Neuroscience: Anatomy, Sensory Processing, and Neurotransmitter Systems

Comprehensive vocabulary flashcards covering neuroanatomy, development, chemical senses (gustation and olfaction), visual, auditory, and somatosensory systems, as well as neurotransmitter synthesis, receptor biophysics, and second-messenger signaling cascades.

Human brain organization

the mammalian plan

nervous system divisions

central nervous system (CNS) and peripheral nervous system (PNS)

Midsagittal Plane

The anatomical plane that divides the brain vertically into equal right and left halves.

Horizontal Plane

The anatomical plane running parallel to the ground that divides the brain into dorsal (top) and ventral (bottom) sections.

Coronal Plane

The anatomical plane oriented perpendicular to the ground and the sagittal plane that divides the brain into anterior (front) and posterior (back) sections.

The central nervous system

Consists of parts of the nervous system that are encased in bone: the brain and spinal cord

• cerebrum, cerebellum, brain stem, spinal cord

spinal cord

-location: attached to the brain stem

-conduit of information (brain-body) from the skin, joints, and muscles of the body.

-spinal cord communicates with the body via spinal nerves. each spinal nerve attaches to the spinal cord by means of two branches, the dorsal root and the ventral root.

• dorsal root brings info into the spinal cord, ventral root brings info away from spinal cord

Peripheral nervous system

-the nervous system except for the brain and spinal cord

• somatic PNS

  • dorsal root ganglia

• visceral PNS

somatic PNS

innervates voluntary movements of skin, joints, muscles. info enters the spinal cord via dorsal roots

dorsal root ganglia

clusters of neuronal cell bodies outside the spinal cord that contain somatic sensory axons

visceral PNS

innervates involuntary sensations of internal organs, blood vessels, and glands

Afferent Axons

Axons that transport incoming sensory action potentials toward a specific anatomical reference point or the central nervous system.

Efferent Axons

Axons that transport outgoing motor signals or regulatory commands away from a reference point or the central nervous system.

The cranial nerves

-12 nerves from the brainstem

-mostly innervate the head

-some are axons from CNS, somatic PNS, or visceral PNS

-OOOTTAFVGVAH

• oh, oh, oh to touch and feel very good velvet aw heck

-SSMMBMBSBBMM

• some say marry money but my brother says big butts matter more

Meninges

-three membranes that surround the brain

• outermost: dura mater

• middle: arachnoid membrane

• innermost: pia mater

ventricular system

-ventricles: the fluid filled caverns and canals inside the brain

-choroid plexus: specialized tissue in ventricles that secretes CSF

-CSF circulates through ventricles; absorbed in subarachnoid space

Glymphatic system

-brain washing machine

-subarachnoid space divided by SLYM (subarachnoid lymphatic-like membrane)

-separates “dirty” CSF collected when asleep from “clean” CSF that flows when awake

-Flow must be unidirectional

Subarachnoid Lymphatic-like Membrane (SLYM)

A delicate anatomical membrane dividing the subarachnoid space that maintains compartmentalization, separating clean cerebrospinal fluid from waste-laden dirty cerebrospinal fluid.

Neural Tube

The embryonic neuroectodermal structure formed by the dorsal fusion of the neural folds, whose epithelial walls differentiate into the entire central nervous system and whose fluid-filled interior gives rise to the ventricular system.

Neural Crest

A strip of ectodermal tissue pinched off laterally during neural tube fusion that gives rise to all neuronal cell bodies and glia of the peripheral nervous system.

Primary Brain Vesicles

The three rostral embryonic swellings that emerge from the early neural tube: the prosencephalon (forebrain), the mesencephalon (midbrain), and the rhombencephalon (hindbrain).

Thalamus

The primary sensory relay and gating center of the diencephalon, through which almost all ascending sensory pathways must pass before reaching the cerebral cortex.

Tectum

The dorsal roof of the midbrain consisting of the superior colliculi (visual orienting and eye movements) and inferior colliculi (relay of ascending auditory information).

Tegmentum

The ventral floor of the midbrain that contains the substantia nigra and red nucleus, both of which contribute to the control and fine-tuning of voluntary movement.

Pyramidal Decussation

The distinct anatomical crossing located in the caudal medulla where descending motor axons within the medullary pyramids cross the midline to control contralateral somatic musculature.

difference in rat and human brain

-convolutions of human cerebrum surface: sulci and gyri

-size of olfactory bulb is much larger in rat

-growth of cerebral hemisphere: temporal, frontal, parietal, occipital

Common features of cerebral cortex in vertebrates

-cell bodies in layers or sheets

-surface layer separated from pia mater, layer 1

-apical dendrites form multiple branches

Brodmann's Areas

Cortical regions delineated and numbered by Korbinian Brodmann based on differences in microscopic cytoarchitecture across cellular layers, correlating directly with specific functional specializations.

Neocortical evolution and structure-function relationships

-cortex amount has evolved, not structure

-3 types of neocortex: primary sensory areas, secondary sensory areas, motor areas

-association areas of cortex — more recent evolution

Spinal cord categories

-cervical

-thoracic

-lumber

-sacrum

-coccyx

basic tastes

-saltiness, sourness, sweetness, bitterness, and umami

• possibly fat, starch, carbonation, calcium, water

-examples of correspondence between chemistry and taste

• sweet—sugars like fructose, sucrose, artificial sweeteners (saccharin and aspartame)

• Bitter—ions like K+ and Mg2+, quinine, and caffeine

-advantage for survival

• poisonous substances are often bitter

how do we distinguish the countless unique flavors of food?

-each food activates a different combination of taste receptors

-distinctive flavor as a result of taste and smell

-other sensory modalities contribute (ex: texture, temperature, etc)

The chemical senses

-Animals depend on the chemical senses to identify nourishment, poison, or potential mate

-oldest and most common sensory system

-Gustation

-Olfaction

Chemoreceptors

organs of taste

-Nasal cavity

• substances passing into the nasal cavity are detectable by olfactory receptors (perceptions of flavor)

-palate

-tongue

-pharynx

-epiglottis

• Palate, pharynx, and epiglottis also have taste-sensitive cells

areas of sensitivity on the tongue

-tip of the tongue

• sweetness

-back of the tongue

• bitterness

-side of the tongue

• saltiness and sourness

This does not mean that we only taste these flavors in these areas, most of the tongue is sensitive to all basic tastes.

Papillae

Epithelial surface projections on the tongue—categorized as fungiform (small front of tongue, larger toward back), foliate (elongated on sides of tongue), and vallate (most posterior)—that each house several hundred taste buds. Each taste bud contains elongated taste receptor cells have preferences for categories of chemicals that can be tasted, lingual stem cells, glia-like basal cells, and terminals of gustatory afferent axons.

threshold concentration

just enough exposure of single papilla to detect taste

Taste Receptor Cells

Modified neuroepithelial cells (50–150 per taste bud) with a lifespan of $2\text{--}24\text{ days}$ (generated from lingual stem cells) that possess apical (apical end is the chemically sensitive part of taste receptor) microvilli projecting through the taste pore to interact with chemical tastants. Interact via electrical synapses and transmitters, some may form an information processing circuit within each taste bud.

Taste responsiveness

-most receptor cells respond primarily to one of basic tastes

-receptor potential: a stimulus induced change in the membrane potential of a sensory receptor

• voltage shift usually from depolarization of taste cell membrane

• most fire one or more fast, sodium dependent action potentials

• transmitter diffuses across synapses, carrying signals from taste cells to gustatory axons

-the taste receptors transmitter depends on taste receptor cell type

• sour—serotonin

• salty, sweet, bitter, umami—ATP

taste receptor cells and gustatory axons preferences

-specialists

• respond exclusively to one of five basic tastes

-generalists

• respond to two or more basic tastes

-contributes to “population code” for taste

Transduction process

-environmental stimulus causes electrical or chemical response in sensory receptor cell

-Mechanisms of taste stimuli may:

• pass directly through ion channels (salt and sour)

• bind to and block ion channels (sour)

• bind to G-protein-coupled receptors and activate second messenger to open ion channels (bitter, sweet, umami)

Salty Taste Transduction

The taste mechanism where salt sensitive taste cells have special Na+ selective channels for extracellular $Na^+$ to directly enter the taste receptor cell via amiloride-sensitive sodium channels, depolarizing the membrane to open voltage-gated sodium and calcium channels and trigger the release of neurotransmitter molecules onto the gustatory afferent axon. Blocked by drug amiloride.

Sour Taste Transduction

-high acidity/low PH

-Protons: causative agents of acidity and sourness

-The taste mechanism wherein acidic $H^+$ protons pass through amiloride-sensitive sodium channels and block intracellular $K^+$ channels, depolarizing the cell membrane to activate voltage-gated calcium channels and trigger exocytosis of serotonin.

Sweet Receptor

-A G-protein-coupled receptor dimer formed by $\text{TAS1R2}$ and $\text{TAS1R3}$

-We differentiate between bitter and sweet because they are expressed in different taste cells

-Tastants bind directly to G-protein-coupled membrane receptors and activate phospholipase C, which increases the synthesis of IP₃. IP₃ then triggers the release of Ca2+ from internal storage sites and opens a taste-specific ion channel, leading to depolarization and transmitter release.

Bitter Receptor

-two different or similar TAS2R proteins

-single-protein monomers

-Tastants bind directly to G-protein-coupled membrane receptors and activate phospholipase C, which increases the synthesis of IP₃. IP₃ then triggers the release of Ca2+ from internal storage sites and opens a taste-specific ion channel, leading to depolarization and transmitter release.

Umami Receptor

-A G-protein-coupled receptor dimer composed of $\text{TAS1R1}$ and $\text{TAS1R3}$ subunits that detects amino acids such as L-glutamate.

-Tastants bind directly to G-protein-coupled membrane receptors and activate phospholipase C, which increases the synthesis of IP₃. IP₃ then triggers the release of Ca2+ from internal storage sites and opens a taste-specific ion channel, leading to depolarization and transmitter release.

Central Gustatory Pathway

The ascending sensory pathway transmitting taste from cranial nerves $\text{VII}$, $\text{IX}$, and $\text{X}$ to the gustatory nucleus in the medulla, ascending ipsilaterally to the ventral posterior medial ($\text{VPM}$) nucleus of the thalamus, and terminating in the primary gustatory cortex.

which 3 cranial nerves carry primary gustatory axons and bring taste info to the brain

-CN 7 (facial)

-CN 9 (glossopharyngeal)

CN 10 (vagus)

Ageusia

The clinical condition marked by the complete loss of taste perception, typically resulting from lesions along the central gustatory pathway.

Gustation

-important to the control of feeding and digestion

-gustatory information is distributed to the hypothalamus and related parts of the basal telencephalon

• these structures seem to be involved in the palatability of foods and the forces that motivate us to eat.

Labeled-Line Hypothesis

A sensory coding theory positing that there are individual taste receptor cells for each stimuli

• in reality, most neurons are broadly tuned

Population Coding

-roughly labeled lines

-responses of a large number of broadly tuned neurons, rather than a small number of precisely tuned neurons, are used to specify the properties of a particular stimulus.

-contribution of temperature, olfactory, and textural features of food

smell function

-warns of harmful substances, combines with taste for identifying foods

-serves as a mode of communication - pheromones used for

• reproductive behavior

• territorial boundaries

• identification of individuals

• signal aggression or submission

-role of human pheromones is unclear

smell - key concepts

-odorants: activate transduction processes in neurons

-olfactory axons constitue 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 have weak smellers compared to many animals

• due to small surface area of olfactory epithelium

Olfactory Receptor Cells

True bipolar sensory neurons embedded in the nasal olfactory epithelium that project ciliated dendrites into mucus and send unmyelinated axons through the cribriform plate to the olfactory bulb.

olfactory pathway

-odorants

-binding to membrane odorant receptor proteins

-G-protein (G_olf) stimulation

-activation of adenylyl cyclase

-formation of cyclic AMP (cAMP)

-binding of cAMP to specific cation channel

-opening of Ca2+ activated chloride channels

-current flow and membrane depolarization (receptor potential)

voltage recording from an olfactory receptor cell during stimulation

odorants generate slow receptor potential in the cilia; the receptor potential propagates down the dendrite and triggers a series of action potentials within the soma of the olfactory receptor cell. Finally, the action potentials but not the receptor potential propagate continuously down the olfactory nerve axon

broad tuning of single olfactory receptor cells

each receptor cell expresses a single olfactory receptor protein and different cells are randomly scattered within a region of the epithelium. Microelectrode recordings from three different cells show that each one responds to many different odors, but with differing preferences. By measuring responses from all three cells, each of the four odors can be clearly distinguished

location and structure of an olfactory bulb

axons of olfactory receptor cells penetrate the cribriform plate and enter the olfactory bulb. After multiple branching, each olfactory axon synapses upon second order neurons within a spherical glomerulus. the second order neurons send axons through the olfactory tract further into the brain

central olfactory pathways

axons of the olfactory tract branch and enter many regions of the forebrain, including the olfactory cortex. The neo cortex is reached only by a pathway that synapses in the medial dorsal nucleus.

Glomeruli (Olfactory Bulb)

a cluster of neurons in the olfactory bulb that receives input only from olfactory receptor cells expressing a particular receptor protein gene

Natural odors and odorant receptors

-most natural odors derived from mixtures of many odorant molecules

• exception: garlic

-olfactory system detects and categorizes up to hundreds of unique odorant molecules simultaneously

• odorant antagonists and agonists.

How humans differentiate smells of chemicals

-most natural odors triggered by mixtures of odorants that activate or inhibit olfactory receptors

-each odor represented by activity of large population of neurons

-neurons responsive to particular odors organized into spatial maps

-timing of action potentials may be essential part of code for particular

Temporal coding in the olfactory system

-temporal patterns of spiking in olfactory neurons

-oscillations of activity when odors are presented to the receptors — relevance unknown

-temporal patterns also evident in spatial odor maps

Diopter

The optical unit measuring the refractive power of a lens or curved interface, defined as the reciprocal of focal distance in meters ($\text{Power} = \frac{1}{\text{Focal Distance (m)}}$).

Dark Current

The steady inward flux of sodium ions ($Na^+$) through $\text{cGMP}$-gated channels into outer segments of rod photoreceptors in the absence of light, maintaining a depolarized resting potential.

Duplex Retina

The functional cellular organization of the vertebrate retina into two separate photoreceptor systems: rods, optimized for high-sensitivity vision in dim light, and cones, optimized for high-acuity and color perception in daylight.

Young-Helmholtz Trichromacy Theory

The theory of color vision stating that the brain calculates perceived color based on the comparative readout and relative activation ratios of three cone photoreceptor types tuned to short (blue), medium (green), and long (red) wavelengths.

Magnocellular LGN Neurons

Large relay neurons in layers 1 and 2 of the lateral geniculate nucleus with large center-surround receptive fields, transient responses, and no wavelength sensitivity, projecting primarily to layer $\text{IVC}\alpha$ to mediate motion vision.

Parvocellular LGN Neurons

Small relay neurons in layers 3 through 6 of the lateral geniculate nucleus exhibiting small receptive fields, sustained responses, and color opponency, projecting to layer $\text{IVC}\beta$ to analyze fine spatial form and color.

Dorsal Stream

The cortical visual processing stream originating in striate cortex and projecting to area $\text{MT}$ and the parietal lobe, specialized for motion perception and spatial guidance of movement.

Ventral Stream

The cortical visual processing pathway originating in striate cortex and projecting through area $\text{V4}$ into the inferior temporal cortex, dedicated to the recognition of visual objects, faces, and color.

Achromatopsia

A clinical syndrome resulting from localized lesions in cortical area $\text{V4}$ characterized by a partial or complete loss of color vision while preserving visual acuity.

Superior Olive

A brainstem auditory structure located in the pons that receives bilateral projections from the cochlear nuclei to compute interaural time and intensity differences for sound localization.

Inferior Colliculus

A midbrain tectal nucleus that integrates ascending auditory signals from the superior olive and lateral lemniscus before relaying them directly to the medial geniculate nucleus of the thalamus.

Medial Geniculate Nucleus (MGN)

The auditory relay nucleus in the dorsal thalamus that receives ascending inputs from the inferior colliculus and projects through the internal capsule to the primary auditory cortex (Brodmann's Areas 41 and 42).

Pacinian Corpuscle

A rapidly adapting encapsulated mechanoreceptor located deep within the dermis with large receptive fields, specialized for detecting high-frequency mechanical vibration.

Meissner's Corpuscle

A rapidly adapting mechanoreceptor located in glabrous skin ridges with small receptive fields, responding selectively to low-frequency vibrations ($1\text{--}10\text{ Hz}$ and $40\text{--}60\text{ Hz}$) and skin deformation.

Merkel's Disk

A slowly adapting tactile mechanoreceptor in the superficial epidermis with small receptive fields, specialized for high-resolution discrimination of fine form, points, and edges.

Ruffini's Ending

A slowly adapting cutaneous mechanoreceptor located deep in the skin that features large receptive fields and responds maximally to sustained stretch of the skin.

PIEZO2

A mechanically activated cation channel protein expressed in primary somatosensory mechanoreceptors that converts mechanical membrane stretch into depolarizing ionic currents.

Dorsal Column-Medial Lemniscal Pathway

The major ascending somatosensory route that carries fine touch, vibration, and proprioception ipsilaterally up the spinal dorsal columns to the gracile and cuneate nuclei of the medulla, decussates via internal arcuate fibers, and ascends via the medial lemniscus to the $\text{VPL}$ nucleus of the thalamus.

Neocortical Layer I (Molecular Layer)

The outermost, cell-sparse lamina of the vertebrate cerebral cortex directly subjacent to the pia mater, occupied predominantly by an axon and dendrite neuropil, including the apical dendrites of deeper pyramidal cells.

Three classes of neurotransmitters

-amino acids

-amines

-peptides

How do we define particular transmitter systems?

by the molecule, synthetic machinery, packaging, reuptake and degradation, and action

first identified neurotransmitter

Acetylcholine (Ach)

The three criteria for neurotransmitters

-Molecule must be synthesized and stored in presynaptic neuron

-must be released by presynaptic axon terminal upon stimulation

-When applied, mimics postsynaptic cell response produced by release of neurotransmitter from the presynaptic neuron

cholinergic

Cells that produce and release ACh

Noradrenergic

The neurons that use the amine neurotransmitter norepinephrine (NE)

Immunocytochemistry

An anatomical method that uses antibodies to study the location of molecules within cells

In Situ Hybridization

An anatomical method that localizes synthesis of protein or peptide to a cell (detect mRNA)

What techniques are used to meet the synthesis and storage criteria for identifying neurotransmitters?

Immunocytochemistry and in situ hybridization

What techniques are used to meet the synaptic mimicry criteria for identifying neurotransmitters?

Microiontophoresis

• a method of applying small amounts of neurotransmitters to cells to asses postsynaptic actions. A microelectrode is used to measure the effects on membrane potential

Methods and challenges with studying transmitter release

-In the PNS, a specific set of cells or axons can be stimulated while taking samples of the fluids bathing their synaptic target. The CNS however, contains a diverse mixture of intermingled synapses using different neurotransmitters. This often makes it impossible to stimulate a single population of synapses containing only a single neurotransmitter.

-One method used to accomplish this is using brain slices kept alive in vitro. They stimulate the synapses and collect and measure released chemicals.

-New methods such as optogenetics

Receptor subtype

the different receptors a neurotransmitter binds to

What is used to identify receptor subtypes

neuropharmacological analysis, ligand-binding methods, molecular analysis

ACh agonists

muscarinic ACh receptors and nicotinic ACh receptors

what antagonist inhibits ACh at nicotinic receptors?

Curare

what antagonist inhibits ACh at muscarinic receptors?

atropine

Three subtypes of glutamate receptors

-AMPA receptors

-NMDA receptors

-Kainate receptors

• each names for a different chemical agonist

• glutamate activates all three receptor subtypes