Action Potential and Signal Propagation

50 vocabulary flashcards covering action potentials, graded potentials, ion concentrations, channel gating, propagation mechanisms, nerve fiber types, demyelination, and local anesthetic action based on the lecture material.

Excitability

The property of certain cells, primarily neurons and muscle cells, to rapidly alter permeability to selective ions upon adequate stimulation, changing the membrane potential ($V_m$).

Extracellular Sodium Concentration ($Na^+$)

The concentration of sodium in the extracellular fluid, which is $142\,mM$.

Intracellular Sodium Concentration ($Na^+$)

The concentration of sodium in the intracellular fluid, which is $10\,mM$.

Extracellular Potassium Concentration ($K^+$)

The concentration of potassium in the extracellular fluid, which is $4\,mM$.

Intracellular Potassium Concentration ($K^+$)

The concentration of potassium in the intracellular fluid, which is $140\,mM$.

Extracellular Calcium Concentration ($Ca^{2+}$)

The concentration of calcium in the extracellular fluid, which is $2.4\,mM$.

Intracellular Calcium Concentration ($Ca^{2+}$)

The concentration of calcium in the intracellular fluid, which is $0.0001\,mM$.

Extracellular Chloride Concentration ($Cl^-$)

The concentration of chloride in the extracellular fluid, which is $105\,mM$.

Intracellular Chloride Concentration ($Cl^-$)

The concentration of chloride in the intracellular fluid, which is $4\,mM$.

Voltage-Gated Ion Channels

Ion channels that open when the membrane potential reaches a specific threshold value.

Stimulus

Anything that elicits a local change in ion channel permeability.

Graded Potential

A local change in membrane potential caused by a change in current, whose amplitude and duration are proportional to stimulus strength.

Ohm's Law for Graded Potentials

Expressed as $V = IR$, stating that a change in current produces a proportional change in membrane potential.

Decremental Conduction

The localized conduction characteristic where charge dissipates as it travels farther from the initial depolarization site.

Threshold Potential

The depolarized membrane potential required to open the specific voltage-gated channels that generate an action potential.

Action Potential

A brief, all-or-none cycle of depolarization and repolarization that occurs when membrane potential reaches threshold.

Voltage-Gated $Na^+$ Channel Activation Speed

Voltage-gated sodium channels open very rapidly in response to threshold depolarization.

Voltage-Gated $K^+$ Channel Activation Speed

Voltage-gated potassium channels open slowly compared to voltage-gated sodium channels.

Inactivation Gate of Voltage-Gated $Na^+$ Channels

A cytoplasmic-side gate that blocks ion flow shortly after opening; the channel cannot open again until it closes at resting membrane potential.

Rising Phase of Action Potential

The rapid depolarization phase driven by the opening of additional voltage-gated $Na^+$ channels and rapid $Na^+$ influx into the cell.

Peak Phase of Action Potential

The phase where voltage-gated $Na^+$ channels become inactivated and voltage-gated $K^+$ channels open.

Repolarization Phase of Action Potential

The phase characterized by the efflux of $K^+$ out of the cell through open voltage-gated $K^+$ channels.

Recovery Phase of Action Potential

The stage where both voltage-gated $Na^+$ and $K^+$ channels are closed, returning the membrane potential to its resting value.

Tetraethylammonium (TEA)

A chemical compound that blocks $K^+$ current, experimentally revealing the isolated $Na^+$ current.

Tetrodotoxin (TTX)

A toxin found in pufferfish that blocks voltage-gated $Na^+$ channels, preventing action potential propagation.

Saxitoxin (STX)

A toxin from red tide dinoflagellates that blocks voltage-gated $Na^+$ channels, revealing the $K^+$ current.

Absolute Refractory Period

The time period following an action potential during which voltage-gated $Na^+$ channels are inactivated and a second action potential cannot be elicited regardless of stimulus strength.

Relative Refractory Period

The time period when the membrane is hyperpolarized, allowing a second response to be elicited only if a stronger stimulus is applied.

One-Way Propagation

The unidirectional conduction of action potentials along neurons maintained by the refractory period.

Axon Diameter Effect

Larger axon diameter provides more intracellular fluid volume and less resistance to local current, increasing action potential propagation speed.

Myelination

An insulating structure around axons (with up to 300 membrane layers) that prevents charge leakage and increases propagation speed.

Nodes of Ranvier

Gaps in the myelin sheath with a very high density of voltage-gated $Na^+$ channels where action potentials are regenerated.

Saltatory Conduction

The propagation process in myelinated axons where action potentials appear to jump from one Node of Ranvier to the next.

Type A$\alpha$ Nerve Fibers

Myelinated somatomotor and proprioception fibers with diameter $12\text{--}20\,\mu m$ and conduction speed $80\text{--}120\,m/s$.

Type A$\beta$ Nerve Fibers

Myelinated touch and pressure fibers with diameter $5\text{--}12\,\mu m$ and conduction speed $35\text{--}75\,m/s$.

Type A$\gamma$ Nerve Fibers

Myelinated motor fibers to muscle spindles with diameter $3\text{--}6\,\mu m$ and conduction speed $12\text{--}30\,m/s$.

Type A$\delta$ Nerve Fibers

Myelinated acute pain, temperature, and fine touch fibers with diameter $2\text{--}5\,\mu m$ and conduction speed $5\text{--}30\,m/s$.

Type B Nerve Fibers

Myelinated preganglionic autonomic fibers with diameter $<3\,\mu m$ and conduction speed $3\text{--}15\,m/s$.

Type C Nerve Fibers

Unmyelinated fibers for aching pain, touch, or postganglionic autonomic functions with diameter $0.3\text{--}1.3\,\mu m$ and speed $0.5\text{--}2.5\,m/s$.

Conduction Speed Comparison

Unmyelinated axons propagate action potentials at $0.5\,m/s$ ($4\,\text{seconds}$ head to toe), whereas myelinated axons propagate at $100\,m/s$ ($0.02\,\text{seconds}$ head to toe).

Pathophysiological Effects of Demyelination

Includes decreased conduction velocity, frequency-related block, total conduction block, ectopic impulse generation, and increased mechanosensitivity.

Crosstalk (Ectopic Impulse Generation)

The phenomenon where charge leaking from a demyelinated neuron triggers an action potential in a neighboring neuron.

Multiple Sclerosis (MS)

An autoimmune disease of the central nervous system (CNS) that destroys oligodendrocytes, resulting in minimal recovery.

Guillain-Barre Syndrome

A peripheral nervous system (PNS) condition characterized by lymphocytic infiltration of Schwann cells following viral infection, with recovery occurring upon remyelination.

Sodium Flux during Action Potential

Approximately 40 million $Na^+$ ions cross the membrane to change $V_m$ by $100\,mV$ in $2\,msec$, representing only $0.06\%$ of intracellular $Na^+$.

Target of Local Anesthetics

Local anesthetics bind to the cytoplasmic side of inactivated voltage-gated $Na^+$ channels, interacting with the inactivation gate to maintain the inactive state.

Local Anesthetic Dissociation Equilibrium

Represented as $\text{RNH}^+ \rightleftharpoons \text{RN} + \text{H}^+$, balancing the ionized form ($\text{RNH}^+$) and the uncharged free base form ($\text{RN}$).

Free Base Form ($\text{RN}$)

The uncharged form of a local anesthetic that is lipid-soluble and capable of crossing the neuronal cell membrane.

Ionized Form ($\text{RNH}^+$)

The active form of a local anesthetic that blocks the voltage-gated $Na^+$ channel from inside the cell.

Effect of Low pH on Local Anesthetic Efficacy

Lowering tissue pH (due to infection or inflammation) shifts equilibrium toward $\text{RNH}^+$, reducing the free base $\text{RN}$ capable of crossing the membrane and reducing effectiveness by over 20-fold at pH 6.