Fundamentals of Neuroscience (Exam 3: Modules 13-19)

The Bending of Stereocilia

(a) At rest, the hair cells are held between the reticular lamina and the basilar membrane, and the stereocilia are attached to the tectorial membrane. (b) When sound causes the basilar membrane to deflect upward, the reticular lamina moves up and inward, causing the stereocilia to bend outward.

Potential mechanisms of odor identification

1. Population coding: Distinct odors are determined by the activity of many OSNs in olfactory epithelium 2. Spatial coding, or olfactory maps 3. Temporal coding

Process of retinofugal projection:

1. optic nerves exit the left and right eyes at the optic disks, then travel through the fatty tissue behind the yes in their bony orbits and then pass through holes in the floor of the skull. The optic nerves from both eyes combine to form the optic chiasm 2. The optic chiasm lies at the base of the brain, just anterior to where the pituitary gland dangles down. At the optic chiasm, the axons originating in the nasal retinas cross from one side to the other. The crossing of a fiber bundle from one side of the brain to the other is called a DECUSSATION. This is because only the axons originating in the nasal retinas cross, it is said that a partial decussation of the retinofugal projection occurs at the optic chiasm. 3. Following the partial decussation at the optic chiasm, the axons of the retinofugal projections form the optic tracts, which run just under the pia along the lateral surfaces of the diencephalon

What percentage of ganglion cells in the retina project to a part of the midbrain tectum called the superior colliculus?

10%

Which layers of the LGN have smaller neurons?

3-6 (the more dorsal layers)

How many distinct layers does each LGN appear to have in a cross section?

6

Shringles

A type of herpes virus commonly known as chickenpox The virus remains in our primary sensory neurons and revives Restricted to the skin innervated by the axons of the affected dorsal root.

Two-point discrimination

Ability to discriminate the detailed features of a stimulus varies tremendously across the body. Two-point discrimination varies at least twentyfold across the body. Fingertip for Braille reading: - Higher density of mechanoreceptors - Enriched in receptor types that have small receptive fields - More brain tissue - Maybe special neural mechanisms for high-resolution discriminations

The Chemical Senses - Taste

Animals depend on the chemical senses to identify nourishment (e.g. food), noxious stimuli (e.g. poison), or potential mates (e.g. pheromones). Chemical sensation -Oldest and most common sensory system across species Chemical senses a) Gustation - taste b) Olfaction- smell c) Other chemoreceptors- chemically sensitive cells are also distributed throughout the body i. Nerve endings in digestive organs. ii. Receptors in arteries to detect oxygen and carbon dioxide levels in the blood. iii. Sensory endings in muscle which can detect lactic acid build-up during exercise.

Somatic sensory cortex

Area 3b is the primary somatic sensory cortex because a) It receives dense inputs from the VP nucleus b) Very responsive to somatosensory stimuli c) Lesions here impair somatic sensation d) Electrically stimulated, it evokes somatic sensory experiences. 3b projection to area 1 sends texture information 3b projection to area 2 emphasizes size and shape.

The Organs of Taste—Tongue

Areas of greatest sensitivity on the tongue -Tip of the tongue i. Sweetness -Back of the tongue ii. Bitterness -Sides of tongues iii. Saltiness and sourness *however, most of the tongue is sensitive to all basic tastes

The Nature of Sound

Audible variations in air pressure, anything moves air molecules can generate a sound Cycle: distance between successive compressed patches of air Sound frequency: number of cycles per second expressed in hertz (Hz)

Retinal Inputs to the LGN layers

Axons arising from M-type, P-type, and nonM-nonP ganglion cells in the two retinas synapse on cells in different LGN layers. At LGN, input from the two eyes is kept separate. In the right LGN, the right eye (ipsilateral) axons synapse on LGN cells in layers 2, 3, and 5. The left eye (contralateral) axons synapse on cells in layers 1, 4, and 6

Primary Auditory Cortex

Axons leaving MGN project to primary auditory cortex (A1, brodmann's area 41).• Structure of A1 and secondary auditory areas: similar to corresponding visual cortex areas (6 layers) A columnar organization on the basis of frequency. In the tonotopic representation in A1, low frequencies are represented rostrally and laterally, whereas high frequencies are represented caudally and medially

Central Olfactory Pathways

Axons of the olfactory tract: branch and enter the forebrain Direct projections from the olfactory bulb synapse in the olfactory cortex, a.k.a. piriform cortex. -The olfactory system is the only sensory system that can bypass the thalamus on its path to the cerebral cortex.

Trajectory of touch-sensitive Ab axons in spinal cord

Aβ axons have two branches: on second-order sensory neurons, rapid unconscious reflexes; ascending input is responsible for perception

Somatotopic map of facial vibrissae on mouse cerebral cortex

Barrel cortex: sensory signals from each vibrissa follicle go to one clearly defined cluster of S1 neurons

Taste receptors for bitterness

Bitter taste receptors: GPCRs consist of proteins from the T2R family. 25 different T2R genes allow for the detection of many different poisonous substances.

Various sizes of primary afferent axons

C fibers: - Mediate pain, temperature, and itch. - Have no myelin and are about 1 μm in diameter. -The slowest axons, conducting at about 0.5-1 m/sec. Ab(beta) mediates touch sensations.

Taste receptors for sourness

Chemical stimulant: acids (H+ ions) Transduction mechanism: flow through H+ ion channel and blocking K+ ion channel Sour taste receptor cells detect high acidity (low pH): -Protons (H+) are causative agents of acidity and sourness and can affect the cell in several ways: 1.Bind to and block special K+ channels which leads to depolarization. 2.Activate and permeate proton channels that allows H+ ions to flow into the cell, leading to depolarization. -Resulting Ca2+ influx leads to neurotransmitter release of serotonin.

Taste receptors for saltiness

Chemical stimulant: salts Transduction mechanism: flow through ion channel Salt-sensitive taste receptor cells: - Special Na+-selective channel are always open -Channel is usually open, so when the concentration of Na+ in mouth increases, depolarization is dependent on extracellular Na+ concentration. -Sufficient receptor potential leads to opening of Na+ and Ca2+ channels, that trigger neurotransmitter release of serotonin. -Blocked by the diuretic drug, amiloride. High levels of salt can also activate sour and bitter receptors (mechanisms still unknown).

The Basic Tastes Continued...

Correspondence between chemistry and taste is usually obvious, but can vary: -Sweet—sugars like fructose, sucrose; proteins like monellin; artificial sweeteners (saccharin and aspartame) taste sweet -Bitter—ions like K+ and Mg2+; complex molecules like quinine, and caffeine a) Advantageous for survival since poisonous substances are often bitter b) Bitterness causes an aversive response, but this can be modified through experience

Cytochrome Oxidase Blobs

Cytochrome oxidase: mitochondrial enzyme used for cell metabolism Blobs: cytochrome oxidase- stained pillars in striate cortex running the full thickness of layers II and III and also in layers V and VI (not IV) Sliced tangentially through layer III, these pillars appear like the spots of a leopard Each blob centered on an ocular dominance column in layer IV. Receive koniocellular inputs from LGN and parvocellular and magnocellular input from layer IVC

Ocular Dominance Columns

David Hubel and Torsten Wiesel (Novel prize 1981) Studied with transneuronal autoradiography from retina, to LGN, to striate cortex Layer IV: the left eye and right eye inputs are laid out as a series of alternating bands, like the stripes of a zebra

Dermatomes

Dermatomes: the area of skin innervated by the right and left dorsal roots of a single spinal segment Dermatomes—one-to-one correspondence with spinal segments

Combination of tastes contribute to flavor

Distinguishing the countless unique flavors of food: 1.Each food activates a different combination of taste receptors. 2.Distinctive smell 3.Other sensory modalities contribute (e.g. texture)

Broad Tuning of Single Olfactory Receptor Cells

Each olfactory receptor protein binds to different odorants more or less readily, making the cell more or less sensitive to different odors. Each odorant can also bind to many different olfactory receptors at different affinities. Stronger odor concentrations also increase the OSNs response to the odorant.

Taste buds contain taste receptor cells

Each papilla has anywhere from one to several hundred taste buds. Each taste bud consists of: 1.Multiple taste receptors cells 2.Basal cells 3.Gustatory afferent axons Taste receptor cells: -Apical end has microvilli which project into the taste pore. i. Microvilli house the receptors. -At bottom of taste bud, taste receptor cells form synapses with the gustatory afferent axons.

Encoding Sound Intensity

Encoding information about stimulus intensity (loudness). -Firing rates of neurons -Number of active neurons More intense, the basilar membrane vibrates with greater amplitude>> membrane potential of activated hair cells more depolarized or hyperpolarized>>action potentials at greater rates.• Intense stimuli produce movements of the basilar membrane over a greater distance>> activation of more hair cells. Loudness perceived is correlated with number of active neurons and their firing rates.

True or False. The six layers of the LGN lie flat on top of each other like a stack of pancakes.

False. The pancakes do not lie flat, however; they are bent around the optic tract like a knee joint. (The shape helps explain the name geniculate because in Latin it means "like a little knee")

Encoding Sound Frequency

From the hair cells to auditory cortex, most neurons are sensitive to their characteristic frequency. From the base to the apex of the cochlea, a progressive decrease occurs in the frequency that produces a maximal deformation of the basilar membrane. Tonotopic maps maintain from basilar membrane to cochlear nucleus. In cochlear nucleus, bands of cells with similar characteristic frequencies increase from anterior to posterior.

Response Properties of Neurons in Auditory Pathway

Ganglion cells fire action potentials only in response to sound within a limited frequency range. Hair cells are excited by deformations of the basilar membrane, and each portion of the membrane is maximally sensitive to a particular range of frequencies. Characteristic frequency: frequency at which a neuron is most responsive—seen in many neurons from cochlea to cortex

Nonthalamic Targets of the Optic Tract

Hypothalamus: role in biological rhythms, including sleep and wakefulness. Midbrain pretectum: control size of the pupil, certain types of eye movement. Midbrain superior colliculus (retinotectal projection): indirect connections with motor neurons in brain stem, commands eye and head movements. Orients the eyes in response to new stimuli.

Olfactory Receptor Cell during Stimulation

If the depolarizing receptor potential reaches threshold, OSN will fire action potentials and transmit information to the CNS.

Organization of the 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: the four more dorsal layers, 3 through 6, contain smaller cells. Innervate by P-cells Koniocellular LGN layers: ventral to each layer. input from nonM-nonP retinal ganglion cells

Depolarization of a hair cell

Ion channels on stereocilia tips are opened when the tip links joining the stereocilia are stretched. The entry of K+ from endolymph depolarizes the hair cell, which opens voltage-gated calcium channels. Incoming Ca2+ leads to the release of neurotransmitter from synaptic vesicles, which then diffuses to the postsynaptic neurite from the spiral ganglion.

What does the pretectum (part of the midbrain) do when direct projections occur?

It controls the size of the pupil and certain types of eye movement

What is the left visual field viewed by?

It is viewed by both the nasal left retina and the temporal right retina.

LGN

LGN, located in the dorsal thalamus, major targets of the two optic tracts. Viewed in cross section, arranged in six distinct layers of cells. The LGN is the gateway to the visual cortex and, therefore, to conscious visual perception. Different types of retinal information are being kept separate.

What is the most ventral layer of the LGN?

Layer 1

Mixing of Information from the Two Eyes

Layer IVC stellate cells project axons radially up mainly to layers IVB and III Whereas all layer IVC neurons receive input from only one eye, most neurons in layers II, III, V, and VI receive some amount of input from each eye. Neurons outside layer IV are organized into alternating bands dominated by the left and right eye. The bands of cells extending through the thickness of the striate cortex are called ocular dominance columns.

Which layers of the LGN have larger neurons?

Layers 1 and 2

Which layers does the left LGN synapse on?

Layers 1, 4, and 6 (contralateral axons)

Which layers does the right LGN synapse on?

Layers 2, 3, and 5 (ipsilateral axons)

What causes blindness in part or all of the visual field in humans and what is important to note about this?

Lesions anywhere in the retinofugal projection from the eye to the LGN to the visual cortex cause blindness. This is important because we know that it is this pathway that mediates conscious visual perception

Where is the superior colliculus located and what is it involved in?

Located in the tectum of the midbrain, it is involved in generating saccadic eye movements, the quick jumps in eye position used to scan across a page while reading.

Central Taste Pathways

Main Pathway: 1) Taste buds gustatory axons: Cranial nerves: - VII - Facial: innervates anterior 2/3 of tongue - IX - glossopharyngeal: innervates posterior 1/3 of tongue. - X - vagus: innervates regions around the throat 2) Cranial nerves/gustatory axons --> gustatory nucleus within the solitary nucleus located in the medulla. 3)Pathways diverge for conscious taste experiences, control of feeding behaviors, palatability of food. a)Conscious taste experiences: -Gustatory nucleus --> ventral posterior medial (VPM) nucleus of the thalamus --> primary gustatory cortex. Stroke or lesion to either areas leads to ageusia (loss of taste).

The Inner Ear—(cont.)

Majority spiral ganglion neurons communicate with the relatively small number of inner hair cells. One spiral ganglion fiber synapses with one inner hair cell, numerous outer hair cells; each inner hair cell feeds about 10 spiral ganglion neurites. Vast majority of the information leaving the cochlea comes from inner hair cells. Amplification by outer hair cells—cochlear amplifier. Motor proteins: only occur in outer hair cells. change length of outer hair cells in respond to sound. Prestin: protein required for outer hair cell movements. When motor proteins change the length of the hair cell, the basilar membrane is pulled toward or pushed away. Furosemide and certain antibiotics (e.g. kanamycin) inactivates the outer hair cell motor proteins and decreases sound transduction.

Receptive field size and adaptation rate

Mechanoreceptors also vary in the persistence of their responses to long-lasting stimuli.

Mechanosensitive ion channels

Mechanoreceptors have unmyelinated axon terminals. Mechanosensitive ion channels convert mechanical force into change of ionic current. Mechanical stimuli may trigger release of second messengers.

Small and large receptive fields

Mechanoreceptors vary in their preferred stimulus frequencies, pressures, and receptive field sizes.

Auditory Pathways Continued...

More projections and brain stem nuclei contribute to the auditory pathways. There is extensive feedback in the auditory pathways. For instance, brain stem neurons send axons that contact outer hair cells, and auditory cortex sends axons to the MGN and inferior colliculus. Each cochlear nucleus receives input from just the one ear on the ipsilateral side; all other auditory nuclei in the brain stem receive input from both ears.

Patterns of Intracortical Connections

Most intracortical connections extend perpendicular to the cortical surface along radial lines. This maintains the retinotopic organization established in layer IV. The axons of some layer III pyramidal cells extend collateral branches that make horizontal connections within layer III. Radial and horizontal connections play different roles in the analysis of the visual world. Layer IVCα projects mainly to cells in layer IVB. Layer IVCβ projects mainly to layer III.

Traveling Wave in the Basilar Membrane

Movement of the endolymph makes the basilar membrane bend near its base, starting a wave toward the apex. The distance the wave travels up depends on the frequency of the sound. If the frequency is high, the stiffer base of the membrane will vibrate a good deal, and the wave will not propagate very far. Low-frequency sounds generate waves that travel all the way up to the floppy apex.

Location and Structure of an Olfactory Bulb

OSN axons synapse in the spherical glomeruli in olfactory bulbs. About 2000 glomeruli in mice (1000 per bulb). The incoming axons synapse onto approximately 100 (250:1) second-order neurons

Smell—Key Concepts

Odorants: activate transduction processes in neurons. Olfactory axons constitute olfactory nerve. Cribriform plate: thin sheet of bone through which small clusters of axons penetrate, coursing to the olfactory bulb Anosmia: inability to smell Humans are weaker "smellers" compared to many animals due to smaller surface area of olfactory epithelium and density of olfactory receptors

Olfaction (Smell)

Olfaction: -Warns of harmful substances in environment -combines with taste for identifying flavor of foods -mode of communication - pheromones are produced by body a) Reproductive behaviors b) Territorial boundaries c) Identification of individuals d) Signal aggression or submission e) Mechanisms and roles of human pheromones still unclear

Spatial coding-olfactory maps

Olfactory maps (sensory maps): -Main olfactory bulb glomerular organization in conserved across individuals and these glomeruli receive input from only one receptor type. -Specific odors result in different spatial patterns of activation in the MOB. -However, there is still much debate on olfactory maps. There is no evidence so far to support that the piriform cortex can "read and understand" the spatial map. Alternatively, this organization may simply be a more efficient way for the brain form connections between related neurons.

Olfactory Sensory neurons (OSN)

Olfactory receptor: - Largest family of mammalian genes discovered with over 1000 genes in rodents. (Buck and Axel 1991). i. Humans have about 350 genes. -7 transmembrane domain GPCRs i. Each GPCR has a unique structure which allows for specific odorant binding. - In most cases, OSNs express only one olfactory receptor gene. - Each GPCR is coupled to the olfactory specific G-protein Golf. - The large family of olfactory receptors suggest that a large number (est. 1 trillion) of odors can be recognized

The Organ of Smell - olfactory epithelium

Olfactory sensory neuron (OSN) axons are unmyelinated and fragile - traumatic blows to head can sever axons and lead to anosmia Conserved structures ∴olfactory acuity determined by: -Surface area of olfactory epithelium: i. 10 cm2 (humans) vs 170 cm2 dogs -Olfactory receptor density i. Dogs have 100X more receptors/cm2 than humans

Important note

Optic nerve fibers cross in the optic chiasm, such that the left visual hemifield is "viewed" by the right hemisphere and the right visual hemifield is "viewed" by the left hemisphere

The Organ of Corti and Associated Structures

Organ of Corti: The auditory receptor cells/hair cells, rods of Corti, and various supporting cells. Hair cells are actually specialized epithelial cells. Has hairy-looking stereocilia extending from top. Hair cells lie between the basilar membrane and the reticular lamina The basilar is at the base of the organ of Corti, the tectorial forms a roof, and the reticular is in the middle. Hair cells form synapses on spiral ganglion which enters the auditory nerve, a branch of the auditory-vestibular nerve.

Retinotopy

Organization whereby neighboring cells in the retina feed information to neighboring places in their target structures (retina, LGN, superior colliculus, striate cortex) The mapping of the visual field onto a retinotopically organized structure is often distorted because visual space is not sampled uniformly by the cells in the retina (Central visual field (fovea) overrepresented in map) Discrete point of light can activate many cells in the target structure due to overlapping receptive fields. Perception is based on the brain's interpretation of distributed patterns of activity—not literal map.

Mechanoreceptor frequency sensitivity

Pacinian corpuscles are most sensitive to vibrations of about 200-300 Hz Meissner's corpuscles respond best around 50 Hz

Mechanoreceptors

Pacinian corpuscles: highest densities in the fingers, lies deep, mm size Ruffini's endings Meissner's corpuscles: one-tenth the size of P. ridges of glabrous skin Merkel's disks: consist of a nerve terminal and a flattened, non-neural epithelial cell

The Organs of Taste—Tongue Continued...

Papillae- bumps on the tongue that contain taste buds. Different types of papillae are located on specific regions of the tongue: 1.Fungiform papillae: mushroom shaped, located on anterior 2/3 of tongue. 2.Vallate papillae: pimple shaped, located on the posterior 1/3 of tongue. 3.Foliate papillae: ridge shaped, located on the sides of the tongue. Threshold concentration: -Just enough exposure to chemical by single papilla required to detect taste.

Cochlea

Perilymph: fluid in scala vestibuli and scala tympani, similar ionic content to CSF. Endolymph: fluid in scala media, similar as intracellular fluid, high K, low Na. Endocochlear potential: endolymph electrical potential 80 mV more positive than perilymph. Enhance auditory transduction.

Structure of the Auditory System

Pinna: cartilage covered by skin, collect sounds Auditory canal: ends at the tympanic membrane Ossicles: transfer movements of the tympanic membrane to the oval window Cochlea: fluid-filled, transforms physical motion of the oval window membrane into a neuronal response

Primary afferent axons

Primary afferent axons enter the spinal cord through the dorsal roots; their cell bodies lie in the dorsal root ganglia

Somatic sensory areas of cortex

Primary somatosensory cortex (S1)=Brodmann's area 3b lies on the postcentral gyrus Adjacent somatic sensory areas -Postcentral gyrus: 3a, 1, 2, -Posterior parietal cortex: areas 5, 7

Outputs of the Striate Cortex

Pyramidal cells send axons out of the striate cortex Layer II, III, and IVBcells project to other cortical areas. Layer V cells project to the superior colliculus and pons. Layer VI cells project back to the LGN.

Audible Sound

Range: 20 Hz to 20,000 Hz, sound velocity is constant. Frequency is the number of sound waves per unit of time or distance. We perceive high-frequency waves as having a higher pitch/tone. Intensity is the air pressure difference between peaks and troughs of the sound waves. We perceive high-intensity waves as louder. Real world: simultaneous combination of different frequency waves at different intensities.

Taste Responsiveness of Taste Cells

Receptor potential: shift in the membrane potential (usually depolarization) when a ligand binds to and activates a taste receptor cell. Typically, the receptor potential opens voltage-gated calcium channels to allow an influx of calcium, which triggers the release of neurotransmitter from the taste cell onto gustatory afferent axons. BUT...the transduction mechanism and neurotransmitter released varies on the type of taste receptor cell!!

Somatotopic map plasticity

Remove digits or overstimulate— examine somatotopy before and after Maps are dynamic. Cortex originally devoted to the amputated digit responded to stimulation of the adjacent digits Representation of the stimulated digits had expanded in comparison with the adjacent, unstimulated ones Amputees: perception of sensations from the missing limb when other body parts are touched

Central visual system

Retina does not pass along information about the patterns of light and dark that fall on it. Rather, the retina extracts information about differences in brightness and color. Information extracted by the retina is analyzed by the central visual system -Neural processing results in perception.• Conscious visual perception originates in the retina, progresses to lateral geniculate nucleus, primary visual cortex, and higher order visual areas

Nonretinal Inputs to the LGN

Retina is not the main source of synaptic input to the LGN, also from brain stem and thalamus. Primary visual cortex provides 80% of the synaptic input to the LGN—role not clearly identified. -"Top-down" modulation may gate "bottom-up" input from LGN to cortex. Brain stem neurons provide modulatory influence on neuronal activity related to alertness and attentiveness Thus, the LGN is more than a simple relay from the retina to the cortex.

The Basic Tastes

Salty: salts. Sour: acids. Sweet: sugars and artificial sweeteners. Bitter: K+, Mg2+, caffeine and quinine. Umami or "delicious" in Japanese: savory taste of the amino acid glutamate; monosodium glutamate often used in processed foods.

Auditory System

Sensory systems. -Sense of hearing: audition. -Sense of balance: vestibular system. Feels the tilts and rotations of the head, sense where our head and body are and how they are moving.

Somatosensory system

Somatic sensation -Enables body to feel, ache, sense temperature and pressure -Somatic sensory system: different from other systems a) Receptors: broadly distributed b) Responds to many kinds of stimuli—at least four senses rather than one: pain, position, temperature, touch

Multiple somatotopic maps

Somatic sensory system has several maps of the body. Areas 3b and 1 of owl monkey—hand area with mirror image maps

Auditory Pathway: first stage

Sound wave moves the tympanic membrane. →Tympanic membrane moves the ossicles. →Ossicles move the membrane at the oval window. →Motion at the oval window moves fluid in the cochlea. →Movement of fluid in the cochlea causes a response in sensory neurons. Outer ear, middle ear, inner ear

Segmental organization of spinal cord

Spinal segments (30)—spinal nerves within four divisions of spinal cord Each segment is named after the vertebra adjacent to where the nerves originate Cervical (C) 1-8, thoracic (T) 1-12, lumbar (L) 1-5, and sacral (S) 1-5.

Lamination of the Striate Cortex

Spiny stellate cells: spine-covered dendrites—layer IVC. Mostly make local connections. Pyramidal cells: spines and thick apical dendrite—layers III, IVB, V, VI. Can make connections to other parts of the brain. Inhibitory neurons: lack spines—all cortical layers—form local connections

Auditory Pathways

Spiral ganglion>>auditory-vestibular nerve>>brain stem. Medulla: dorsal cochlear nucleus and ventral cochlear nucleus ipsilateral to the cochlea. Ventral cochlear nucleus >> superior olive on both sides of the brain stem. Olivary neurons >>lateral lemniscus >> inferior colliculus of the midbrain. Dorsal cochlear nucleus bypasses superior olive. All ascending auditory pathways converge onto the inferior colliculus>>MGN>>auditory cortex.

Cytoarchitecture of the Striate Cortex

Starting at the white matter, the cell layers are named by Roman numerals VI, V, IV, III, and II. Layer I, just under the pia mater, is largely devoid of neurons and consists almost entirely of axons and dendrites of cells in other layers. Three sublayers into layer IV, labeled IVA, IVB, and IVC. Layer IVC is further divided into two tiers called IVCα and IVCβ

Receptive Fields at LGN

Study the action potential discharges of a geniculate neuron in response to visual stimuli and map its receptive field. Receptive fields of LGN neurons: almost identical to the ganglion cells that feed them Magnocellular LGN neurons: large center-surround receptive fields with transient response, like M-cells. Parvocellular LGN cells: small center-surround receptive fields with sustained response, like P-cells. The neurons are activated by only one eye and ON-center and OFF-center cells are intermixed.

Maps of Expression of Olfactory Receptor Proteins

Subpopulations of olfactory receptor genes are expressed in non-overlapping regions of the main olfactory epithelium (MOE). Within each region the individual olfactory receptors are randomly dispersed.

Taste receptors for sweetness and umami

Sweetness: Detect sweet molecules: sugars, proteins, artificial sweeteners -Requires T1R2 + T1R3 receptors -Same second messenger system as bitter taste receptor cells, but activate unique gustatory afferent axons Umami: Detect amino acids (e.g. glutamate) Requires T1R1 + T1R3 receptors

The Visual Pathway that Mediates Conscious Visual Perception

Targets of the optic tract: mostly in LGN Optic radiation: projection from LGN to the primary visual cortex.

The Organs of Taste

Taste is primarily a function of the tongue, but regions of the pharynx, palate, and epiglottis have some sensitivity to chemicals as well Chemicals entering mouth can enter through pharynx to contribute to perception of flavor through olfaction

Temporal coding

Temporal coding in the olfactory system -Oscillatory activity when animals breathe in (in this case 2 spikes per inhalation) -Temporal pattern of action potential firing changes when odors are presented to the animal. -Similar to the theory of spatial coding, it is difficult to show that the temporal coding is actually used to identify a stimulus.

The Middle and Inner Ear

Tensor tympani and stapedius muscle contraction make the ossicle chain more rigid. So to diminish sound conduction. The attenuation reflex: Response when onset of loud sound. Function: To adapt the ear to continuous sound at high intensities. thus increasing the dynamic range we can hear. Adapts ear to loud sounds, protects inner ear. Suppresses low frequencies more, enables us to understand speech better.

Columnar organization of S1's area 3b.

Thalamic inputs to S1 terminate mainly in layer IV. S1 neurons with similar inputs and responses are stacked vertically into columns extend across cortical layers Within the area of each finger representation are alternating columns of cells with rapidly adapting (green) and slowly adapting (red) sensory responses.

Dorsal column-medial lemniscal pathway

The ascending branch of the large sensory axons (Aβ) enters the ipsilateral dorsal column of the spinal cord The axons of the dorsal column terminate in the dorsal column nuclei, at the junction of the spinal cord and medulla. some of the longest axons From this point onward, decussate to ventral posterior (VP) nucleus of the thalamus, and primary somatosensory cortex, or S1.

The Middle Ear

The fluid in the inner ear resists being moved much more than air does. Sound force amplification (20 times) Pressure: force per surface area. (1) ossicles' levering effect raise the force. (2) the surface area of the oval window is smaller than the area of the tympanic membrane.

Right and Left Visual Hemifields

The full visual field is the entire region of space seen with both eyes looking straight ahead Objects appearing to the left of the midline are in the left visual hemifield, and to the right of the midline are in the right visual hemifield Binocular visual field: central portion of both visual hemifields is viewed by both retinas. Objects in the binocular region of the left visual hemifield will be imaged on the nasal retina of the left eye and the temporal retina of the right eye. Fibers from the nasal portion of the left retina cross at the optic chiasm, all the information about the left visual hemifield is directed to the right side of the brain. The left visual hemifield is "viewed" by the right hemisphere and the right visual hemifield is "viewed" by the left hemisphere.

Hair cell receptor potentials.

The hair cell depolarizes or hyperpolarizes, depending on the direction in which the stereocilia bend. A sound wave causes the stereocilia to bend back and forth, the hair cell generates a receptor potential that alternately hyperpolarizes and depolarizes.

Parallel Pathways

The magnocellular pathway: cortical neurons are direction selective, analysis of object motion and the guidance of motor actions. The parvo-interblob pathway: projects to layer II and III interblob regions. have small orientation-selective receptive fields, analysis of fine object shape. The blob pathway: projects to the cytochrome oxidase blobs in layers II and III. neurons in the blobs are color selective, analysis of object color.

The Retinofugal Projection

The neural pathway that leaves the eye, beginning with the optic nerve. Three structures before they form synapses in the brain stem: optic nerve, optic chiasm, and optic tract. Optic chiasm: anterior of pituitary, the axons originating in the nasal retinas cross from one side to the other (partial decussation).

Anatomy of the Striate Cortex

The primary visual cortex (V1 / striate cortex) is Brodmann's area 17 and is located in the occipital lobe of the primate brain Striate refers to the fact that area V1 has an unusually dense stripe of myelinated axons

Response of Basilar Membrane to Sound

The response of the basilar membrane establishes a place code in which different locations of membrane are maximally deformed at different sound frequencies. Systematic organization of sound frequency within an auditory structure is called tonotopy. Tonotopic maps exist on the basilar membrane and within each of the auditory relay nuclei, the MGN, and auditory cortex.

What do the neurons of the LGN give rise to?

They give rise to axons that project to the primary visual cortex and this projection from the LGN to the cortex is called the optic radiation

Taste receptors for bitterness, sweetness, and umami

Transduction mechanism: GPCR binding Rely on dimers (two proteins that are attached to one another) of T1R and T2R families of receptor proteins. -Ligand binding activates the GPCR, leading to IP3 production via Phospholipase C (PLC). -IP3 opens the taste cell specific Na+ channel and the release of Ca2+ from internal storage sites. -These cells do not have neurotransmitter filled vesicles, so increased Ca2+ opens an ATP-permeable channel which allows ATP to flow out of the cell and activate purinergic receptors on the gustatory afferent axons.

Mechanisms of Taste Transduction

Transduction: the process by which an environmental stimulus causes an electrical response in sensory receptor cells. Potential transduction mechanisms by taste stimuli (tastants) : 1.Pass directly through ion channels 2.Bind to and block ion channels 3.Bind to G-protein-coupled receptors and activate second messenger to open ion channels

Cochlear Implants

Treat certain forms of deafness by using electronic devices to bypass the middle ear and the hair cells, and activate the auditory nerve axons directly.

True or False. At the LGN, input from the two eyes is kept separate.

True

True or False. The anatomical organization of the LGN supports the idea that the retina gives rise to streams of information that are processed in parallel.

True

True or False. The right LGN receives information about the left visual field.

True

True or False. The right and left lateral geniculate nuclei, located in the dorsal thalamus, are the major targets of the two optic tracts.

True

True or False. Axons arising from M-type, P-type, and nonM-nonP ganglion cells in the two retinas synapse on cells in different LGN layers.

True

The Inner Ear (cochlea+ labyrinth)

Two holes: oval window, round window Three fluid-filled chambers: scala vestibuli, scala media, and scala tympani Two membranes: Reissner's membrane, basilar membrane Organ of Corti (auditory receptor neurons), tectorial membrane

Inputs to the Striate Cortex

Two overlapping retinotopic maps, one from the magnocellular LGN (IVCα) and the other from the parvocellular LGN Magnocellular LGN neurons project primarily to layer IVC. Parvocellular LGN neurons project to layer IVC. Koniocellular LGN axons make synapses primarily in layers I and III.

Neural Coding of Taste

Two possibilities: 1.Labelled lines hypothesis: individual tastes are encoded at each level of circuit (taste receptor cell -> primary gustatory axons -> ... ->cortex) 2.Population coding (most plausible) : responses of a large number of broadly-tuned neurons, rather than a small number of precisely-tuned neurons, at different levels of circuit are used to specify the properties of a particular case

Components of the Middle Ear

Tympanic membrane Three ossicles: malleus, incus, stapes Footplate transmits sound vibrations to the fluids of the cochlea Eustachian tube: connects nasal cavities

Touch

Types and layers of skin -Hairy and glabrous (hairless—e.g., palms) -Epidermis (outer) and dermis (inner) Functions of skin: -Protects -Prevents evaporation of body fluids -Provides direct contact with world Mechanoreceptors -Most somatosensory receptors are mechanoreceptors (sensitive to physical distortion).

Transduction Mechanisms of Vertebrate Olfactory Receptor Cells

Unlike taste, all olfactory neurons utilize the same transduction mechanism: 1.Odorants bind to olfactory GPCR 2.Stimulates the olfactory specific G-protein Golf 3.Activates adenylyl cyclase 4.Converts ATP to cAMP 5.cAMP binds to a cyclic nucleotide gated cation channel 6.Open cation channel allows influx of Na+ and Ca2+ which depolarizes the OSN and... 7.Ca2+ opens the Ca2+ -activated Cl- channels 8.Cl- efflux amplifies membrane depolarization

Cortical somatotopy—homunculus

Wilder Penfield: Electrical stimulation of the S1 surface can map somatic sensations localized across the body Somatotopy: mapping of the body's surface sensations onto the brain Tonotopy, Retinotopy...

Mapping of Olfactory Receptor Neurons

Within and between the olfactory bulbs, there is very complex circuitry containing inhibitory and excitatory connections. Activity within the olfactory bulbs can also be modulated by input from higher brain areas (e.g. cortex, amygdala)

retinofugal projection

a neural pathway that carries information away from the eye (beginning at the optic nerve) (away from the retina)

lateral geniculate nucleus (LGN)

a thalamic nucleus that relays information from the retina to the primary visual cortex

Visual Field Deficits from Lesions in the Retinofugal Projection

a)A transection of the left optic nerve would render a person blind in the left eye only b)A transection of the left optic tract would lead to blindness in the right visual field. c)A midline transection of the optic chiasm would affect only the fibers that cross the midline. The visual field viewed by the nasal retinas—that is, the peripheral visual fields on both sides

Central Taste Pathways Continued...

b) control of feeding behaviors: - gustatory nucleus --> areas of the brainstem, mainly the medulla, which control swallowing, salivation, gagging vomiting, digestion and respiration. c) palatability of food: - Gustatory nucleus --> hypothalamus and parts of the limbic system, like the amygdala. These areas are involved in palatability and motivation to eat. Lesions to either area lead to changes in food preferences and over/undereating.

What are the three structures that the ganglion cell axons "fleeing" the retina pass through? (in order) Before they form synapses in the brain stem

optic nerve -> optic chiasm -> optic tract

What is the gateway to the visual cortex and, therefore, to conscious visual perception?

the LGN

What do most optic tract axons innervate?

the lateral geniculate nucleus (LGN) of the dorsal thalamus

Binocular visual field

the portion of the visual field viewed by both eyes (the central portion of both visual hemifields is viewed by BOTH retinas)