Fundamentals of Neuroscience Exam 3

receptive field

• small area of retina where light changes a neuron’s firing rate

  • light anywhere else outside this would have no effect on firing rate

OFF bipolar cells

• depolarize in the dark

• hyperpolarize in the light

  • light effectively turns them off

• have ionotropic glutamate receptors

  • glutamate released by photoreceptors is excitatory, activates ionotropic receptor

• in light, absence of glutamate causes ionotropic receptors to close

  • preventing sodium influx, hyperpolarizing mem potential

• CHANNELS CLOSE IN ABSENCE OF GLUTAMATE

ON bipolar cells

• depolarize in light

  • they are turned on by light

• hyperpolarize in dark

• have g-protein-coupled receptors

• glutamate released is inhibitory, hyperpolarizes

• in dark, glutamate released activates receptors and cation channels close

  • stopping influx of sodium and calcium, hyperpolarizing mem

• in light, absence of glutamate results in channels being open and allowing cation influx, depolarizing mem potential

• CHANNELS CLOSE IN PRESENCE OF GLUTAMATE

photoreceptor response

• hyperpolarization = less glutamate released

• depolarization = more glutamate released

• hyperpolarize in light

• depolarize in dark

receptive field center

• circular area of retina providing direct photoreceptor input

receptive field surround

• surrounding area of retina providing input via horizontal cells

antagonistic center-surround receptive fields

• response of a bipolar cell’s membrane potential to light in the receptive field center is opposite to that of light in the surround

on-center/off-center bipolar cell

• depolarized by light in receptive field center

• hyperpolarized by light in receptive field surround

bipolar cell indirect pathway

• bipolar cells are connected via horizontal cells to photoreceptors that surrounds central cluster

• when photoreceptor hyperpolarizes in response to light, horizontal cells hyperpolarize (inhibitory synaptic effect)

  • depolarizes central photoreceptor, counteracting hyperpolarizing effect of light shined directly on it

ganglion cell receptive fields

• have same center-surround receptive field organization as bipolar cells

• on-center and off-center x cells receive input from corresponding type of bipolar cell

• will fire APs regardless of exposure to light

• on and off cells not responsive to changes in illumination in center and surround

  • responsive to differences that occur within receptive fields

center of receptive field

• result of direct innervation between photoreceptors, bipolar cells, and ganglion cells

surround of receptive field

• result of indirect communication among retinal neurons via horizontal and amacrine cells

• has opposing effect on bipolar or ganglion cell compared to effect of center region

on-center ganglion cell

• will be depolarized

• respond w/ barrage of action potentials when a small spot of light is on center of receptive field

off-center ganglion cell

• will fire more action potentials when a dark spot covers receptive field center

• fire fewer action potentials when small spot of light is projected

ganglion cells emphasize contrast

(for off-center ganglion cell)

• more output when darkness completely covers center and partially covers surround

  • only partial inhibition from surround

• comparatively less output when darkness completely covers surround and center

  • more inhibiton from surround

M-type ganglion cell

• large

• 5% of cell pop

• larger receptive fields

• low res vision

• conduct APs more rapidly in optic nerve

• more sensitive to low-contrast stimuli

• respond to stimulation of receptive field centers w transient burst of APs

• lack color opponency, responses are not color-specific

P-type ganglion cell

• smaller

• 90% of cell pop

• smaller receptive field

• respond to stimulation of receptive field centers w sustained discharge as long as stimulus is on

• color-opponency

color opponency in ganglion cells

• some P cells and nonM-nonP cells are sensitive to differences in the wavelength of light

• red vs green (R+G-)

  • absorb diff but overlapping wavelengths of light

  • red wavelengths partially absorbed by green cones and inhibits response of neuron

• blue vs yellow (B+Y-)

  • very little blue is absorbed by surround, strong stimulus

• white light contains all visible wavelengths

  • center and surround equally activated

  • cancel each other out, no response

color-opponent cells

• response to one color in the receptive field center is cancelled by showing another color in the receptive field surround

ipRGCs

• intrinsically photosensitive retinal ganglion cells

• use melanopsin as photopigment

• function as normal ganglion cells that receive input from rods and cones and send axons out optic nerves

• they are also photoreceptors

  • depolarize to light

  • large receptive fields

  • not used in fine pattern vision

• explains why subset of blind people synchronize behavior to daily changes in sunlight

visual pathway

• conscious visual perception originates in retina

• lateral geniculate nucleus

• primary visual cortex

• higher order visual areas

retinofugal projection

• leaves the eye, beginning w/ optic nerve

• ganglion cell axons fleeing retina pass through before they form synapses in the brain stem:

  • optic nerve

  • optic chiasm

  • optic tract

optic nerves

• exit the left and right eyes at the optic disks

• travel through the fatty tissue behind the eyes ii their bony orbits

• pass through holes in the floor of the skull

optic chiasm

• optic nerves combine to form this

• lies at the base of the brain, anterior to where pituitary gland dangles down

• axons originating in nasal retinas cross from one side to the other (partial decussation: only axons cross)

decussation

• crossing of a fiber bundle from one side of the brain to the other

optic tract

• after partial decussation at optic chiasm, axons of retinofugal projections form x

• run under pia along lateral surfaces of diencephalon

• most x axons innervate LGN

visual field

• entire region of space seen w/ both eyes looking straight ahead

binocular visual field

• central portion of both visual hemifields viewed by both retinas

left visual hemifield

• objects to left of midline

• objects in binocular region of x imaged on:

  • nasal retina of left eye

  • temporal retina of right eye

• viewed by right hemisphere

right visual hemifield

• objects to right of midline

• objects in binocular region of x imaged on:

  • temporal retina left eye

  • nasal retina right eye

• viewed by left hemisphere

optic radiation

• projection from LGN to primary visual cortex

  • neurons in LGN give rise to axons that project to primary visual cortex

lesions in retinofugal projection

• left optic nerve

  • blind in left eye only

• left optic tract

  • blindness in right visual field

• midline transection of optic chiasm

  • blind only in fibers that cross midline

  • peripheral visual fields on both sides (viewed by nasal retinas)

lateral geniculate nucleus

• located in dorsal thalamus

• major target of optic tracts

• six distinct layers of cells

• gateway to visual cortex —> conscious visual perception

• layers = different types of retinal info being kept separate

right LGN

• receive input from right eye axons in layers 2, 3, and 5

• receive left eye axons in layers 1, 4, and 6

1, 4, 6 CONTRALATERAL

2, 3, 5 IPSILATERAL

left LGN

• receive input from left eye axons in layers 2, 3, and 5

• receive input from right eye axons in layers 1, 4, 6

1, 4, 6 CONTRALATERAL

2, 3, 5 IPSILATERAL

organization of LGN

• inputs segregated by eye and ganglion cell type

magnocellular LGN layers

• layers 1 and 2

• contain larger neurons

• innervate by M-cells

parvocellular LGN layers

• more dorsal layers 3-6

• contain smaller cells

• innervate by P-cells

koniocellular LGN layers

• ventral to each layer

• input from nonM-nonP retinal ganglion cells

nonretinal inputs to LGN

• retina is not main source of synaptic input to x

• also receive from brain stem and thalamus

• primary visual cortex provides 80% of synaptic input to x

  • top-down modulation gates bottom-up input

• brain stem neurons provide modulatory influence on neuronal activity related to alertness and attentiveness

primary visual cortex

• aka V1/striate cortex

• brodmann’s area 17

• located in occipital lobe of primate brain

• has unusually dense stripe of myelinated axons (striate)

cytoarchitecture of striate cortex

• starting at white matter: cell layers VI, V, IV, III, and II

• layer I under pia mater

  • devoid of neurons, amost entirely axon and dendrites of cells in other layers

  • 3 sublayers of layer IV: IVA, IVB, and IVC

    • two tiers of IVC: IVCalpha IVCbeta

spiny stellate cells

• spine-covered dendrites

• layer IVC

• mostly make local connections

pyramidal cells

• spines and thick apical/top dendrite

• layers III, IVB, V, VI

• can make connections to other parts of the brain (farther)

inhibitory neurons

• lack spines

• all cortical layers

• form local connections

inputs to striate cortex

• 2 overlapping retinotopic projection maps

  • one from magnocellular LGN

    • project primarily to IVCalpha

  • other from parvocellular LGN

    • project to IVCbeta

• koniocellular LGN axons follow diff path and make synapses in layers I and III

intracortical connections

• radial connections extend perpendicular to cortical surface along across layers

  • from white matter to layer I

  • pattern maintains retinotopic organization in layer IV

  • ex: cell from VI receives info from same part of retina as cell above it in layer IV

• horizontal connections btwn axons in layer III w/ each other via collateral branches

• IVCalpha (receives magno) project mainly to cells in layer IVB

• IVCbeta (receives parvo) project mainly to III

• III and IVB axon may form synapses w/ dendrites of pyramidal cells of all layers

ocular dominance columns

• bands of cells extending thru thickness of striate cortex

• experiment by hubel and wiesel:

  • studied transneuronal autoradiography from retina to LGN, to striate cortex

  • found layer IV: left eye and right eye inputs are laid out as a series of alternating band, like zebra stripes

mixing of info from 2 eyes

• IVC stellate cells project axons radially up mainly to layers IVB and III

• all IVC neurons receive input from only one eye,

• most neurons in layers II, III, V, and VI receive some amt of input from both eyes

• neurons outside IV are organized into alternating bands dominated by the left and right eyes

outputs of striate cortex

• pyramidal cells send axons out of striate cortex

• layers II, III, IVB cells project to other cortical areas

• layer V cells project to superior colliculus and pons

• layer VI cells project back to LGN

layer V

• of striate cortex

• cells project to superior colliculus and pons

layer VI

• of the striate cortex

• cells project back to LGN

cytochrome oxidase blobs

• mitochondrial enzyme used for cell metabolism

• x are cytochrome oxidase-stained pillars in striate cortex running the full thickness of layers II, III, V, and VI (NOT IV)

  • each x centered on an ocular dominance column in layer IV

  • receive koniocellular inputs from LGN

  • receive parvo and magno input from IVC

magnocellular pathway

• parallel pathway

• cortical neurons are direction selective

• analysis of object motion and guidance of motor actions

parvo-interblob pathway

• parallel pathway

• projects to II and III interblob regions

• have small orientation-selective receptive fields

• analysis of fine object shape

blob pathway

• nonM and nonP ganglion cells project to koniocellular

• project to cytochrome oxidase blobs in II and III

• neurons in blobs are color selective

• analysis of object color

chemical senses

• animals depend on x to identify nourishment, noxious stimuli, or potential mates

• oldest and most common sensory system

• gustation

• smell

• other chemoreceptors

chemoreceptors

• chemically sensitive cells distributed throughout the body

  • nerve endings in digestive organs

  • receptors in arteries to detect O2 and CO2 levels in the blood

  • sensory endings in muscle

    • can detect lactic acid build-up during exercise

basic tastes

• salty

  • sides of tongue has greates sensitivity

• sour

  • sides of tongue

• sweet

• bitter

  • K+, Mg2+, caffeine, quinine

• umami

  • savory taste of amino acid glutamate (MSG in processed foods)

sweet

• fructose

• sucrose

• monellin protein

• artificial sweetners: saccharin and aspartame

• tip of tongue has greatest sensitivity for x

bitter

• K+

• Mg2+

• quinine

• caffeine

• advantageous for survival; poison often bitter

• causes aversive response

  • can be modified thru experience (acquired taste)

• greatest sensitivity on back of tongue

combo of tastes contribute to flavor

• each food activates a combo of taste receptors

• distinctive smell

• other sensory modalities contribute (texture)

organs of taste

• tongue (primarily)

• pharynx

  • chemicals can enter thru to contribute to perception of flavor through olfaction

• palate

• epiglottis

papillae

• bumps on the tongue that contain taste buds

• diff types:

  • fungiform papillae

  • vallate papillae

  • foliate papillae

• each has 1-100s of taste buds

fungiform papillae

• mushroom shaped

• located on anterior 2/3 of tongue

vallate papillae

• pimple shaped

• located on posterior 1/3 of tongue

foliate papillae

• ridge shaped

• located on sides of the tongue

threshold concentration

• just enough exposure to chemical by single papilla required to detect taste

taste bud

• 1-100s on papilla

• each consists of:

  • multiple taste receptor cells

  • basal cells

  • gustatory afferent axons

taste receptor cells

• apical end has microvilli

• at bottom of taste bud, x cells form synapses w/ gustatory afferent axons

microvilli

• at apical end

• project into the taste pore

• house the receptors

receptor potential

• shift in the membrane potential (usually depolarization) when a ligand binds to and activates a taste receptor cell

• typically opens voltage-gated calcium channels to allow influx of calcium

  • triggers release of NT from taste cell onto gustatory afferent axons

    • transduction mechanism and NT released varies on type of taste receptor cell

transduction

• process by which environmental stimulus causes an electrical response in sensory receptor cells

• potential x mechanisms by taste stimuli:

  • pass directly thru ion channels

  • bind to and block ion channels

  • bind to G-protein-coupled receptors and activate second messenger to open ion channels

transduction mechan for salts

• flow through ion channel

• x-sensitive taste receptor cells:

  • special Na+ -selective channel are always open

    • usually open, so when conc of Na+ in mouth increases, depolarization is dependent on extracellular Na+ concentration

  • sufficient receptor potential leads to opening of Na+ and Ca2+ channels

    • trigger NT release of SEROTONIN

    • blocked by diuretic, amiloride

• high lvls of x can activate sour and bitter receptors

transduction mech for sour

• flow through H+ ion channel

• blocking K+ ion channel

• x taste receptor cells detect high acidity

  • H+ can affect cell

    • bind to and block special K+ channels

      • leads to depolarization

    • activate and permeate H+ channels that allow H+ ions to flow into the cell

      • leads to depolarization

  • resulting Ca2+ influx leads to NT release of SEROTONIN

transduction mech for bitter, sweet, and umami

• rely on dimers of T1R and T2R families of receptor proteins

  • ligand binding activates GPCR

    • leads to IP3 production via phospholipase C (PLC)

  • IP3 opens taste cell specific Na+ channel and release of Ca2+ from internal storage sites

  • cells don’t have NT filled vesicles

    • inc Ca2+ opens an ATP-permeable channel

      • allows ATP (acts as NT) to flow out of the cell and activate purinergic receptors on gustatory afferent axons

IP3

• activation of GPCR produces x via phospholipase C (PLC)

• opens taste cell specific Na+ channel

• release of Ca2+ from internal storage sites

bitter taste receptors

• GPCRs consist of proteins from T2R family

• 25 diff T2R genes allow for detection of poisonous substances

sweet taste receptors

• detect sweet molecules:

  • sugars

  • proteins

  • artificial sweeteners

• requires T1R2 + T1R3 receptors to perceive x

  • same 2nd messenger system as bitter taste receptor cells but activate unique gustatory afferent axons

umami taste receptors

• detect amino acids

  • glutamate

• requires T1R1 + T1R3 receptors to perceive x

main taste pathway

• taste buds —> gustatory axons

  • 3 cranial nerves carry primary gustatory axons:

    • VII - facial: innervates anterior 2/3 of tongue

    • IX - glossopharyngeal: innervates posterior 1/3 of tongue

    • X - Vagus: innervates regions around throat

• cranial nerves/gustatory axons —> gustatory nucleus

  • cranial nerves carry taste axons that enter brain stem, bundle together, synapse w/in gustatory nucleus

    • part of solitary nucleus located in the medulla

• pathways diverge for:

  • conscious taste experiences

  • control of feeding behaviors

  • palatability of food

facial nerve

• VII

• innervates anterior 2/3rds of tongue

glossopharyngeal nerve

• IX

• innervates posterior 1/3 of tongue

vagus nerve

• X

• innervates regions around throat

conscious taste experiences

• gustatory nucleus —> VPM nucleus of thalamus —> primary gustatory cortex

• stroke or lesion to either area leads to ageusia (loss of taste)

control of feeding behaviors

• gustatory nucleus —> areas of brain stem (mainly medulla)

  • control swallowing, salivation, gagging, vomiting, digestion, and respiration

palatability of food

• gustatory nucleus —> hypothalamus and parts of limbic system (amygdala)

• areas involved in x and motivation to eat

• lesions to either area lead to changes in food preferences and over/undereating

population coding

• possibility of neural coding of taste

• most plausible

• responses of a large # of broadly-tuned neurons, rather than small # of precisely-tuned neurons, at different lvls of circuit are used to specify properties of a particular taste

  • taste receptor cells are less specific in their responses and may be excited by salt and sour

  • primary taste axons even less specific

labeled lines hypothesis

• possibility of neural coding of taste

• individual tastes are encoded at each level of circuit

  • ex: specific neurons that respond to sweet w/ rapid firing of these cells and not cells of other tastes from taste receptors to cortex

olfaction

• warns of harmful substances in environment

• combines w/ taste for identifying flavor of foods

• mode of communication

  • pheromones produced by body

    • reproductive behaviors

    • territorial boundaries

    • identification of individuals

    • signal aggression or submission

• humans weaker smellers compared to animals

  • due to smaller surface area of olfactory epithelium and density of olfactory receptors

odorants

• activate transduction process in neurons

olfactory neurons

• constitute olfactory nerve

• express only one olfactory receptor gene (one neuron-one receptor)

cribiform plate

• thin sheet of bone through which small clusters of axons penetrate

  • coursing to olfactory bulb

ansomia

• inability to smell

• can result from:

  • viral infections

  • aging

  • neurodegenerative diseases

olfactory acuity

• determined by:

  • surface area of olfactory epithelium

    • 10 cm2 in humans vs 170 cm2 in dog

  • olfactory receptor density

    • dogs have 100x more receptors/cm2 than humans

olfactory receptor

• largest family of mammalian genes discovered w/ over 1000 genes in rodents

  • humans have 350 genes that code for x

• family of GPCR

  • each has 7 transmembrane alpha helices

  • each GPCR has unique structure allows for specific odorant binding

  • each GPCR coupled to olfactory specific G-protein G_olf

• large family of x suggest large number of odors can be recognized ~1 trillion

transduction mech of olfactory receptor cells

all neurons utilize same transduction mechanism

• odorants bind to olfactory GPCR —>

• stimulates olfactory specific protein G_olf —>

• activates adenylyl cyclase —>

• converts ATP to cAMP —>

• cAMP binds to a cyclic nucleotide gated cation channel —>

• open cation channel allows influx of Na+ and Ca2+ which depolarizes OSN and —>

• Ca2+ opens Ca2+ activated Cl- channels —>

• Cl- efflux amplifies membrane depolarization

olfactory receptor cell during stimulation

• if depolarizing receptor potential reaches threshold, OSN will fire APs and transmit info to CNS

olfactory bulb

• OSN axons synapse in spherical glomeruli in x

  • 2000 glomeruli in mice (1000/bulb)

• incoming axons synapse onto approx 100 second-order neurons

• w/in and btwn olfactory bulbs is complex circuitry containing inhibitory and excitatory connections

• activity w/in bulbs can be modulated by input from higher brain areas (cortex, amygdala)

maps of expression of olfactory receptor proteins

• subpops of olfactory receptor genes are expressed in non-overlapping regions of main olfactory epithelium (MOE)

  • within each region individual olfactory receptors are randomly dispersed