Chapter 12: Electrical Signals in Neurons Flashcards

A set of 120 vocabulary flashcards covering ion channels, resting membrane potential, graded potentials, action potentials, propagation, and synaptic transmission based on Chapter 12 lecture notes.

Electrochemical Gradient

The combined concentration (chemical) difference and electrical difference that drives the passive movement of specific ions across the plasma membrane when ion channels are open.

Leak Channels

Gated ion channels whose gates randomly alternate between open and closed positions, found in nearly all cells including the dendrites, cell bodies, and axons of all neuron types.

Potassium Ion Leak Channels

Ion channels in plasma membranes that are much more numerous and leakier than sodium ion leak channels, giving the membrane a much higher permeability to $\text{K}^+$ than to $\text{Na}^+$.

Ligand-Gated Channel

An ion channel that opens or closes in response to the binding of a chemical stimulus (ligand), located in dendrites of sensory neurons (e.g., pain receptors) and dendrites/cell bodies of interneurons and motor neurons.

Acetylcholine (ACh)

A neurotransmitter ligand that opens cation channels, allowing $\text{Na}^+$ and $\text{Ca}^{2+}$ to diffuse inward and $\text{K}^+$ to diffuse outward.

Mechanically-Gated Channel

An ion channel that opens or closes in response to mechanical stimulation such as vibration, touch, pressure, or tissue stretching.

Voltage-Gated Channel

An ion channel located in the axons of all neuron types that opens in response to a change in membrane potential (voltage) to generate and conduct action potentials.

Upper Motor Neuron

A type of motor neuron in which a stimulus in the brain triggers a graded potential in its dendrites and cell body, leading to an action potential that synapses with a lower motor neuron in the CNS.

Lower Motor Neuron

A type of motor neuron that receives neurotransmitter signals from an upper motor neuron, generating a graded potential and action potential to directly stimulate skeletal muscle fibers.

Perception

The conscious awareness of a sensation, which is primarily a function of the cerebral cortex.

Resting Membrane Potential

An electrical potential difference (voltage) that exists across the plasma membrane of an excitable cell under resting conditions.

Current

The flow of charged particles; in living cells, it is constituted by the movement of ions across the plasma membrane through ion channels.

Recording Microelectrode

A specialized electrode whose tip is inserted inside a cell to detect internal charge when measuring membrane potential.

Reference Electrode

An electrode placed outside a cell in the extracellular fluid when measuring membrane potential.

Voltmeter

An instrument connected to recording microelectrodes and reference electrodes to measure the electrical potential difference across a plasma membrane.

Typical Neuronal Resting Membrane Potential

A baseline voltage value of $-70\,\text{mV}$ in neurons, indicating that the inside of the cell is negative relative to the outside.

Polarized Cell

A cell that exhibits a membrane potential across its plasma membrane.

Extracellular Fluid (ECF) Ion Composition

The fluid outside cells, rich in sodium ions ($\text{Na}^+$) and chloride ions ($\text{Cl}^-$).

Cytosol Ion Composition

The intracellular fluid inside cells, where the main cation is $\text{K}^+$ and dominant anions are organophosphates (such as ATP) and amino acids in proteins.

Trapped Anions

Nondiffusible intracellular anions attached to molecules such as ATP and large proteins that cannot follow $\text{K}^+$ out of the cell, sustaining internal negativity.

Electrogenic Na+-K+ ATPase

A membrane pump that expels $3\,\text{Na}^+$ for every $2\,\text{K}^+$ imported, contributing a small negative voltage ($-3\,\text{mV}$) to the resting membrane potential.

Graded Potential

A small deviation from the resting membrane potential that makes the membrane either more polarized or less polarized, varying in amplitude based on stimulus strength.

Hyperpolarizing Graded Potential

A graded potential response that makes the membrane potential inside more negative than the resting level.

Depolarizing Graded Potential

A graded potential response that makes the membrane potential inside less negative than the resting level.

Decremental Conduction

The mode of travel by which graded potentials gradually die out as they spread along the membrane due to charge loss through leak channels.

Summation

The process by which two or more graded potentials add together to become larger in amplitude.

Postsynaptic Potential

A graded potential that occurs in the dendrites or cell body of a neuron in response to a neurotransmitter.

Receptor Potential

A graded potential that forms in sensory receptors in response to a stimulus.

Glycine Effect on Ligand-Gated Channels

Binding of the neurotransmitter glycine opens $\text{Cl}^-$ channels, allowing $\text{Cl}^-$ ions into the cell and producing a hyperpolarizing graded potential.

Action Potential (AP)

A sequence of rapidly occurring events that decrease and reverse the membrane potential and then restore it to the resting state.

Depolarizing Phase of Action Potential

The phase during which the negative membrane potential becomes less negative, reaches zero, and becomes positive ($+30\,\text{mV}$) due to rapid inward movement of $\text{Na}^+$.

Repolarizing Phase of Action Potential

The phase during which the membrane potential is restored to the resting state of $-70\,\text{mV}$ due to slowing of $\text{Na}^+$ inflow and outflow of $\text{K}^+$.

After-Hyperpolarizing Phase

The phase following repolarization where voltage-gated $\text{K}^+$ channels remain open, temporarily making the membrane potential more negative (about $-90\,\text{mV}$).

Action Potential Threshold

The critical depolarization level (about $-55\,\text{mV}$ in many neurons) required to trigger an action potential.

Subthreshold Stimulus

A weak depolarizing stimulus that fails to bring the membrane potential to threshold and cannot generate an action potential.

Threshold Stimulus

A stimulus that is just strong enough to depolarize the membrane potential to threshold ($-55\,\text{mV}$) to initiate a single action potential.

Suprathreshold Stimulus

A stimulus strong enough to depolarize the membrane above threshold, triggering a higher frequency of action potentials of identical amplitude.

All-or-None Principle

The principle stating that if a stimulus reaches threshold, an action potential occurs completely; if threshold is not reached, no action potential occurs at all.

Voltage-Gated Na+ Channel Activation Gate

The gate on a voltage-gated $\text{Na}^+$ channel that is closed at rest and opens rapidly when threshold depolarization is reached.

Voltage-Gated Na+ Channel Inactivation Gate

The gate on a voltage-gated $\text{Na}^+$ channel that is open at rest and closes shortly after activation to halt $\text{Na}^+$ inflow.

Positive Feedback in Action Potential Generation

The process where initial $\text{Na}^+$ inflow depolarizes the membrane further, causing even more voltage-gated $\text{Na}^+$ channels to open.

Resting State of Voltage-Gated Na+ Channel

The state of a voltage-gated $\text{Na}^+$ channel where the inactivation gate is open and the activation gate is closed, preventing $\text{Na}^+$ inflow.

Activated State of Voltage-Gated Na+ Channel

The state of a voltage-gated $\text{Na}^+$ channel where both activation and inactivation gates are open, allowing $\text{Na}^+$ ions to rush into the cell.

Inactivated State of Voltage-Gated Na+ Channel

The state of a voltage-gated $\text{Na}^+$ channel where the inactivation gate is closed, blocking $\text{Na}^+$ flow shortly after activation.

Number of Na+ Ions in Single Action Potential

Approximately $20,000\,\text{Na}^+$ ions cross the membrane during a single action potential, which are easily returned outside by $\text{Na}^+\text{--}\text{K}^+$ ATPases.

Voltage-Gated K+ Channel Kinetics

Channels that open more slowly than voltage-gated $\text{Na}^+$ channels and alternate between closed and open states without having an inactivated state.

Refractory Period

The period of time after an action potential begins during which an excitable cell cannot generate another action potential in response to a normal threshold stimulus.

Absolute Refractory Period

The time interval during which even a very strong stimulus cannot initiate a second action potential, coinciding with $\text{Na}^+$ channel activation and inactivation.

Absolute Refractory Period in Large Axons

A brief period lasting about $0.4\,\text{msec}$, enabling large-diameter axons to transmit up to $1000$ impulses per second.

Absolute Refractory Period in Small Axons

A duration lasting up to $4\,\text{msec}$, limiting small-diameter axons to a maximum of $250$ impulses per second.

Relative Refractory Period

The period of time during which a second action potential can be initiated, but only by a larger-than-normal stimulus, occurring when voltage-gated $\text{K}^+$ channels remain open.

Propagation

The non-decremental mode of action potential conduction along an axon membrane that maintains impulse strength over long distances using positive feedback.

Direction of Action Potential Propagation

Action potentials propagate unidirectionally from the trigger zone to axon terminals because preceding membrane regions are in the absolute refractory period.

Tetrodotoxin (TTX)

A lethal neurotoxin present in Japanese puffer fish that blocks action potentials by inserting into voltage-gated $\text{Na}^+$ channels so they cannot open.

Local Anesthetics

Drugs like procaine (Novocaine) and lidocaine that block pain by preventing voltage-gated $\text{Na}^+$ channels from opening, halting action potential propagation to the CNS.

Effect of Cold on Axonal Propagation

Localized cooling lowers the propagation speed of action potentials along axons, partially blocking pain sensations.

Continuous Conduction

Step-by-step depolarization and repolarization of each adjacent segment of the plasma membrane, occurring in unmyelinated axons and muscle fibers.

Saltatory Conduction

The special mode of action potential propagation along myelinated axons where electrical current leaps from one node of Ranvier to the next.

Nodes of Ranvier

Unmyelinated gaps in the myelin sheath along an axon containing high densities of voltage-gated channels where current flows across the axolemma.

Energy Efficiency of Saltatory Conduction

Conduction mode that uses less ATP because minimal ion exchange occurs across small membrane regions at nodes, requiring less pump activity from $\text{Na}^+\text{--}\text{K}^+$ ATPases.

Myelination Factor in Propagation

The presence of a myelin sheath significantly increases the propagation speed of action potentials relative to unmyelinated axons.

Axon Diameter Factor in Propagation

Larger-diameter axons propagate action potentials faster than smaller-diameter axons due to their greater surface area.

Temperature Factor in Propagation

Warmer temperatures increase action potential propagation speeds, while cooling decreases propagation speeds.

A Fibers

Myelinated nerve fibers with the largest diameters ($5\text{--}20\,\mu\text{m}$) and speeds of $12\text{--}130\,\text{m/s}$, carrying touch, pressure, position, thermal, pain, and motor signals to skeletal muscles.

B Fibers

Myelinated nerve fibers with intermediate diameters ($2\text{--}3\,\mu\text{m}$) and speeds up to $15\,\text{m/s}$, conducting visceral sensory impulses and preganglionic autonomic motor impulses.

C Fibers

Unmyelinated nerve fibers with the smallest diameters ($0.5\text{--}1.5\,\mu\text{m}$) and speeds of $0.5\text{--}2\,\text{m/s}$, conducting pain, touch, pressure, heat, cold, and postganglionic autonomic signals.

Frequency Code

The primary mechanism for encoding stimulus intensity based on the frequency at which action potentials are generated at the trigger zone.

Sensory Neuron Recruitment

The process where a stronger stimulus activates or recruits a larger number of sensory neurons to convey greater intensity.

Presynaptic Neuron

A nerve cell that carries a nerve impulse toward a synapse and sends the signal.

Postsynaptic Cell

The cell that receives a signal at a synapse, which may be a postsynaptic neuron or an effector cell (muscle cell or gland cell).

Axodendritic Synapse

A synapse formed between the axon terminal of a presynaptic neuron and a dendrite of a postsynaptic neuron.

Axosomatic Synapse

A synapse formed between the axon terminal of a presynaptic neuron and the cell body (soma) of a postsynaptic neuron.

Axoaxonic Synapse

A synapse formed between the axon terminal of a presynaptic neuron and the axon of a postsynaptic neuron.

Comparison of Neuronal vs Muscle Resting Potential

The typical resting membrane potential of a neuron is $-70\,\text{mV}$, while it is closer to $-90\,\text{mV}$ in skeletal and cardiac muscle fibers.

Duration of Nerve Impulse vs Muscle Action Potential

Nerve impulses last $0.5\text{--}2\,\text{msec}$, skeletal muscle action potentials last $1.0\text{--}5.0\,\text{msec}$, and cardiac/smooth muscle action potentials last $10\text{--}300\,\text{msec}$.

Conduction Distance of Graded vs Action Potentials

Graded potentials allow short-distance communication due to decremental conduction, whereas action potentials propagate for long-distance communication.

Origin of Graded Potentials

Graded potentials arise mainly in the dendrites and cell body of a neuron.

Origin of Action Potentials

Action potentials arise at the trigger zone and propagate along the axon.

Channel Types for Graded Potentials

Graded potentials rely on ligand-gated or mechanically-gated ion channels.

Channel Types for Action Potentials

Action potentials rely on voltage-gated channels for $\text{Na}^+$ and $\text{K}^+$.

Amplitude Range of Graded Potentials

Graded potential amplitudes vary from less than $1\,\text{mV}$ to more than $50\,\text{mV}$ depending on stimulus strength.

Amplitude of Action Potentials

Action potential amplitudes follow an all-or-none rule, typically measuring about $100\,\text{mV}$.

Polarity Characteristics of Graded Potentials

Graded potentials may be hyperpolarizing (inhibitory) or depolarizing (excitatory).

Polarity Characteristics of Action Potentials

Action potentials always consist of a depolarizing phase followed by a repolarizing phase and return to resting potential.

Refractory Period in Graded Potentials

Graded potentials lack a refractory period, which allows summation to occur.

Refractory Period in Action Potentials

Action potentials possess a refractory period, preventing summation from occurring.

Leak Channel Mechanism

Ion channel mechanism where gates randomly open and close to permit ion movement down electrochemical gradients.

Ligand-Gated Channel Mechanism

Ion channel mechanism where a chemical stimulus (such as acetylcholine) binds to open the gate for ion passage.

Mechanically-Gated Channel Mechanism

Ion channel mechanism where mechanical distortion opens the gate to allow passage of specific cations.

Voltage-Gated Channel Mechanism

Ion channel mechanism where a shift in membrane voltage (e.g., from $-70\,\text{mV}$ to $-50\,\text{mV}$) triggers gate opening.

Voltmeter Measurement Diagram

Diagram illustrating the placement of recording microelectrodes inside and reference electrodes outside a neuron to read the $-70\,\text{mV}$ resting potential.

Resting Membrane Potential Factors Diagram

Diagram illustrating $\text{K}^+$ leakage, trapped intracellular anions, and $\text{Na}^+\text{--}\text{K}^+$ ATPase pump activity maintaining the resting potential.

Hyperpolarizing Graded Potential Trace

Membrane potential voltage recording showing a temporary hyperpolarizing deviation from $-70\,\text{mV}$ down to $-80\,\text{mV}$.

Depolarizing Graded Potential Trace

Membrane potential voltage recording showing a temporary depolarizing deviation from $-70\,\text{mV}$ up to $-60\,\text{mV}$.

Summation of Graded Potentials Graph

Graph demonstrating two closely timed depolarizing stimuli summing to produce a larger combined response.

Phases of Action Potential Graph

Graph showing the depolarizing, repolarizing, and after-hyperpolarizing phases along with the absolute and relative refractory periods.

Stimulus Strength vs AP Generation Graph

Graph displaying subthreshold, threshold, and suprathreshold depolarizations and their resulting action potential responses.

Propagation Modes Diagram

Diagram comparing continuous conduction along unmyelinated axons with saltatory conduction leaping between nodes of Ranvier on myelinated axons.

Chemical Aspect of Electrochemical Gradient

The passive concentration force driving ions to move from areas of higher concentration to areas of lower concentration.

Electrical Aspect of Electrochemical Gradient

The electrical charge attraction driving positively charged cations toward negative areas and negatively charged anions toward positive areas.