Gustation & Vision { Kayla Coggburn, Grace Davis, Carmen Matthews, and Charlie Williams Gustation • Gustation is taste • Gustation provides information about the foods and liquids that we put into our mouths • We’re able to taste things because of taste buds, which are sensory structures with specialized epithelial cells • The tongue contains basal cells, which divide in order to produce other cells that mature to become gustatory cells Taste Sensations • Humans have four primary taste sensations: sweet, salty, sour, and bitter • There are also two additional, less common taste sensations: Umami and water • Umami is a pleasant taste that is associated with parmesan cheese and soup broth. Steps to Taste • 1. Dissolved chemicals bind to the receptor proteins of the gustatory cells. • 2. Every taste sensation has a different receptor mechanism • 2A. Salty & Sour: Chemically gated ions which result in depolarization • 2B. Sweet, Bitter, and Umami: G proteins called gustducins • 3. Neurotransmitters are released by the receptor cell, resulting in taste receptor stimulation. • 4. The release of the neurotransmitters generates action potential, which leads to taste sensations and reception. Aging and Gustation • Humans start life with over 10,000 taste buds, but the number drops around 50. • As people age they lose sensitivity to foods. Foods that young people find spicy, older people find bland and with too little flavor. Vision-Structures of the Eye • Accessory structures of the eye include: eyelids, superficial epithelium of the eye, and all the structures that are associated with production, secretion, and removal of tears. • Structures that make the exterior of the eye: Palpebral fissure - Gap that separates free space of the upper and lower eyelids Medial canthus & lateral canthus – Where the eyelids do connect Eyelashes – Hairs that prevent objects from entering eye surface Tarsal glands – Secretes a lipid that prevent eyelids from sticking together Conjunctiva – Epithelium that covers inner surfaces of the eyelids & outer layer of the eye Palpebral conjunctiva – Covers inner surface of eyelids Ocular conjunctiva – Covers anterior surface of the eye Cornea – A transparent part of the outer layer of the eye The Eye • The eyes are an irregular spheroid shape, and weigh about 8 oz. • The eye is cushioned by orbital fat • The eye contains three layers 1. Outer layer – fibrous tunic a) Contains the sclera and cornea 2. Middle layer – vascular tunic b) Contains blood vessels, lymphatic vessels, and intrinsic muscles 3. Inner layer – neural tunic (retina) c) Contains a pigmented part and a neural part The Retina • The retina holds photoreceptors, which are responsible for detecting light • The retina has around 130 million photoreceptors • There are two types of photoreceptors: 1. Rods – Do NOT determine colors. Allow people to see light 2. Cones – Color vision • Cones don’t allow you to see color unless there is a strong enough light source to stimulate them • Rods and cones synapse with 6 million bipolar cells, which then synapse with ganglion cells to see color and light • The lens lets our eye focus by changing its shape • An astigmatism may occur if the cornea or lens is curved in an odd way, which causes distortion when looking at something Photoreceptors, Rods & Cones • Photoreceptors detect photons, which are the basic unit of visible light • The process of detecting and receiving light is called photoreception • Humans are receptive to wavelengths 700-400nm (visible light spectrum, ROY G BIV) (Red photons = greatest wavelength, least energy; Violet photons = shortest wavelength, most energy) • Rods are able to give the central nervous system the information needed about the presence or absence of photons. (Seeing light) • Cones give the information of the wavelength of photons. (Seeing color) Photoreception • Sodium ion channels are chemically regulated in the outer portion of the photoreceptor. • In dark: the channels are left open with the presence of cyclic-GMP. Since the channels are open, the transmembrane potential is lower than normal, which makes the photoreceptor releasing neurotransmitters continuously across the inner segment synapses. • That inner layer is simultaneously pumping sodium ions out of the cytoplasm. This is called the dark current. • This is how people are able to vaguely see when the lighting is low Rhodopsin-based Photoreception • Begins when a photon hits a rhodopsin molecule • 4 Steps in rhodopsin-based photoreception: Step 1 – Opsin is activated. Step 2 – Opsin activates transducin, which then activates phosphodiesterase Step 3 – Cyclic-GMP levels decline, and the gated sodium channels close Step 4 – The dark current is reduced, and the rate of neurotransmitters that are released decreases Recovery • After a photon is absorbed, the whole rhodopsin molecule has to be broken down and the put back together • Bleaching is when a rhodopsin molecule starts to break down into retinene and opsin • Retinene must be enzymatically converted by ATP • Opsin is inactive throughout the process of bleaching • Cyclic-GMP is produced, and the sodium ion channels are reopened Color Vision • There are three types of cones: 1. Blue cones 16% of all cones 2. Green cones 10% of all cones 3. Red cones 74% of all cones • Every type of cone has its own form of opsin, and different sensitivities to different wavelength ranges. • The way that the cones are stimulated is the basis for color vision • We see color because of the information received from all three cone types. Yellow is a combination of highly stimulated green cones, less stimulated red cones, and non-stimulated blue cones White is a combination of all three cones stimulated equally Color Blindness • Color blindness occurs when someone has one or more sets of cones that don’t function • The cones could be absent, or just not able to produce the pigment needed • There is inherited color blindness that involves one to two cones. This is not uncommon. • 10% of men have some form of color blindness, but only 0.67% of women do. • Only 1 person out of 300,000 have total color blindness. Where they have no cones at all. Visual Pathway • The visual pathway starts at the photoreceptors and ends at the visual cortex • The message must be sent across two synapses before it can be read. 1. Photoreceptor to bipolar cell 2. Bipolar cell to ganglion cell • M cells give information about the general form and shape of an object, its motion, and shadows. • The activation of M cells indicates that light has been detected and has arrived in a general area. • P cells give information about edges, detail and color. Diseases • Astigmatism – A condition that causes blurred vision from one of three reasons: 1. Irregular shape of the cornea 2. Irregular shape of the cover of the eye 3. Curvature of the lens inside the eye • Conjunctivitis – An inflammation of the conjunctiva (transparent layer of tissue that surrounds the inner part of the eye and the white part) Otherwise known as “pink eye”. It can be caused by: 1. Allergies 2. Bacteria • Cataract – A cloudy, or yellow, part of the clear and transparent lens of the eye. Can be caused by: 1. Diabetes 2. Smoking 3. Certain Drugs 4. Nutritional deficiency Careers • Ophthalmologist - A medical doctor who specializes in eye and vision care • Optometrist – A healthcare professional who provides vision care. Things like sight testing and correction, diagnosis of the problem, treatments, and manages changes in vision. • Optician – A technician who designs, verifies and fits glasses lenses, frames and contact lenses