nm, λmax ~420 nm Green (M) cones, range ~400 – 650 nm, λmax ~535 nm Red (L) cones, range ~400 – 700 nm, λmax ~560 nm Retinal Vitamin A derivitive Ligand to opsin Responsible for “tuning” the absorption range of the opsin molecule Two states dependent on light absorption Color perception Dependent on the ratio of stimulation of the various cone types in response to different wavelengths of light Example: Light with wavelength of 560 nm Red cone: maximally stimulated (100%) Green cone: ~80% stimulation Blue cone: ~0% stimulation Perceived color: greenish yellow Receptive fields The area in which a photon can stimulate a single ganglion cell Represents the number of photoreceptor cells that converge on a single ganglion cell More cells = large receptive field Less cells = smaller receptive field Convergence and acuity Acuity is dependent on the number of photoreceptor cells per ganglion Low convergence yields more acuity High convergence yields less acuity The degree of convergence correlates to receptive field Low convergence yields smaller receptive fields & greater acuity High convergence yields larger receptive fields & reduced acuity Dark adaptation Ability for our eyes to adjust to low amounts of available light after exposure to high amounts of light Dependent on rods and cones Cones adapt quickly, but require more light Rods adapt slowly, but require less light Gives rise to photopic (cone dependent) and scotopic vision (rod-dependent)