Human Physiology: Internal Environment, Cell Transport, Membrane Potentials, and Synaptic Transmission

Flashcards covering core terminology and concepts from lectures on internal environment, body fluids, homeostasis, cellular transport mechanisms, resting membrane potential, action potentials, glial cells, and synaptic physiology.

Protoplasm

All the physical matter contained within a cell, encompassing both the cytoplasm and the nucleus, composed of water (up to 80%), inorganic ions, carbohydrates, lipids, and functional or structural proteins.

Great Oxygenation Event

A major evolutionary event occurring approximately 2,330,000 million years ago when atmospheric and oceanic oxygen levels increased rapidly, setting off a cascade that led to multicellular life.

Extracellular Fluid (ECF)

The fluid environment outside cells representing about 1/3 (33%) of total body water, rich in sodium ($Na^+$), chloride ($Cl^-$), bicarbonate ($HCO_3^-$), oxygen, glucose, fatty acids, and amino acids.

Intracellular Fluid (ICF)

The fluid contained inside cells representing about 2/3 (66%) of total body water, characterized by high concentrations of potassium ($K^+$), magnesium ($Mg^{2+}$), and phosphate ions.

Transcellular Fluid

A specialized subcompartment of extracellular fluid (~1% of total body water) enclosed in chambers lined by epithelial cells, including cerebrospinal fluid, peritoneal fluid, intra-ocular fluids, endolymph, and perilymph.

Homeostasis

The concept described by Walter Cannon in 1929 stating that the human body maintains nearly constant physical and chemical conditions within its internal environment.

Negative-Feedback System

A biological control system requiring a sensor/receptor, a set-point reference value, a signal-multiplying control center, and an effector mechanism that acts to bring a parameter back toward the set-point.

Positive-Feedback System

A control loop in which an initial change triggers responses that further amplify the stimulus, often leading to instability or a "vicious cycle," but essential in processes like blood clotting and action potential generation.

Rheostasis

A concept proposed by Nicholas Mrosovsky in 1990 describing higher-order physiological regulation where set-point values undergo organized, regulated changes (programmed or reactive) rather than remaining static.

Allostasis

The physiological process of achieving stability through change by altering set-points in anticipation of environmental perturbations, regulated predominantly by higher-order brain regions like the limbic system and prefrontal cortex.

Allostatic Load

The severe pathophysiological strain (Type 2 allostasis) resulting from long-term, continuous activation of allostatic mechanisms over weeks, months, or years.

Adaptive Control

A feed-forward, delayed negative feedback mechanism used by the brain to evaluate trial results and refine motor movements on subsequent attempts.

Subcortical Level (Palioencephalon)

The region of the central nervous system that controls subconscious activities, comprising the medulla, pons, mesencephalon (brainstem), thalamus, hypothalamus, basal ganglia, amygdala, and cerebellum.

Cerebral Cortex (Telencephalon)

The most recently evolved brain region that functions in association with lower centers as a storehouse of memory, thought, fine voluntary motor control, speech, and processing of sensory inputs.

Electrical Synapse

A bidirectional synapse formed by gap junctions containing connexin channels, allowing small molecules and electrical currents to pass directly between adjacent cells.

Chemical Synapse

A one-way functional junction where presynaptic depolarization triggers calcium-dependent release of neurotransmitter molecules into a ~250 Å synaptic cleft to bind postsynaptic receptors.

SNARE Proteins

Vesicular proteins (synaptobrevin, synaptotagmin) and presynaptic membrane proteins (syntaxin, SNAP-25) that entwine to pull synaptic vesicles close to the plasma membrane for exocytosis in the presence of calcium.

Rab Proteins

Small GTP-binding proteins acting as molecular switches that assemble with the SNARE fusion complex when GTP-bound and dissociate following GTP hydrolysis to GDP.

Axon Hillock

The cone-shaped origin of the axon featuring a 7-fold higher density of voltage-gated sodium channels than the soma, requiring only +10 to +20 mV depolarization to reach excitation threshold and trigger an action potential.

Excitatory Postsynaptic Potential (EPSP)

A local depolarizing potential produced when neurotransmitter binding opens postsynaptic cation channels (such as $Na^+$), shifting the intracellular potential closer to threshold.

Inhibitory Postsynaptic Potential (IPSP)

A local hyperpolarizing potential caused by neurotransmitter binding that opens anion channels ($Cl^-$ influx) or $K^+$ channels ($K^+$ efflux), driving the membrane potential further from threshold.

Presynaptic Inhibition

Inhibition achieved when a third neuron releases an inhibitory neurotransmitter (primarily GABA) onto a presynaptic terminal, opening $Cl^-$ channels to reduce calcium-dependent neurotransmitter release.

Electrotonic Conduction

The non-action-potential passive propagation of electrical currents along dendrites and the neuronal cell body toward the axon hillock.

Spatial Summation

The additive integration of multiple postsynaptic potentials resulting from simultaneous stimulation across several distinct synapses on a single postsynaptic neuron.

Temporal Summation

The additive integration of successive postsynaptic potentials produced at a single synapse by high-frequency presynaptic firing within 1 to 20 milliseconds.

Neuroplasticity

The ability of neurons and neural networks to reorganize their structure, functions, or connections in response to intrinsic/extrinsic stimuli, learning, or injury.

Neuromodulation

The slow regulation of neuronal groups by neuromodulators (e.g., acetylcholine, dopamine, histamine, serotonin, neuropeptides) acting primarily through GPCRs to alter excitability and synaptic growth.

Neurohormone

A signaling molecule synthesized by neurons and released directly into the circulatory system or cerebrospinal fluid, exerting long-lasting actions via metabotropic or intracellular receptors.

Amphipathic Phospholipid

A lipid molecule possessing both a hydrophilic (polar) head facing aqueous environments and hydrophobic (nonpolar) fatty acid tails that form the internal barrier of the cell membrane.

Flippase, Floppase, and Scramblase

Membrane translocases: P-type flippase moves phospholipids inward using ATP; ABC floppase moves phospholipids outward using ATP; scramblase moves phospholipids bidirectionally in a calcium-dependent, energy-independent manner.

Aquaporins

A family of at least 13 distinct transmembrane protein channels that selectively permit the rapid passage of water molecules across cell membranes.

Glycocalyx

A loose carbohydrate coat covering the outer cell surface, composed of glycoproteins, glycolipids, and proteoglycans, providing a negative surface charge, cell adhesion, hormone reception, and immune identity (MHC).

Simple Diffusion

The passive, unassisted movement of uncharged, lipophilic molecules directly through the phospholipid bilayer down their concentration gradient, obeying Fick's law.

Facilitated Diffusion

Carrier- or channel-mediated passive transport of water-soluble substances down an electrochemical gradient, exhibiting maximum transport rate ($V_{max}$) due to carrier protein saturation.

Primary Active Transport

The movement of solutes against an electrochemical gradient directly driven by exergonic chemical energy, most commonly the hydrolysis of ATP.

Sodium-Potassium Pump ($Na^+/K^+$ ATPase)

An electrogenic primary active transport pump that hydrolyzes 1 ATP to export 3 $Na^+$ ions and import 2 $K^+$ ions, crucial for controlling cell volume and establishing resting membrane potential.

Secondary Active Transport

Transport of a solute against its gradient powered by energy stored in an ion gradient (usually $Na^+$) created by primary active transport; includes symporters (cotransporters) and antiporters (exchangers).

Phagocytosis

A form of endocytosis ("cellular eating") in which a cell projects pseudopodia to engulf large solid particles or microorganisms into a phagocytic vesicle for defense.

Pinocytosis

A form of endocytosis ("cellular drinking") where small membrane invaginations internalize droplets of extracellular fluid and dissolved solutes, particularly involved in intestinal fat absorption.

Transcytosis

A cellular transport route combining endocytosis, vesicular movement across the cytoplasm, and exocytosis to rapidly shuttle bulk molecules across cell layers (often via caveolae).

Astrocytes

The most abundant glial cells in the CNS that maintain the extracellular microenvironment, regulate ion/metabolite concentrations, and contribute to the blood-brain barrier.

Oligodendrocytes

Glial cells in the central nervous system responsible for forming myelin sheaths around multiple central neuronal axons.

Schwann Cells

Glial cells in the peripheral nervous system that wrap single peripheral axons with insulating myelin sheaths, exhibit phagocytic activity, and promote PNS axonal regeneration.

Microglia

Specialized resident macrophage glial cells of the CNS that perform immune defense and constantly scavenge for infections, damaged cells, and debris.

Ependymal Cells

Ciliated epithelial-like glial cells lining the ventricles of the brain and central canal of the spinal cord that produce and circulate cerebrospinal fluid (CSF).

Action Potential

A rapid, all-or-none electrical impulse originating at the axon hillock, characterized by depolarization from resting potential (-70 mV) to positive values (+20 to +30 mV) via voltage-gated $Na^+$ channel opening.

Absolute Refractory Period

The period following action potential firing during which voltage-gated $Na^+$ channels are inactivated and cannot be reopened, preventing the generation of a second action potential regardless of stimulus strength.

Relative Refractory Period

The timeframe following the absolute refractory period during which voltage-gated $K^+$ channels remain open, allowing a second action potential to be initiated only by a suprathreshold stimulus.

Saltatory Conduction

The rapid and energy-efficient mode of nerve impulse propagation in myelinated axons where the action potential jumps from one Node of Ranvier to the next.

Sphingomyelin

An electrical insulating lipid abundant in Schwann cell and oligodendrocyte membranes that decreases membrane ion flow approximately 5000-fold.

Action Potential with Plateau

A specialized action potential waveform in cardiac and smooth muscle cells where rapid initial depolarization by fast $Na^+$ channels is prolonged by slow voltage-activated $Ca^{2+}-Na^+$ channels and delayed $K^+$ channel opening.

Pacemaker Current

A slow upward-deflecting membrane current occurring during Phase 4 in cardiac nodal tissue that causes automatic, self-induced depolarization due to the absence of fast voltage-gated $Na^+$ channels.