Toxicology of the Liver and the Nervous System

Vocabulary practice flashcards covering hepatic architecture, mechanisms and zones of hepatotoxicity, neuronal physiology, and toxicological patterns of nervous system injury.

Hepatotoxin

A substance or xenobiotic that causes injury or damage to the liver (for example, acetaminophen).

Liver Lobule

The basic histological unit of the liver (~$50{,}000$–$100{,}000$ lobules per liver) consisting of hepatocytes arranged around a central vein and bordered at the periphery by portal triads.

Portal Triad

A microscopic structural landmark located at the angles of liver lobules that contains a hepatic arteriole branch, a hepatic portal vein branch, and a bile ductule.

Hepatic Acinus

The functional microvascular unit of the liver parenchyma, divided into three concentric zones (Zone 1, Zone 2, and Zone 3) distinguished by nutrient and oxygen gradients extending from the portal triad to the central vein.

Zone 1 (Periportal Region)

The oxygen-rich (~$12\%$) region of the hepatic acinus where hepatocytes are mitochondria-rich, high in phase II conjugating enzymes (such as GSTs and UGTs), and carry out gluconeogenesis, fatty acid oxidation, and urea synthesis; selectively targeted by iron, arsenic, phosphorus, and allyl alcohol.

Zone 2 (Midzonal Region)

The intermediate zone of the hepatic acinus characterized by intermediate oxygen levels (~$7\%$); selectively targeted by toxicants such as beryllium ($\text{Be}$) and furosemide.

Zone 3 (Centrilobular Region)

The oxygen-poor (~$4\%$), hypoxic zone surrounding the central vein, rich in cytochrome P450 (phase I) enzymes; preferentially damaged by toxicants requiring bioactivation, including ethanol, acetaminophen, chloroform, and carbon tetrachloride.

Steatosis (Fatty Liver)

A typically reversible hepatic lesion resulting from disrupted lipid metabolism and intracellular triglyceride accumulation; induced by obesity, ethanol, valproic acid, and enzyme inhibitors such as puromycin and ethionine.

Hepatic Necrosis

Uncontrolled cell death resulting from physical or chemical insult, characterized morphologically by cell swelling, membrane rupture, and inflammatory infiltration, and biochemically by the release of alanine aminotransferase (ALT) and $\gamma$-glutamyltranspeptidase (GGT) into the serum.

Hepatic Apoptosis

Regulated, programmed cell death characterized morphologically by cell shrinkage, chromatin condensation, nuclear fragmentation, and cytoplasmic blebbing in the absence of inflammatory cell infiltration.

Necrosis vs. Apoptosis

Two distinct pathways of cell demise: necrosis features compromised membrane integrity, cell swelling, lysis, and inflammatory response; apoptosis proceeds through cellular shrinkage, chromatin condensation, membrane blebbing, and phagocytic clearance without inflammation.

Canalicular Cholestasis

A reduction in bile volume formed or impairment in the secretion of solutes into bile, resulting in elevated serum levels of bile salts and bilirubin, jaundice, and discolored dark urine; caused by cyclosporin A, estrogens, and manganese ($\text{Mn}$).

Bile Duct Damage

Hepatic injury characterized clinically by a sharp rise in serum alkaline phosphatase activity alongside cholestatic symptoms; caused by antibiotics such as amoxicillin.

Sinusoidal Damage

Occlusion or destruction of hepatic sinusoids carrying blood through the liver, precipitating hepatomegaly and circulatory shock accompanied by decreased blood pressure and systemic hypoxia; caused by high doses of acetaminophen, anabolic steroids, and pyrrolizidine alkaloids.

Cirrhosis

Irreversible end-stage liver pathology wherein ongoing hepatocyte death triggers progressive accumulation of collagen fibers and fibrotic scar tissue; linked to chronic ethanol use, androgens, excessive vitamin A, and arsenic.

Hepatocellular Carcinoma

Malignant hepatic neoplasia linked to exposure to androgen abuse, dietary contamination with aflatoxins, and radioactive thorium dioxide.

Neuron Anatomy

The principal structural regions of a nerve cell, consisting of receptive dendrites, a soma (cell body) containing the nucleus, an axon insulated by myelin and Schwann cells punctuated by nodes of Ranvier, and terminal synapsing arborizations.

Patterns of Neurotoxic Injury

The four main cellular patterns of toxic damage to nervous tissue: neuronopathy (loss of the entire neuron), axonopathy (damage to the axon with distal degeneration), myelinopathy (demyelination or intramyelinic edema), and transmission toxicity (functional disruption of synaptic signaling).

Kinesin

An ATP-driven motor protein that moves vesicles, organelles, and proteins along microtubules in an anterograde (forward) direction from the neuronal cell body toward axon terminals.

Dynein

An ATP-dependent motor protein that carries cargoes along microtubules in a retrograde (backward) direction from the periphery back toward the neuronal soma.

Axonal Transport Rates

The physiological velocity categories of intra-axonal movement: slow transport (~$1\,\text{mm/day}$ for cytoskeletal elements), intermediate transport (~$50\,\text{mm/day}$ for mitochondria), and fast transport (~$400\,\text{mm/day}$ for proteins).

Axotomy and Axonal Regeneration

The multi-step response triggered by physical severing of an axon: distal stump and myelin fragment, macrophages phagocytose debris, proliferating Schwann cells align into cords to guide sprouting axonal buds, and the regrowing axon is re-ensheathed.

Delayers of Axonal Degeneration

Experimental and physiological interventions that retard the rate of post-injury axonal degeneration: lowering ambient temperature, inhibiting proteolysis with calpain II inhibitors, and blocking the influx of extracellular $\text{Ca}^{2+}$.

Myelin

A lipid- and protein-rich insulating membrane synthesized by oligodendrocytes in the CNS and Schwann cells in the PNS that wraps axons to dramatically increase the velocity of electrical impulse conduction.

Neuronopathy

A toxicant-induced, irreversible loss of whole neurons caused by agents such as lead, aluminum, arsenic, manganese, ethanol, methyl mercury, and MPTP.

Methyl Mercury

A neurotoxic organometal that induces extensive neuronopathy and encephalopathy by disrupting glycolysis, aerobic respiration, neurotransmitter release, and protein and nucleic acid synthesis.

MPTP

A chemical contaminant generated during meperidine synthesis that is selectively taken up and metabolized within dopaminergic neurons, destroying them and precipitating irreversible Parkinson's disease-like pathology.

Axonopathy

A neurotoxic injury characterized by targeted chemical damage to axons causing 'chemical transection' and degeneration of the distal axon segment, manifesting as symmetric sensory and motor deficits in hands and feet; caused by organophosphorus esters, acrylamide, and zinc pyridinethione.

Tri-o-cresyl Phosphate (TOCP)

An organophosphorus ester that inhibits acetylcholinesterase ($\text{AChE}$) and induces axonopathy, historically responsible for causing 'Ginger-Jake syndrome.'

Intramyelinic Edema

A specific form of myelinopathy featuring fluid separation of myelin lamellae from the axon without primary axonal loss; classically induced by the disinfectant hexachlorophene.

Demyelination

A myelinopathy characterized by the progressive stripping and loss of myelin from structurally intact axons, causing marked deceleration of nerve conduction velocities; triggered by lead and disulfiram.

Cocaine Transmission Toxicity

A form of transmission-associated neurotoxicity wherein cocaine blocks presynaptic dopamine reuptake transporters, dramatically prolonging the synaptic residence time of dopamine and exaggerating postsynaptic stimulation.

Amphetamine

A central stimulant that interferes with neurotransmission by competing for transport via plasma membrane dopamine carriers and disrupting vesicular dopamine storage, leading to elevated synaptic dopamine levels.

Nicotine

A botanical alkaloid and pesticide that binds nicotinic cholinergic receptors, elevating blood pressure and heart rate at pharmacological doses and causing fatal asphyxiation during toxic overdose.

Atropine

An acetylcholine antagonist that competitively blocks cholinergic receptors, utilized clinically as an antidote to treat acute poisoning by anticholinesterase organophosphorus pesticides.

Excitatory Amino Acid Receptors (EAARs)

Membrane receptors responsive to glutamate and related acidic amino acids, comprising ionotropic receptors (ligand-gated channels permitting $\text{Ca}^{2+}$ influx) and metabotropic receptors (G-protein coupled receptors that modulate intracellular $\text{cAMP}$).

Chinese Restaurant Syndrome

A transient reaction following the ingestion of large amounts of monosodium glutamate (MSG), characterized clinically by a burning sensation across the face, neck, and chest.