Special Senses I (01.22.2025) PDF

Special Senses DEN 7101 Fiona Britton, Ph.D.  Special Senses Specialized Organ Sensory Receptor Taste taste bud taste receptor Smell olfactory epithelium olfactory cell Vision retina rods &...

Special Senses DEN 7101 Fiona Britton, Ph.D.  Special Senses Specialized Organ Sensory Receptor Taste taste bud taste receptor Smell olfactory epithelium olfactory cell Vision retina rods & cones Hearing cochlea hair cell Equilibrium vestibular apparatus hair cell  Taste (Gustation) Gustation = the action of tasting Learning Objectives Describe the location and cellular composition of taste buds. Describe key features of taste receptors as sites of sensory transduction. Name the 5 taste modalities. Know examples of various ion channels and GPCR chemoreceptors that activate taste transduction. Describe the sensory transduction mechanism (receptor potential generation) & the neural pathways by which action potentials reach the gustatory region of the cortex. Name abnormalities in taste sensations.  Taste Buds Taste buds are the specialized sense organs for taste. ~5000 taste buds Location: papillae distributed on the dorsal surface of the tongue oral mucosa of the palate, pharynx & epiglottis 3 types of papillae: Fungiform anterior 2/3’s of the tongue; highest density is at the tip Circumvallate large papillae arranged in a chevron at the rear of tongue Foliate present on the posterolateral tongue  Taste Buds Each taste bud contains: 50–100 taste receptors Numerous basal cells & support cells All arranged around a central taste pore on the dorsal surface of the tongue. Taste receptor has microvilli that project into the taste pore. Exposes the microvilli to chemicals dissolved in saliva. Taste receptor cells are replaced every 9-10 days, via differentiation of basal cells, which migrate from adjacent lingual epithelium surrounding the taste bud.  Taste Receptors Taste receptor = sensory receptor modified epithelial cell that responds to chemical stimuli (tastants) => chemoreceptors => site of sensory transduction Taste receptors are innervated by primary afferent sensory nerve fibers (1st -order) that penetrate the basal lamina. These nerve fibers receive synaptic input from the taste receptors. These nerve fibers branch extensively. Each nerve fiber receives input from ~5 taste buds. Each taste bud is innervated by ~50 sensory nerve fibers.  Sensory Transduction in Taste Receptors The apical membrane of taste receptors contain various ligand gated ion channels & GPCRs. Chemical stimuli activate the ion channels and/or GPCR signaling pathways. Results in the generation of a receptor potential (depolarization) Intracellular Ca2+ increases either i) via the opening of voltage- gated Ca2+ channels or ii) via Ca2+ release from intracellular stores. Increased Ca2+ causes synaptic vesicles to fuse with the basolateral membrane for synaptic transmission mediated by serotonin. Serotonin binds to receptors on afferent sensory neurons and From: Taste Receptors and the Transduction of Taste Signals. an AP is elicited.  Taste: Neural Sensory Pathways gustatory cortex Taste receptors synapse with primary afferent sensory neurons (1st order) from branches of 3 cranial nerves. 1. facial (VII) –from fungiform papillae. 2. glossopharyngeal (IX) –from posterior 1/3rd of NTS in tongue. medulla oblongata 3. vagus (X) from palate, pharynx & elsewhere Facial nerve (VII) Vagus All sensory nerves synapse with 2nd-order neurons Glossopharyngeal nerve (X) in the NTS (nucleus of the tractus solitarius) in the nerve (IX) medulla. NTS relays information to the thalamus (3rd order). Thalamus projects to the gustatory cortex.  Taste Modalities 5 different chemoreceptors (taste receptors) sense chemical signals from bitter, sweet, sour, salty and umami substances. 5 basic tastes Common stimuli sweet sucrose salty NaCl sour acid bitter quinine umami (‘oo-ma-me’, savory/meaty) monosodium glutamate  Taste receptors for the 5 modalities of taste Salt & Sour chemicals activate ligand-gated ion channels Mediated mainly via epithelial sodium channels (ENaC). Salt sensed by Na+ ions; Sour triggered by H+ ions. Sweet, Bitter & Umami chemicals activate GPCRs When stimulated, each of these LGIC or Sweet: T1R family GPCR generates a receptor potential and Bitter: T2R family (coupled to gustducin causes neurotransmitter release. - a Gs-protein) Umami: Glutamate receptor (mGluR4)  Sensory Coding for Taste Each taste has its own distinct nerve response pattern. Taste is coded in the summation of neural activity (APs) from each cranial nerve (facial [VII], glossopharyngeal [IX], vagus [X]. The response of different taste receptors to a specific taste modality and the subsequent summation of the generated APs encode taste. Intensity of lights as an example of summed neural activity from each cranial nerve.  Abnormalities in Taste ▪ Ageusia - absence of taste ▪ Hypogeusia - diminished taste sensitivity ▪ Dysgeusia - unpleasant perception of taste (metallic, foul, rancid) Causes: Damage to facial or glossopharyngeal nerve neurological disorders e.g. Bell palsy, multiple sclerosis, infections Vitamin B3 or zinc deficiency Poor oral hygiene can cause problems with taste sensitivity Adverse side effect of drugs can diminish saliva secretion e.g. xerostomia Aging Tobacco use  Smell (Olfactation) Learning Objectives Describe the structure and function of the neural elements in the olfactory epithelium and olfactory bulb. Explain how odorant receptors are activated. Explain the mechanism by which signal transduction occurs in olfactory receptors. Name abnormalities in odor sensations.  Structure of the olfactory bulb The olfactory bulb has a specialized arrangement of neurons. Olfactory sensory neurons (OSNs) are the sensory receptors for olfactory transduction. OSNs are primary sensory afferents (1 st-order) The glomerulus is the site where OSN synapse with dendrites of mitral cells. Mitral cells are 2nd -order sensory neurons, whose axons enter the olfactory tract and ascend to the olfactory cortex.  Structure of the olfactory epithelium Olfactory epithelium is a specialized portion of the nasal mucosa. 1. Olfactory sensory neurons (OSNs) sensory receptors for olfactory transduction Each OSN projects a dendrite which terminates in olfactory cilia into the mucus layer of the nasal cavity epithelium. Odorant receptors are on these cilia. OSN axons pass through the cribriform plate (bone) to the olfactory bulb. 2. Supporting columnar cells Secrete mucus that provides the appropriate environment for odor detection. 3. Basal stem cells Generate new olfactory sensory neurons as needed.  Olfactory Receptors The olfactory system is an extremely discriminative & sensitive chemosensory system. < 1 million distinct odors There are ~ 1000 human genes for olfactory receptors = 3% of the human genome Floral Roses Ethereal Pears Twelve qualities with 500-1000 odorant receptors. Musky Musk Evaluated by “professional noses” in perfume, wine, Camphor Eucalyptus cosmetic industries. Putrid Rotten eggs Pungent Vinegar All olfactory receptors are G-protein-coupled receptors (GPCRs) Odorants bind to GPCRs on the cilia and initiate the sensory transduction cascade of events leading to generation of APs in the sensory olfactory nerve.  Olfactory receptors: Sensory Transduction A smell = chemical in the air dissolved in mucus. Odorants bind reversibly to GPCRs a G s group of proteins called Golf. G-protein α-subunit dissociates and activates adenylyl cyclase to catalyze cAMP production. cAMP acts as a 2 nd messenger to open Na + /Ca2+ ion channels. Inward diffusion of Na + and Ca2+ produces depolarization = receptor potential.  Olfactory receptors: Sensory Transduction Receptor potentials are a graded response depending on the concentration of odorants. Receptor potentials can be summated to produce specific patterns of activity for each stimulating odorant contributes to neural coding of odors. When the RP exceeds its threshold stimulus, APs in the olfactory nerve (CN1) are generated. APs are propagated to synapses in the glomeruli within the olfactory bulb. Olfactory nerve AP frequency is proportional to the concentration of odorants. AP can be attenuated by rapidly adapting receptors.  Olfactory receptors: Sensory Transduction The glomeruli is the site where axons of OSN synapse with numerous dendrites from mitral cells and tufted cells. Millions of olfactory axons converge upon only a few thousand glomeruli within each bulb to synapse with about 75,000 mitral cells. The mechanism of convergence/divergence of olfactory neurons with mitral cells generate subsequent APs. This complex pattern of neuronal integration discriminates the various odorant molecules. Sensory coding for smells are created from the activation of multiple receptors and neurons. The axons of mitral cells (2 nd order neurons) enter the olfactory tract and ascend to the olfactory cortex. Abnormalities in Olfaction ▪ Anosmia - inability to smell ▪ Hyposmia - diminished olfactory sensitivity ▪ Hyperosmia - enhanced olfactory sensitivity (common in pregnancy) ▪ Dysosmia - distorted sense of smell Causes include: Damage to olfactory nerves (head trauma, tumors). Sinus infections Nasal congestion Nasal polyps Poor dental hygiene  Vision Learning objectives Be familiar with the various layers of the eye. Explain how refraction and accommodation bring light rays to a focus on the retina. Explain the refractive deficits responsible for hyperopia and myopia. Describe the functional organization of the retina. Describe the electrical responses and the sequence of events involved in phototransduction. Describe the process involved in color vision and the types of color blindness Define visual acuity, and age-related macular degeneration. Trace the neural pathways that transmit visual information from photoreceptors to the visual cortex. Basic mechanism of Vision The eyes convert photon energy (electromagnetic waves in the visible spectrum), into APs in the optic nerve that are conducted to the cerebral cortex, where they produce the sensation of vision. Optical component Visual image is focused on photoreceptors on the retina. Neural component The visual image is transduced into a pattern of graded receptor potentials.  Basic Anatomy of the Eye A fluid-filled sphere enclosed by 3 layers of tissue. Fluids: aqueous humor & vitreous humor Outer layer Sclera. Protective white layer of the eyeball. Light cannot pass. Cornea. Transparent anterior portion through which light rays enter the eye. Middle layer - 3 distinct but continuous structures: Iris. Colored portion of eye, in front of the lens. Ciliary body. Adjusts the refractive power of the lens. Choroid. Nourishing vascular layer Inner layer Retina -Neural tissue containing photoreceptor cells Lines the posterior 2/3rds of the choroid.  Formation of images on the retina Light rays diverge in all directions from their source. For optimal vision, light rays that reach the pupil must be focused to a point on the retina Refraction of light Light rays bend as they pass through transparent materials of different densities. Refraction is the mechanism that permits the formation of focused images on the retina. Refraction achieved by the a) cornea b) lens  Formation of images on the retina Properties of light Light rays striking a concave Light rays striking a convex surface converge at a surface diverge. single point called a focal point. For optimal sight, the focal point should be on the retina. The focal length is the distance between the lens and the retina.  Formation of images on the retina Both the cornea and the lens have convex surfaces which focus light rays onto the retina. The cornea is responsible for most of the refraction. However, the lens can adjust its shape (curvature) to adjust the focal length when focusing on objects at different distances. Termed accommodation.  Accommodation of the Lens Accommodation. Process by which the curvature of the lens is adjusted. i.e. the refractive power of the lens is adjusted Mechanism: Focal length is adjusted through the contraction of ciliary muscles. The lens is suspended by zonule fibers attached to ciliary muscle. Viewing near objects: Ciliary muscles contract which relaxes zonule fiber tension on the lens. The lens become more rounded, curvature increases, refractive power increases. Viewing distant objects: Ciliary muscles relax increasing zonule fibers tension, lens flattens, lesser curvature Refractive power is greatest when the lens curvature is greatest.  Defects of the image-forming mechanism Correct focus = Emmetropia Myopia = nearsightedness Hyperopia = farsightedness Light rays focus in front of the retina. Light rays focus beyond the retina. Distant objects are blurry. Near objects are blurry. Corrected with biconcave lens - cause light Corrected with biconvex lens - adds to rays to diverge so rays are brought to focus the refractive power of the lens. on the retina.  ANS control of Pupil Diameter The amount of light entering the eye is adjusted by the size of the pupil. Pupil size is controlled by the iris. The iris consists of: 1. inner circular muscle layer controlled by parasympathetic neurons - constricts the pupil in bright light (miosis) 2. outer radial muscle layer controlled by sympathetic neurons - dilates the pupil in dim light (mydriasis)  The Retina Retina is the specialized sensory organ of the eye. The retina is the neural portion of the eye. 5 types of neurons in the retina: photoreceptors bipolar cells ganglion cells horizontal cells amacrine cells Retina has 3 distinct layers where the cell bodies of these neurons are stacked: Outer - contains rods and cones Middle - contains bipolar cells Inner - contains ganglion cells Structure of the retina  Photoreceptors: Rods & Cones Rods are the photoreceptors for night vision. Cones are the photoreceptors for color vision. Outer segment Composed of membranous disks in regular stacks (rods) or in flattened sacs (cones). Contain a photosensitive pigment that react to light to initiate receptor potentials. Inner segment Contains the cell nucleus that synthesizes photosensitive compounds. Synaptic terminal Region of synaptic contact with bipolar or horizontal cells.  Photoreceptors: Rods & Cones Photosensitive pigments Each photosensitive pigment is a compound of Molecular structure of a typical 1. a retinal molecule photosensitive pigment Retinal exists in 2 configurations In the dark; 11-cis retinal In the light; 11-trans retinal And 2. a light sensitive opsin protein The type of opsin differs in rods and cones. The photosensitive pigment of rods is rhodopsin The opsin present will determine which wavelength of light is optimally absorbed by the photoreceptor. FYI: Vitamin A is needed for the synthesis of retinal. Vit A deficiency → visual abnormalities.  Photoreceptors: Rods & Cones Photosensitive pigments Rods contain a single photopigment = rhodopsin Cones can have 3 types of photopigment (opsins). Each opsin is sensitive to light of different wavelengths referred to as blue, green, and red cones Short (S), medium (M), and long (L) wavelength cones Color vision is trichromatic Color perception is determined by the relative Absorption spectra of neuronal AP frequencies from each of these 3 photopigments. Solid curves = 3 cones. kinds of cone opsins; dashed Any spectral color can be produced by mixing curve = rod rhodopsin various proportions of these colors.  Phototransduction In rods, the photosensitive pigment is rhodopsin. Rhodopsin is coupled with a trimeric Gs protein called transducin. Light stimulation of rhodopsin leads to a structural change in 11-retinal (cis- to trans-) and a conformational change in opsin. Transducin G protein is stimulated, the  subunit dissociates and in turn activates phosphodiesterase enzyme which degrades cyclic GMP (cGMP). Thus exposure to light decreases cGMP levels.  Phototransduction cGMP is the ligand for cGMP-gated ion channels present in the outer segment membrane of both rods & cones. These are Na+ -permeable ion channels which generate the receptor potential (RP) in photoreceptors. In the dark: cGMP levels are high cGMP binds to and opens Na+-permeable channels Na+ (and other cations) enter Depolarization of the photoreceptor cell (=> RP) Exposure to light PDE hydrolyzes cGMP, decreasing cGMP levels Na+-permeable channels close Photoreceptor produces a hyperpolarizing RP. The Process of Phototransduction Photoreceptor in the Dark Fyi: The neurotransmitter is glutamate  The Process of Phototransduction Photoreceptor in the Light Neural Pathway for Vision  Neural Pathway for Vision Optic nerve (CN II) formed by the axons of retina ganglion cells. CN II exits each eye and combines in front of the brainstem to form the optic chiasm. 2 optic tracts emerge from the optic chiasma to the associated lateral geniculate nucleus (LGN) of the thalamus. Terminate in the primary visual cortex. Left visual field: Light strikes the nasal retina of the left eye & the temporal retina of the right. Ganglion cell axons from the left nasal retina decussate to the right brain at the optic chiasm while those from the right temporal retina stay on the same side. Right visual field: The same is true for light from the  Anatomical distribution of Rods & Cones The fovea is a depression in the retina where eyesight is sharpest. Cones Low density throughout the retina, with a high density in the fovea. Rods Present at high density throughout the retina, but absent in the fovea. Optic disk. The region of the retina where the optic nerve leaves the eye. No photoreceptors in this area so does not respond to light = blind spot  Visual Acuity The fovea is the point on the retina where visual acuity is greatest Each cone photoreceptor synapses on a single bipolar cell, which in turn, synapses on a single ganglion cell, providing a direct pathway to the brain. In the fovea, the density of cones is high and the 1-1 relationship with bipolar and ganglion cells allows this region to mediate high visual acuity. Fovea literally means “pit” The fovea is in the center of a yellow pigmented region of the retina called the macula. Age-related macular degeneration is a disease in which there is gradual deterioration of visual acuity Visual Acuity The degree to which the details of objects are perceived. Defined as the shortest distance by which 2 lines can be separated and still be perceived as 2 lines. Visual acuity is often determined using Snellen letter charts viewed from a distance of 20 ft (6 m)  Color Blindness Ishihara charts are the most common test for color blindness Color blindness can be an inherited abnormality Red/green color blindness most common. Inherited as recessive and X-linked 8% males /0.4% of females Color blindness can also occur in individuals with lesions of the visual cortex. Transient blue-green color weakness occurs as a side effect in individuals taking Viagra for the treatment of ED because the drug inhibits the retinal phosphodiesterase.