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Withinthe macula, the fovea is the region of greatest visual acuity; itcontains only cones.Reception of light by the photoreceptors initiates a sequenceof events that results in reduced Na+ permeability and hyperpolarization of the photoreceptor cell membrane. This resultsin inhibition of the release of either inhibitory or excitatoryneurotransmitters at synapses between photoreceptors andbipolar and horizontal cells. Ultimately, the generated signalsare transmitted by the ganglion cells, whose axons form theoptic nerves. Nerve fibers originating in the nasal hemiretina(nasal half of the retina) of each eye cross at the optic chiasm(the site of partial crossing of the two optic nerves), whileSensory Physiology63Cerebral cortex: postcentral gyrusPosterior limb of internal capsuleVentral posterolateral (VPL) nucleus of thalamusMesencephalon (cerebral peduncles)Medial lemniscusSpinothalamic tractLower part of medulla oblongataReticular formationGracile nucleusCuneate nucleusFasciculus gracilisFasciculus cuneatusDorsal (posterior) spinal root ganglionProprioception, positionLarge myelinated fibersCervical part of spinal cordTouch, pressure, vibrationPain, temperature Small myelinated andunmyelinated fibersLateral spinothalamic tract: pain, temperatureVentral (anterior) spinothalamic tract: touch, pressureLateral cervical nucleusSpinocervical tractLumbar part of spinal cordFigure 5.3 Somesthetic System of the Body The primary somatosensory cortex (postcentralgyrus) receives information regarding proprioception, touch, pressure, vibration, pain, and temperatureoriginating at receptors below the head through the illustrated tracts by way of the thalamus.
Pain-, temperature-, and pressure-related signals are conducted through the spinothalamic and spinoreticular tractsof the anterolateral system (red and blue lines). The fasciculus gracilis and fasciculus cuneatus, as well asthe lateral cervical system, convey proprioception-, touch-, and vibration-related signals (blue and purplelines).fibers from the temporal hemiretina remain in the ipsilateraltract (same side). The optic tracts carry the signals to synapsesat the lateral geniculate nuclei, from which signals are transmitted to the visual cortex in the occipital lobe of the cerebralcortex.THE AUDITORY SYSTEMThe auditory system detects sound, transduces it into electrical signals, and transmits it to the brain, where the complextask of interpretation of sound, for example as speech, takesplace.
The process begins at the external ear, which directssound to the tympanic membrane (eardrum) (Fig. 5.7). Thismembrane separates the external ear from the air-filled middleear, where sound is amplified. Vibration of the tympanicmembrane is conducted to the maleus, incus, and stapes, thesmall bones of the middle ear known collectively as the ossicles.
The stapes is connected to the oval window, one of twomembranous “windows” between the middle ear and innerear. Thus, vibration of the ossicles is transmitted to the innerear by the stapes.64The Nervous System and MuscleCerebral cortex: postcentral gyrusVentral posteromedial (VPM) nucleus of thalamusInternal capsuleMidbrain (cerebral peduncles)Ventral trigeminal lemniscusPontine reticular formationPonsDorsal trigeminal lemniscusTrigeminal mesencephalic nucleusTrigeminal motor nucleusPrincipal sensory trigeminal nucleusTouch, pressurePain, temperatureProprioceptionTrigeminal (semilunar) ganglionOphthalmic n.Maxillary n.Sensory rootandMotor root of mandibular n.Medullary reticular formationSpinal trigeminal tractSpinal trigeminal nucleusCervical part of spinal cordFacial (VII) n.Vagus (X) n.Dorsolateral fasciculus (of Lissauer)Substantia gelatinosa (Iamina II)Figure 5.4 Somesthetic System of the Head The primary somatosensory cortex (postcentralgyrus) receives information regarding proprioception, touch, pressure, pain, and temperature originating atreceptors in the head through the illustrated tracts by way of the thalamus.
Proprioceptive signals are carriedby nerve fibers (purple lines) originating in cell bodies in the mesencephalic nucleus of the trigeminal nerve(CN V), whereas touch, pressure, pain, and temperature produce signals carried by nerve fibers (blue andred lines) to cell bodies in the trigeminal ganglion of CN V.Sensory PhysiologyB. Section through retinaA. EyeballIrisAxons at surface of retina passing viaoptic nerve, chiasm, and tract to lateralgeniculate bodyLensCorneaSuspensory ligamentInner limiting membraneCiliary bodyAnteriorchamberOraserrataGanglion cellPosteriorchamberMüller cell (supporting glial cell)containingaqueous humorAmacrine cellBipolar cellHorizontal cellRodConePigment cells of choroidVitreous humorRetinaChoroidOptic nerveScleraFoveaC.
Rod in darkD. Rod in lightSynaptic ending fully polarizedSynapticendingdepolarizedSynaptic barPhotonsof lightRhodopsinMetabolicenergyNucleusLumirhodopsinMetarhodopsinCentriole (basal body)CurrentflowNa+permeabilityincreasedRetinene⫹OpsinRetinene⫹OpsinVitamin ANa+ permeability decreasedVitamin ACirculationFigure 5.5 Visual Receptors Light enters the eye through the cornea and lens (A). Light focused ontothe retina by the lens is absorbed by rhodopsin in photoreceptors (rods and cones, B). In light, the cGMPsecond messenger system is activated, resulting in closure of membrane Na+ channels, hyperpolarizing thecell (illustrated for rod cells in D).
Subsequent inhibition of neurotransmitter release at synapses with bipolarand horizontal cells is the next step in the transmission of light-generated signals to the optic nerve. In dark,the rod or cone is depolarized (C).6566The Nervous System and MuscleCentral darker circle represents macular zoneGGAABOverlappingvisual fieldsLightest shades represent monocular fieldsBHEach quadrant a different colorHR RCCPChoroidPPeripheryProjection onleft retinaChoroidMaculaStructure of retina (schematic):A Amacrine cellsProjection on left dorsalB Bipolar cellslateral geniculate nucleusC ConesG Ganglion cellsH Horizontal cellsP Pigment cellsR RodsOptic(II) nervesOptic chiasmProjection on right retinaOptic tractsLateralgeniculatebodiesProjection on right dorsallateral geniculate nucleusCalcarine fissureProjection on right occipital lobeProjection on left occipital lobeFigure 5.6 Retinogeniculostriate Visual Pathway Rods and cones detect light in the retina, withcones specifically mediating color perception and rods detecting light.
In the central macular zone of theretina, only cones are found; this is the region with greatest acuity (top left). Axons of ganglion cells carryafferent signals through the optic nerve. At the optic chiasm, fibers originating in the nasal half of the retinacross to the contralateral tract, whereas fibers from the temporal half remain in the ipsilateral tract. Fibersproject to synapses in the lateral geniculate nucleus (visual fields are inverted at this point). Signals areconveyed from the lateral geniculate nucleus to the primary visual cortex.The fluid-filled inner ear is a complex structure that includesthe vestibule, the cochlea, and the semicircular canals.
Thevestibule and cochlea consist of a membranous labyrinthwithin a bony labyrinth (Fig. 5.8). The membranous labyrinthof the cochlea includes three ducts known as the scala vestibuli, scala tympani, and scala media (cochlear duct). The fluidof the scala media, the endolymph, resembles intracellularfluid in its composition, whereas the periplymph of the othertwo ducts is similar to other extracellular fluids.Transduction of Sound into Electrical SignalsThe spiral organ of Corti is the site of transduction of soundinto electrical signals. Mechanoreceptors known as hair cellsSensory PhysiologyFrontal sectionLimbs of stapesProminence of lateral semicircular canalIncusTegmen tympaniMalleus (head)Epitympanic recess67Facial nerve (VII) (cut)Base of stapes in oval (vestibular) windowVestibuleSemicircular ducts, ampullae, utricle, and sacculeFacial nerve (VII) (cut)Vestibular nerveCochlear nerveAuricleInternal acoustic meatusVestibulocochlearnerve (VIII)HelicotremaExternalacoustic meatusNasopharynxScala vestibuliTympanic membraneTympanic cavityPromontoryRound (cochlear) windowPharyngotympanic (auditory) tubeCochlear ductcontaining spiralorgan (Corti)CochleaScala tympaniNote: Arrows indicate course of sound wavesFigure 5.7 Peripheral Pathways for Sound Reception The process of auditory perception beginswhen sound reaches the auricle (external ear) and is directed through the external acoustic meatus to thetympanic membrane, causing it to vibrate.
The vibration is transmitted by the bones of the air-filled middleear (maleus, incus, and stapes) to the fluid-filled inner ear via the oval window. Within the inner ear, theorgan of Corti of the cochlea contains auditory receptors (see Fig. 5.8).have stereocilia with their distal ends extending to and embedded in the tectorial membrane (see Fig. 5.8). When sound isconducted into the fluid of the inner ear, vibration of thebasilar membrane results in bending of these stereocilia. Theendolymph is unusual in terms of its electrical potential(approximately +80 mV relative to the perilymph), resultingin a charge difference across the apical membrane of approximately 170 mV. Bending of the stereocilia results in changesin cation conductance of the apical cell membrane of haircells; bending in one direction produces hyperpolarization,and bending in the other direction causes depolarization.
Asa result, there is an oscillatory release of neurotransmitter at68The Nervous System and MuscleA. Membranous labyrinth within bony labyrinth(path of sound waves)Cochlear nerveSemicircular canalsUtricleSacculeScala vestibuliCochlear duct (scala media)Scala tympaniRound windowOval window and stapesB.
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