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The Special Senses. Sensory Reception. Sensory reception is a great example of the diversity of neuronal function.
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Sensory Reception • Sensory reception is a great example of the diversity of neuronal function. • Said another way, if we want to know how our brains work, the senses are a good place to start. They are probably simple versions of things like personality and reasoning.
Chemical Senses • Chemical senses – gustation (taste) and olfaction (smell) • Their chemoreceptors respond to chemicals in aqueous solution • Taste – to substances dissolved in saliva • Smell – to substances dissolved in fluids of the nasal membranes
Taste Buds • Most of the 10,000 or so taste buds are found on the tongue • Taste buds are found in papillae of the tongue mucosa
Taste Sensations • There are five basic taste sensations • Sweet – sugars, saccharin, alcohol, and some amino acids • Salt – metal ions • Sour – hydrogen ions • Bitter – chemicals, toxins, medicines • Umami – monosodium glutamate (MSG)
Taste Transduction • Taste receptors work just like neurons. Their axon hillock has to get to -55mV. • Na+ influx in salty tastes • H+ in sour tastes (by directly entering the cell, by opening cation channels • Sweet by closing K+ leak channels) • Bitter by releasing Ca2+ from intracellular stores. • Umami by opening glutamate channels
Gustatory Pathway • Cranial Nerves VII, IX, and X carry impulses from taste buds to the medulla • These impulses then travel to the thalamus, and from there fibers branch to the: • Gustatory cortex (taste) • Hypothalamus and limbic system (appreciation of taste) Throat
Think about taste • How many taste receptors have we discussed? • How many tastes do you think you can sense? • What do you think the mechanism for sensing so many tastes with 5 receptors be? Nobody really knows.
Think about taste • Black vs. green olives • I hate tomatoes but I am ok with spaghetti sauce. • I am not particular about taste, I have eaten spaghetti thousands of times. • Cold pizza, hot pizza, instant cold pizza. • Wine gargle • Meat substitutes • Why do some people like salt with their food (fat) • Sugar on cottage cheese. I ask Ella and Celi to try all their foods, but I will never try this.
Influence of Other Sensations on Taste • Taste is 80% smell • We also sense heat, texture, and pain. • Temperature and texture enhance or detract from taste
Sense of Smell • Olfactory receptor cells are bipolar neurons with radiating olfactory cilia
Physiology of Smell • Olfactory receptors respond to several different odor-causing chemicals • You can smell about 10,000 smells • When bound to ligand these proteins initiate a G protein mechanism, which uses cAMP as a second messenger • cAMP opens Na+ and Ca2+ channels, causing depolarization of the receptor membrane that then triggers an action potential
Olfactory Transduction Process Na+ Odorant binding protein Odorant chemical Active Inactive Na+ influx causes depolarization ATP Adenylate cyclase cAMP Depolarization of olfactory receptor cell membrane triggers action potentials in axon of receptor Cytoplasm
It must be the combination of different receptors that allows us to smell a variety of smells. Carnation Rose Skunk
It must be the combination of different receptors that allows us to smell a variety of smells.
Think about smell • One thing to note is that you smell and taste different types of molecules. • Taste = small ions and small molecules • Smell = usually large organic molecules • http://leonlab.bio.uci.edu/odorants.cfm • Here again, as with taste, you have more smells than you have receptors. • How? • Why do we smell?
Olfactory Pathway • Olfactory receptor cells synapse with mitral cells • Glomerular mitral cells process odor signals • Mitral cells send impulses to: • The olfactory cortex • The hypothalamus, amygdala, and limbic system
Eye and Associated Structures • 70% of all sensory receptors are in the eye • Accessory structures include eyebrows, eyelids, conjunctiva, lacrimal apparatus, and extrinsic eye muscles
Conjunctiva • Transparent membrane that: • Lines the eyelids as the palpebral conjunctiva • Covers the whites of the eyes as the ocular conjunctiva • Lubricates and protects the eye • Conjuctivitis (pink eye)
Lacrimal Apparatus • Consists of the lacrimal gland and associated ducts • Lacrimal glands secrete tears • Tears • Contain mucus, antibodies, and lysozyme
Extrinsic Eye Muscles • Six straplike extrinsic eye muscles • Enable the eye to follow moving objects • Maintain the shape of the eyeball • Four rectus muscles originate from the annular ring • Two oblique muscles move the eye in the vertical plane
Summary of Cranial Nerves and Muscle Actions • Names, actions, and cranial nerve innervation of the extrinsic eye muscles
Structure of the Eyeball • A slightly irregular hollow sphere with anterior and posterior poles • The wall is composed of three tunics – fibrous, vascular, and sensory • The internal cavity is filled with fluids called humors
Iris • The colored part of the eye • Pupil – central opening of the iris • Regulates the amount of light entering the eye during: • Close vision and bright light – pupils constrict • Distant vision and dim light – pupils dilate • Changes in emotional state – pupils dilate when the subject matter is appealing or requires problem-solving skills • Pinhole effect
Retina • A delicate two-layered membrane • Pigmented layer – the outer layer that absorbs light and prevents its scattering • Neural layer, which contains: • Photoreceptors that transduce light energy • Bipolar cells and ganglion cells • Amacrine and horizontal cells
The Retina: Photoreceptors • The primary component of the neural layer are the… • Rods: • Respond to dim light • Are used for peripheral vision • Cones: • Respond to bright light • Have high-acuity color vision • Are concentrated in the fovea centralis We’ll come back to rods and cones when we learn physiology
Blood Supply to the Retina • The neural retina receives its blood supply from two sources • The outer third receives its blood from the choroid • The inner two-thirds is served by the central artery and vein Choroid Central
Macular Degeneration • Macular Degeneration is when the visual center of the retina is damaged. • There are two types; wet and dry. • Wet = blood • Dry = metabolic breakdown products of the retina
Inner Chambers and Fluids • The lens separates the internal eye into anterior and posterior segments Anterior Posterior
Anterior Segment Anterior Posterior • Composed of two chambers • Anterior – between the cornea and the iris • Posterior – between the iris and the lens • Aqueous humor • A plasmalike fluid that fills the anterior segment • Supports, nourishes, and removes wastes
Inner Chambers and Fluids • The posterior segment is filled with a clear gel called vitreous humor that: • Transmits light • Supports the posterior surface of the lens • Holds the neural retina firmly against the pigmented layer • Contributes to intraocular pressure (glaucoma and detached retina)
Anterior Segment Anterior Posterior
Lens • A biconvex, transparent, flexible, avascular structure that: • Allows precise focusing of light onto the retina • Is composed of epithelium and lens fibers • Lens epithelium – anterior cells that differentiate into lens fibers • Lens fibers – cells filled with the transparent protein crystallin • With age, the lens becomes more compact and dense and loses its elasticity
Light • Our eyes respond to a small portion of this spectrum called thevisible spectrum • Different cones in the retina respond to different wavelengths of the visible spectrum
Refraction and Lenses • When light passes from one transparent medium to another its speed changes and it refracts (bends) • Light passing through a convex lens (as in the eye) is bent so that the rays converge to a focal point
Refraction and Lenses • When a convex lens forms an image, the image is a horizontal and vertical reflection.
