Human Physiology: Membrane Transport, Chemical Signaling, and Neural Integration

Comprehensive vocabulary flashcards covering membrane transport mechanisms, chemical messengers and signal transduction, endocrine system regulation, neurophysiology and action potential dynamics, and synaptic transmission.

Chemical Driving Force

A directional force that drives molecules down their concentration gradient from an area of higher concentration to an area of lower concentration, where the rate of transport depends on the size of the concentration gradient.

Electrical Driving Force

A directional force acting exclusively on charged particles ($\text{ions}$), determined by the principle that opposite charges attract and like charges repel across a cell membrane.

Membrane Potential ($V_m$)

The difference in electrical potential (voltage) across the plasma membrane, where the sign of the potential designates the net charge located inside the cell relative to the outside.

Equilibrium Potential ($E_X$)

The specific membrane potential ($V_m$) at which the electrical driving force exactly balances and opposes the chemical driving force for an ion, producing an electrochemical gradient of zero and no net ion movement ($E_{\text{Na}} = +60\,\text{mV}$, $E_{\text{K}} = -94\,\text{mV}$).

Simple Diffusion

The spontaneous passive movement of membrane-permeant molecules directly across the lipid bilayer down their electrochemical gradient without requiring a transport protein.

Facilitated Diffusion

A form of passive mediated transport wherein molecules cross the plasma membrane down their electrochemical gradient via specific transmembrane carrier proteins, exhibiting transport maximums due to carrier saturation.

Primary Active Transport

A transport process in which transmembrane pumps directly hydrolyze $\text{ATP}$ ($\text{ATPase}$ activity) to transport solutes across a membrane against their electrochemical gradients.

Secondary Active Transport

Transport driven indirectly by the energy stored in an electrochemical gradient that was previously created and maintained by primary active transport (e.g., sodium-linked glucose transport or sodium-proton exchange).

Osmosis

The passive net flow of water molecules across a semipermeable membrane down its concentration gradient (or toward regions of higher solute concentration), unaffected directly by membrane potential ($V_m$).

Osmolarity

The total concentration of all solute particles (both permeant and impermeant) in a solution, measured in osmoles or milliosmoles per liter ($\text{mOsm}$).

Tonicity

The concentration of impermeant solutes relative to intracellular fluid ($\text{ICF}$), which determines whether water will move into or out of a cell and thereby alter cell volume.

Pinocytosis

A nonspecific form of endocytosis in which a cell engulfs droplets of extracellular fluid containing dissolved molecules by forming small endosomes.

Transcytosis

The transcellular transport of macromolecules across an epithelial cell layer through a sequential process involving endocytosis at one membrane followed by exocytosis at the opposing membrane.

Paracrine

A functional class of chemical messenger secreted into the extracellular fluid by a cell to act on nearby target cells via simple diffusion.

Autocrine

A type of chemical messenger that binds to receptors on and alters the activity of the very same cell that secreted it.

Neurohormone

A chemical messenger secreted by neurosecretory neurons into the bloodstream to reach distant target cells throughout the body.

Catecholamines

A group of amine messengers derived from tyrosine containing a catechol group and an amine group ($-\text{NH}_2$), including dopamine, norepinephrine, and epinephrine.

Eicosanoids

A class of lipophilic paracrine messengers mostly derived from arachidonic acid via either the cyclooxygenase or lipoxygenase pathways, including prostaglandins, prostacyclins, thromboxanes, and leukotrienes.

Up-Regulation

An increase in the total number of target-cell receptors compared to normal, occurring in response to chronically low messenger concentrations to increase target sensitivity.

Receptor Agonist

A ligand that binds to a specific receptor and produces a typical biological or cellular response.

Receptor Antagonist

A ligand that competes with agonists for receptor binding sites and prevents a cellular response when binding occurs.

Neurohypophysis

The posterior lobe of the pituitary gland consisting of nervous tissue, which releases the neurohormones oxytocin and vasopressin ($\text{ADH}$) from axon terminals originating in the hypothalamus.

Adenohypophysis

The anterior lobe of the pituitary gland made of glandular tissue, which synthesizes and secretes tropic hormones in response to hypothalamic tropic hormones delivered via the hypothalamic portal vein.

Tropic Hormone

A hormone whose primary function is to regulate the secretion of another hormone from an endocrine target tissue.

Parathyroid Hormone (PTH)

A hormone secreted by the parathyroid glands that increases blood $\text{Ca}^{2+}$ levels.

Adrenocorticoids

A class of steroid hormones secreted by the adrenal cortex, categorized into mineralocorticoids (primarily aldosterone), glucocorticoids (primarily cortisol), and sex hormones (primarily androgens).

Chromaffin Cells

Modified neuroendocrine cells located within the adrenal medulla that secrete catecholamines consisting of approximately $80\%$ epinephrine, $20\%$ norepinephrine, and $<1\%$ dopamine.

Islets of Langerhans

Endocrine cell clusters located in the pancreas containing alpha cells (glucagon), beta cells (insulin), delta cells (somatostatin), and F cells (pancreatic polypeptide).

Permissiveness

A type of endocrine interaction where the presence of one hormone is necessary for another hormone to exert its full physiological effects.

Primary Hypersecretion

An endocrine disorder characterized by excess hormone secretion arising directly from an abnormality within the endocrine gland that produces the hormone itself, rather than from excess stimulation.

Nerve

A discrete organ composed of a bundle of axons traveling together in the peripheral nervous system ($\text{PNS}$).

Ganglion

A cluster of neuronal cell bodies ($\text{soma}$) located within the peripheral nervous system ($\text{PNS}$).

Graded Potential

A small, decremental change in membrane potential ($V_m$) that varies in size with stimulus strength and decays as it spreads away from the site of origin.

Temporal Summation

The additive effect produced when multiple graded potentials are generated at the same synapse in rapid succession before previous potentials have fully decayed.

Spatial Summation

The additive effect produced when graded potentials originating from different synapses on the same postsynaptic neuron occur simultaneously and summate at the axon hillock.

Absolute Refractory Period

The period during and immediately after an action potential during which a second action potential cannot be generated under any circumstance, primarily due to inactivation gates closing on voltage-gated $\text{Na}^+$ channels.

Relative Refractory Period

The interval following the absolute refractory period during which an action potential can be triggered, but only in response to a suprathreshold stimulus stronger than that required at rest, largely due to open voltage-gated $\text{K}^+$ channels.

Saltatory Conduction

The rapid mode of action potential propagation along myelinated axons, in which the electrical impulse jumps from one Node of Ranvier to the next.

Tetrodotoxin (TTX)

A potent neurotoxin found in pufferfish that selectively blocks voltage-gated $\text{Na}^+$ channels, preventing the generation and propagation of action potentials.

Electrical Synapse

A junction between two excitable cells where plasma membranes are bridged by gap junctions ($\text{connexons}$), allowing bidirectional ion flow and direct electrical coupling with minimal synaptic delay.

Chemical Synapse

A specialized junction where a presynaptic neuron releases neurotransmitter molecules across a synaptic cleft ($30\text{--}50\,\text{nm}$) to bind receptors on a postsynaptic target cell.

Selective Serotonin Reuptake Inhibitors (SSRIs)

A class of pharmaceutical agents (e.g., Prozac, Lexapro, Paxil, Zoloft) used to treat clinical depression by blocking the reuptake molecules that return exocytosed serotonin into the presynaptic axon terminal.

Excitatory Postsynaptic Potential (EPSP)

A graded depolarizing change in the postsynaptic membrane potential ($V_m$) that moves the membrane closer to threshold, commonly produced by the opening of channels permeable to $\text{Na}^+$ and $\text{K}^+$ or the closing of $\text{K}^+$ channels.

Inhibitory Postsynaptic Potential (IPSP)

A graded hyperpolarizing or stabilizing change in the postsynaptic membrane potential ($V_m$) that moves the membrane further from threshold, typically mediated by the opening of $\text{K}^+$ or $\text{Cl}^-$ channels.

Presynaptic Facilitation

A form of synaptic modulation occurring at an axoaxonic synapse where activity in an upstream neuron enhances the influx of $\text{Ca}^{2+}$ in the axon terminal of another neuron, increasing neurotransmitter release onto the postsynaptic cell.

Acetylcholinesterase (AChE)

An enzyme located in the synaptic cleft or associated with pre- or postsynaptic membranes that terminates cholinergic signaling by rapidly breaking down acetylcholine ($\text{ACh}$) into acetate and choline.