Neurophysiology and Cellular Transport

Vocabulary practice flashcards covering membrane potentials, tonicity, osmolarity, epithelial transport, and neurophysiology based on the review lecture.

Absolute Refractory Period

The period of time during an action potential when voltage-gated $\text{Na}^+$ channels are inactivated and no stimulus, regardless of its strength, can trigger another action potential.

Relative Refractory Period

The phase following an action potential during which a larger-than-normal depolarizing stimulus is required to initiate a new action potential, caused by after-hyperpolarization (membrane potential dipping toward $-80\,\text{mV}$) and partially inactivated $\text{Na}^+$ channels that have an increased threshold (e.g., $-45\,\text{mV}$ instead of $-55\,\text{mV}$).

Chemical Disequilibrium

The condition in which the intracellular fluid (ICF) and extracellular fluid (ECF) do not share the same chemical composition or individual ion concentrations.

Electrical Disequilibrium

The state where the intracellular fluid contains a slight excess of anions and the extracellular fluid contains an equal slight excess of cations, creating an electrical charge separation across the membrane.

Membrane Potential

The electrical charge difference between the intracellular and extracellular fluids across the plasma membrane, averaging $-70\,\text{mV}$ at rest.

Osmotic Equilibrium

The physiological state where the total concentration of solute particles per volume of solution is identical between the intracellular fluid (ICF) and extracellular fluid (ECF), approximately $300\,\text{mOsM}$ ($300\,\text{mosmol/L}$).

Aquaporin Channel

A water-specific membrane channel present in all cells that allows the passive movement of water molecules across the cell membrane.

Osmosis

The net movement of water molecules across a selectively permeable membrane from an area of lower solute concentration to an area of higher solute concentration down an osmotic gradient.

Penetrating Solute

A solute (such as urea) that freely permeates the cell membrane, diffusing across until its concentration is equalized between the ICF and ECF without establishing a permanent osmotic gradient.

Non-Penetrating Solute

A solute particle that cannot cross the plasma membrane, remaining confined to its compartment and creating an osmotic driving force for water movement.

Osmolarity

The concentration of total solute particles (including both penetrating and non-penetrating solutes) expressed as osmoles per liter of solution ($\text{mOsM}$ or $\text{mosmol/L}$).

Tonicity

A physiological descriptor of how a solution affects cell volume at equilibrium, determined solely by the concentration gradient of non-penetrating (NP) solutes.

Osmolality

The concentration of solute particles per kilogram of solvent, practically interchangeable with osmolarity in human physiology due to the dilute nature of biological fluids.

Transcellular Transport

The movement of substances directly through an epithelial cell, crossing both apical and basolateral membranes without vesicular trafficking.

Paracellular Transport

The movement of substances across an epithelium through the intercellular spaces between adjacent cells across tight junctions.

Transcytosis

A transport process where macromolecules cross an endothelial or epithelial layer via a combination of endocytosis into caveolae, vesicular transport through the cytoplasm, and exocytosis.

Equilibrium Potential

The membrane potential that exactly balances and opposes the concentration gradient for a single permeable ion.

Nernst Equation

The mathematical formula $E_{\text{ion}} = \frac{61}{z}\log\left(\frac{[\text{ion}]_{\text{out}}}{[\text{ion}]_{\text{in}}}\right)$ used to calculate the equilibrium potential for a single ion at $37\,^\circ\text{C}$.

Goldman-Hodgkin-Katz (GHK) Equation

The equation used to calculate the resting membrane potential ($V_m$) by considering the concentration gradients and relative membrane permeabilities ($P$) of all ions that cross freely.

Pseudounipolar Neurons

Sensory neurons that possess a single process extending from the cell body which functions as an axon, formed during development by the fusion of the dendrite with the axon.

Bipolar Neurons

Sensory neurons involved in specialized senses (such as smell and vision) featuring two relatively equal processes extending from opposite sides of the cell body.

Anaxonic Neurons

Central nervous system interneurons that possess multiple dendritic arborizations but lack an identifiable axon.

Multipolar Neurons

Neurons having multiple dendrites and a single axon, including highly branched CNS interneurons and typical efferent neurons having 5 to 7 dendrites that branch 4 to 6 times.

Graded Potential

A local change in membrane potential whose amplitude is proportional to the strength of the input stimulus and which decays with distance from the site of origin due to cytoplasmic resistance and current leakage.

Ionotropic Receptors

Ligand-gated ion channels activated directly by neurotransmitters that mediate rapid, short-acting synaptic potentials.

Metabotropic Receptors

G-protein coupled receptors activated by neuromodulators that initiate intracellular cascades or regulate ion channels to produce slow synaptic potentials and long-term effects.

Presynaptic Modulation

Regulation of synaptic transmission occurring when a modulatory neuron forms an axo-axonic synapse on a presynaptic axon terminal, altering the amount of neurotransmitter released via facilitation or inhibition.

Postsynaptic Modulation

A regulatory mechanism that alters synaptic efficacy by modifying the responsiveness of the postsynaptic cell, often by changing receptor density or sensitivity (e.g., long-term potentiation).

Anterograde Axonal Transport

Microtubule-dependent movement from the soma toward the axon terminal driven by kinesin motor proteins, occurring via fast transport ($400\,\text{mm/day}$ for vesicles and mitochondria) or slow transport ($8\,\text{mm/day}$ for cytoplasmic proteins).

Retrograde Axonal Transport

Microtubule-dependent transport from the axon terminal back toward the cell body driven by dynein motor proteins, operating as fast transport at a rate of $200\,\text{mm/day}$ for vesicles and mitochondria.