Neuroscience Exam 2: Neurotransmitters, Brain Development, and Visual Systems

Comprehensive practice flashcards in vocabulary style covering neurotransmitter synthesis and receptors, central nervous system neuroanatomy, embryonic neural development and axon guidance, and the physiology and circuitry of the visual system.

Tyrosine Hydroxylase (TH)

The rate-limiting enzyme in catecholamine biosynthesis that catalyzes the conversion of tyrosine into L-dihydroxyphenylalanine (L-DOPA).

Glutamic Acid Decarboxylase (GAD)

The enzyme responsible for converting the excitatory amino acid neurotransmitter glutamate into the primary inhibitory neurotransmitter GABA.

Vesicular H⁺-ATPase

A membrane protein on synaptic vesicles that uses ATP to pump protons ($H^+$) into the vesicle lumen, establishing an electrochemical proton gradient that powers vesicular neurotransmitter transporters.

Neuropeptides

Large, non-classic neurotransmitters synthesized in the cell soma via the rough ER and Golgi apparatus, packaged into large dense-core vesicles (70–250 nm), transported along microtubules by kinesin, and released away from the active zone in response to high-frequency action potential bursts with diffuse intracellular $Ca^{2+}$ elevation.

Nitric Oxide (NO)

A gaseous non-classic messenger synthesized on demand from arginine by nitric oxide synthase; it diffuses freely across membranes without vesicle storage and does not act via classical cell-surface receptors.

Autoreceptor

A presynaptic receptor located on an axon terminal that senses the neuron's own released neurotransmitter to provide negative feedback and modulate further release.

Ionotropic Receptors

Ligand-gated ion channels that directly open or close an ion pore upon neurotransmitter binding, mediating rapid, brief, and localized changes in postsynaptic membrane potential.

Metabotropic Receptors

G protein-coupled receptors (GPCRs) and receptor tyrosine kinases that lack an intrinsic ion channel and initiate slower, longer-lasting intracellular signaling cascades to modulate ion channels, protein phosphorylation, and gene transcription.

AMPA Receptor

A tetrameric ionotropic glutamate receptor composed of subunits with 3 transmembrane domains around a pore; binding of glutamate triggers rapid influx of $Na^+$ to produce fast postsynaptic membrane depolarization.

NMDA Receptor

A tetrameric ionotropic glutamate receptor blocked by $Mg^{2+}$ at resting membrane potential; removal of the magnesium block via membrane depolarization permits influx of $Ca^{2+}$ and $Na^+$ (and efflux of $K^+$) to initiate downstream plasticity cascades.

P2X Receptor

An ionotropic purine receptor composed of 3 subunits, each possessing 2 transmembrane spanning domains, which opens a channel permeable to $Na^+$, $K^+$, and $Ca^{2+}$.

Endocannabinoids

Endogenous lipid messengers (such as 2-AG and anandamide) synthesized on demand in postsynaptic neurons following $Ca^{2+}$ elevation that travel retrogradely across the synapse to activate presynaptic CB1 GPCRs, inhibiting voltage-gated calcium channels and reducing transmitter release.

CREB

cAMP response element-binding protein; a transcription factor activated by phosphorylation via protein kinases (such as PKA, CaMKIV, and RSK) that binds CRE sites in DNA to induce transcription of genes essential for synaptic plasticity and memory (including BDNF and GluR1).

PMP22 Duplication

A genetic abnormality causing overexpression of peripheral myelin protein 22, resulting in the Charcot-Marie-Tooth disease type 1A (CMT1A) phenotype marked by distal limb weakness, disrupted Schwann cell myelin compactness, and slowed nerve conduction velocity.

Primary Brain Vesicles

The three initial embryonic brain swellings of the rostral neural tube: the prosencephalon (forebrain), mesencephalon (midbrain), and rhombencephalon (hindbrain).

Prosencephalon

The embryonic forebrain primary vesicle that subdivides into the telencephalon (giving rise to the cerebral cortex, hippocampus, amygdala, and basal ganglia) and the diencephalon (giving rise to the thalamus, hypothalamus, and optic cups).

Mesencephalon

The midbrain primary brain vesicle that matures to contain the superior colliculi, inferior colliculi, and substantia nigra.

Rhombencephalon

The hindbrain primary vesicle that divides into the metencephalon (giving rise to the pons and cerebellum) and myelencephalon (giving rise to the medulla oblongata).

Association Cortex

Cortical areas outside primary sensory and motor regions that integrate information from multiple sensory modalities, coordinate complex processing, and add cognitive or emotional context.

Primary Sensory Cortex

The initial cortical destination that receives modality-specific sensory input relayed from thalamic sensory relay nuclei.

Substantia Nigra

A darkly pigmented dopaminergic nucleus located in the midbrain (mesencephalon) that projects to the striatum and is vital for motor control.

Inside-Out Cortical Neurogenesis

The developmental principle wherein earlier-born cortical neurons stop migrating early to form deep layers (near the white matter core), while later-born neurons migrate along radial glia past them to occupy superficial layers.

Sonic Hedgehog (Shh)

A morphogen secreted by the embryonic notochord and ventral floor plate that establishes a ventral-to-dorsal concentration gradient to regulate Class I and Class II transcription factors, specifying spinal interneurons and motor neurons.

Retinoic Acid (RA)

A posterior/caudal morphogen that establishes a caudalizing gradient and regulates the concentration-dependent activation of nested Hox gene expression along the rostral-caudal axis.

Axonal Growth Cone

The motile, actin-rich tip of a developing axon that extends filopodia and lamellipodia to sense attractive and repulsive guidance cues via surface receptors.

Netrin

A chemoattractive guidance factor secreted by the ventral floor plate that diffuses to form a gradient, binding to DCC receptors on commissural axon growth cones to pull them toward the ventral midline.

Slit

A chemorepulsive molecule concentrated at the ventral midline floor plate that binds Robo receptors to propel commissural axons away from the midline and prevent recrossing.

Comm (Commissureless)

A regulatory protein that sequesters Robo receptors away from the growth cone surface before crossing the midline, allowing the axon to approach and cross the midline without premature Slit repulsion.

Wnt4 / Frizzled3

A morphogen gradient and its corresponding attractive receptor that guides commissural axons rostrally toward the brain after they cross the ventral midline.

Neurexin and Neuroligin

A trans-synaptic adhesion pair where presynaptic neurexin triggers postsynaptic scaffolding and neurotransmitter receptor clustering, while postsynaptic neuroligin instructs presynaptic active zone formation and synaptic vesicle clustering.

Rhodopsin

A G protein-coupled photopigment embedded in the disc membranes of rod outer segments, composed of the protein opsin covalently bound to the light-absorbing chromophore 11-cis retinal.

Transducin ($G_t$)

The heterotrimeric G protein in rod photoreceptors whose $\alpha$-subunit exchanges GDP for GTP upon activation by metarhodopsin II, subsequently dissociating to activate phosphodiesterase (PDE).

Phosphodiesterase (PDE)

The visual effector enzyme activated by transducin ($G_{t\alpha}\text{-GTP}$) that hydrolyzes cyclic GMP (cGMP) into GMP, reducing cGMP levels to close cGMP-gated cation channels.

Dark Current

The steady inward flow of $Na^+$ and $Ca^{2+}$ ions through open cGMP-gated channels in darkness that maintains the resting photoreceptor membrane potential depolarized at approximately $-30\,\text{mV}$ and sustains continuous glutamate release.

Guanylate Cyclase-Activating Proteins (GCAPs)

$Ca^{2+}$-sensitive regulatory proteins in photoreceptors that detect the drop in intracellular $Ca^{2+}$ during light exposure, stimulating guanylyl cyclase to produce cGMP and reopen CNG channels during rod recovery.

Fovea

The specialized central region of the retinal macula packed exclusively with cones, lacking rods, and specialized for high-acuity color vision.

Optic Disc

The site on the retina where ganglion cell axons converge and exit the eyeball as the optic nerve (CN II); because it contains no photoreceptors, it forms a physiological blind spot.

ON-Center Bipolar Cell

A retinal interneuron expressing metabotropic mGluR6 receptors that is hyperpolarized by glutamate in the dark and depolarizes in response to light-induced decreases in glutamate release.

OFF-Center Bipolar Cell

A retinal interneuron expressing ionotropic AMPA receptors that depolarizes in response to glutamate in darkness and hyperpolarizes when light reduces photoreceptor glutamate release.

Horizontal Cells

GABAergic retinal interneurons that provide lateral inhibition across photoreceptors, establishing the inhibitory surround of bipolar and ganglion cell receptive fields to enhance visual edge contrast.

AII Amacrine Cells

Retinal interneurons that receive excitatory input from rod bipolar cells and relay visual signals through gap junctions to ON cone bipolar cells while releasing glycine to inhibit OFF cone bipolar cells.

Magnocellular (M) Ganglion Cells

Large retinal ganglion cells characterized by extensive dendritic trees and large receptive fields, specialized for detecting motion, temporal changes, and low-contrast stimuli.

Parvocellular (P) Ganglion Cells

Small retinal ganglion cells with compact dendritic arborizations and small receptive fields, specialized for high-acuity form, fine spatial detail, and color vision.

Lateral Geniculate Nucleus (LGN)

The primary thalamic visual relay nucleus consisting of 6 layers: layers 1–2 are magnocellular, layers 3–6 are parvocellular, ipsilateral retinal axons project to layers 2, 3, and 5, and contralateral retinal axons project to layers 1, 4, and 6.

Koniocellular Layers

Thin, interlaminar zones situated between the six main layers of the LGN that receive non-M, non-P retinal ganglion cell inputs and are involved in color processing.

V1 Cortical Processing Flow

The canonical sensory circuitry of the primary visual cortex wherein LGN axons terminate predominantly in layer 4, layer 4 projects to layers 2/3 (which project to V2 and other cortical targets), and layers 2/3 project to layers 5/6 (which provide subcortical and thalamic feedback).

Dorsal Stream

The 'where' or 'how' visual processing pathway originating mainly from magnocellular inputs that extends toward the parietal cortex (including MT/V5) to process motion, spatial location, depth, and visually guided action.

Ventral Stream

The 'what' visual processing pathway driven by parvocellular and magnocellular inputs that extends toward the inferior temporal cortex (including V4) to analyze color, form, shape, and object identification.

Cortical Magnification

The disproportionately large area of primary visual cortex (V1) dedicated to processing inputs from the central fovea compared to its actual surface area on the retina.

Simple Cell

A primary visual cortex (V1) neuron that responds optimally to a static bar or edge of light at a specific orientation within a fixed position in its receptive field, created by the convergent input of aligned center-surround receptive fields.

Complex Cell

A V1 neuron that responds selectively to an oriented bar of light moving across its receptive field in a specific preferred direction, synthesized from the sequential firing of converging simple cells.

Ocular Dominance Columns

Perpendicular slabs of neurons in primary visual cortex (V1) that preferentially respond to input originating from one specific eye, experimentally visualized by Hubel and Wiesel via transneuronal autoradiography with radioactive proline.

Cytochrome Oxidase Blobs

Peg-like, cylinder-shaped clusters of neurons in V1 layers 2 and 3 that stain richly for cytochrome oxidase and process color information.

Cortical Module

An approximately $1\,\text{mm}^3$ unit of primary visual cortex containing complete sets of ocular dominance columns, orientation columns covering 180 degrees, and cytochrome oxidase blobs to process orientation, color, motion, and binocular disparity for a discrete region of the visual field.

Bitemporal Hemianopia

The loss of peripheral/temporal visual fields in both eyes resulting from a midline lesion at the optic chiasm (such as from a pituitary tumor) that selectively severs crossing fibers from the nasal hemiretinas.

Homonymous Hemianopia

The loss of vision in the entire contralateral visual hemifield in both eyes resulting from unilateral damage to the post-chiasmatic optic tract, LGN, optic radiations, or primary visual cortex.

Deuteranopia

A congenital form of red-green color blindness caused by the absence of functional medium-wavelength (green, M) cone photopigment opsins.

George Wald

The Nobel laureate recognized for discovering the biochemical mechanisms of phototransduction, including how rhodopsin changes shape upon light absorption and its structural dependence on vitamin A.

Haldan Keffer Hartline

The Nobel laureate who elucidated the electrical properties of the retina, center-surround receptive field antagonism, and lateral inhibition using the horseshoe crab (Limulus).

Ragnar Granit

The Nobel laureate who recorded electrical impulses from single retinal cells and demonstrated that cones exhibit distinct wavelength-selective sensitivities corresponding to three primary colors.

David Hubel and Torsten Wiesel

The Nobel laureates who uncovered the columnar functional organization of the primary visual cortex (V1), establishing the concepts of ocular dominance columns, orientation selectivity, and simple versus complex receptive fields.