Neuroscience 3000 Test 3 Study Guide
Neuroscience 3000 Test 3 Study Guide NEUROSC 3000 - 020
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NEUROSC 3000 - 020
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This 51 page Study Guide was uploaded by Victoria Gonzalez on Tuesday December 8, 2015. The Study Guide belongs to NEUROSC 3000 - 020 at Ohio State University taught by Robert Boyd in Summer 2015. Since its upload, it has received 150 views. For similar materials see Introduction to Neuroscience in Neuroscience at Ohio State University.
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1 Neuroscience 3000 Midterm 3 Study Guide Victoria Gonzalez Chapter 9: The Eye 1. Overview of vision a. Light enters as waves b. Major advancement allowing for reading, looking at art c. Half of the cortex is devoted to vision d. Retina: located at the back of the eye; part of the brain i. The retinal contains photoreceptors which convert light into neural activity; processing begins here ii. Eyes have two overlapping retinas: 1. One is specialized for low light levels 2. The other is specialized for high light levels and the detection of colour iii. The retina detects differences in light intensity, not absolute values iv. Retinal axons are bundled to form optic nerves 1. Optic nerves distribute action potentials to various brain regions that perform different functions: a. Biological rhythms, eye movement, optics e. First stop is lateral geniculate nucleus (LGN) in the thalamus i. Then to the cortex where it is interpreted and remembered 2. Light a. Electromagnetic radiation (wave of energy) visible to our eyes b. Wavelength: distance between two peaks c. Frequency: waves per second i. Higher frequency, higher energy d. Amplitude: peak minus trough e. Only a small part of electromagnetic spectrum is visible for humans: 400-700 nm 3. Optics: the study of light rays and their interactions a. Reflection: most of light we see; light rays bounce off surface b. Absorption: transfer of light energy, compounds absorb light of specific wavelengths and reflect others 2 c. Refraction: bending of light when traveling from one medium to another d. Ray: wave of electromagnetic radiation in vacuum traveling as a straight line 4. Eye anatomy a. Pupil: allows light to enter; it’s dark because of retinal pigments b. Iris: eye colour; iris has two muscles that control pupil size c. Cornea: transparent glassy surface, no blood vessels d. Sclera: tough, white, wall of eyeball e. Extraocular muscles: three pairs of muscles that move eye f. Conjunctiva: membrane that folds back from the inside of eyelids g. Optic nerve: carries axons from retina to the brain h. Retina i. Optic disc: where the optic nerve exits the retina 1. Blood vessels are found here 2. It is a blind spot because it has no photoreceptors ii. Macula: responsible for central vision 1. Has no blood vessels iii. Fovea: center of the retina 1. Thinnest part of the retina; looks dark i. Ciliary muscle: controls the shape of lens j. Aqueous humor: between cornea and lens; nourishes cornea k. Vitreous humor: inside eyeball; keeps eyeball spherical 3 4 5. Eye disorders a. Extraocular muscles: input from one eye is suppressed; imbalance i. Esotropia: cross-eyed; direction of the two eyes cross ii. Exotropia: wall-eyed; direction of the two eyes diverge b. Cataract: clouding of the lens c. Glaucoma: increased intraocular pressure; leads to blindness d. Detached retina: retina pulls away from the wall of the eye e. Retinitis pigmentosa: loss of peripheral and night vision from photoreceptor degeneration f. Macular degeneration: loss of central vision 6. Image formation by the eye a. The cornea refracts most of the light i. Refraction is used to focus light reaching the cornea into a small point on the retina ii. Light is slowed by the aqueous humor causing it to bend b. Focal distance: distance from the refractive surface to the point where parallel light rays converge i. Focal distance for parallel rays: 2.4 cm (0.024 m) ii. The cornea has a refractive power of 42 diopters 1. Focal distance depends on curvature of cornea a. Tighter curves make for shorter focal distances c. Lens also bends light (12 diopters) to help form sharp images of close objects (9 meters) i. Accommodation: changing lens shape 1. Ciliary muscles contract and swell to make the lens more curved giving it more refracting power d. Hyperopia: farsightedness; can’t focus near objects; eye is too short i. Corrected by placing a convex glass in front of eye e. Myopia: nearsightedness; can’t focus far objects; eye is too long i. Corrected by placing a concave glass in front of eye f. Pupillary light reflex: pupil changes size in accordance to light i. Produced by retina-brain stem connections ii. It is consensual: shining a light in one eye changes both pupils 5 iii. Constriction of pupil increases depth of focus; far images are more in focus g. Visual field: space seen with both eyes when focused on one point h. Visual acuity: ability to distinguish between nearby points i. Depends on pacing of photoreceptors in retina and the precision of the eyes refraction ii. 20/20 vision = 0.083 degrees 6 7. Anatomy of the retina a. Most direct path: photoreceptors bipolar cells ganglion cells i. Modifiers to this pathway: 1. Horizontal cells receive input from photoreceptors and project laterally to surrounding bipolar cells and photoreceptors 2. Amacrine cells receive input from bipolar cells and project laterally to bipolar cells, ganglion cells, and other amacrine cells b. Photoreceptors (rods and cones) are the only light sensitive cells i. Other cells are influenced by light via direct/indirect synapses with photoreceptors c. Ganglion cells are the only source of output from the retina i. Ganglion cells are the only retinal neurons that fire action potentials d. Laminar organization: retinal cells are in layers i. Layers are named in reference to middle of the eyeball 1. Ganglion cell layer: ganglion cell bodies 2. Inner plexiform layer: synaptic contacts between bipolar, amacrine, and ganglion cells 3. Inner nuclear layer: bipolar, horizontal, amacrine 4. Outer plexiform layer: synaptic contacts between bipolar, photoreceptors, and horizontal cells 7 5. Outer nuclear layer: cell bodies of photoreceptors 6. Layer of photoreceptor outer segments: contains light sensitive part of retina (rods and cones) 7. Pigmented epithelium e. Photoreceptors: we have about 100 million photoreceptors i. Made up of four parts: outer segment, inner segment, cell body, and synaptic terminal) 1. The outer segment contains stacked disks that have light-sensitive photopigments to absorb light ii. Humans have a duplex retina: two complementary systems in each eye (rods and cones) iii. Both rods and cones are mesotopic: read intermediate light f. Rods: many more rods (92 million in each retina) i. Long, cylindrical; contain many disks ii. Used for night vision 1. Scotopic: night time lighting iii. 100x more sensitive to light iv. Only located in the peripheral; none in the fovea g. Cones: fewer number (5 million in each retina) i. Short, conical; contain fewer disks ii. Cones function in daylight 1. Photopic: daytime lighting iii. Ability to see colour iv. Located mainly in the fovea; few in the peripherals 8 h. Peripheral retina i. Peripheral retina: mostly rods; few cones 1. More sensitive to light because rods are responsible for low light ii. Peripheral retina has more receptor cells per ganglion cell iii. Peripheral retina is poor for details i. Fovea i. Fovea contains only cones; no rods ii. Specialized for high resolution and colour 8. Phototransduction in rods a. 20x more rods than cones b. Membrane potential (Vm) of rod outer segments is -30 mV c. Depolarization is due to special Na channels opening i. Dark current: movement of positive charge across the membrane which happens in the dark ii. Sodium channels that open are gated by cGMP 1. cGMP is made by guanylate cyclase in photoreceptors iii. Light reduces level of cGMP, Na channels close 1. Photoreceptor cells hyperpolarize in light d. Light is the “agonist” for a GPCR (G-protein-coupled receptor) e. Light is absorbed by rhodopsin in the rod outer segment i. Rhodopsin is made up of: 1. Opsin: receptor of light a. G-protein coupled receptor with 7 transmembrane alpha helices, retinal bound to opsin 2. Retinal (vitamin A) f. Light input changes retinal shape and opsin is activated (bleaching: changes the wavelengths absorbed by rhodopsin) g. Transductin stimulated, PDE activated, cGMP broken down h. Amplification allows detection of single photon 9 9. Phototransduction in cones a. Cones contain one of three opsins that give photopigments different wavelength sensitivities b. Rods: no colour (500 nm) c. Blue cones; short cones (430 nm) d. Green cones; medium (530 nm) e. Red cones; long cones (560 nm) f. Young: all colours are created by mixing red, green, and blue light i. Retina senses this g. Young-Helmholtz trichromacy theory: brain assigns colours based on comparison of cone readouts h. Blue-green during the day, dashboard lights (red to affect cones) 10. Dark and light adaptation a. Day to night dark adaptation takes about 20 minutes b. Dark adaptation is due to: i. Pupil dilation; allows more light to enter the eye ii. Regeneration of unbleached rhodopsin 10 c. Light adaptation in cones takes 5-10 minutes, Ca 2+ is important i. In the dark, calcium comes into cone through channel ii. Calcium inhibits guanylyl cyclase which is responsible for making cGMP (cGMP opens sodium channels) 11. Retinal processing a. Only retinal ganglion cells fire action potentials, others change membrane potential in smaller, measured ways b. Ganglion cell responses are affected by horizontal and bipolar cells 12. Bipolar cells: depolarize = release more light; hyperpolarize = release less a. Off-type bipolar cells: hyperpolarize in light i. Light turns them off; release less glutamate neurotransmitter ii. “Sign conserving” (does what photoreceptors do) iii. Have ionotropic (transmitter-gated) channels for glutamate b. On-type bipolar cells: depolarize in light i. Turned on by light; release more neurotransmitter ii. “Sign inverting” iii. Have G protein coupled glutamate receptors c. Receive input from one photoreceptor in fovea, thousands in peripheral retina, horizontal cells too d. Receptive field: are of retina that when stimulated by light changes Vm, center and surround, 1mm = 3.5 degrees, fraction of degree in center, several in periphery 13. P and M ganglion cells a. P ganglion cell: 90% i. Contact one to a few cone bipolar cells ii. They are small, have a small concentric receptive field 11 iii. Carry information about colour iv. Sustained slowly adapting response v. Weak response to movement vi. Carry information about shape b. M ganglion cell: 5% i. Synapses with many bipolar cells ii. Has a large concentric receptor field iii. Transient rapidly adapting response iv. Responds to movement across field v. Low contrast stimuli c. NonM-nonP: 5% i. Some carry colour information 14. Colour opponency a. P and some nonM-nonP cells are sensitive to colour b. Colour-opponent cells: respond in center to one wavelength and is cancelled by another wavelength in the surround c. Blue-blue cones in field d. Yellow-red and green cones e. M cells have no opponency because they receive input from all cone types in both center and surround f. Overall ganglion cell response is to send information about light vs. dark, red vs. green, blue vs. yellow 15. Intrinsically photosensitive retinal ganglion cells (ipRGC) a. Use melanopsin as a photopigment b. Depolarize in light c. Large dendritic fields d. Dendrites are sensitive to light e. Provide input to subcortical areas to synchronize response to light changes (circadian rhythms) 12 16. Parallel processing: different visual attributes are processed simultaneously using distinct pathways a. Information about depth b. Light and dark information c. Ganglion cells have different receptive fields and response properties d. M cells have low resolution but a large field e. P cells have small fields, fine detail f. P and nonM-nonP cells red-green, blue, and yellow information Chapter 10: The Central Visual System Learning Objectives: Understand the anatomy of the central visual system including the LGN, and primary visual cortex (V1) Know the “flow” of information through the path from the retina to the layers of V1 Understand the visual information channels Know the role of extrastriate areas in vision 1. Central visual system a. Must sense shape, colour, position, movement, and binocular vision i. Neurons are sensitive to each of these factors b. Retina extracts some information 13 c. Conscious visual perception pathway from retina to lateral geniculate nucleus (LGN) of thalamus to primary visual cortex i. Primary visual cortex is also called: V1, striate cortex d. Information is separated into parallel pathways for analyzing different features of input e. V1 is connected to over two dozen temporal and parietal lobe areas 2. Retinofugal projection: neural pathway that leaves the eye a. Fugal: pathway that’s direct away from a structure (away from retina) b. Retinofugal projection is made of: i. Optic nerve: one for each eye ii. Optic chiasm: where the two optic nerves combine and axons from nasal retinas cross from one side to the other iii. Optic tracts: run under pia along lateral diencephalon c. Partial decussation: axons from nasal retinas cross from one side to the other, axons from temporal retinas do not cross 3. Visual hemifields a. Visual field: everything left of the midline is the left visual hemifield b. Binocular visual field: overlapping section viewed by both retinas, center of both hemifields c. Left visual hemifield on nasal retina of left eye and temporal of right eye d. Left visual hemifield is viewed by right hemisphere; right visual hemifield is viewed by the left hemisphere 14 4. Optic tract a. Most connect to lateral geniculate nucleus (LGN) of dorsal thalamus b. Optic radiation: projection from LGN to primary visual cortex i. This is the pathway that mediates conscious visual perception c. Damage at various levels cause specific deficits i. If left optic nerve is cut, vision in left eye is lost ii. If left optic tract is cut, the right visual field is lost iii. If optic chiasm is cut, peripheral vision is lost d. Axons connected to the hypothalamus synchronize sleep and wakefulness with the light-dark cycle and other biological rhythms e. Axons connected to pretectum control pupil size and eye movement f. Axons connected to superior colliculus (tectum) orient eyes in response to new stimulus i. 10% of ganglion cells connect here; retinotectal projection 15 5. Lateral geniculate nucleus are the major targets of the optic tracts a. Located in the dorsal thalamus, one on each side b. Six layers; bent like a stack of pancakes around the optic tract i. 1 is most ventral (inside, bottom pancake) c. Optic radiation: from retinal ganglion cells to V1 d. Input from left visual field goes to right LGN e. Layers of LGN store different information: i. Right eye axons synapse in layers 2, 3, and 5 ii. Left eye axons in layers 1,4, and 6 iii. Layer 1 and 2 cells are larger, called magnocellular 1. M-type RGC to magnocellular iv. Layer 3-6 cells are smaller, parvocellular 1. P-type RGC to parvocellular v. NonM-NonP-type to koniocellular layers; tiny neurons between main layers, discovered later 16 f. Receptive fields of LGN neurons are the same as those of the ganglion cells (RGC) that feed them g. ON-center ad OFF-center cells are mixed in all layers h. Small koniocellular cells center-surround with light/dark or colour opponency i. Major input to LGN is from V1, role is not clear, also from the brain stem, modulate responses to stimuli, flash of light when startled 6. Anatomy of striate cortex/primary visual cortex/V1 a. LGN’s major target is the primary visual cortex (V1) i. Located in the occipital lobe of the brain b. Six layers, total of 2 mm thick i. Layer 1 has no neurons c. Spiny stellate cells in layers IVC (axons make local connections, Golgi type II) d. Pyramidal cells in other layers (thick apical dendrite, basal dendrites, project to other regions) e. Inhibitory neurons in all layers, local only 7. Retinotopy a. Neighboring retinal cells inform neighboring target cells in the LGN and striate cortex b. Map is usually distorted because central field is over represented i. Many more neurons that receive information from central field than from periphery (more ganglion cells near fovea) c. A single point of light can activate many cells in the retina and many more cells in the target structure (cortex); causes overlap 17 d. No little pictures, just firing patterns 8. LGN inputs to layer IVC of the striate cortex a. Layers: I, II, III, IVA, IVB, IVCa, IVCb, V, VI, white matter b. Magnocellular (M-type) to IVCα c. Parvocellular (P-type) to IVCβ d. Koniocellular (nonM-nonP type) to layers II and III e. Ocular dominance columns: region of striate cortex receiving information predominantly from one eye 9. Innervation of other cortical layers a. Layer IVC neurons project radially to layers IVB and III where left and right eye inputs mix i. IVCα (Magno) projects to IVB ii. IVCβ (Parvo) projects to layer III b. In layers III and IVB an axon may form synapses with pyramidal cell dendrites from all layers 18 10. Blobs a. Blobs: patches of cytochrome-oxidase-rich neurons i. Cytochrome oxidase: midochrondiral enzyme used in metabolism b. Run through layers II and III, V and VI of the striate cortex c. Lined up with a layer IV ocular dominance columns d. Blobs receive direct input from LGN from the koniocellular cells (layer III) e. Blobs receive input from IVC of striate cortex (parvocellular and magnocellular) 11. Physiology of the striate cortex a. Receptive fields in layer IVC are similar to magnocellular and parvocellular LGN inputs b. Small, center-surround receptive fields c. IVCα is insensitive to wavelength d. IVCβ center-surround color opponency e. Outside IVC is more complex 12. Binocularity a. Ocular dominance columns in IV b. Inputs from both eyes are separate at first and mixed in the more superficial layers c. Some have binocular input; some have monocular input 19 13. Orientation selectivity a. Most fields in retina, LGN, and IVC are circular and respond best to spot of light the same size as receptive field center (center-surround) b. Outside IVC better response to more complex stimuli 14. Orientation columns a. Orientation selectivity: neurons that respond to bars in a particulat orientation i. In the perpendicular orientation, response is much weaker b. Radial column of cells through several layers having same preferred orientation c. Tangential (different layered) cells have shifted orientation selectivity d. 180 degree shift in 1mm e. Orientation-selective neurons specialized for analysis of shape 15. Direction selectivity: respond when a bar of light at an optimal orientation moves perpendicular in one direction; not in the other direction a. Subset of cells that are orientation selective b. Receive input from magnocellular cells of LGN c. Analysis of object motion 20 16. Simple cells: a neuron in V1 that has an elongated orientation-selective receive field with ON and OFF subregions a. V1 neurons receiving a converging input from three or more LGN cells with receptor fields that are aligned along an axis b. Binocular: get information from both eyes c. Sensitive to stimulus orientation; direction selective d. Relatively insensitive to wavelength (no colour) 17. Complex cells: a type of neuron that has orientation- selective receptor a. Don’t have distinct ON and OFF regions b. Complex cells may receive input from several simple cells c. Binocular d. Direction selective e. Relatively insensitive to wavelength (no colour) 21 18. Parallel pathways a. Three channels or pathways process visual input i. Magnocellular pathway 1. M type RGC to magno of LGN to IVCα to IVB 2. IVB: binocular receptive fields here, simple and complex cells 3. IVB: direction and orientation selectivity 4. M channel role is analysis of object motion and guidance of motor actions ii. Parvocellular-interblob pathway 1. P type ganglion cells to parvo of LGN to IVCβ to layer II and III interblob regions 2. Not generally direction sensitive or wavelength sensitive 3. Binocular, simple, and complex 4. Orientation selective 5. Analysis of fine object shape iii. Blob pathway 1. NonM-nonP to koniocellular of LGN to blobs in layers II and III 2. Colour, monocular, no orientation selectivity 3. Circular receptive fields, complex mix of colour opponency and center-surround organizations 4. Analysis of object colours 22 19. Summary a. Several parallel pathways i. M: motion ii. P-Interblob: shape iii. Blob: colour b. Patch of cortex analyzes visual input (cortical module) c. 2 x 2 mm of cortex has: i. 2 sets of ocular dominance columns ii. 16 blobs iii. Interblobs iv. All 180 degrees of orientation 20. Beyond straite a. Two dozen extrastriate areas b. Two large streams of processing i. Dorsal: visual control of action and motion analysis, extension of V1 magnocellular pathway 1. MT (V5): detects motion a. All cells are direction selective b. The direction of motion columns like orientation columns seen in V1 2. Medial superior temporal (MST)-linear motion, radial motion, circular motion 3. Information used for navigation, directing eye movements, motion perception 23 ii. Ventral: perception and object recognition, extension of V1 parvo-interblob and blob pathways 1. V4: receives input from parvo-interblob and blob pathways a. Orientation and colour selective b. Shape and colour 2. Area IT: colours and abstract shapes, some respond to faces c. Each stream receives some input from all pathways 21. Perception a. Grandmother cells: extremely selective receptive fields i. Not yet proven b. Parallel processing c. Objects stimulate synchronous activity in widely separated areas 24 Chapter 11: Auditory and Vestibular Systems 1. Introduction a. Audition: sense of hearing i. Detect sound ii. Perceive and interpret nuances of sound b. Vestibular system: sense of balance; where we are in space i. Head and body location ii. Head and body movements 2. The nature of sound a. Sounds are audible variations in air pressure b. Frequency: number of waves/cycles per second expressed in units called hertz (Hz) i. Cycle: distance between successive compressed patches c. Range: 20 Hz to 20,000 Hz d. Pitch: i. High pitch = high frequency ii. Low pitch = low frequency e. Intensity: high intensity are louder than low intensity 3. Structure of the auditory system a. Pinna: outer ear; funnels in the sound waves (especially from in front of us) b. Auditory canal: entrance to the internal ear c. Tympanic membrane (eardrum): vibrates with the sound waves and moves the bones in the inner ear d. Ossicles: bones in the inner ear that amplify the sound e. Oval window: connects ossicles to cochlea f. Chochlea: transforms physical motion of the oval window into a neuronal response g. Auditory pathway: i. Sound wave moves tympanic membrane ii. Tympanic membrane moves the ossicles iii. Ossicles move the membrane at the oval window iv. Motion at the oval window moves fluid in the cochlea v. Movement of fluid in the cochlea causes a response in sensory neuron vi. Auditory receptors in the cochlea vii. Brain stem neurons viii. Medial geniculate nucleus in the thalamus ix. Primary auditory cortex (A1) in the temporal lobe h. 25 4. The outer ear a. Pinna and auditory canal 5. The middle ear a. Tympanic membrane and the ossicles b. Eustachian tube: connects the air in the nasal cavities with the air in the middle ear c. Sound is amplified by the ossicles i. Amplification is needed to increase pressure; more pressure is needed to move cochlear fluid vs. moving air ii. Pressure: force/surface area iii. 20x greater pressure at oval window than tympanic membrane d. Attenuation reflex i. Response where onset of loud sound causes tensor tympani muscle and stapedius muscle to contract 1. When muscles contract, the ossicles become more rigid and sound conduction is diminished ii. Sound attenuation is greater at low frequencies iii. Functions: 1. Adapt ear to loud sounds (high intensities) 2. Understand speech better (attenuation suppresses low frequencies more) 3. Turns on when we speak; we don’t hear out own voices as loudly 6. The inner ear a. Includes cochlea and labyrinth (part of vestibular system) b. Cochlea anatomy i. Fluid in the cochlea is divided in three chambers: 1. Scala vestibuli: meets the oval window 2. Scala media 3. Scala tympani: meets the round window ii. Basilar membrane: separates scala tympani and scala media 1. Organ of corti: sits upon basilar membrane; contains auditory receptor cells iii. Perilymph: fluid in scala vestibule and scala tympani 1. Fluid is similar to extracellular fluid iv. Endolymph: fluid in scala media 1. Similar to intracellular fluid v. Endochochlear potential: endolymph’s electric potential is about 80 mV more positive than perilymph 1. Enhances auditory transduction 26 c. Physiology of the cochlea i. Pressure at oval window pushes perilymph into scala vestibuli causing the round window membrane to bulge out ii. Any motion at the oval window must be accompanied by a complementary motion at the round window d. The basilar membrane is flexible and bends in response to sound i. Basilar membrane is wider at the apex than at the base ii. Membrane is more stiff at the base than the apex iii. Endolymph movement bends the basilar membrane near the base creating a wave that moves towards apex 1. Higher frequency wave vibrates the base a lot 2. Low frequency moves farther towards apex 3. Tonotopy: organization of sound frequency within an auditory system e. Organ of corti and associated structures i. Organ of corti: contains auditory receptor cells that change mechanical energy into a change in membrane potential 1. Consists of: hair cells, rods of corti, and supporting cells ii. Each auditory receptor (hair cell) has 10-300 hair- looking stereocilia 1. Hair cells are not neurons 2. Hair cells synapse on neurons in the spiral ganglion in the modiolus 3. Spiral ganglion cells are bipolar; connect hair cells with the auditory nerve f. Transduction of hair cells i. The basilar membrane moves in response to a motion at the oval window causing the whole structure to move 1. The basilar membrane, rods of Corti, reticular lamina, and hair cells are rigidly connected 27 ii. Lateral motion of the reticular lamina relative to the tectorial membrane bends stereocilia on the outer hair cells one way or another 1. All the cilia move as a unit; they are connected 2. When they bend in one direction, cell depolarizes; in the other direction, cell hyperpolarizes iii. The tip of each sterocilium has an ion channel that opens and closes by the bending of stereocilia 1. When cilia are pointing straight up, a small amount of K+ moves from endolymph to hair cell 2. When cilia moves in one direction, channel opens more, increase amount of inward K+ current 3. Displacement in the other direction, closes channels more, reducing inward K+ movement iv. Entry of K+ into hair cells causes depolarization (while it causes hyperpolarization in most neurons; endolymph has a high K+ concentration) 1. Depolarization activates voltage-gated calcium channels 2. Entry of calcium triggers the release of glutamate g. Innervation of hair cells i. One spiral ganglion fiber connects to one inner hair cell ii. One spiral ganglion fiber connects to many outer hair cells iii. Even though there are more outer hair cells, most of the information leaving the cochlea comes from inner hair cells 28 29 h. Amplification by outer hair cells i. Cochlear amplifier: outer hair cells amplify the movement of the basilar membrane during low- intensity (quiet) sound stimuli 1. Motor proteins: change the length of outer hair cells a. Prestin: primary motor protein required to move outer hair cell ii. Soft sounds are amplified more than loud sounds 7. Central auditory processes a. Spiral ganglion ventral cochlear nucleus superior olive inferior colliculus medial geniculate nucleus auditory cortex b. All ascending auditory pathways converge onto the inferior colliculus c. Response properties of neurons in auditory pathway i. Characteristic frequency: frequency at which neuron is most responsive (from cochlea to cortex) ii. Response properties become more complex and diverse beyond the brain stem iii. Binaural neurons (both ears) are present in the superior olive 1. Important in sound localization 8. Encoding sound intensity and frequency a. Information about sound intensity is coded by: i. Firing rates of neurons (higher with higher intensity/louder) ii. Number of active neurons (more with higher intensity/louder) b. Stimulus frequency i. Neurons are most sensitive at their characteristic frequency 30 31 c. Tonotopic maps on the basilar membrane i. Auditory nerve fibers connected to hair cells near the apex (top) of the hair cell have low frequencies ii. Auditory nerve fibers connected to hair cells near the base of the hair cell have high frequencies iii. There are tonotopic maps on: 1. Basilar membrane 2. Spiral ganglion 3. Cochlear nucleus d. Phase locking: consistent firing of a cell at the same phase of a sound wave i. Low frequencies: phase-locking on every cycle or some fraction of cycles 1. Makes it easy to determine the frequency of the sound ii. High frequencies: the response is not fixed to a particular phase of the wave 1. Sound waves travel too fast to represent their timing 32 e. Indicating sound frequency: i. At low frequencies: phase locking is used ii. At intermediate frequencies: phase locking and tonotopy iii. At high frequencies: tonotopy is used 9. Mechanisms of sound localization a. Localization of sound in the horizontal plane i. Interaural time delay: the time it takes for a sound to reach from ear to ear ii. Interaural intensity difference: sound at high frequency from one side of the ear 1. Our heads cast a sound shadow 2. There is no sound shadow at low frequencies because sound waves diffract around the head iii. Duplex theory of sound localization: time delay + intensity 1. Interaural time delay: lower frequencies (20- 2,000 Hz) 2. Ineraural intensity difference: higher frequencies (2,000-20,000 Hz) b. Binaural neurons are present in the superior olive; they receive information from both ears and play a role in sound localization i. Sound wave enters ear, activates cochlear nucleus, and is sent to the superior olive ii. Delay lines: arrival of the spike from one side to the olivary neuron is delayed just enough that it coincides with the arrival of the spike from the other side; spikes summate to make EPSP 33 1. Phase locking is essential for comparing timing of inputs (only at low frequencies) c. Localization of sound in vertical plane i. Comparing input to both ears is not useful in vertical plane 1. Interaural delay and interaural intensity do not change ii. Vertical sound localization is based on reflections from pinna 10. Auditory cortex a. Primary auditory cortex (A1) is in the temporal lobe i. Secondary auditory cortex surrounds A1 b. Principles of auditory cortex i. Tonotopy, columnar organization of cells with similar binaural interaction ii. Bilateral ablation of auditory cortex = deafness 34 1. Ablation of one side preserves function; both ears send output to cortex in both hemispheres a. Lesion in striate cortex: complete blindness in one visual hemifield 2. Unilateral loss leads to impaired sound localization iii. There are different frequency bands that are processed in parallel by tonotopic structures c. Neuronal response properties i. Frequency tuning: similar characteristic frequency ii. Isofrequency bands: group similar characteristic frequency, diversity among cells 35 11. The vestibular system a. Importance of the vestibular system: balance, equilibrium, posture, head, body, eye movement b. Vestibular labyrinth: structure near cochlea; contains hair cells i. Vestibular system also uses hair cells to detect changes ii. Vestibular labyrinth includes two structures: 1. Otholith organs: gravity, tilt/angle of the head, linear acceleration a. Each otolith organ contains a sensory epithelium called a macula i. Macular hair cells respond to tilt ii. When the head tilts, gravity pulls on otoconia which moves the gelatinous cap and causes the cilia to bend 2. Semicircular canals: changes in head rotation a. Structure: three arching structures angled 90 degrees between them in each ear i. Each arching structure is paired with another on opposite side of head ii. Helps sense all the possible head rotation angles b. Head rotation: shaking or nodding head c. Angular acceleration: caused by sudden rotational movements 36 c. Vestibulo-ocular reflex (VOR): retinas remain stable independent of head motions i. Function: line of sight is fixed on a visual target ii. Mechanism: senses rotations of head and commands compensatory movements of eyes in the opposite direction iii. Works well even in the dark or when your eyes are closed iv. Connections from semicircular canals, to vestibular nucleus, to cranial nerve nuclei excite extraocular muscles 12. Hearing and balance a. Nearly identical sensory receptors (hair cells) 37 b. Movement detectors: periodic waves, rotational, and linear force c. Auditory system: senses external environment d. Vestibular system: senses movements of itself Chapter 12: Somatic Sensory System 1. Somatic sensory system a. Receptors all over the body rather than receptors grouped together b. Many stimuli; we think of it as four senses: i. Touch ii. Temperature iii. Pain iv. Body position 2. Touch a. Skin can be hairy or glabrous b. Largest sensory organ c. Protects, prevents evaporation of body fluids d. Can detect a dot only 0.006mm x 0.04 mm 3. Mechanoreceptors of the skin a. Mechanoreceptors: physical distortion (bending/stretching) b. Each has unmyelinated axon branches with mechanosensitive ion channels c. Pacinian corpuscle: largest, most studied receptor d. Ruffini’s endings e. Meissner’s corpuscles f. Merkel’s discs: have a nerve terminal, flattened epithelial cell g. Krause end bulbs: nerve terminal like balls of string h. Small receptive fields: i. Meissner’s corpuscles ii. Merkel’s disks i. Large receptive fields: i. Pacinian corpuscles ii. Ruffini’s endings j. Fast adapting: respond quickly at first and then stop firing i. Meissner’s ii. Pacinian k. Slowly adapting: generate a sustained response during long stimulus i. Merkel’s discs ii. Ruffini’s endings l. Hair 38 i. Slow or fast adapting ii. Vary in stimulus frequencies, pressures, receptive field sizes iii. Hair is also a sensory instrument iv. Follicles are innervated by free nerve endings, bending of hair causes changes in action potential firing rate v. Rats have facial whiskers (vibrissae) m. Pacinian corpuscles are most sensitive to 200 – 300 Hz vibrations i. Meissner’s at 50 Hz n. Selectivity of a mechonoreceptive axon depends mainly on structure of ending i. Pacinian corpuscle formed from 20-70 layers of connective tissue (viscous liquid in between) with nerve terminal in middle 1. When deformed, mechanosensitive channels open, sodium and calcium enter depolarizing receptor potential o. Prolonged stimulation causes decrease in receptor potential, release reverses this 39 4. Two point discrimination a. Varies 20 fold across body b. Fingertips are the most sensitive i. More mechanoreceptors ii. Small receptive fields iii. More brain power iv. High resolution mechanisms 40 5. Primary afferent axons a. Bring information to brain stem or spinal cord b. Enter at dorsal roots c. Varying diameters and size which are correlated with type of receptor d. C fibers are the slowest: unmyelinated 6. Spinal cord a. 30 pairs of dorsal and ventral roots b. Spinal nerves pass through notches in vertebral column c. Four groups of spinal segments i. Cervical (C) 1-8 ii. Thoracic (T) 1-12 iii. Lumbar (L) 1-5 iv. Sacral (S) 1-5 41 7. Dermatomes a. Dermatome: area of skin innervated by the right and left dorsal roots of single spinal segment b. Bands on body surface c. One to one relationship between spinal segments and dermatomes d. Adjacent dorsal roots innervate overlapping areas i. If a dorsal root is cut, dermatome does not lose all sensation e. Shingles: (herpes zoster) virus infects all neurons of a single dorsal root ganglion 8. Sensory spinal cord 42 a. Dorsal horn, ventral horn, intermediate zone, columns (white matter) b. Primary afferent axons synapse on second-order sensory neurons within the dorsal horn c. Large Aβ axons synapse on: i. Second-order sensory neurons in the dorsal horn ii. Send axon to the brain 9. Dorsal column-medial lemniscal pathway: touch pathway a. Pathway for touch and vibration are different from pain and temperature pathway b. Aβ sensory axons travel to brain on ipsilateral side through the dorsal column i. Relay touch information and limb position c. Second-order axons also in dorsal column d. Dorsal column axons connect to dorsal column nuclei at medulla-spinal cord junction e. Dorsal column neuron axons decussate (cross to contralateral side) when they reach the medial lemniscus (in medulla white matter) i. Sensory information now on contralateral side f. Medial lemniscus travel through medulla, pons, and midbrain to ventral posterior (VP) nucleus of the thalamus g. Thalamus to primary somatosensory cortex (S1) 43 10. Sensory information is modified in the brain stem and thalamus a. Dorsal column nuclei and thalamic nuclei aren’t simply relays but alter information as it passes through b. Inhibitory interactions take place c. Lateral inhibition: nearby cells inhibit one another i. Promotes contrast enhancement: amplification of difference in the activity of neighboring neurons d. Cortex influences input also 11. Trigeminal touch pathway a. Trigeminal nerves (cranial nerve V): somatic sensation of the face i. Enter the brain at the pons b. Innervate face, mouth, dura matter, tongue c. Some sensation from VII, IX, and X d. Large diameter sensory nerves e. Synapse on ipsilateral trigeminal nucleus then decussate f. Then thalamus g. Then S1 cortex 44 12. Somatosensory cortex a. Most processing in cortex b. Located in parietal lobe c. S1 is located in the postcentral gyrus on 3b d. 1, 2, 3a also process somatosensory information i. Posterior parietal cortex (5, 7) e. Area 3b is considered primary somatosensory cortex because: i. It receives input from ventral posterior (VP) of thalamus ii. It is highly responsive to somatosensory input, but no other senses iii. S1 lesions impair somatic sensation iv. Electrical stimulation causes sensation f. Areas and functions: 45 i. 1: texture information ii. 2: size and shape information iii. 3a: body position iv. 3b: primary visual g. Thalamus 3a and 3b h. 3b 1 and 2 i. Structure: i. Has layers 1. Thalamic input: layers I to IV 2. Layer IV neurons project to other layers ii. Cells with similar characteristics are stacked vertically 13. Cortical Somatotopy: mapping of the body’s surface sensations onto a structure in the brain a. Mapped by S1 stimulation or by recording from S1 neurons after somatic stimulation, maps similar b. Receptive fields form orderly map i. Similar to retina and auditory system c. Map is called homunculus d. Map is not continuous and not to scale e. Size of cortical sensory region was determined by destiny and importance of input f. Importance of body part varies between species g. Sensory signals from each vibrissa follicle in mice go to one clearly defined cluster (barrel) of S1 neurons h. Somatotopy has multiple maps within each part of the sensory cortex 14. Somatotopic map plasticity a. “Use it or lose it” i. If neurons are no longer getting input from a part of the body (lost limb), that part of the brain changes what it responds to b. Map changes based on loss or increased use c. Can use functional imaging to study in humans d. Phantom limb: a feeling coming from a missing limb when other body parts are touched i. Can be invoked by stimulating regions whose S1 representation border those of the missing limb ii. Cortical regions that used to correspond to the limb, now respond to stimulation in the face e. For violinists more cortex is devoted to the fingers of their left hand 15. Posterior parietal cortex a. Have large receptive fields b. Elaborate stimulus preferences 46 c. Integrates with visual, attention, and movement d. Damage to posterior parietal cortex: i. Agnosia: inability to recognize objects ii. Astereoagnosia: inability to recognize objects by feeling them iii. Neglect syndrome: part of the body or part of the world (ex: an entire visual field) is ignored e. Perception and interpretation of spatial relationships, body image, and coordinated body movements f. Somatosensory system is integrated with other systems; especially the visual system 16. Pain a. Nociceptors: free, unmyelinated axons b. Use a different pathway from mechanoceptors c. Pain: the feeling, or conscious awareness d. Nociception: a sensory process that can be controlled by higher centers; provides signals that trigger pain e. Some people can’t sense pain f. Pain transduction i. Stimuli have potential to cause tissue damage ii. Types of damaging stimuli include oxygen deprivation, some chemicals, temperature extremes, mechanical stress iii. Mechanically gated channels open and the cell will depolarize when the tissue is stretched or deformed iv. Some (K+) channels open in response to proteases, ATP, potassium, bradykinin, hydrogen v. Pain and heat 1. 37-43 degrees Celsius is warm a. Above 43 C is painful because tissues burn 2. Warmth and scalding use different neural mechanisms a. Warmth: non-nociceptive thermoreceptors 3. Lactic acid production leads hydrogen to build up in extracellular fluid; H+ activates the hydrogen-gated channels on nociceptors 4. Histamine (from mast cells) depolarizes nociceptors g. Nocioceptors i. Transduction of painful stimuli occurs at the free endings of C fibers (unmyelinated) and Aδ fibers (lightly myelinated) ii. Most nocioreceptors are polymodal: respond to various stimuli (thermal, chemical, and mechanical) 47 1. Mechanical: strong pressure 2. Thermal: extreme hot or cold 3. Chemical iii. In most tissues, bone, meninges; not in the brain h. Hyperalgesia: a reduced threshold for pain; spontaneous sensitivity i. Primary hyperalgesia: area of damaged tissue ii. Secondary hyperalgesia: sensitive surrounding area iii. Peripheral and CNS mechanisms are used iv. The “inflammatory soup” makes nociceptors more sensitive: 1. Bradykinin: depolarizes nociceptors; makes heat-activated ion channels more sensitive 2. Substance P: peptide made by nociceptors; causes vasodilation and histamine release from mast cells 3. Prostaglandins: increase the sensitivity of nociceptors a. Anti-inflammatory drugs: inhibit enzymes that synthesize prostaglandins v. A mechanism for hyperalgesia is the cross-talk between the touch and the pain pathways in the spinal cord i. Primary afferents and spinal mechanisms i. First pain: fast and sharp 1. Due to Aδ fibers (lightly myelinated) ii. Second pain: dull, longer lasting 1. Due to C fibers (unmyelinated) iii. Aδ and C fibers have cell bodies in dorsal root ganglia 48 1. Dorsal horn zone of Lissauer substantia gelatinosa iv. Glutamate is the neurotransmitter used v. Substance P is used for moderate to intense pain vi. Capsaicin stimulates the release of substance P 1. Can be analgesic by depleting substance P from nerve endings vii. Visceral nociceptor axons enter the spinal cord; same as cutaneous viii. Cross-talk can produce referred pain: nociceptor activation is perceived as a cutaneous sensation j. Touch vs. pain Touch Pain Special structures for Free nerve endings input Fat myelinated AB fibers, Slow, lightly myelinated fast Aδ and unmyelinated C fibers Fibers connect deep in Fibers run in the zone of dorsal horn of spinal cord Lissauer to substantia gelatinosa k. Ascending pain pathways i. Spinothalamic pathway 1. Pain and temperature in the body 2. Decussate immediately and run in ventrally in the lateral spinothalamic tract 3. They don’t synapse until reaching the thalamus 4. They don’t communicate with medial lemniscus 49 l. Trigeminal pain pathway: pain and temperature from face/head i. Small diameter fibers in trigeminal nerve synapse on neurons in spinal trigeminal nucleus in the brain stem ii. These second-order neurons decussate and connect to the thalamus via trigeminal lemniscus iii. Input from spinothalamic tract and trigeminal lemniscus spread over larger area of thalamus than the medial lemniscus iv. Touch and pain are still segregated in the VP nucleus v. Spinothalamic axons synapse in intralaminar nuclei of the thalamus vi. Projections to large areas of the cortex, more so than mechanosensory pathways 17. Regulation of pain a. 20% of the population has some form of chronic pain b. The perception of pain varies between people c. Afferent regulation i. Hyperalgesia ii. Reduced by AB fiber activity iii. Gate theory of pain: an interneuron inhibits the pain projection neuron 1. Interneuron is excited by sensory/touch axons (AB, Aa) 2. Interneuron is inhibited by pain axon (C) 50 d. Descending regulation i. Periventricular and periaqueductal gray matter (PAG) neurons synapse on raphe nucleus, suppress pain ii. Raphe neurons (serotonergic) project to dorsal horn and depress nociceptive neuron activity e. Opioids i. Come from poppy, morphine, codeine, heroin ii. Power analgesics, also produce mood changes, nausea, drowsiness, mental stupor, and constipation iii. They bind to opioid receptors, widely expressed in pain pathways f. Endogenous opioids i. Endorphins are endogenous opioids, also expressed in pain pathways 1. Produce analgesia when injected in PAG, raphe nucleus, or dorsal horn ii. Naloxone is an antagonist 1. It blocks glutamate release to hyperpolarize neurons 2. Causes pain by binding to opioid receptors iii. There is a placebo effect mediated through endogenous opioids 18. Temperature a. Thermoreceptors are sensitive to temperature because of their specific membrane mechanisms b. Hypothalamus and spinal cord contain temperature sensitive neurons to help maintain a stable body temperature i. But thermoreceptors determine perception of temperature 51 c. Sensitivity varies across skin i. Some spots are sensitive to hot or cold; but not both ii. Separate receptors code for hot vs. cold d. The sensitivity to a change in temperature depends on the type of ion channels the neuron expresses i. There are six distinct TRP channels; one neuron one channel 1. Trpv1: above 43C and capsaicin/chili peppers 2. Trpm8: menthol or below 25C e. The CNS doesn’t know what kind of stimulus caused receptors to fire f
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