BIOL11821 Molecule to Man: Autonomic Nervous System, Receptor Superfamilies, Ion Channels, and Quantitative Pharmacology

Comprehensive vocabulary flashcards covering autonomic nervous system divisions, neurotransmitter biosynthesis and degradation, receptor superfamilies, voltage-gated ion channels, and quantitative pharmacology equations and parameters.

Autonomic Nervous System (ANS)

A branch of the peripheral nervous system that regulates involuntary physiological processes essential for maintaining homeostasis and responding to the environment, including heart rate, digestion, and glandular secretion. It comprises the sympathetic, parasympathetic, and enteric divisions.

Enteric Nervous System

A specialized division of the autonomic nervous system located within the gastrointestinal tract that coordinates digestive functions independently of the central nervous system.

Preganglionic Neuron

An autonomic motor neuron whose cell body resides in the central nervous system (brainstem or spinal cord) and projects its axon outward to synapse in an autonomic ganglion.

Postganglionic Neuron

An autonomic neuron whose cell body is located within an autonomic ganglion outside the central nervous system and whose axon projects directly to target tissues such as smooth muscle, cardiac muscle, or glands.

Paravertebral Ganglia

Sympathetic ganglia arranged in paired chains running parallel on either side of the vertebral column, receiving short preganglionic axons originating from the thoracic and lumbar spinal cord.

Prevertebral Ganglia

Sympathetic ganglia located anterior to the spine near major abdominal arteries, supplying organs of the gut, renal and hepatic vessels, bladder, and genitalia.

Chromaffin Cells

Endocrine cells in the adrenal medulla that are functionally analogous to postganglionic sympathetic neurons lacking axons; they possess nicotinic acetylcholine receptors and release adrenaline and noradrenaline directly into circulation in a $4:1$ ratio.

Choline Acetyltransferase (ChAT)

The enzyme located in the presynaptic cytoplasm that catalyzes the synthesis of acetylcholine from choline and acetyl-CoA.

Acetylcholinesterase (AChE)

An enzyme located in the synaptic cleft that rapidly terminates cholinergic signaling by hydrolyzing acetylcholine into acetate and choline.

Tyrosine Hydroxylase

The rate-limiting enzyme in catecholamine biosynthesis that catalyzes the conversion of the amino acid tyrosine to L-DOPA.

DOPA Decarboxylase

The cytosolic enzyme that converts L-DOPA into dopamine during catecholamine biosynthesis.

Dopamine $\beta$-Hydroxylase

The enzyme responsible for converting dopamine into noradrenaline.

Phenylethanolamine N-Methyltransferase (PNMT)

An enzyme primarily expressed in the adrenal medulla that converts noradrenaline into adrenaline by adding a methyl group to the terminal amine.

Norepinephrine Transporter (NET)

A high-affinity presynaptic transporter mechanism responsible for reuptake of noradrenaline from the synaptic cleft into the presynaptic neuron to terminate signaling.

Catechol-O-Methyltransferase (COMT)

An enzyme located intracellularly and in synapses that metabolizes catecholamines by transferring a methyl group to one of the catechol ring hydroxyl groups.

Monoamine Oxidase (MAO)

A mitochondrial enzyme that breaks down monoamine neurotransmitters (including dopamine, noradrenaline, and adrenaline) inside cells.

Chronotropic Effect

An influence that alters heart rate; positive chronotropy increases heart rate (e.g., via sympathetic stimulation of $\beta_1$ adrenoceptors), whereas negative chronotropy decreases heart rate (e.g., via parasympathetic vagal stimulation).

Inotropic Effect

An influence that alters the force or strength of myocardial contraction; positive inotropy enhances contractility, while negative inotropy reduces it.

Frank-Starling Law of the Heart

The physiological principle stating that within normal physiological limits, increased ventricular filling stretches cardiac muscle fibers, causing an increase in the force of ventricular contraction and stroke volume.

Signal Transduction

The process by which a receptor binds a specific extracellular chemical messenger and translates that recognition event into a cellular response via an altered molecular signal.

G Protein-Coupled Receptor (GPCR)

A major superfamily of cell-surface receptors possessing seven transmembrane $\alpha$-helices that couple to intracellular heterotrimeric G proteins to regulate downstream effectors.

Rhodopsin-like Family (Family A)

The largest family within the GPCR superfamily, containing over 600 members across 19 subfamilies, including adrenoceptors, muscarinic receptors, and olfactory receptors.

Orphan Receptor

A cloned receptor whose amino acid sequence and structure place it within an established receptor family, but whose endogenous physiological ligand remains unidentified.

Pentameric Ligand-Gated Ion Channel (pLGIC)

A superfamily of rapid-acting ionotropic receptors composed of five protein subunits arranged symmetrically around a central ion pore, each subunit having four transmembrane domains with the second forming the channel lining.

Ionotropic Glutamate Receptor (iGluR)

A superfamily of tetrameric ligand-gated cation channels (including NMDA, AMPA, and kainate receptors) whose subunits possess three transmembrane helices and a re-entrant pore loop entering and exiting on the cytoplasmic face.

Receptor Tyrosine Kinase (RTK)

A class of enzyme-linked cell-surface receptors with a single transmembrane domain whose intracellular region contains intrinsic tyrosine kinase activity activated by ligand-induced receptor dimerization and autophosphorylation.

Nuclear Receptor

An intracellular receptor located in the cytoplasm or nucleus that binds lipophilic ligands, sheds chaperone proteins, dimerizes, and binds hormone response elements on DNA to alter gene transcription.

Adenylyl Cyclase

A membrane-bound enzyme that catalyzes the conversion of intracellular ATP into cyclic AMP ($\text{cAMP}$) and pyrophosphate ($\text{PP}_i$) upon activation by $G_s$ $\alpha$ subunits.

Protein Kinase A (PKA)

A cytosolic serine/threonine kinase that exists as an inactive tetramer consisting of two regulatory and two catalytic subunits; binding of four $\text{cAMP}$ molecules releases the active catalytic subunits.

Phospholipase C (PLC)

A membrane-associated enzyme activated by $G_q$ $\alpha$ subunits that cleaves phosphatidylinositol 4,5-bisphosphate ($\text{PIP}_2$) into diacylglycerol ($\text{DAG}$) and inositol 1,4,5-trisphosphate ($\text{IP}_3$).

Inositol 1,4,5-Trisphosphate ($\text{IP}_3$)

A water-soluble second messenger produced by PLC that diffuses into the cytoplasm and binds to ligand-gated calcium channels on the endoplasmic reticulum to release stored calcium.

Diacylglycerol ($\text{DAG}$)

A hydrophobic lipid second messenger produced by PLC cleavage of $\text{PIP}_2$ that remains in the plasma membrane and, together with calcium, activates protein kinase C.

Voltage Sensor

The fourth transmembrane segment ($\text{S4}$) of a voltage-gated ion channel subunit, populated with positively charged amino acids that shift in response to changes in membrane potential to open or close the pore gate.

Pseudo-Subunit

One of four repeated structural domains within the single continuous polypeptide $\alpha$ subunit of voltage-gated sodium ($\text{Na}_V$) or calcium ($\text{Ca}_V$) channels, each mimicking a 6-transmembrane potassium channel subunit.

$\alpha_2\delta$ Subunit

An accessory subunit of voltage-gated calcium channels synthesized as a single polypeptide and post-translationally cleaved into two chains linked by a disulfide bridge, acting as a therapeutic target for certain pain and epilepsy drugs.

Selectivity Filter

The narrowest portion of an ion channel pore lined with specific amino acids and charges that discriminates between permeant and non-permeant ions based on their ionic radius, charge, and hydration shell energetics.

Equilibrium Dissociation Constant ($K_D$)

A quantitative measure of drug affinity defined as the ratio of dissociation rate constant to association rate constant ($\frac{k_{-1}}{k_{+1}}$), representing the free drug concentration at which $50\%$ of total receptor sites are occupied at equilibrium.

Maximal Binding ($B_{max}$)

The total density or capacity of specific drug-binding sites in a tissue preparation, usually expressed in units of femtomoles per milligram of protein ($\text{fmol/mg protein}$).

Hill-Langmuir Equation

The fundamental mathematical equation describing equilibrium ligand binding: $B = \frac{B_{max}[D]}{K_D + [D]}$, predicting a rectangular hyperbolic relationship between bound ligand and free ligand concentration.

Non-Specific Binding (NSB)

The fraction of radioligand that binds to non-receptor components (such as membrane lipids or filter fibers), characterized as a low-affinity, non-saturable pool determined by adding an excess of unlabelled competitor.

Cheng-Prusoff Equation

The mathematical relationship used to calculate the absolute equilibrium dissociation constant ($K_i$) of an unlabelled competing ligand from its experimental $IC_{50}$: $K_i = \frac{IC_{50}}{1 + \frac{[D]}{K_D}}$.

$$IC_{50}$$

The concentration of a competing, unlabelled ligand that reduces the specific binding of a radioligand by $50\%$ in a competition binding assay.

Efficacy

The inherent ability of a drug, once bound to a receptor, to induce the conformational change that activates the receptor and initiates an intracellular response.

Potency ($EC_{50}$)

The molar concentration of an agonist required to produce $50\%$ of that specific drug's maximal attainable response ($E_{max}$) in a functional assay.

Partial Agonist

A ligand that binds to a receptor but possesses lower efficacy than the endogenous full agonist, producing a submaximal biological response ($E_{max} < 100\%$) even when occupying all available receptors.

Competitive Antagonist

A drug that selectively binds to the orthosteric binding site of a receptor with affinity but possesses zero efficacy, reversibly blocking agonist access without altering the maximal attainable agonist response if agonist concentration is sufficiently increased.

Spare Receptors

A condition in a cellular system where maximum biological response ($E_{max}$) is achieved at a ligand concentration that occupies only a fraction of the total available receptor pool ($B_{max}$), resulting in an $EC_{50}$ value lower than the drug's equilibrium dissociation constant ($K_D$).