Peripheral Nervous SystemFriday, December 07, 201212:12 PMSensory Receptors (located in sense organs)Classified based on stimuli they respond to by generating electrical signals Photoreceptors (70% of sensory receptors) - located in the eyeball (sense organ)Eye --> composed of:1. Trilayered wall-Fibrous layer (sclera & cornea)-Vascular layer (choroid, ciliary body & iris)-Sensory layer (pigmented layer & neural layer - retina)i. Pigmented Layer - abuts the choroidFunction:Cells in this layer provide nutrients to the neurons in the neural layer, including the photoreceptorsCells in this layer synthesize the visual pigment, retinal using vitamin ACells in this layer engulf/destroy pathogens and cell debrisii. Neural Layer - retinaComposed of 3 layers of neuronsPhotoreceptors - layer abutting the pigmented layer, cells in this pigmented layer supply the photoreceptors with nutrients. Hence, if the neural layer detached from the pigmented layer (retinal detachment) the photoreceptors die--> blindness2 types of photoreceptors:-Rods - respond to dim light-Cones - respond to bright light, sensitive in bright light, used in viewing colored objects, give high-acuity (sharp) color visionTypes of cones:-Blue-Green-RedTrichromatic Theory of Vision - several colors are viewed by the activation of different combinations of the 3 types of cones*(rods are more sensitive to light than the cones) Lateral to the optic disc is the macula lutea --> contains mostly cones. The center of the macula lutea, called the fovea centralis contains only cones --> used for high focus in bright light for detail viewing (sharpest color vision)Rods are located lateral to the macula lutea, toward the sides of the eye wall (better if you look from the corners of the eyes in the dark).Bipolar neuronsGanglion cells (neurons) - axons of the ganglion cells bundle to form the optic nerve (CN II) --> optic nerve exits the eye through an area in the back wall called the optic disc (blind spot) --> optic disc lacks photoreceptorsGanglion cells are the only neurons in the retina that can generate/transmit action potentials by their axons (optic nerves)2. 2 Fluid compartments (humors)-Aqueous-Vitreous3. Lens --> located between the 2 humors, positioned in an upright position by the suspensory ligament (ciliary zonule) Ear-->Tympanic membrane --> separates the external auditory canal & middle ear.The malleus abuts the apex of the cone-shaped tympanic membrane & abuts the incus which abuts the stapes (stapes sits on the oval window)(oval window is the bony structure that separates the middle ear and inner ear. Arrangement of the ossicles between the tympanic membrane and the oval window (malleus, incus & stapes) Outer Ear (air-filled)-Pinna-External auditory canalMiddle Ear (air-filled)-Ossicles (malleus, incus & stapes)Inner Ear-Bony labyrinth (fluid-filled)Contains Perilymph, membranous labyrinth sacs/ducts-Vestibule - houses the 2 membranous sacs: saccule & utricle (contain equilibrium receptors that detect vertical and horizontal movement of the head)Equilibrium receptors in the saccule and utricle are called maculae.-Semicircular canals - houses the membranous labyrinth ducts called semicircular ducts, expanded ends of these ducts are called Ampullae house equilibrium receptors for the rotational movements of the head.Equilibrium receptors in the ampullae --> Cristae Ampullares-Cochlea - contains the membranous labyrinth duct called the cochlear duct --> houses the organ of corti (composed of basilar membrane surrounded by endolymph, tectorial membrane = gel-like (between these 2 membranes are 2 cell types: supporting cells & hair cells))Hair cells - mechanoreceptors for hearingHair-like extensions from the hair cells called stereocilia are trapped in the tectorial membrane.-Membranous labyrinth (Endolymph)-Saccule -Utricle-Semicircular ducts The afferent fibers of the cochlear nerve are wrapped around the bases of the hair cells. Olfaction - sense of smellChemoreceptors called olfactory cells are located in the olfactory epithelium, located in the roof of the nasal cavity. The dendrites of the olfactory cells (which are bipolar neurons) have cilia called olfactory hairs/cilia. Olfactory cilia are covered by a thin layer of mucus. The axons of the olfactory cells bundle to form olfactory nerves (CNI). The axons of a second order called mitral cells bundle to form olfactory tracts.2 Criteria for an odorant:1. Volatile - gaseous form2. Must dissolve in thin mucus coating the olfactory hairsThe dissolved chemical attaches to the olfactory hairs to set off electrical currents, that are transferred to the olfactory nerve where action potentials develop --> mitral cells and the axons which form the olfactory tracts transmit information to the olfactory relay center in the thalamus called the medial dorsal nucleus (frontal & temporal lobes) Mouth-->Taste buds & Gustatory cellsGustation - sense of tasteGustatory cells are specialized epithelial cells with microvilli called gustatory hairsGustatory cells are located in taste buds, which are located in papillaeGustatory cortex located in the insulaTaste is 80%
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