Chapter 15 The Special Senses 15-* Special Senses Olfaction Taste Visual system Hearing Balance Olfaction Seven primary odors now recognized, but average person can recognize 4000 different odors. Perceived by olfactory epithelium. Dendrites of olfactory neurons have enlarged ends (olfactory vesicles). Cilia (olfactory hairs) of olfactory neuron embedded in mucus. Odorants dissolve in mucus. Odorants attach to receptors, cilia depolarize and initiate action potentials in olfactory neurons. One receptor may respond to more than one type of odor. Olfactory epithelium is replaced as it wears down. Olfactory neurons are replaced by basal cells every two months. Unique: most neurons are permanent cells (aren't replaced if they die). Odorant Binding to Membrane of Olfactory Hair Neuronal Pathways of Olfaction Olfactory sensory pathway: olfactory neurons (bipolar) in the olfactory epithelium pass through cribiform plate to olfactory bulbs and synapse with tufted cells or mitral cells. These extend to the olfactory tract and synapse with association neurons. Association neurons also receive input from brain, so information can be modified before it reaches the brain. Information goes to olfactory cortex of the frontal lobe without going through thalamus (only major sense that does not go through thalamus). Three regions in frontal lobe affect conscious perception of smell and interact with limbic system. Lateral olfactory area: conscious perception of smell Medial olfactory area: visceral and emotional reactions to odors Intermediate olfactory area: effect modification of incoming information. Taste Types of papillae Filiform. Filament-shaped. Most numerous. No taste buds. Rough in texture and help to manipulate food. Vallate. Largest, least numerous. 8-12 in V along border between anterior and posterior parts of the tongue. Have taste buds. Fungiform. Mushroom-shaped. Scattered irregularly over the superior surface of tongue. Look like small red dots interspersed among the filiform. Have taste buds. Foliate. Leaf-shaped. In folds on the sides of the tongue. Contain most sensitive taste buds. Decrease in number with age. Taste bud: supporting cells surrounding taste (gustatory) cells. Taste cells have microvilli (gustatory hairs) extending into taste pores Replaced about every 10 days Taste bud: supporting cells surrounding taste (gustatory) cells. Taste cells have microvilli (gustatory hairs) extending into taste pores Replaced about every 10 days Taste Taste Types Salty. Most sensitive receptors on tip of tongue. Shares lowest sensitivity with sweet. Anything with Na+ causes depolarization plus other metal ions. Craved by humans. Sour. Most sensitive receptors on lateral aspects of the tongue. Sweet. Most sensitive receptors on tip of tongue. Shares lowest sensitivity with salty. Sugars, some carbohydrates, and some proteins (NutraSweet: aspartame). Craved by humans. Bitter. Most sensitive receptors on posterior aspect. Highest sensitivity. Sensation produced by alkaloids, which are toxic. Umami (Glutamate). Scattered sensitivity. Caused by amino acids. Craved by humans. Taste Texture affects the perception of taste Temperature affects taste perception Very rapid adaptation, both at level of taste bud and within the CNS Taste influenced by olfaction Different tastes have different thresholds with bitter being the taste to which we are most sensitive. Many alkaloids (bitter) are poisonous. Actions of the Major Tastants Neuronal Pathways for Taste Chorda tympani (part of VII): carry sensations from anterior one-third of tongue (except from circumvallate papillae Cranial nerve IX and X carry information from posterior one-third tongue, circumvallate papillae, superior pharynx, epiglottis. Information goes to medulla oblongata where decussation takes place and information projects from there to the thalamus. Then projects to taste area of cortex (extreme inferior end of the postcentral gyrus) Light Visible light: portion of electromagnetic spectrum detected by human eye Structure and Function of the Retina Sensory retina: three layers of neurons: photoreceptor, bipolar, and ganglionic Cell bodies form nuclear layers separated by plexiform layers, where neurons of adjacent layers synapse with each other Pigmented retina: single layer of cells; filled with melanin. With choroid, enhances visual acuity by isolating individual photoreceptors, reducing light scattering Rods Bipolar photoreceptor cells; black and white vision. Found over most of retina, but not in fovea. More sensitive to light than cones. Use pigment rhodopsin Signal Transmission in the Retina Photoreceptors and bipolar cells only generate graded potentials (EPSPs and IPSPs) Light hyperpolarizes photoreceptor cells, causing them to stop releasing the inhibitory neurotransmitter glutamate Bipolar cells (no longer inhibited) are then allowed to depolarize and release neurotransmitter onto ganglion cells Ganglion cells generate APs that are transmitted in the optic nerve 1 cGMP-gated channels open, allowing cation influx; the photoreceptor depolarizes. Voltage-gated Ca2+ channels open in synaptic terminals. 2 Neurotransmitter is released continuously. 3 4 Hyperpolarization closes voltage-gated Ca2+ channels, inhibiting neurotransmitter release. 5 No EPSPs occur in ganglion cell. 6 No action potentials occur along the optic nerve. 7 Neurotransmitter causes IPSPs in bipolar cell; hyperpolarization results. Na+ Ca2+ Ca2+ Photoreceptor cell (rod) Bipolar cell Ganglion cell In the dark 1 cGMP-gated channels are closed, so cation influx stops; the photoreceptor hyperpolarizes. Voltage-gated Ca2+ channels close in synaptic terminals. 2 No neurotransmitter is released. 3 Lack of IPSPs in bipolar cell results in depolarization. 4 Depolarization opens voltage-gated Ca2+ channels; neurotransmitter is released. 5 EPSPs occur in ganglion cell. 6 Action potentials propagate along the optic nerve. 7 Photoreceptor cell (rod) Bipolar cell Ganglion cell Light Ca2+ In the light Cones Bipolar receptor cells. Responsible for color vision and visual acuity. Numerous in fovea; fewer over rest of retina. As light intensity decreases so does our ability to see color. Visual pigment is iodopsin: three types that respond to blue, red and green light Overlap in response to light, thus interpretations of gradation of color possible: several millions Neuronal Pathways 15-* Visual Fields Binocular vision: visual fields partially overlap yielding depth perception Hearing and Balance External ear: hearing. Terminates at eardrum (tympanic membrane). Includes auricle and external auditory canal Middle ear: hearing. Air-filled space containing auditory ossicles Inner ear: hearing and balance. Interconnecting fluid-filled tunnels and chambers within the temporal bone Inner Ear Oval window communicates with vestibule which communicates with the scala vestibuli of the cochlea Scala vestibuli extends from oval window to helicotrema at cochlear apex Second cochlea chamber (scala tympani) from helicotrema to round window Scala vestibuli and tympani filled with perilymph Inner Ear Wall of scala vestibuli is vestibular membrane Wall of scala tympani is basilar membrane Cochlear duct (scala media): space between vestibular and basilar membranes. Filled with endolymph Width of basilar membrane increases from 0.04 mm near oval window to 0.5 mm near helicotrema Near oval window basilar membrane responds to high-frequency vibrations Near helicotrema responds to low-frequency vibrations Inner Ear Spiral organ (organ of Corti): cells in cochlear duct Contain hair cells (sensory cells) with hair-like projections at the apical ends. These are microvilli called stereocilia. Basilar region of hair cells covered by synaptic terminals of sensory neurons Cell bodies of afferent neurons grouped into cochlear (spiral) ganglion Afferent fibers form the cochlear nerve Cochlea Semicircular canals Vestibulocochlear nerve Vestibule Hair cells arranged in rows. Of inner hair cells (responsible for hearing) and outer hair cells (regulate tension on basilar membrane) Hair bundle: stereocilia of one inner hair cell Tip link (gating spring) attaches tip of each stereocilium in a hair bundle to the side of the next longer stereocilium. As stereocilia bend, they open K+ gates (mechanically gated ion channel) Inner Ear Effect of Sound Waves on Cochlear Structures Neuronal Pathways for Hearing Balance Static labyrinth consists of utricle and saccule of the vestibule (mostly simple cuboidal) Evaluates position of head relative to gravity Detects linear acceleration and deceleration (as in a car) Kinetic labyrinth associated with the semicircular canals Evaluates movement of the head in three dimensional space Static Labyrinth Macula: has a specialized epithelium of supporting columnar cells and hair cells with numerous stereocilia (microvilli) and one cilium (kinocilium) embedded in gelatinous mass weighted by otoliths Utricle has macula oriented parallel to base of skull Saccula has macula oriented perpendicular to base of skull Gelatinous mass moves in response to gravity bending hair cells and initiating action potentials Otoliths stimulate hair cells with varying frequencies Patterns of stimulation translated by brain into specific information about head position or acceleration Kinetic Labyrinth Three semicircular canals filled with endolymph: transverse plane, coronal plane, sagittal plane Base of each expanded into ampulla with sensory epithelium (crista ampullaris) Cupula suspended over crista hair cells. Acts as a float displaced by fluid movements within semicircular canals Displacement of the cupula is most intense when the rate of head movement changes, thus this system detects changes in the rate of movement rather than movement alone. Neuronal Pathways for Balance
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