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Special senses


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Special senses

  1. 1. Special Senses • Have highly localized receptors that provide specific information about the environment Five special senses • Smell Interaction of chemicals with sensory receptors • Taste Interaction of chemicals with sensory receptors • Sight Interaction of light with sensory receptors • Hearing Interaction of mechanical stimulation with sensory receptors • Balance Interaction of mechanical stimulation with sensory receptors Olfaction • Sense of Smell • Response to airborne molecules, called odorants, entering the nasal cavity At least 7 (perhaps 50) primary odors exist • Camphoraceous (e.g., moth balls) • Musky-fish • Floral • Pepperminty • Ethereal (e.g., fresh pears) • Pungent-pepper • Putrid-rotten • Olfactory neurons have very low thresholds and accommodate rapidly • Olfactory Epithelium and Bulb Olfactory neurons in the olfactory epithelium are bipolar neurons • Distal ends have olfactory hairs Olfactory hairs have receptors that respond to dissolved substances • Approximately 1000 different odorant receptors • Receptors activate G proteins, which results in ion channels opening and depolarization • guanosine nucleotide-binding proteins Neuronal Pathways for Olfaction • Axons from the olfactory neurons extend as olfactory nerves to the olfactory bulb, where they synapse with interneurons • Axons from interneurons form the olfactory tracts, which connect to the olfactory cortex Olfactory bulbs and cortex accommodate to odors. Taste Sensory structures that detect taste stimuli are taste buds. Most taste buds are located in the epithelium of papillae. Taste buds are found on the • Tongue • Palate • Lips • Throat There are four types of papillae • Three contain taste buds • The fourth and most numerous has no taste buds, but gives the tongue its roughness Histology of Taste Buds Taste buds consist of • Taste cells Have taste hairs that extend into taste pores
  2. 2. Nonsensory cell types: • Basilar cells • Supporting cells Function of Taste Receptors on the hairs detect dissolved substances Five basic types of taste exist: • Salty • Sodium ions • Sour • Acids • Sweet • Sugars, some other carbohydrates, and some proteins • Bitter • Alkaloids (bases) • Umami • Elicited by the amino acid glutamate and related compounds Function of Taste All taste buds can sense the five primary tastes, but tend to be most sensitive to one Sensitivity to bitter substances is the highest (Poisons) • Taste is strongly influenced by olfactory sensations • Nasal congestion can dampen the taste sensation Tongue can detect other stimuli besides taste • Temperature • Texture Neuronal Pathways for Taste • The facial nerve carries taste sensations from the anterior two-thirds of the tongue. • The glossopharyngeal nerve carries taste sensations from the posterior one-third of the tongue • The vagus nerve carries taste sensations from the epiglottis The neural pathways for taste extend from the medulla oblongata to the thalamus and to the cerebral cortex Visual System Consists of • Eye • eyeball • optic nerve Accessory Structures • eyebrows, eyelids, conjunctiva, lacrimal apparatus, and extrinsic eye muscles • Sensory Neurons Accessory Structures • Eyebrows Prevent perspiration from entering the eyes and help shade the eyes • Eyelids Consist of five tissue layers Protect the eyes from foreign objects Help lubricate the eyes by spreading tears over their surface Lubricating glands associated with the eyelids • Meibomian glands and sebaceous glands • Ciliary glands lie between the hair follicles • Eyelashes Project from the free margin of each eyelid Initiate reflex blinking • Conjunctiva Covers the inner eyelid and the anterior part of the eye
  3. 3. Accessory Structures • Lacrimal Apparatus Consists of the lacrimal gland, lacrimal canaliculi, and a nasolacrimal duct Lacrimal glands secrete tears • Tears • Contain mostly water, with some salts, mucus, and lysozyme • Enter the eye via superolateral excretory ducts • Exit the eye medially via the lacrimal canaliculi • Drain into the nasolacrimal duct Accessory Structures Extrinsic Eye Muscles • Six strap-like muscles Enable the eye to follow moving objects Maintain the shape of the eyeball Four rectus muscles originate from the annular ring Two oblique muscles move the eye in the vertical plane Anatomy of the Eye Eyeball - A slightly irregular hollow sphere with anterior and posterior poles The eyeball is composed of three layers Fibrous layer • Sclera • Cornea Vascular layer • Choroid • Ciliary body • Iris Nervous layer • Retina The internal cavity is filled with fluids called humors Fibrous Layer • Sclera Posterior 4/5ths of the eye White connective tissue that maintains the shape of the eyeball Provides a site for muscle attachment Cornea Anterior 1/5th of the eye Transparent and refracts light that enters the eye Vascular Layer Choroid • A vascular network • Many melanin-containing pigment cells • Appears black in color • Prevents the reflection of light inside the eye Ciliary body Ciliary ring A thickened ring of tissue surrounding the lens Composed of smooth muscle bundles (ciliary muscles) Anchors the suspensory ligament that holds the lens in place Contraction of ciliary muscles changes the shape of the lens Ciliary process-produces aqueous humor Pupil-light enters the eye through this. Iris Smooth muscle regulated by the autonomic nervous system. -Regulates the amount of light by controlling the size of pupil Two muscles of Iris Sphincter pupillae Close vision and bright light: pupils constrict. Circular group. Dilator pupillae- radial group. • Distant vision and dim light: pupils dilate
  4. 4. • Changes in emotional state: pupils dilate when the subject matter is appealing or requires problem-solving skills • Controls the amount of light entering the pupil • Color is determined by the amount of melanin present • Large amounts of melanin: brown or black • Less melanin: light brown, green, or grey • Even less melanin: blue Nervous Layer Retina • The inner layer of the eyeball • Has over 126 million photoreceptor cells, which respond to light Macula (fovea centralis) • Area of greatest sensitivity to light • Highest concentration of photoreceptor cells Optic disc • Location through which nerves exit and blood vessels enter the eye • No photoreceptor cells • The “blind spot” of the eye Chambers of the Eye Composed of three chambers Anterior chamber Between the cornea and the iris Posterior chamber Between the iris and the lens Viterous chamber Much larger then the other two chambers Posterior to the lens Aqueous Humor Fills the anterior and posterior chambers Supports, nourishes, and removes wastes for the cornea, which has no blood vessels Produced by the ciliary processes as a blood filtrate Returned to the circulation through the scleral venous sinus Vitreous Humor Fills the vitreous chamber Contributes to intraocular pressure Helps maintain the shape of the eyeball Holds the lens and retina in place Functions in the refraction of light in the eye Lens A biconvex, transparent, flexible, avascular structure that: Allows precise focusing of light onto the retina Is composed of epithelium and lens fibers Lens epithelium: anterior cells that differentiate into lens fibers Lens fibers: cells filled with the transparent protein crystallin With age, the lens becomes more compact and dense and loses its elasticity Functions of the Complete Eye Properties of Light Electromagnetic spectrum All energy waves from short gamma rays to long radio waves Visible spectrum Portion of the electromagnetic spectrum that can be detected by the human eye • Refraction- Bending of light • Light striking a concave surface refracts outward (divergence) • Light striking a convex surface refracts inward (convergence)
  5. 5. • Converging light rays meet at the focal point and are said to be focused Functions of the Complete Eye Focusing system of the Eye (light refracting) A.Cornea Responsible for most of the convergence B. Aqueous humor C. Lens Adjusts the convergence by changing shape D. Vitreous humor Distant and Near Vision Distant vision: looking at objects 20 feet or more from the eye Near vision: looking at objects less than 20 feet from the eye • Relaxation of the ciliary muscles causes the lens to flatten, producing the emmetropic eye Emmetropia - Normal resting condition of the lens Far point of vision • Point at which the lens does not have to thicken for focusing to occur • Normally 20 feet or more from the eye Near point of vision Closest point an object can come to the eye and still be focused When an object is less than 20 feet from the eye, the image falling on the retina is no longer in focus Three events must occur to bring the image into focus Accommodation by the lens • Contraction of the ciliary muscles causes the lens to become more spherical • Change in the lens shape enables the eye to focus on objects that are less than 20 feet away Constriction of the Pupil Increases the depth of focus Convergence of the eyes Medial rotation of the eyes Structure and Function of the Retina Pigmented layer of the retina provides a black backdrop for increasing visual acuity • Rods and cones synapse with bipolar cells • Bipolar cells synapse with ganglion cells, which form the optic nerve Rods • Responsible for non-color vision and vision in low illumination (night vision) • Rod-shaped photoreceptive part of the rods contains about 700 double-layered membranous discs • Discs contain rhodopsin • Rhodopsin -A purple pigment consisting of the protein opsin covalently bound to a yellow photosensitive pigment called retinal (derived from Vit. A) • Exposure to light activates rhodopsin • Rhodopsin is split by light into retinal and opsin, eventually resulting in an action potential • Light adaptation is caused by a reduction of rhodopsin • Dark adaptation is caused by rhodopsin production Cones • Responsible for color vision and visual acuity
  6. 6. • Three types, each with a different type of iodopsin photopigment • Pigments are most sensitive to blue, red, and green light • Perception of many colors results from mixing the ratio of the different types of cones that are active at a given moment • Most visual images are focused on the fovea centralis and macula • Fovea centralis has a very high concentration of cones • In the remaining macula there are more cones than rods • Most rods are in the periphery of the retina • Bipolar and ganglion cells in the retina can modify information sent to the brain • Interneurons in the inner layers of the retina enhance contrast between the edges of objects Neuronal Pathways for Vision • Ganglion cell axons form the optic nerve, optic chiasm, and optic tracts • Extend to the thalamus and synapse • Then the neurons form the optic radiations that project to the visual cortex • Depth perception is the ability to judge relative distances of an object from the eyes and is a property of binocular vision • Binocular vision results because a slightly different image is seen by each eye Hearing and Balance Three parts of the ear are • External ear Extends from the outside of the head to the tympanic membrane • Middle ear Air-filled chamber medial to the tympanic membrane • Inner ear Set of fluid-filled chambers medial to the middle ear The external and middle ear are involved with hearing The inner ear functions in both hearing and equilibrium Auditory Structures and Their Functions External Ear Auricle -Fleshy part of the external ear External acoustic meatus -Passageway that leads to the tympanic membrane -Lined with hairs and ceruminous glands. -Ceruminous glands produce cerumen (earwax) Tympanic membrane (eardrum) -Thin connective tissue membrane that vibrates in response to sound -Transfers sound energy to the middle ear ossicles -Boundary between outer and middle ears Middle Ear -A small, air-filled, mucosa-lined cavity -Flanked laterally by the eardrum -Flanked medially by the oval and round windows Contains three small bones: the malleus, incus, and stapes Transmit vibratory motion of the eardrum to the oval window Dampened by the tensor tympani and stapedius muscles
  7. 7. Auditory tube (pharyngotympanic or eustachian tube) -Connects the middle ear to the pharynx -Equalizes pressure in the middle ear cavity with the external air pressure Inner Ear Bony labyrinth Interconnecting, fluid-filled tunnels and chambers within the temporal bone Contains Vestibule and semicircular canals: primarily involved in balance Cochlea: involved in hearing Membranous labyrinth Series of membranous sacs within the bony labyrinth Filled with a potassium-rich fluid called endolymph Space between the bony labyrinth and membranous labyrinth is filled with perilymph Cochlea • Spiral-shaped canal within the temporal bone • Divided into three compartments by the vestibular and basilar membranes • Scala vestibuli and scala tympani contain perilymph • Cochlear duct contains endolymph and the spiral organ • Spiral organ consists of inner hair cells and outer hair cells, which attach to the tectorial membrane • Hair cells have hairlike projections at their apical ends, which are very long microvilli called stereocilia Auditory Function Pitch is determined by the frequency of sound waves Volume is determined by the amplitude of sound waves Timbre is the resonant quality (overtones) of sound Auditory Function • Hearing involves • Sound waves funneled by the auricle down the external acoustic meatus cause the tympanic membrane to vibrate • Tympanic membrane vibrations pass along the auditory ossicles to the oval window of the inner ear Movement of the stapes in the oval window causes the perilymph, vestibular membrane, and endolymph to vibrate and produces movement of the basilar membrane Movement of the basilar membrane causes bending of the stereocilia of inner hair cells in the spiral organ Hearing involves • Bending of the stereocilia pulls on gating springs and opens K+ channels • K+ ions enter the hair cell and result in depolarization of the cell • Depolarization causes the release of glutamate, generating action potentials in the sensory neurons associated with hair cells • The round window dissipates sound waves and protects the inner ear from pressure buildup Neuronal Pathways for Hearing • Axons from the vestibulocochlear nerve synapse in the medulla • Neurons from the medulla project axons to the inferior colliculi, where they synapse
  8. 8. • Neurons from this point project to the thalamus and synapse • Thalamic neurons extend to the auditory cortex • Efferent neurons project to cranial nerve nuclei responsible for controlling muscles that dampen sound in the middle ear Static Balance Evaluates the position of the head relative to gravity and detects linear acceleration and deceleration Vestibule contains The utricle and saccule in the inner ear Contain maculae made of hair cells Hairs are embedded in an otolithic membrane Consists of a gelatinous mass and crystals called otoliths Moves in response to gravity Dynamic Balance • Evaluates movements of the head Semicircular Canals • Three semicircular canals at right angles to one another are present in the inner ear • The ampulla of each semicircular canal contains the crista ampullaris • Has hair cells with hairs embedded in a gelatinous mass, the cupula • When the head moves, endolymph within the semicircular canals moves the cupula Neuronal Pathways for Balance • Axons from the maculae and the cristae ampullares extend to the vestibular nucleus of the medulla • Fibers from the medulla run to the spinal cord, cerebellum, cortex, and nuclei that control the extrinsic eye muscles • Balance also depends on proprioception and visual input