Neurobiology Exam 1 (KILL ME)

Describe the historical progression from holistic views of brain function to modern neuronal localization.

Hippocrates identified the brain as the center of sensation and intelligence. Aristotle held that the brain was a cooling organ balancing the heart. Galen conducted early anatomical dissections. Vesalius published detailed anatomical drawings. Descartes hypothesized the pineal gland moved body fluids. Functional localization evolved through Bell and Magendie, who demonstrated that spinal roots are functionally split into motor ventral roots and sensory posterior/dorsal roots. Flourens used broad ablations, whereas Paul Broca localized speech production to a specific region via post-mortem brain examination.

Contrast Reticular Theory with the Neuron Doctrine, detailing the methodological breakthrough that resolved this debate.

Camillo Golgi proposed the Reticular Theory, suggesting the nervous system is a continuous physical network. Santiago Ramón y Cajal established the Neuron Doctrine, arguing the nervous system consists of individual, discrete signaling cells. Cajal utilized Golgi's silver nitrate staining technique to visualize individual, isolated neurons under a microscope, proving physical discontinuity.

Compare the structural components of the neuronal cytoskeleton and detail the mechanisms, directionality, motor proteins, and clinical significance of axonal transport.

Microtubules are large longitudinal tracks used for molecular transport. Neurofilaments are intermediate filaments providing structural framework and maintaining axon shape. Microfilaments are actin structures aiding shape and directional guidance. Anterograde transport uses kinesin motor proteins to move soma-synthesized vesicles along microtubules toward the axon terminal. Retrograde transport uses dynein motor proteins to transport waste, signals, and endocytosed materials backward from the terminal to the soma. Neurotropic viruses (like Rabies and Herpes simplex) enter axon terminals and hijack retrograde transport along microtubules to infect the cell body in the soma.

Detail the primary classes of neuroglia in the CNS and PNS, explaining their specific biophysical and physiological roles.

Astrocytes (CNS) maintain Blood-Brain Barrier (BBB) integrity, store energy as glycogen, buffer extracellular $K^+$ ions, convert excess glutamate into non-toxic glutamine, guide neurons during development, and assist in injury isolation and cleanup. Oligodendrocytes (CNS) produce and maintain myelin sheaths wrapping multiple axons. Schwann Cells (PNS) produce myelin sheaths for individual peripheral axons. Microglia (CNS) act as immune defense cells that become phagocytic during injury to engulf debris, execute synaptic pruning, and play roles in neurodegenerative conditions like Alzheimer's disease. Ependymal Cells line brain ventricles and secrete cerebrospinal fluid (CSF).

Compare Gray's Type I and Type II synapses and explain how dendritic spine structural abnormalities lead to clinical deficits.

Gray's Type I synapses feature asymmetrical membrane differentiations (thick postsynaptic density), round vesicles, are located primarily on dendrites, and are functionally excitatory. Gray's Type II synapses feature symmetrical membrane differentiations, flattened vesicles, are located on the soma or axon hillock, and are functionally inhibitory. Inhibitory Type II inputs placed near the soma/hillock block depolarizing signals from distal dendritic Type I synapses before reaching threshold, known as a somatic veto. Abnormal spine growth, dysmorphic spine shapes, or reduced spine density disrupt synaptic integration and correlate directly with intellectual disabilities, mental issues, and motor deficits.

Explain the biophysical origin of the resting membrane potential ($V_m$). Contrast the Nernst and Goldman-Hodgkin-Katz (GHK) equations, detailing the exact role of the $Na^+/K^+$ ATPase.

The resting membrane potential, (Vm = -65mV), is established by semi-permeable ion channels and an uneven distribution of charges across the membrane. The Nernst Equation calculates the equilibrium potential (E ion) for a single ion species: E_{X} = \frac{0.058}{z} \log \frac{[X]_{out}}{[X]_{in}}$. The ion ratios ($[X]_{out}:[X]_{in}$) are $K^+ = 1:20$, $Na^+ = 10:1$, $Ca^{2+} = 10,000:1$, and $Cl^- = 11.5:1$. The Goldman-Hodgkin-Katz (GHK) Equation calculates the actual membrane potential ($V_m$) by incorporating concentration gradients and relative membrane permeabilities ($P_K > P_{Na} > P_{Cl}$) simultaneously. The Na^+/K^+ pump actively uses 1 ATP to move 3Na+ out and 2 K+ in to maintain these ionic concentration gradients.

Define ionic driving force and explain how Ohm's Law governs neuronal current flow across the membrane.

Ohm's Law adaptation is $I_{ion} = g_{ion} (V_m - E_{ion})$, where current ($I$) depends on membrane conductance ($g$, inverse of resistance) and driving force ($V_m - E_{ion}$). At a rest of -65mV, the K^+ driving force is -65 mV - (-80mV) = +15mV, creating an outward driving force pushing K^+ out. At the same rest, the Na+ driving force is (-65mV} - (+62mV) = -127mV, creating a strong inward driving force pulling Na+ in.

Walk step-by-step through the conductance changes, ion movements, and feedback loops during an action potential waveform.

The action potential begins when depolarization reaches threshold, initiating a positive feedback loop as voltage-gated Na^+ channels open. During the rising phase and overshoot, an inward Na^+ current rapidly depolarizes the membrane toward E_{Na} (+62mV). During the peak and falling phase, voltage-gated Na^+ channels undergo time-dependent inactivation while delayed rectifier K^+ channels fully open, causing an outward K^+ current that repolarizes the membrane. Finally, in the undershoot (AHP), high K^+ conductance hyperpolarizes V_m toward E_K (-80mV) before the delayed K^+ channels close.

Contrast the absolute and relative refractory periods in terms of channel state conformations, ionic conductance, and firing limitations.

During the absolute refractory period, voltage-gated Na^+ channels are completely inactivated. No stimulus strength can elicit another action potential, which sets the maximum firing rate ceiling. During the relative refractory period, Na^+ channels reset to closed states, but delayed K^+ channels remain open. This elevated K^+ conductance hyperpolarizes V_m and shunts depolarization, meaning a stronger-than-normal stimulus is required to reach threshold.

Explain how axon diameter and myelination alter passive cable properties to increase action potential conduction velocity.

A larger axon diameter expands the internal core area, which decreases internal longitudinal resistance (r_i) and accelerates passive signal spread. Myelination occurs when glia wrap axons to increase membrane resistance (r_m) by reducing current leak, and to decrease membrane capacitance (C_m). This enables saltatory conduction, where action potentials jump between unmyelinated Nodes of Ranvier where voltage-gated Na^+ channels are densely concentrated, regenerating the signal between insulated passive spreads. Myelinated Type A fibers (5-20um) conduct fast pain up to 120m/s}, while unmyelinated Type C fibers (0.5-1.5um) conduct slow lingering pain up to 2m/s.Type B fibers have light myelination and a diameter of 2-3um. They conduct signals at speeds up to 15m/s and function in intermediate autonomic signaling

Outline the step-by-step molecular mechanism of neurotransmitter release from the presynaptic terminal upon action potential arrival.

An action potential first depolarizes the axon terminal membrane. This causes voltage-gated Ca^2+ channels to open, resulting in Ca^2+ influx. The Ca^2+ influx triggers synaptic vesicle exocytosis via SNARE complexes within 200 microseconds. Neurotransmitters then diffuse across the synaptic cleft. The action terminates via enzymatic breakdown, active reuptake, or diffusion.

Compare and contrast ionotropic and metabotropic postsynaptic receptors in structure, activation speed, mechanism, and biological outcome.

Ionotropic receptors are ligand-gated ion channels where direct transmitter binding opens the channel pore. This produces fast, short-duration ion flux, such as with AMPA, NMDA, $GABA_A$, and Nicotinic ACh receptors. Metabotropic receptors are GPCRs where a transmitter binds a 7-transmembrane receptor, which activates a G-protein that regulates second messengers (cAMP, PKA) or ion channels. This produces slower, longer-lasting modulatory effects.

Describe spatial/temporal EPSP summation and explain the biophysical mechanism of shunting inhibition.

Spatial summation integrates simultaneous EPSPs from separate synapses at the soma. Temporal summation integrates rapid successive EPSPs from a single synapse before prior signals decay. Shunting inhibition occurs when an inhibitory input (GABA/Glycine) opens Cl^- channels. Because V_m is near E_Cl, the open Cl^- channels increase membrane conductance (g_Cl), acting as a low-resistance path that drains depolarizing current from incoming EPSPs before reaching the hillock.

State Dale's Principle and the criteria for establishing a neurotransmitter.

A neurotransmitter must be synthesized/stored presynaptically, released upon presynaptic stimulation, and produce a specific postsynaptic response. Dale's Principle states that a neuron releases the same transmitter combination at all of its synapses.

Synthesize key transmitter classes, receptors, agonists, and antagonists.

• Cholinergic class uses Acetylcholine (ACh) with Nicotinic and Muscarinic receptors; agonists are Nicotine and Muscarine, and antagonists are Curare and Atropine.

• Adrenergic class uses Catecholamines (DA, NE, Epi) with $\alpha$ and $\beta$ receptors; Isoproterenol is an agonist, and Propranolol is an antagonist.

• Excitatory Amino Acids use Glutamate with AMPA, NMDA, and Kainate receptors; AP5 blocks NMDA.

• Inhibitory Amino Acids use GABA/Glycine with GABA_A and GABA_B receptors; Baclofen is a GABA_B agonist, and Bicuculline is a GABA_A antagonist.

• Unconventional transmitters include Endocannabinoids (Anandamide, 2-AG) utilizing retrograde CB1 receptors

Describe how patch-clamp recording functions and detail what it revealed about single-channel behavior during action potentials.

Patch-clamp recording utilizes a micropipette to form a high-resistance gigaseal on a membrane patch to record single-channel current fluctuations. This technique confirmed that single Na^+ channels open abruptly in an all-or-none fashion, remain open for approx. 1ms, and enter an inactivated state until repolarization occurs. It also showed that the extracellular pore loop acts as a selectivity filter by stripping hydration spheres from passing ions.

Compare Immunocytochemistry, In-Situ Hybridization, and Micro-iontophoresis in mapping neurochemical pathways.

Immunocytochemistry (ICC) uses fluorescent antibodies to locate specific enzymes or transmitter proteins. In-Situ Hybridization (ISH) uses labeled RNA probes to detect mRNA transcripts, which confirms gene expression/synthesis in the soma. Micro-iontophoresis applies candidate chemical transmitters onto postsynaptic membranes to verify that localized application mimics endogenous synaptic activity.

Explain the dual voltage-and-ligand gating mechanism of NMDA receptors and its role in Ca^2+ entry and excitotoxicity.

NMDA receptors require both glutamate binding and postsynaptic depolarization. At rest, extracellular Mg^2+ blocks the channel pore. AMPA-mediated depolarization expels the Mg^2+ block, allowing Na^+ and Ca^2+ influx. While Ca^2+ influx drives intracellular second-messenger cascades, excessive glutamate accumulation leads to pathological Ca^2+ overload and cellular death, known as excitotoxicity

Detail the full enzymatic pathway for catecholamine biosynthesis, highlighting the rate-limiting enzyme.

The pathway begins with Tyrosine being converted to L-DOPA via Tyrosine Hydroxylase (TH). L-DOPA is converted to Dopamine via Dopa Decarboxylase. Dopamine is converted to Norepinephrine via DBH. Norepinephrine is converted to Epinephrine via PNMT. Tyrosine Hydroxylase (TH) is the rate-limiting enzyme that governs overall production speed via end-product feedback inhibition.