Neurobiology Breakdowns of Systems for Exam III
Neurobiology Breakdowns of Systems for Exam III NSC 201
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This 26 page Study Guide was uploaded by Aleks Lomakin on Tuesday January 12, 2016. The Study Guide belongs to NSC 201 at University of Rochester taught by NORDEEN E in Fall 2015. Since its upload, it has received 55 views. For similar materials see BASIC NEUROBIOLOGY in Neuroscience at University of Rochester.
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SPINAL CONTROL OF MOVEMENTS Chapter 13 Sensorimotor Function o Hierarchical Organization Highest level association cortex Lower level muscles Signals flow between the two levels o Sensory input Motor output o Learning changes the nature and locus of sensorimotor control Spinal Cord o Core of gray matter (neurons), surrounded by white matter (axons) Doral horn sensory inputs Intermediate horn interneurons Ventral horn motor neurons o Each segment has bilateral pair of spinal nerves Mix of sensory and motor Sensory afferents enter via dorsal root Dorsal root ganglion (with long receptors) Motor neurons exit via ventral root One goes out the left and one out the right dermatome o Dermatome each segment of spinal cord innervates a specific body part (control specific muscles) Organization 31 pairs of nerves Top legs Lower back of legs o Sensorimotor Spinal Circuits Descending cortical inputs interneurons in gray matter Interneurons excite/inhibit lower motor neurons Motor Units o Smallest unit of motor activity o Motor neuron (alpha motor neuron) + all muscle fibers it contacts They all contract together Number of fibers varies per unit (innervation ratio) Distal muscles fine control, low ratio Proximal muscles forceful movement, high ratio o Activated in order from small to large Smallest ones are activated first, and as more become needed, activates larger and larger ones Smaller for smaller movements (larger larger movements) o Neuromuscular Junctions ACh (released by motor neurons) motorend plate (on each muscle fiber) contraction (muscles generate force) Two muscles needed on the side of the bone One to pull it down and one to pull up They come in pairs o Muscle contraction Each alpha motor neuron AP muscle fiber AP in all muscle fibers in the motor unit Twitch a contraction elicited by single spike in a single alpha MN Temporal summation twitches sum as alpha MH firing rate increases, increasing force and smoothness of contraction (EPSPs) Tetanus maximum fore developed by saturating summation at high MH firing rate Rate and temporal pattern of an alpha MN activity controls the strength of contraction o Basic Types of Motor Units Skeletal muscle fibers Slow capable of sustained contraction due to vascularization o Doesn’t generate much force o Standing (keeping back straight) o Red muscle (many capillaries) Fast muscle fibers fatigue quickly o Not as much capillary action to them o Lots of glycogen stored o Run out of energy quickly o White muscle Individual muscles have a mix of both Motor unit has one or the other Fast or slow Big or small Motor pool has both (muscles that have both fast and slow fibers) Will go out to either fast or slow Activates them when needed Flexor and Extensor Muscles o Flexor bends or flexes a joint o Extensor straighten or extend a joint o Synergistic muscles two muscles whose contraction produces the same movement Biceps and brachiali muscle (do the same task) Used when bicep alone cannot do the work needed o Antagonistic muscles two muscles that act in opposition Biceps and triceps Muscle Innervation o Skeletal muscle has both extrafusal and intrafusal fibers Extrafusal cause muscle contraction Intrafusal found within muscle spindles One of two types of muscle proprioceptors Muscle spindles in parallel with muscle fibers and signal muscle length Receives its own motor input from gamma motor neurons to keep the spindle responsive to changes in length of the extrafusal muscle Recurrent Collateral Inhibition o Muscle fibers and MN need rest after activity o When lower outer MN comes out of the muscle twitch sends collateral to excite Renshaw cell inhibits that neuron Causing the next contraction in a different neuron Inhibits itself so that its fibers don’t do all the work (spread out) Stretch Reflex o Reflex involuntary and nearly instantaneous movement in response to external stimuli o Produce immediate compensatory contraction to present external forces from altering the intended position of the body Sends commad to outer MN and at the same time to gamma MN so it is the same length o Monosynaptic response one synapse commands the alpha MN to contract again (how you maintain posture) Withdrawal Reflex o Reflexes may be monosynaptic or polysynaptic Many polysynaptic reflexes involved in reciprocal innervation When flexors excited, extensors inhibited o Polysynaptic reflex involving reciprocal innervation Complex Sensorimotor Reflexes o Breathing, feeding, etc. Things that have the ability to be rhythmic and patterned (cyclical) o Walking inverted pendulum gait; body vaults over the stiff limb with each step Swing flexors active, lifting limb off the substrate Stance extensors active, placing limb on the substrate (weight bearing) Swingstance transition flexors and extensors coactivated o Central pattern generator Flexors and extensors controlled by pools of interneurons and motor neurons o Mutual inhibition creates alternating activity in flexors and extensors o Descending excitation activates CPG to initiate and sustain locomotion Sensorimotor Programs o System comprises a hierarchy of central sensorimotor programs o Levels of the system have patterns of activity programmed to them Complex movements produces by activating programs o Development of Sensorimotor programs Programs established with or without practice Practice generate and changes the locus of programming BRAIN CONTROL OF MOVEMENTS Chapter 14 Principles of Sensorimotor Function o Hierarchical organization Association cortex at the highest level and muscles at the lowest Signals flow between levels over multiple pathways o Motor output guided by sensory input o Learning changes the nature and locus of sensorimotor control Conscious automatic When you learn one skill really well (riding a bike) then it drops in hierarchy and you can learn something new (riding a bike and texting) Sensorimotor Association Cortex o At the top of sensorimotor hierarchy o 2 major areas of sensorimotor association cortex: posterior parietal and dorsolateral prefrontal Each composed of several different areas with different functions o Posterior parietal Before a movement is initiated Current position of body parts Location of external objects of interest Receives input from dorsal streams (“where” information) of the somatosensory, auditory and visual systems PPAC decides which sound you should respond to o Out of 3 inputs, decides which one is important Special sensory systems send two major streams Electrical stimulation experience the intent to perform a particular action Damage to PPAC Apraxia (left) inability to make a requested movement (cannot form intent) Contralateral neglect (right) inability to respond to stimuli contralateral to the lesion o The opposite side therefore does not exist Outputs to: dorsolateral prefrontal association cortex, secondary motor cortex and frontal eye field o Dorsolateral prefrontal Receives input from and projects to PPAC Given an intent to move anticipates consequences of various movements and form a plan of action Along with ventrolateral PFC and the end point of the ventral streams (“what” information) Outputs to secondary motor cortex, primary motor cortex and frontal eye field Secondary Motor Cortex o Inputs from association cortex (mainly DLPFAC) o 3 major areas Premotor (PMA) Nearly all cells show extrinsic (movementrelated) activity Electrical stimulation generates complex movement patterns such as hand shaping or reaching For cued movements o Prior to movement, cells active M1<PMA o During movement, cell active M1>PMA o Suggests that PMA devising specific movement strategies prior to execution Supplementary Cingulate o Converts general plans of actions specific set of instructions Active during imagining or planning movements o Outputs to primary motor cortex Primary Motor Cortex (M1) o Located in precentral gyrus of the frontal lobe o Major point of convergence of cortical sensorimotor signals o One of the major points of departure of signals from cortex o Controls the execution of movement o Functional Organization Somatotopic manner (body map) Controls body parts that are capable of intricate movements (hands and face) Each site receives feedback from receptors in the muscles and the joints that site influences o Coding of Movement Controls voluntary movements Intrinsic Space Hypothesis Controls muscles (lowlevel movement dynamics) Controlling parameters such as movement force Electrical microstimulation of motor cortex twitch in individual muscle (as long as stimulation is weak) Movement dynamics o Muscle force o Movement velocity o Joint position Extrinsic Space Hypothesis Controls movements (higherlevel more abstract kinematic aspects of movement) Direction, range and speed of movement Activation of the homunculus at a given site with natural duration and amplitude stimulation elicits complex, speciestypical movements involving that body part o M1 neurons prefer a particular direction of movement o Directional tuning is broad Proves intrinsic hypothesis not true because you move body parts, you don’t move one muscle at a time o One activates arm and one activates leg Movement Directiona Coding Neural representation of movement direction is best expressed by a population code o Directional vector sum all of the “votes” made by each neuron added together to create one overall direction of movement Mirror Neurons Fire when you observe a movement Represent motor acts, but don’t actually execute the movement Useful in understanding actions and learning them Used to read emotions and sensations of others to empathize Cortical Loops and Descending Tracts o 2 major cortical loops: One through basal ganglia and secondary motor cortex (selects ad initiates action) Another through the cerebellum and primary motor cortex that modulates and sequences muscle contractions while a movement is in progress o 4 major descending pathways (primarily from M1) Two in dorsolateral region of the spinal cord Two in the ventromedial region The Basal Ganglia o A collection of interconnected, midline, nuclei located lateral to the thalamus o Striatum (caudate nucleus and putamen), globus pallidus (external and internal), subthalamic nucleus and substania nigra Grouped together and have one major function in terms of the motor system o Pathways Two major functional pathways: direct and indirect/hyperdirect Proper function is balance Direct excitation Facilirates motor (or cognitive) programs in the cerebral cortex that are adaptive for the present task Indirect/hyperdirect inhibition Inhibits the execution of competing motor programs Thalamus is a “yesman” (good decision) Pathway is like multiplication (+) x (+) = (+) (+) x () = () () x () = (+) o Parkinson’s disease Characterized by slowness or absence of movement (bradykinesia), rigidity and a resting tremor (hands, fingers) Balance is tipped in favor of the indirect inhibition pathway Always voting “no” Therapy Ldopa Increased dopamine synthesis in living cells at substantia nigra alleviates some of the symptoms o Huntington’s disease Characterized by involuntary jerky movements of the body, especially of the extremities and face Loss of striatal neurons Direct excitatory pathway is preferred Cerebellum o Thin sheet of folded cortex Folia series of shallow ridges o 10% brain mass o 50% of neurons o Cerebellar Cortex Subdivided into lobes, lobules, vermis (midline region) and lateral hemispheres o Cerebellar nuclei Embedded deep in the white matter of the cerebellum Communicate cerebellar cortical output to other brain centers including motor cortex, descending motor pathways, and vestibular nuclei (balance) o Motor Loop Layer V sensorimotor cortical cells (secondary and primary motor cortex, somatosensory cortex, posterior parietal cortex) project to pontine nuclei (in pons) Ponetine nucleus sends input to cerebellar cortex Function Modulate and sequence muscle contractions for voluntary movement Evaluate disparities between intention and action Correct output of cortical and subcortical motor systems while movement is in progress o Sensorimotor adaptation Cerebellum reprograms movements to compensate for sensory disturbances Adapting which wearing glasses that bend visual perception knows what is straight even if the eyes are not able to see straight Important for learning new motor skills Practice improves efficiency, speed and precision o Disorders Ataxia intention tremor; disturbances in rate and regularity of alternating movements Asynergia prolonged reaction time; decomposition of multijoint movements Dysmetria undershoot or overshoot of movement toward objects Descending Motor Pathways o 4 major descending pathways Dorsolateral tracts Corticospinal (direct) directs tract Corticorubrospinal synapses in the brain stem Terminate in one contralateral spinal cord segment Distal muscles Limb movemnts Ventromedial tracts Corticospinal (direct) directs tract Corticobrainstemspinal synapses in the brain stem Terminate in multiple, bilateral spinal cord segments Proximal muslces Posture and whole body movements o Dorsolateral Tracts Corticospinal tract Descends from primary motor cortex through the medullar pyramids then crosses the midline and synapses directly into the cord Wrist, hands, fingers, toes Direct pathway Corticorubrospinal tract Synapses at red nucleus and crosses before medulla Face, arms, legs Direct pathway and an indirect pathway o Ventromedial Tracts Corticospinal tract Descends ipsilaterally and directly to the spinal cord, then branches and innervates interneuron circuits bilaterally in multiple spinal segments Cortisobrainstemspinal tract Synapses on various brainstem structures and descends bilaterally, carrying information from both hemispheres Synapse on small interneurons in the cord that synapse on motor neurons of the trunk and proximal limb muscles VISUAL SYSTEM OUTLINE Chapters 9 & 10 The Eye o Cornea refractive o Lens refractive for near vision Accommodation when flatseeing far and when bentseeing close o Iris muscle; regulates amount of light entering the eye o Retina tissue upon which an image is projected; contained photoreceptors and is associated with neuronal circuitry Macula central region with highest density of photoreceptors Fovea infolding of macula Optic disk “blind spot”; exit point of optic nerve Brain fills in blind spot by surroundings Pigment epithelium basement membrane; light absorption and metabolic support Visual field defines where you can no longer see Left visual field on the right side of retina (and vise versa) Upper visual field on bottom of retina (and vise versa) Visual acuity the ability of the eye to distinguish two points near each other Laminar organization (LIGHT)Ganglion cells—(amacrine cells)bipolar cells— (horizontal cells)photoreceptorspigmented epithelium o Ganglion cells fire action potentials in response to light (impulsesoptic nervebrain) o Amacrine cells receive input from bipolar cells, spread signal laterally to ganglion cells, bipolar cells and other amacrine cells o Horizontal cells receive input from bipolar cells, spread signal laterally to bipolar cells and photoreceptors If the photoreceptor is missed, the light goes to the pigmented epithelium and is absorbed o Some visual information is lost o Extraocular muscles eye movement Voluntary saccadic/smooth Reflextive image stabilization and head movements The Receptor Mosaic o 125 million photoreceptors in primate retina o 2 classes of receptors Rods Best at medium wavelengths (achromatic) o All have same photopigments Low thresholds function in low light o Scotopic conditions o Small amount of photons needed to activate it (~1) Long, cylindrical outer segment which contains many disks Highest density in peripheral retina (no rods in fovea) Cones Chromatic (3 types of cones depending on wavelength) o Short (blueviolet; rare) o Medium (green) o Long (red) High thresholds function in bright light o Photopic conditions o Many photons needed to activate it (~1020) Short, tapering outer segments with fewer disks Clustered densely in central retina (fovea) Innervation ratio photoreceptors:ganglion cells 1:1 centrally Central vision processed by fovea is overrepresented by output of retina o Fovea Process visual information from 1 degree visual space Central vision very detailed, periphery is not Highest density of receptors, bipolar and ganglion cells Bipolar and ganglion cell bodies minimize scattering to photoreceptors by creating a pit o Pulled to the side so light can come in o Photoreceptors Modified epithelial cells (short receptors have no axons) Outer segment transduction Transduce lightalter membrane currents Inner segment transmission Synapses with bipolar and horizontal cells Transduction currentsrelease of glutamate Transduction o Rhosopsin photopigment Cisretinalchromophore that captures photons Opsin protein that determines spectrum of photons captured o In darkness (unstimulated) cGMPgated Na channels are opendepolarization “dark current” Rods and cones Photoreceptors release more NT in dark than in light glutamate o In light (stimulated) Cisretinaltransretinal dissociates opsin Opsin activated Gprotein transducingPDEbreaks down cGMP cGMPNa channels closeinward current decreaseshyperpolarization Receptive Fields o Photoreceptor receptive fields Degrees of visual space (hundredths to tenths) Radially symmetrical spot defined by crosssectional area Response depends on luminance o Bipolar and ganglion cell receptive fields Circular Opponent response between center and surround depending on response of bipolar cells to photoreceptors “oncenter” o light on in center o stars (like light) “offcenter” o lights off o polka dots (don’t like light) o Opponent receptive fields Ganglion cell RFs match those of their bipolar inputs Bipolar cells Direct input from photoreceptors center Indirect inhibitory inputs from an annulus of receptors around center (horizontal cells) surround Two types o Off center ionotropic glutamate receptors (Na influx) o On center metabotropic glutamate receptors (K efflux) Retinal Ganglion Cell Properties o M cells Peripheral retina (~100,000) Large cell bodies and large RFs Course resolution Inputs from 1,000s of rods and cones Rapid adaptation and conduction of APs Motion response (better than stationary) Largescale, lowcontrast moving patterns o P cells Central retina (~1 million, fovea) Small cell bodies and small RFs Inputs from ~1 cone (color) Slow adaptation and conduction of APs Stationary targets (better than moving) Smallscale, highcontrast fine patterns Retina PCV (primary visual cortex) o Project reversed and upsidedown image on monocular visual field onto each retina o Optic nerves project bilaterallyLGN (apart of visual thalamus) Temporal hemiretina (views contralateral half) ipsilateral LGN Nasal hemiretina (views ipsilateral half) contralateral LGN Crosses the midline in optic chiasm o LGN on each side –(ipsilaterally)PVC o Info from each visual hemisphere processed in contralateral hemi of VC Lateral Geniculate Nucleus o Anatomy Organized into 6 distinct layers 1 and 2 magnocellular “Mtype” large cells (MRGC input) 3—6 parvocellular “Ptype” small cells (PRGC input) Monocular layers (receive input from one eye only) Layers 1, 4, 6 contralateral nasal hemiretina Layers 2, 3, 5 ipsilateral temporal hemiretina Retinotopically organized layers o Physiology Layers are retinotopic “space map” of contralateral visual hemi field in each layer Cells are monocular segregation of RGC input from each eye in overlapping layers RFs are circular/opponent Visual Cortex o Cortical areas processing visual info in occipital, parietal and temporal lobes o Occupies more than 50% of neocortex Vision important for behavior o PVC in occipital lobe o Central 4 degrees of visual space overrepresented in visual cortex Primary Visual Cortex (PVC) o Anatomy Neocortex: 6 layers (interconnect vertically cortical columns) I fibers from cells in deeper layers intracortical connections II & III outputs to other cortical regions (ipsilateral & contralateral) IV (granular layer) input from thalamus to other cortical areas V output to subcortical structures (midbrain, brainstem, spinal cord) VI output to thalamus (LGN) Inputs to PVC LGN Layer IVC (of PVC) o Mcells IVC alpha o Pcells IVC beta V1 is retinotopic o Inputs from 2 eyes representing a given point in the visual field terminate sidebyside Ocular Dominance Columns Beyond layer IVC, signals are binocular Input from one eye is generally dominant over the other o Physiology Receptive Fields of Cortical Neurons Cells in layer IVC: monocular and center/surround organization Neurons beyond layer IVC simple cells Rectangular RFs On and off regions (similar to cells in layer IVC) Orientationselective and location sensitive o V. Complex cells which have no distinct on/off regions, but produce on/off responses throughout receptive fields Many are binocular Simple Cells are Edge Detectors Constructed from convergence of center/surround o Small spot in center strong response o Large spot in surround weak response Optimal stimulus for elongated RF: edge, bar or grating precisely oriented and placed at boundary of on/off region Direction and velocity selective response for visual stimulus moving across RF Cortical Modules in Visual Cortex Pairs of ocular dominance columns o Blobs and many orientation columns Higherorder neurons that prefer same orientation as simple sensitive to the line over larger areas of visual space (complex cells) to the line’s length (hypercomplex) and/or to binocular disparity In PVC small spot of light activates thousands of V1 neurons 2x2mm chunk of striate cortex o This is the cortical module processes information about a small patch of visual field o Necessary without it there is a blind spot o Sufficient neural machinery required to form the color of objects Blobs Receptive Fields Staining reveals blobs of densely packed cells o Most apparent in superficial layers II and III Blobs o Wavelength sensitive o Monocular o Lack orientation and direction selectivity o Koniocellular and parvocellular input (from LGN) o Coloropponent and center/surround Doubleopponent cells o Process color Interblobs o Binocular o Orientation selective and direction selective o Simple and complex cells Blob cell firing rate is higher than that of interblob cells Doral Stream Visual motion and visual control of action o Motion and space Area MT (V5) processing of object motion o Receives input from V2 and V3 also o Neurons with large receptive fields respond to stimulus movement in a narrow range of direction selectivity Area MST cells selective for linear motion, radial motion and circular motion o Global motion patterns o Selective for apparent movement heading o Navigation in space (distinguishing selfmotion from objectmotion) Parietal cortex lesions lead to selective impairments of spatial processing o Akinetopsia, stimultanagnosia, optic ataxia Ventral Stream Perception of visual world and recognition of objects o Color and form Area V4 o Input from blob and interblob o Large receptive fields orientation selective and color selective o Important for shape perception and color perception Area IT o Major output of V4 o Color and abstract shapes are good stimuli o Responsive to faces (fusiform face area) CHEMICAL SENSES: OLFACTION Chapter 8 Sense of Smell o Critical survival role Finding/evaluating food prior to ingestion Avoiding toxic aerosols (doesn’t smell right get rid of it) o Odors and odor objects Olfactory system identifies odor objects of biological significance Odorants Small, volatile molecules (alchols, ethers, fatty acids) Large, complex molecules (musk, steroids) Olfactory Epithelium o Olfactory sensory neurons (OSNs) found in olfactory epithelium of nasal cavity ~20 million in humans Turn over ~28 days (differentiation of basal cells) Long receptor type (sends axons) o Located at the back of the nose o Secretes mucous odorants dissolve in Contains proteins, antibodies, enzymes, salts, odorant binding proteins (enhance concentration of odorants) o Composed of: OSNs, basal (stem) cells, supporting cells Supporting cells similar to glia; help produce mucous and give structure Basal cells source of new receptor cells that continually grow, die and regenerate o Olfactory Bulbs OSN axons form the olfactory nerve (CN I) which projects into this bulb (OB) Axons penetrate a think layer of bone (cribriform plate) Olfactory receptor (OR) genes Humans express ~350 Each OSN expresses one OR subtype (distribution is confined to a single zone) o Most subtypes broadly tuned o Odorant can activate many OR subtypes Sensitivity to odors o OSNs odor generalists (spectrum of responses to a variety of odors) odor objects activate more than one type of OSN (but to a different extent) o Odorant concentration is important o OSN cannot uniquely code for either the identity or the strength of odorants Olfactory receptor proteins 7 transmembranedomain Gprotein coupled receptors, similar to sweetbitterumami taste receptors Each OSN typically expresses only one OR gene Glomerulus OSNs expressing a particular OR converge onto one glomerulus Input layer of each bulb Mapping of receptor cell into here is precise o Homologous chemical senses overlapping o Different odorants elicit activity in different population of glomeruli (partially overlapping) o Odor concentration mechanism for intensity (discrimination of odors) Receives input from only receptor cells of a particular type Odorant processing o Parallel inputoutput pathways (mitral/turfed cells) o Lateral inhibition sharpens mitral cell selectivity to improve odorant discrimination o Organization similar to the retina Chemotopic Organization Each glomerulus is an anatomical and functional unit, processing input from OSNs expressing a single OR Glomerulus have ~25,000 OSN terminals Zones in the olfactory epithelium expressing same OR genes are topographically mapped in the OB Olfactory Transduction o Odorants odorant receptor protein stimulate G protin (G ) increased olf cAMP opens cation channels (influx of Na and Ca) Caactivated Cl channels open (efflux of Cl) depolarization o Currents generate graded receptor potentials APs in OSN axons o Termination Odorants diffuse away (broken down by enzymes) cAMP activates processes that terminate transduction o Adaptation/desensitization Response to prolonged or repetitive stimulation If you shorten the time between smells, the second smell gets shorter and shorter Central Projections o Olfactory tract projects bilaterally to medial temporal lobe structure including the piriform cortex and amygdala (2 pathways from medial lobe) Limbic emotional response Thalamusorbitofrontal cortex conscious perception o The only system that does not pass through thalamus before the cortex CHEMICAL SENSES: TASTE Chapter 8 Taste o Taste a sensation, taste receptor cells in oral cavity o Flavor a perception, multisensory (sight, sound, touch, smell as well as taste) o 5 basic tastes innate Ionic: sour, salty Nonionic: sweet, bitter, umami o Survival Nutrients generally attractive (sweet, salty) Antinutrients repulsive (sour, bitter) Not as helpful for homeostatic properties Bitter tastants are often poisonous in nature learned that they are likely to kill you o Chemistry Some perceptions directly related to taste Salts salty or acids sour Wide variety of unrelated chemicals (sweet sensation) Sugars o Sugar substitutes can fool taste into thinking there is nutritional value there, but the brain is left confused because it received no boost (splenda) Brain will remember this Certain proteins o Transduction Only system with both direct and indirect transduction Most taste receptors are sensitive to two different areas Direct transduction ionic tastes Salty (nutrient) Na ions directly permeate Na channes depol Sour (“antinutrient” Go through Na channels and block K channels (preventing efflux) depol Short receptors (usually no AP) causes AP from depol that happens towards the top of the cell and moves towards the bottom end o A little depol inc in magnitude AP Indirect transduction nonionic tastes (G proteins) Metabotropic receptors look for a tastor (signal molecule) and activate receptors based on what taste was in the food Bitter, sweet or umami binding G protein/PLC/IP3 cascade o IP3 elevates internal [Ca] (release of Ca from internal storage) transmitter release o Opens Caactivated Na channeldepolopens Cav channels o Early processing of tastes Receptor potential magnitude type and concentration of tastant Most cells like more than one tastant Primary gustatory afferents (PGE) go to many cells and many buds Taste selective go to so many cells and eventually pick the right one; sensitive to one taste over another 90% respond to 2+ tastes o Central pathways 7, 9, and 10 cranial nerves solitary nucleus of the medulla 7 (facial nerve) 2/3 of tongue, palate 9 (glossopharyngeal nerve) 1/3 of tongue 10 (vagus nerve) throat, glottis, epiglottis, pharynx Medullaventral posterior (nucleus of thalamus) primary and secondary gustatory cortex (ipsilateral) Ventral posterior medial nucleus (VPM) portion of the thalamus that deals with head sensory information o Central respresentation 4 of 5 primary tastes (excluding umami) overlap extensively Population code how population of neurons respond No individual cell tells you enough information (must look at the population) Completely different than vision and auditory systems in which there are maps to tell you exactly where the sense came from o Pathways from primary gustatory cortex Forebrain taste/flavor perception, where chemical signals first merge to form percept of flavor Medullary motor nuclei feeding behavior: swallowing, chewing, gagging, vomiting, salvation, respiration Hypothalamus, amygdala motivations and hedonic value of food (good food good experience) The Tongue o Taste primary function of the tongue Also present on the roof of the mouth and in the back of the throat o Taste buds 2,000—5,000 on human tongue Papillae give a rough appearance Vallate, foliate, fungiform Not equally spread out everywhere, but can taste any of the 5 tastes Taste buds primary gustatory axons brain stem thalamus cerebral cortex o Taste receptor cells Specialized epithelial structure of the taste bud (along with basal stem cells) 50—100 per taste bud Juices from food sink down into the crevices on the tongue taste Short receptors Replaced every 10 days due to damage done to them by food, mechanical movments Basal stem cells new taste cells Functional morphology Apical pole microvilli (taste pore mucous of oral cavity) o Large surface area to maximize tastant contact Basolateral pole contains organelles o Synapses primary gustatory afferents (project to brain via central gustatory pathway) AUDITORY SYSTEM OUTLINE Chapter 11 The Ear o Sound propagation Wave auditory canal strikes ear drum ossicles (little bones) vibrate oval window vibrates fluid in cochlea starts to move vibrations dissipate through window o Cochlea 3 fluidfilled chambers scala vestibuli, scala media, scala tympani Basilar membrane separates scali tympani from scali media Wider at apex than at base (“flipper”) Distance that the wave travels up the membrane depends on its frequency o High frequency more vibrations, wave doesn’t propagate as far o Low frequency wave travels all the way up to floppy apex Entire foundation of hair cells move (move as a unit) Organ of Corti Two types of hair cells o Inner (~3,500)extend below tectorial membrane o Outer (~14,000) extend in the gelatinous Hair cells (short receptors) cannot generate APs because no axons o Stereocilia on upper surface o The up and down motion of basilar membrane is coverted to sidetoside motion of sterocilia Transduction by Hair Cells Mechanical energy change in membrane polarization o “Mechanotransduction” o Can be converted to electrical or chemical signal Recordings of stereocilia from hair cells o When bent one direction depolarization o When bent opposite direction hyperpolarization o Stereocilia ~500nm in length Movement of 1/1000 of their direction perceptible sound o Ion channels open/close depending on bending Stereocilia 1 or 2 ion channels Hair cell ~100 ion channels Outer hair cells amplifiers (detect soft stimuli) Inner hair cells release glutamate 3:1 ratio of inner vs. outer (majority of information comes from inner hair cells) o Tonotopic Organization Most structure of the auditory system are arrayed according to frequency Frequency sensitivity basilar membrane portions deformed by certain frequencies Phase locking the consistent firing of a cell at the same phase of a sound wave Peaks, troughs, or other constant location on a wave Frequencies determine the mechanism used Low frequency phase locking (able to keep up with wave) Intermediate frequency phase locking and tonotopy o Volley principle High frequency tonotopy o Relied on to indicate sound frequency Auditory Cortex o Located in temporal lobe o Includes Core (primary, A1) 10 belt regions (secondary) Each organized based on frequency (tonotopic) o Ear Primary Auditory Cortex Axons of the auditory nerve fibers synapse in the cochlear nuclei on the same side Projections lead to the superior olives on both sides of the brain stem Cochlear nuclei and superior olives inferior colliculi ipsilateral medial geniculate nuclei (thalamus) ipsilateral primary auditory cortex o Tonotopic representation Low frequencies rostrally and laterally represented High frequencies caudally and medially represented o Visual interactions Vision can affect the was sounds are perceived (McGurk effect) Sound Localization o Duplex theory of sound localization Interaural time delay sound coming from the right reaches you right eat first, and then your left ear second Higher frequencies less delay (travel faster) Interaural intensity difference if the sound comes from the right, the left ear receives a lower intensity of sound than the right ear o Sensitivity of binaural neurons to sound localization Binaural neurons first present in superior olive Receive input from both sides of the brain stem (interaural time delay) use axons as delay lines to measure small time differences precisely o Pitch perception Small are anterior to the PAC has neurons that response to pitch (rather than frequency) Harmonics differ between males and females Brain knows male voices have lower pitch Damage to the Auditory System o Lesions of Auditory Cortex Unilateral disrupts ability to localize sounds in contralateral hemifield Bilateral localization and pitch discrimination o Deafness Conductive damage to ossicles Nerve damage to hair cells
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