Peripheral Nervous System Friday December 07 2012 12 12 PM Sensory 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 choroid Function Cells in this layer provide nutrients to the neurons in the neural layer including the photoreceptors Cells in this layer synthesize the visual pigment retinal using vitamin A Cells in this layer engulf destroy pathogens and cell debris ii Neural Layer retina Composed of 3 layers of neurons Photoreceptors 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 blindness 2 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 vision Types of cones Blue Green Red Trichromatic 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 neurons Ganglion 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 photoreceptors Ganglion 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 Vitreous 3 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 canal Middle 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 hearing Hair 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 smell Chemoreceptors 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 form 2 Must dissolve in thin mucus coating the olfactory hairs The 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 cells Gustation sense of taste Gustatory cells are specialized epithelial cells with microvilli called gustatory hairs Gustatory cells are located in taste buds which are located in papillae Gustatory cortex located in the insula Taste is 80 smell
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