Chapter 16 Special Senses

LECTURE OUTLINE CHAPTER 16 MariebThe Nervous System: General and Special SensesLecture OutlineI. ReceptorsA. All sensory receptors are transducers of some sort, that is, they changean incoming stimulus of pressure, vibration, light, etc., into electrochemicalneuron impulses. Each is specific in that it can transduce onlycertain types of stimuli into neuron action potentials.B. Perception - conscious awareness of a sensationC. Sense organs as transducers1. chemoreceptors2. mechanoreceptors3. thermoreceptors4. nociceptorsD. Interpretation of Sensory Information1. receptive field - that region monitored by a receptor cell2. labeled line - route between receptor and appropriate corticalarea3. sensory coding - all the qualities of the action potential patternscharacteristic of that sense4. tonic receptors - continuously monitor stimuli5. phasic receptors - monitor and report changes in stimulusE. Central Processing and Adaptation1. adaptation - loss of sensitivity after exposure to astimulus.2. peripheral adaptation - result of sensor fatigue , receptorbecomes less sensitive to stimulusa. fast adapting receptors - typical of phasic receptors, loose"reportability" quicklyb. slow adapting receptors - typical of tonic receptors, slightadaptive lossF. Sensory Limitations1. there are many stimuli for which we do not have receptors2. stimuli may be out of range for the appropriate receptors - uvlight for example3. our awareness depends on interpretation which may be inexact.II. General SensesA. Exteroceptors - detect the external environment (temperature,pressure, light, etc.)B. Interoceptors provide information about internalconditions, most often tension or stretch in smooth muscle ortendons.1. nociceptor stimulation results in pain.heat, mechanical stress and chemicals can allcause pain by stimulating different classes of receptors.1

a. fast pain - deep cuts, etc. - referred immediately toprimary sensory cortex, usually trigger reflexesb. slow pain - later, persistent, indistinct source, thalamicc. referred pain - visceral, "incorrect" source perceived2. thermoreceptors - detect heat or absence thereof, phasic,same pathways as pain3. mechanoreceptors - stimulated by anything moving orphysical perturbation.a. tactile receptors - touch, pressure, vibrationi. encapsulated receptorsa) Meissners corpuscles - fine touchb) Pacinian corpuscles - deep pressurec) Ruffini corpuscles - tension, crude touchii. unencapsulated receptorsa) free nerve endings - painb) Merkels discs - fine touchc) root hair plexes - hair movementb. baroreceptors are sensitive to internal pressures bloodpressure, lung stretch, digestive tract tensionc. proprioceptors are kinesthetic monitors of tendon musclestretch.4. chemoreceptors - respond to specific chemicals either insolution (taste) or volatilized (smell) or internally inmonitoring blood composition. Internal chemoreceptors monitorblood composition - Na+, pH, osmolarity, etc.III. Special sensesA. Olfaction (Smell)distant chemical sense, many chemicals can be detected at a distance aslong as they are gaseous.1. receptors -sensory epithelium, olfactory receptors and glands,supporting cells and basal cells - covers superior nasal conchaeand septum.2. olfactory pathways - receptor neurons pass into the craniumthrough the cribiform plate and synapse in the olfactory bulbs.These olfactory neurons are unique in that they are the only onesknown to routinely replace themselves or to reach the cerebrumwithout synapsing in the thalamus.3. olfactory discrimination - Although difficult to describe, thenumber of different odors recognizable is immense.B. Gustation (Taste) - taste is chemical also but requires that thedissolved chemicals make contact with the tongue or mouth.1. gustatory receptors - gustatory chemoreceptors on thesides of taste buds. Taste buds are incorporated in thepapillae which are described by their shapes,a. filiform - thin, thread like projectionsb. fungiform - shaped like mushrooms.2

c. circumvallate - large target-shaped bumps near theback of the tongue.2. gustatory pathways - cranial nerves VII, IX and X to thenucleus solitarius in medulla oblongata to gustatory cortex3. gustatory discrimination - six tastesa. sour - (H+)b. sweet - (organic)c. salt - (metallic)d. bitter - (alkaloids)e. waterf. umami - (savory)C. Equilibrium and Hearing – ears1. external eara. pinnab. tympanic membrane2. middle eara. auditory (Eustachian) tubeb. auditory ossicles - amplify force of vibrationi. malleus (hammer)ii. incus (anvil)iii. stapes (stirrup)c. tensor tympani muscle - stiffens tympanic membraned. stapedius muscle - dampens movement of stapes3. inner eara. labyrinth - membranous and bonyi. endolymph & perilymphii. saccule w/ macula & hair cellsiii. utricleiv. vestibule - membranous area connected to bothb. cochleai. cochlear duct containing theii. Organ of Corti –iii. tympanic & vestibular ducts,iv. tectorial membrane,v. hair cells with stereociliavi. round windowvii. oval windowc. semicircular canalsi. three mutually perpendicular- posterior, lateraland anteriorii. ampulla - where ends of canals meet vestibuleiii. maculae - hair cells with otolithsD. Pathways for Vestibular Sensation1. vestibular branch of cranial nerve VIIIa. synapses in pons / medulla oblongatab. relays information to vestibular apparatus of cerebellum3

c. relays information to cerebral cortexd. sends commands to motor nuclei in brain stem and spinalcordE. Receptor Function in the Inner Ear1. cochleaa. modiolus central hub of cochleab. cochlear duct sandwiched betweenc. vestibular duct andd. tympanic ducte. Organ of Cortii. basilar membrane on which sitii. hair cells with stereociliaiii. tectorial membrane above the hair cellsiv. sound causes hair cells to bounce and touch thetectorial membrane causing transduction.F. Auditory Pathways1. afferent neurons in cochlear branch of cranial nerve VIII.2. these synapse to neurons relayed to opposite side of midbrain3.auditory reflexes occur from here, axons synapse to thalamus4. projection fibers to auditory cortex from thalamusIV.. Vision – eyeA. Accessory Structures1. eyelids – palpebraea. medial ( with lacrimal caruncle) and lateral canthus -corners of eyesb. eyelashes - with sebaceous glands (of Zeis)b. tarsal (Meibomian) glands - on edges of eyelidsc. tarsal plate - tough connective tissue on eyelidsd. conjunctiva - mucous membrane lining eyelids2. lacrimal apparatusa. lacrimal gland - superior and lateral to eyeb. lacrimal puncta - holes near nose to drain tearsc. lacrimal canaliculi- drain tears tod. nasolacrimal ducts - empties to nasal cavityB. Eye1. superficial structuresa. cushioned by fat pads2. fibrous tunic - tough outer layera. sclera - white part of fibrous tunicb. cornea - transparent avascular anterior partc. limbus - boundary between the above3 vascular tunic (uvea)a. choroid - heavily vascularb. iris with pupil hole, circular sphincter muscles andradial dilator muscles4

c. ciliary body - muscles attached to suspensory ligament,regulates focus of lens4. nervous tunica. retina1. outer pigmented layer2.. inner layer -photoreceptorsa) rods black/white vision, motion detectionb) cones – color vision, intense light3. horizontal cells - modulate between rods and conesand bipolar cells4. bipolar cells - synapse with rods and cones5. ganglion cells - synapse with bipolar cells6 amacrine cells - synapse with both bipolar andganglion cellsb. ora serrata - anterior edge of retinac. macula lutea – fovea centralis - all cones, best visiond. optic disc – blind spot, where optic nerve exits eyee. optic nerve5. lensa. ciliary bodyb. suspensory ligamentc. anterior chamber with aqueous humor1) anterior compartment2) posterior compartmentd. posterior chamber with vitreous body (humor)e. cataractV. Visual PathwaysOptic Chiasma - optic nerves partially cross in sphenoid(right side of the field of each eye combining and going to thelateral geniculate (knee) on the right, those from the left tothe left, of the thalamus which then relays the data to thevisual centers. )VI. Cortical IntegrationThe visual cortex must combine and integrate the incominginformation into a composite which is perceived as depth.(This is a remarkable feat when one considers that the imageis tiny, curved, inverted, the two images slightly out offrame, and each missing a large portion at opposite edgesof the field and having a hole in the image.)VII. The Brain Stem and Visual ProcessingA. Optic nerve to midbrain or bypass to hypothalamusB. Motor commands control eye movementsC. Hypothalamus to epithalamus - affect diurnal rhythmsD. Collaterals to visual cortex5