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TULANE / NSCI / NSC 3320 / Where does primary neuron come from?

Where does primary neuron come from?

Where does primary neuron come from?


School: Tulane University
Department: NSCI
Course: Systems Neuroscience
Professor: Laura schrader
Term: Spring 2016
Cost: 25
Name: week 4 notes
Description: lecture notes from week 4
Uploaded: 02/09/2016
7 Pages 55 Views 4 Unlocks

Monday, February 1, 2016

Where does primary neuron come from?

Week 4


• sensory systems:

• primary neuron comes in from periphery and synapses in brainstem or spinal cord • secondary neuron (in brainstem or spinal cord) projects to thalamus

• tertiary neuron goes to sensory cortex

• most of the olfactory input goes directly to cortex

• perception of an environmental stimulus:

• sensory receptors: transduce stimulus into action potentials

• photoreceptors: light

• mechanoreceptors: mechanical information

• chemoreceptors: chemical signals

• thermal receptors: hot or cold Don't forget about the age old question of What is the performance of theatre?

• proprioceptors: position of limbs and joints in space

• nociceptors - pain

• properties of stimulus

What does secondary neuron mean?

• modality: which sensory neurons activated Don't forget about the age old question of Which factors are held constant when using the ceteris paribus assumption?

• ex: which taste we’re talking about

• location: sensory regions organized according to incoming signals

• adjacent sensory input processed in adjacent columns

• preserves the topographic organization of receptors

• not really associated with taste

• duration: duration of action potentials Don't forget about the age old question of What is the symbolism of the hammer and sickle?

• intensity: encoded by number of action potentials or fibers activated

• hypotheses of sensory information processing (evidence for both, likely a combination of the  two)

• 1. labeled line: coding model in which peripheral neurons that respond the most robustly to  a given sensory modality carry the information via segregated pathways (wire from  periphery to cortex)

What do sensory receptors mean?

• ONE cell/nerve sends information to cortex

• 2. ensemble code: stimulus and intensity encoded by broadly tuned ensemble of neurons • MULTIPLE neurons We also discuss several other topics like Is nominal data mutually exclusive?

• 5 taste sensations (used to be thought that tastes were sensed on different regions of the  tongue, but actually just which receptors are present)

• sweet - sugars (fructose, sucrose)

• triggered by organic ions

• bitter

• triggered by organic ions

• poisonous taste

• quinine receptors

• umami

• triggered by organic ions

• sour

• triggered by H+ ions (acidic)

• salty

• triggered by Na+ ions


Monday, February 1, 2016 If you want to learn more check out What are the levels of awareness?

• oleogustus

• triggered by fatty acids We also discuss several other topics like What is an auxillary olfactory sense organ in the back of the mouth cavity?

• recent discovery (unsure if transduce as individual taste or not)

• taste is a combination of many things

• olfactory sense also helps with taste (why mouth and tongue so close to nasal cavity) • regions of pharynx, palate and epiglottis also have taste receptors (in addition to tongue) • somatosensory information from mouth

• heat sensed by cranial nerve V (trigeminal)

• papillae: taste sensitive structures

• each papillae has 1-100 tastebuds (tastebuds inside divot)

• each taste bud has

• 50-150 receptor cells

• these taste cells synapse onto gustatory afferent axons

• gustatory afferent axons

• basal cells

• taste pore: senses tastent and activates cells

• receptors for taste on taste cells

• substance binds to receptor by:

• depolarizing taste receptor and allowing calcium influx

• OR causes release of calcium from intracellular stores

• neurotransmitter for sour and salty is SEROTONIN

• bitter, sweet and umami use GPCR (ATP)

• responsiveness of taste cells and gustatory axons:

• 2 different cells episode to NaCl, quinine, HCl and sucrose

• both cells respond differently to different substances

• when cell depolarized, caused increase in action potentials in their axons • when hyper polarized, inhibits AP

• most agree that each cell has a primary taste which is dependent on receptor type in cell • type 1 cells: express Na channels (for salty)

• Na fluxes in through cell, depolarizes and releases serotonin

• type 2: express GPCR (bitter, sweet and umami)

• activate PLC which turns PIP2 into iP3 - increases calcium stores which allows for  release of nt

• type 3: express PKD2L1 (acid sensing sour)

• unsure if this protein is a receptor for the H+ ions OR the cells that express this protein  are just sensitive to hydrogen

• taste transduction:

• salty: simple influx of sodium

• sour: influx of hydrogen closes potassium channels —> hyperpolarization • taste receptor proteins: GPCR (NOT neurons)

• 30 bitter receptors - T2R

• poison (bitter, bad)

• attractive tastes - T1-R1, 2 and 3 (receptors come together as a dimer) • sweet: T1R2 and T1R3

• high energy

• T1R3 may also be involved in tasting calcium

• umami: combination of T1R1 and T1R3

• tastant induced activity in receptor cells in various KO animals (which are genetically modified  to lack the receptor for that taste


Monday, February 1, 2016

• KO animals that lack the receptors show NO response to the taste

• data supports specific receptor theory

• evidence for ensemble coding

• cranial nerves carry various taste sensations

• CT and GSP are branches of facial nerve

• large component of sweet and salty carried here (because these tastes on the anterior  portion of the tongue)

• GP: glossopharyngeal nerve

• carries info from the back 1/3 of tongue

• mainly bitter taste

• SLN is a branch of the vagus nerve

• info from back of throat

• water taste  

• response of individual cells in that nucleus are clearly broadly tuned

• in the cortex, some cells respond broadly to each taste

• acid or sour taste carried in all of the nerves  

• evidence fo labeled line coding: basic tastes are represented in spatial map in primary taste  cortex

• shows activation of cortex in response to different taste - spatial map

• hotspots in cortex which respond to different tastes (but probably respond to other tastes  as well)

• Information from taste carried in cranial nerves (7, 9 and 10)

• Cranial nerve 5 carries somatosensory information

• Taste bud has various types of cells which senses gustatory info to nucleus of solitary tract of  the brainstem

• In rodent, goes to Parabrachial nuclei, goes to amygdala and hypothalamus and VPM • VPM goes to gustatory cortex orbiotofrontal cortex (part of prefrontal cortex and social  interactions)

• process:

• info synapses in gustatory portion of nucleus in solitary tract of medulla • info synapses in ventroposteriormedial portion of thalamus (2nd order neuron) • the project axons to primary gustatory nucleus in insular cortex  


• smell: mode of communication

• important signals:  

• reproductive behavior

• territorial boundaries

• identification (kin relations, danger, taste)

• aggression

• olfaction and taste both chemosensory and often interact

• receptor cells are NEURONS

• strong connection between odor and memory

• electro-olfactogram: allows receptor potentials on olfactory epithelium to be measured • tube on nose measure response to various odorants

• electrode placed using endoscopy, kept in place by frame

• outlet of stimulator carefully brought into nostril


Monday, February 1, 2016

• measures:

• amplitude of EOG is logarithmically related to odorant concentration

• EOG amplitude different at different positions of olfactory epithelium

• longer duration of stimulation/high concentration, slower return of EOG to baseline • odorants produce selective desensitization in olfactory epithelium

• olfactory epithelium: senses smells and odors

• stratified

• 3 cell types

• olfactory receptor neurons: bipolar cells (continually regenerate)

• cilia reaches down into mucus at the top of sinuses to sense odorants

• send their axons (olfactory nerve) to the cribiform plate

• these on synapse in olfactory bulb at the ventral part of the brain

• supporting cells (similar to glia): have Bowman glands that produce mucus • mucus: dissolves odorants before they reach the receptor cells

• water-based with mucopolysaccharides, proteins, antibodies, enzymes,  odorant binding proteins and salts

• basal cells: source of new receptor cells (stem cells)

• cell turnover every 4-8 weeks —> neurogenesis (one of the only brain areas that  does this)

• signal transduction in olfactory receptor cells

• odorant molecules bind to odorant receptors (GPCR - Golf)

• GPCR activates adenylyl cyclase, which uses ATP to produce cAMP

• cAMP activates cycles nucleotide gated channel

• this channel opens and causes influx of calcium and sodium, which leads to depolarization • depolarization causes opening of chloride channels, which causes Cl to leave and cause  MORE depolarization (Cl leaves because these receptors cells have a very high Cl  concentration)

• if this is enough to trigger action potential in the cilia, propagates down the olfactory nerve • methods of termination of the olfactory response:

• odorant diffuses

• enzymes in mucus break the smell down

• termination of signaling (no ore odor)

• adaptation (to smell)

• masking (via other smells)

• random scatter of cells expressing receptor subtypes

• broadly tuned-rebonds to many odorants BUT show a preference (respond most to a  specific smell

• one gene for specific olfactory receptor

• each glomerulus receives input from cells expressing the same olfactory receptor • no organization - randomly placed in epithelium

• 1991: Buck and Axel clones receptor genes from rodents (which have 1000s of receptors -  we only have around 300)

• convergence of primary neurons (which synapse onto second order neurons in glomeruli) • multiple primary neurons synapse on SINGLE mitral cells in glomeruli

• 25000 primary neurons onto 100 secondary mitral and tufted cells

• olfactory nerve: cranial nerve I

• precise mapping of axons of cells expressing specific receptor genes to glomeruli - orderly  processing

• specific cells synapse into 2 glomeruli


Monday, February 1, 2016

• modulation and processing of info in glomeruli by inhibitory and excitatory interneurons • modulatory inputs also from other brain areas

• olfactory maps:

• specific scents display different pattern of neuronal activation in olfactory bulb - also  happens in rodents

• specific glomeruli activated by specific scents

• regions of olfactory bulbs: olfactory maps (regional activation)

• pathway:

• primary olfactory neurons synapse on mitral cells in olfactory tract

• tract divides into medial and lateral olfactory stria

• medial: project to anterior olfactory nucleus —> travels to septal nuclei OR to  contralateral bulb to inhibit mitral cell activity there

• lateral: projects to amygdala, uncus, pyriform, olfactory tubercle and entorhinal cortex • central pathways:

• one directly to olfactory cortex (lateral):

• pyriform, entorhinal cortex:

• emotion/motivation/memory track - why smell is so associated with memory • one synapses in olfactory tubercle:

• olfactory tubercle —> medial dorsal nucleus —> orbitofrontal cortex

• perception of actual smell - THIS one goes to the thalamus

• cortical processing of odors

• different odors activate unique but overlapping neurons in pyriform cortex • activate diffuse cells

• no “nice” arrangement of info in pyriform cortex - no spatial organization • individual mitral cells in glomeruli likely project to broad areas of cortex • spike timing in glomeruli: possible mechanism for odor discrimination

• frequency and latency of spikes sent from various cells

• response from 2 different mitral cells

• cells respond fairly regularly, but have different latencies in response to odorant

Auditory Systems  

• properties of sound waves:

• frequency: pitch

• intensity: loudness

• anatomy of the ear/process of sound

• ossicles: little bones (smallest in the body)

• sound waves move through tympanic membrane

• ossicles move the membrane at oval window

• motion at oval window moves fluid in the cochlea

• movement of this fluid causes response in sensory neurons

• footplate (at stapes of oval window)moves in and out (like piston) transmitting sound  vibrations to the inner ear at the oval window

• ossicles amplify changes in pressure to transmit to the fluid of cochlea (alters both force  and SA)

• pressure = force/area

• bones increase force and decrease surface area at oval window

• organ of corti: contains auditory receptor cells


Monday, February 1, 2016

• endolymph:

• within scala media

• high potassium

• low sodium (intracellular)

• about 80mV more positive than the perilymph —> endocochlear potential • perilymph: low potassium, high Na (extracellular)

• scala vestibuli - on top

• scala media - middle (contains endolymph)

• stria vascularis: active transport to maintain concentration gradients

• membrane on side of scala media

• scala timpani - bottom

• distance the wave travels depends on the frequency

• high frequency: the base will vibrate a lot - dissipate most of the energy and not travel far • lower frequency: travels down membrane

• response of the basilar membrane establishes a place code in which the different  locations on the membrane are maximally deformed by different frequencies • neural coding of pitch

• schematic model of cochlear function

• basilar membrane: wider and less stiff at apex at base

• organ of corti - auditory epithelium

• hair cells are the sensory receptor cells of the auditory system

• outer hair cells lie toward the apex

• inner hair cells more towards modiulus

• rods of corti offer support

• up and down oscillations that bend hair cells - critical step in transduction of sound to  neural signal

• hair cell apical surface contain 50-100 stiffened cilia known as stereo cilia • arranged in ascending height

• hair cell movement causes stereo cilia to flex - this closes channels in cilia membranes • depolarization of hair cell:

• K channels at tops of stereo cilia open when tip links that join the streeocilia are  stretched

• K ENTERS to depolarize cell - opens voltage gated calcium channels

• ca causes release of neurotransmitter

• nt diffuses into spiral ganglion

• 95% of spiral ganglion neurons communicate with small number of inner hair cells • only 5% receive input from outer hair cells

• inner hair cells innervate about 10 spiral ganglion cells (1:10)

• snapse in dorsal and ventral cochlear nucleus of medulla IPSILATERALLY • multiple pathways

• cells in ventral cochlear nucleus send axons projecting to superior olive on both sides of  brainstem

• axons ascend in lateral lemniscus to inferior colliculus

• dorsal nucleus bypasses superior olive

• ALL ascending inputs converge at inferior colliculus

• main pathway:

• spiral ganglion —> ventral cochlear nucleus (via auditory nerve)

• ventral cochlear nucleus —> superior olive

• superior olive —> inferior colliculus (via lateral lemniscus)


Monday, February 1, 2016

• inferior colliculus —> medial geniculate nucleus

• medial geniculate nucleus —> auditory nucleus

• IF it goes to dorsal instead of ventral nucleus: bypasses superior olive

• other projections/brainstem nuclei also involved (ex; inferior to superior colliculus pathways  integrate visual and auditory info)

• LOTS of feedback - brainstem neurons send axons to contact outer hair cells —>  ATTENUATION reflex

• each cochlear nucleus received input from one ear on IPSILATERAL side • all other auditory info from BOTH ears

• only way for brainstem to produce damage in one ear is if cochlear nucleus or auditory  nerve damaged on one side

• properties of sound

• frequency

• tonotopic mapping

• basilar membrane:

• base: high frequencies

• apex: low frequencies

• cochlear nucleus: clusters of cells with characteristic frequencies

• posterior: low frequencies

• anterior: high frequencies

• auditory cortex: characteristic frequencies on parts of cortex

• phase locking

• low frequencies: phase locking on every cycle or some fraction of cycles • combined with tonotopic detection of frequency (<3 kHz)

• high frequencies: relies on tonotopy ALONE

• intensity: encoded by number of axons and action potentials fired

• source:

• horizontal:

• interaural time delay - processed by medial superior olive

• interaural intensity difference - processed by lateral superior olive and medial  nucleus of trapezoid body (MNTB)

• vertical:

• reflections off of pinna

• processing by cochlear nucleus and superior olivary nucleus


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