The Special Senses Objectives Describe the sensory organs of smell, and olfaction. Identify the accessory and internal structures of the eye, and explain their function. Explain how light stimulates the production of nerve impulses and vision. Describe the structures of the external and middle ear and explain how they function. Describe the parts of the inner ear and their roles in equilibrium and hearing. Olfactory organs Contain olfactory epithelium with olfactory receptors, supporting cells, basal cells Olfactory receptors are modified neurons Olfaction detects dissolved chemicals as they interact with odorant binding proteins Olfaction Olfactory pathways No synapse in the thalamus for arriving information Olfactory discrimination Can distinguish thousands of chemical stimuli CNS interprets smells by pattern of receptor activity Olfactory receptor population shows considerable turnover Number of receptors declines with age Gustation Taste receptors Clustered in taste buds Associated with lingual papillae Taste buds Gustatory cells extend taste hairs through a narrow taste pore Gustatory pathways Taste buds are monitored by cranial nerves Synapse with the medulla oblongata, then thalamus and the primary sensory cortex Gustatory discrimination Primary taste sensations Sweet, sour, salty, bitter Receptors also exist for umami and water Taste sensitivity shows significant individual differences, some of which are inherited The number of taste buds declines with age 82
Vision Accessory structures of the eye Eyelids (palpebrae) separated by the palpebral fissue Eyelashes Tarsal glands Lacrimal apparatus External structures of the eye Conjunctiva covers most of eye Cornea is transparent anterior portion Lacrimal apparatus Secretions from the lacrimal gland contain lysozyme Tears form in the lacrimal glands, wash across the eye and collect in the lacrimal lake Pass through lacrimal punctae, lacrimal canaliculi, lacrimal sac, and nasolacrimal duct The eye Three layers Outer fibrous tunic Sclera, cornea, limbus Middle vascular tunic Iris, ciliary body, choroid Inner nervous tunic Retina Internal structures of the eye Ciliary body Ciliary muscles and ciliary processes, which attach to suspensory ligaments of lens Retina Outer pigmented portion Inner neural part Rods and cones Retina Retina contains rods and cones Cones densely packed at fovea (center of the macula lutea) Retinal pathway Photoreceptors to bipolar cells to ganglion cells, to the brain via the optic nerve Axons of ganglion cells converge at blind spot (optic disc) 83
Eye anatomy Ciliary body and lens divide the anterior cavity of the eye into posterior (vitreous) cavity and anterior cavity Anterior cavity further divided Anterior chamber in front of eye Posterior chamber between the iris and the lens Fluids in the eye Aqueous humor circulates within the eye Composition is much like CSF Diffuses through the walls of anterior chamber Passes through canal of Schlemm Re-enters circulation Vitreous humor fills the posterior cavity. Not recycled permanent fluid Lens Posterior to the cornea and forms anterior boundary of posterior cavity Posterior cavity contains vitreous humor Lens helps focus Light is refracted as it passes through lens Accommodation is the process by which the lens adjusts to focus images Normal visual acuity is 20/20 Color sensitivity Integration of information from red, blue, and green cones Colorblindness is the inability to detect certain colors Retinal adaptation Dark adapted most visual pigments are fully receptive to stimulation Light adapted pupil constricts and pigments bleached. Seeing in stereo Vision from the field of view transfers from one side to the other while in transit Depth perception is obtained by comparing relative positions of objects from two eyes Visual circadian rhythm Input to suprachiasmic nucleus affects the function of the brainstem Circadian rhythm ties to day-night cycle, and affects metabolic rates 84
Equilibrium and Hearing Both equilibrium and hearing are provided by receptors of the inner ear Anatomy of the ear external ear Auricle or pinnae surrounds the ear External acoustic meatus ends on tympanic membrane Middle ear Communicates with pharynx via auditory tube (Eustachian) Middle ear encloses and protects the auditory ossicles Inner ear Membranous labyrinth contains endolymph Bony labyrinth surrounds and protects membranous labyrinth Vestibule Semicircular canals Cochlea Components of the inner ear Vestibule contains the utricle and saccule Semicircular canals contain the semicircular ducts Cochlea contains the cochlear duct Windows Round window separates the perilymph from the air spaces of the middle ear Oval window connected to the base of the stapes Basic receptors of inner ear are hair cells Equilibrium Anterior, posterior, and lateral semicircular ducts are continuous with the utricle Each duct contains an ampulla with gelatinous cupula and sensory receptors Saccule and utricle connected by a passageway between the endolymphatic duct Terminates in the endolymphatic sac Saccule and utricle have hair cells clustered in maculae Cilia contact the otolith (statoconia) 85
Hearing Cochlear duct lies between the vestibular duct and the tympanic duct Hair cells of the cochlear duct lie within the Organ of Corti Intensity is the energy content of a sound Measured in decibels Pathway of sound Sound waves travel toward tympanic membrane, which vibrates Auditory ossicles conduct the vibration into the inner ear Tensor tympani and stapedius muscles contract to reduce the amount of movement when loud sounds arrive Movement at the oval window applies pressure to the perilymph of the cochlear duct Pressure waves distort basilar membrane Hair cells of the organ of Corti are pushed against the tectoral membrane Neural pathway Sensory neurons of hearing are located in the spiral ganglion of the cochlea Afferent fibers form the cochlear branch of cranial nerve VIII Synapse at the cochlear nucleus 86