2 X. The Sensory System A. Introduction 1. Receptors detect environmental changes and stimuli.2. Different receptors are sensitive to and receive differentsensory stimuli.a. Chemoreceptors – medulla in the lower brain stem, aorticand carotid bodies detect O2, CO2, and H+ levels in the blood.Receptors of the small intestine detect H+ levels in thelumen.b. Pain receptors – located in the skin and internal organs.c. Thermoreceptors – sense heat (temperature) in the skin andcontrol body temperature in the hypothalmus of the brain.d. Mechanoreceptors – sense stretch and pressure within theskin, muscles, and joints.e. Photoreceptors – light sensitive neurons in the retina of theeye.
5 3. Sensationa. Sensations are feelings resulting from sensory stimulation.b. A particular part of the sensory cortex always interpretsimpulses reaching it in the same way.c. The brain projects a sensation back to the region ofstimulation.4. Sensory adaptations are adjustments made by sensoryreceptors to continuous stimulation in which impulses aretriggered at slower and slower rates.B. Pain receptors1. Pain receptors are free nerve endings stimulated by tissuedamage.2. The only receptors in visceral organs that provide sensation arepain receptors, (visceral pain).3. The sensations produced from visceral receptors are likely tofeel as if they are coming from some other part or location.This is called “referred” pain.
6 4. Somatic pain originates in the skin, skeletal muscles, joints and tendons.5. Stimulated nerve fibers from amputated limbs can tend to resultin “phantom pain”.6. Pain experience has two components: pain stimulus and reactionto pain stimulus which determines the amount of suffering.
7 C. Sense of Smell (Olfaction) 1. Olfactory organsa. The olfactory organs consist of receptors and supporting cellsin the nasal cavity.b. Olfactory receptors are neurons with cilia that are sensitive togaseous/dissolved chemicals.c. Nerve impulses travel from the olfactory receptors through theolfactory nerves, olfactory bulbs, and olfactory tracts tointerpreting centers in the olfactory portion of the cerebralcortex.You perceive odors (smells) in your brain!2. Olfactory stimulationa. Olfactory impulses may result when various gaseous moleculescombine with specific binding sites on the cilia of the receptorcells.b. Olfactory receptors adapt rapidly.
9 D. Sense of Taste 1. Taste receptors a. Taste buds consist of receptor cells and supporting cells locatedin papillae on the tongue.b. Taste cells have taste hairs that are sensitive to particularchemicals dissolved in water.c. Taste hair surfaces seem to have receptor sites to whichchemicals combine.2. Taste sensation (gustation)a. The five primary taste sensations are: sweet, sour, salty, bitter,and umami.b. Various taste sensations result from the stimulation of one ormore sets of taste receptors.c. Much of taste also involves the sense of smell.
11 E. Sense of Hearing1. The external ear composed of three parts collects sound wavescreated by vibrating objects.a. The visible “ear” that collects and directs sound is the auricle orpinna.b. The external auditory meatus directs sound into the skull.c. The tympanic membrane vibrates to begin the conversion ofsound energy to mechanical energy.2. Middle ear (air filled space)a. The three auditory ossicles, (malleus, incus, & stapes) of themiddle ear conduct and multiply the energy of vibration to theoval window of the inner ear.b. Eustachian tubes connect the middle ear to the throat andfunction to help maintain equal air pressure on both sides ofthe eardrums so they are free to vibrate in response to sound.
15 3. Inner eara. The inner ear consists of a complex system of interconnectedtubes and chambers, the osseous (bony) filled with perilymph,and membranous (membrane) labyrinths filled with endolymph.b. The organ of Corti contains the hearing receptors (hair cells) thatare stimulated by vibrations in the fluids of the inner ear.c. Different frequencies of vibrations are thought to stimulatedifferent receptor hair cells.4. Steps in hearing:a. Sound waves collected by auricle (pinna)b. Waves pass through External Auditory Meatusc. Tympanic Membrane vibratesd. Malleus connected on the medial side of Tympanic Membranemoves.e. Malleus moves the incusf. Incus moves the stapesg. Stapes pushes on the oval window. Movement of stapes is 20times greater than that of the Tympanic Membrane
16 h. Perilymph in Scala Vestibuli moves i. Perilymph wave energy causes the Basilar Membrane in thecochlear duct to move up and down.j. Organ of Corti hair cells within the cochlear duct (containingendolymph) move up and downk. Hair cells touch the Tectorial Membranel. Hairs bend and send nerve impulses down the cochlear branchof the Vestibulocochlear Nerve to the temporal lobe of thecerebrum.m. Different frequencies cause different regions of the BasilarMembrane to vibrate causing different regions of Organ ofCorti hair cells to be stimulated = perception of differentfrequencies.o. Movement of perilymph in the Scala Vestibuli goes to the endof the cochlea and moves the perilymph in the Scala Tympaniwhich pushes on the Round Window so the wave energy is lost,hearing stops.
22 3. Other parts that help with the maintenance of equilibrium include the eyes and mechanoreceptors associated with certain jointscalled proprioceptors.G. Sense of Sight1. Visual accessory organs include the eyelids, lacrimal apparatus(lacrimal gland produces tears, Nasolacrimal duct drains the eye tothe nasal cavity), and extrinsic muscles to move the eye.
24 2. Structure of the Eyea. The wall of the eye has an outer, middle, and inner layer thatfunctions as follows:1.) The outer white layer (sclera) is fibrous, protective, andshapes the eye. Its transparent anterior portion (cornea)protects and refracts (bends) light entering the eye.Astigmatism is blurry vision in parts of the visual field causedby unequal curvatures “waviness” in the cornea.2.) The middle layer (choroid) is the vascular layer and contains abrown pigment that helps to absorb light to avoid visualconfusion.3.) the inner layer (retina) contains the visual receptor cellsb. The lens is a transparent, elastic structure whose shape iscontrolled by the action of the ciliary muscles that are part ofthe ciliary body. The lens changes shape to refract light.c. The iris is a muscular diaphragm that controls the amount of lightentering the eye. The pupil is the hole in the middle of the iris.
25 d. Spaces within the eye are filled with fluids that help to maintain the shape of the eye.1.) Anterior cavity (in front of the lens) is filled with aqueoushumor2.) Posterior cavity (larger and behind the lens) is filled byvitreous humor.3. Refraction of lighta. Light waves are refracted primarily by the cornea and lens.b. The lens must be thickened (accommodation) to focus onobjects closer than 20 feet away by use of the ciliary muscles.4. Visual receptorsa. The visual receptors are called rods and cones1.) Rods (more sensitive to low light) provide black & white visionthat is poor in detail2.) Cones require higher intensity light to provide highly detailedcolor vision.
28 b. Visual pigments1.) A light-sensitive pigment in rods decomposes in the presenceof light to trigger nerve impulses that our visual cortexperceives as vision2.) Color vision seems to be related to the presence of three setsof cones (blue, green, red) containing different light-sensitivepigments. Cones are most highly concentrated at the MaculaLutea of the retina. This is the area of most acute colorvision and light hits it when you look directly at objects.5. Stereoscopic visiona. Stereoscopic vision involves the perception of distance anddepth (depth perception)b. Stereoscopic vision occurs because of the formation of twoslightly different retinal images that the brain superimposes andinterprets as one image in three dimensions
29 6. Visual nerve pathwaysa. Nerve fibers from the retina form the optic nerves. Nerve fibersexit the back of the eye at the optic disc causing the “blind spot”.Because nerve fibers fill this area, no cones or rods are present.b. Some nerve fibers from each eye cross over in the optic chiasm.This ensures that all areas of the visual field perceived by botheyes are processed by one side of the brain into a coherentimage.
30 a. If elongate (too long): 7. Shape of the eyeballa. If elongate (too long):1.) The focus point of the image is in front of the retina, theimage that hits the retina is out of focus.2.) This is Myopia “short sighted vision” you can’t see far awayso you are described as being “nearsighted”.b. If eyeball is (too short):1.) The focus point of the image occurs past (behind) the retinaso the image that hits the retina is out of focus.2.) This is hyperopia “farsightedness” You can’t see closeobjects!
31 8. Aginga. As you get older the lens gets harder and will not thicken as oraccommodate as much for close vision. This is usually correctedby glasses or contact lenses.