T8

Question 1

The main excitable cells are the ones that

Answer 1

that have the ability to produce rapid, transient changes in their membrane potential when excited.

Question 2

T or F. Membrane potential does contradict electroneutrality.

Answer 2

F. Membrane potential does not contradict electroneutrality.

Question 3

Membrane Potential and Electroneutrality:

Answer 3

Neurons maintain a resting membrane potential, where the inside of the neuron is more negatively charged relative to the outside. This difference is due to:
- Higher concentrations of potassium ions (K++) inside the cell.
- Higher concentrations of sodium ions (Na++) and chloride ions (Cl–) outside the cell.
- Negatively charged proteins inside the neuron.

Despite this local charge difference (which is key for electrical signaling), the bulk solutions inside and outside the cell remain electrically neutral. This means that, although there is a charge separation across the membrane, the overall number of positive and negative ions in the intracellular and extracellular fluids is balanced.

Question 4

Local Charge Imbalance Across the Membrane:

Answer 4

Inside of the neuron is more negative relative to the outside due to the distribution of ions, which creates the membrane potential (typically around -70 mV at rest)

This difference in charge is caused by a small layer of ions just inside and just outside the cell membrane, creating a localized electrical field.

Question 5

Bulk Solutions Remain Neutral:

Answer 5

The bulk of the cytoplasm inside the neuron and the extracellular fluid outside the neuron are both electrically. This means that the total number of positive and negative charges within the intracellular space and the extracellular space are equal

The negative charge inside the neuron (relative to outside) is due to unequal distribution of ions right near the membrane, but in the overall, larger volume of the cell and surrounding space, the numbers of positive and negative charges balance out.

Question 6

Electroneutrality (def)

Answer 6

Electroneutrality applies to the entire bulk fluid both inside and outside the neuron, where the number of cations (positive ions) equals the number of anions (negative ions) in each compartment.

Question 7

Example of electroneutrality

Answer 7

Example:
+ Inside the neuron, potassium ions (K+ ++) are the predominant cations, and negatively charged proteins and other anions balance this out.
+ Outside the neuron, sodium ions (Na+ ++) and chloride ions (Cl− −) are the main ions, also in roughly equal amounts.
+ Across the thin membrane, there is a tiny imbalance that creates the electrical potential difference (more Na+ ++ outside, more K+ ++ inside), but the bulk of the solutions remain neutral.

Question 8

Phospholipid bilayer (membrane capacitance)

Answer 8

Phospholipid bilayer: This structure separates the intracellular and extracellular fluids, which contain ions (charged particles). The hydrophobic interior of the bilayer acts as the insulating layer, while the charged heads on both sides interact with the surrounding fluids. In this analogy, the lipid bilayer acts as the dielectric in a capacitor.

Question 9

Intracellular and extracellular fluids (mem capacitance)

Answer 9

Intracellular and extracellular fluids: These serve as the conductive media, equivalent to the two plates of a capacitor. Each side of the membrane can hold opposite charges, much like the metal plates in a capacitor hold positive and negative charges.

Question 10

Membrane potential (mem capacitance)

Answer 10

Membrane potential: The voltage across the membrane (difference in charge between the inside and outside of the cell) corresponds to the electric potential between the plates of a capacitor. The membrane potential is created by the separation of charges (primarily due to ion concentration differences) across the membrane.

Question 11

Charge separation (mem. capacitance)

Answer 11

Charge separation: When ions (such as Na⁺, K⁺, and Cl⁻) accumulate on opposite sides of the membrane, the bilayer acts like a capacitor, storing charge. The total charge stored depends on the capacitance of the membrane.

Question 12

Capacitance of the membrane (mem. capacitance)

Answer 12

Capacitance of the membrane: This is determined by the area of the membrane, its thickness, and the properties of the lipid bilayer (specifically, its dielectric constant). The capacitance of a typical cell membrane is about 1 µF/cm².

Question 13

Summary membrane capacitance (phospho bilayer)

Answer 13

In summary, this analogy shows that the cell membrane behaves like a capacitor by separating charges across its surface and storing electrical potential. This stored charge is crucial for processes like action potential propagation in neurons and other electrically excitable cells.

Question 14

Passive membrane properties (general charged ions)

Answer 14

the movement of ions and molecules across them without the expenditure of energy. -maintaining cellular homeostasis, establishing resting membrane potentials, and facilitating signal transduction

Question 15

Passive membrane properties (charged ions)

Answer 15

Selective Permeability: Cell membranes allow certain substances to pass while restricting others, primarily due to lipid bilayer structure and specific proteins.

Ion Channels:
1. Leaky Channels: Always open, allowing passive movement of ions (e.g., K+^++ flows out).
2. Gated Channels: Open in response to stimuli, facilitating passive transport when open.

Resting Membrane Potential (RMP): Typically, around -70 mV in neurons, arising from uneven ion distribution (higher K+^++ inside, higher Na+^++ outside).
Maintained by passive K+^++ movement out of the cell.

Concentration Gradients:
Driven by the movement from high to low concentration until equilibrium is reached.

Diffusion:
1. Simple Diffusion: Small, nonpolar molecules cross the lipid bilayer.
2. Facilitated Diffusion: Larger or polar molecules pass through protein channels.

Electrochemical Gradients:
Movement of ions is influenced by both concentration and electrical gradients, creating an electrochemical gradient.

Capacitance: The cell membrane acts as a capacitor, storing charge and enabling action potentials in excitable cells.

Question 16

Intracellular and extracellular mediums (passive membrane properties)

Answer 16

Intra and extracellular mediums are composed by several charged ions:

Cations. Positive charged:
K + (Potassium)
Na+ (Sodium)
Ca 2+ (Calcium)

Anions. Negative charged:
Cl- (chloride)
Other protein molecules

Question 17

Neuronal potentials
Membrane potential

Answer 17

Membrane potential: this is a general term that describes the voltage across the membrane at any point in time (-90 to +60 mV).

Question 18

Neuronal potentials
Resting Potential

Answer 18

Resting potential: Membrane potential at resting state, meaning that the neuron it is not sending or receiving signals (-60 to -70 mV).

Question 19

Ion channels (passive membrane properties)

Answer 19

Ion channels
Ion channels: allow ions to cross the cellular membrane.
Leaky Ion channels: Ion channels that have a permeability and constant resistance.

Question 20

Membrane Potential (General Term)

Answer 20

The membrane potential refers to the voltage difference across the cell membrane at any given time. It is the electrical potential inside the cell relative to the outside.

The membrane potential is dynamic and can change in response to various stimuli, such as the opening of ion channels, action potentials, or other changes in ionic concentration.

Question 21

Membrane Potential (General Term)
Typical values

Answer 21

Typical Values: The membrane potential can fluctuate, especially in excitable cells like neurons and muscle cells, where it can go from highly negative (hyperpolarized) to positive values (depolarized) during activities like action potentials.

Question 22

Factors Influencing Membrane Potential:

Answer 22

Factors Influencing Membrane Potential:

Ion Channels: The opening and closing of ion channels (e.g., Na⁺, K⁺, Cl⁻, Ca²⁺) allow ions to flow in and out, altering the membrane potential.

Ionic Gradients: The difference in concentrations of ions inside vs. outside the cell (Na⁺ is higher outside, K⁺ is higher inside, etc.) influences the membrane potential.

External Stimuli: Membrane potential is affected by stimuli like synaptic inputs (in neurons) or mechanical changes (in sensory cells).

Question 23

Resting Potential (Specific Condition)

Answer 23

The resting potential is the specific value of the membrane potential when a cell is at rest — meaning it is not actively sending signals or undergoing action potentials.

The resting potential is a stable, baseline voltage that the cell maintains in the absence of external stimuli or activity.

The resting potential is usually a negative value, typically around -60 mV to -70 mV in neurons, though it can vary between different cell types.

Question 24

How Resting Potential is Maintained:

Answer 24

Selective Permeability: The membrane is more permeable to K⁺ ions at rest, which tends to move out of the cell, contributing to the negative resting potential.

Na⁺/K⁺ Pump: The sodium-potassium pump (Na⁺/K⁺-ATPase) actively pumps 3 Na⁺ ions out and 2 K⁺ ions in, helping to maintain the concentration gradients and negative resting potential.

Leak Channels: K⁺ leak channels are open at rest, allowing potassium ions to flow out of the cell more easily than other ions, contributing to the resting potential.

Question 25

Leaky Ion Channels

Answer 25

Passive ion channels are integral membrane proteins that allow specific ions to flow across the cell membrane continuously, without the need for an external signal or energy input. They are crucial for maintaining the resting membrane potential and overall homeostasis of cells, especially in excitable tissues like neurons and muscle cells.

Always Open: generally open at all times, allowing ions to pass freely based on their concentration gradients.

Selective Permeability: selective for specific ions. For example:
+ Potassium (K+^++) channels primarily allow potassium ions to move across the membrane.
+ Sodium (Na+^++) channels allow sodium ions to enter the cell.

Question 26

Concentration Gradient

Answer 26

Ions move through leaky channels down their concentration gradients (from areas of high concentration to areas of low concentration). For example, K+ ++ ions tend to move out of the cell because they are more concentrated inside, while Na+ ++ ions can enter the cell because they are more concentrated outside.
Leaky ion channels play a crucial role in establishing and maintaining the resting membrane potential of cells. In neurons, the permeability of the membrane to K+++ ions, through leaky potassium channels, is significantly higher than that for Na+++ ions. As K+++ ions leave the cell, it contributes to a negative internal charge, creating the resting membrane potential (typically around -70 mV).

Question 27

Concentration Gradient
Role in Action Potentials

Answer 27

Role in Action Potentials: During an action potential, the transient opening of voltage-gated ion channels alters the membrane potential. However, the presence of leaky ion channels helps to return the membrane to its resting state after depolarization by allowing K+^++ ions to flow out of the cell.

Question 28

Examples of Leaky Ion Channels:

Answer 28

Potassium (K+^++) Channels: These are the most common leaky channels in neurons. They allow K+^++ ions to move out of the cell, which is essential for maintaining a negative resting membrane potential.

Sodium (Na+^++) Channels: There are fewer leaky sodium channels compared to potassium channels, but they still allow some Na+^++ ions to enter the cell, contributing to the overall membrane potential.

Chloride (Cl−^-−) Channels: Some leaky channels allow chloride ions to move across the membrane, which can also influence the membrane potential, especially in certain types of neurons.

Question 29

Leakage ion channels

Answer 29

K + (Potassium) channels: many channels.
Na+ (Sodium) channels: few channels.

Potassium permeability is much higher than the other channels, so potassium dominates the resting potential.

Question 30

Ion channel with membrane model

Answer 30

Conductance is a measure of how easily ions or electrical current can flow through a material or structure.
In the context of ion channels and biological membranes, conductance refers to the ease with which ions (like Na⁺, K⁺, Ca²⁺, or Cl⁻) move through the ion channels embedded in the cell membrane.
It is the reciprocal of resistance, which means that high conductance corresponds to low resistance and vice versa. Conductance is typically measured in siemens (S).

Mathematical Definition of Conductance: G= 1/R → G is the conductance (in siemens), R (in ohms).

Question 31

Maintain Resting Membrane Potential: (ion channels leaky)

Answer 31

They play a critical role in establishing and maintaining the resting membrane potential of cells, especially neurons. The constant movement of ions through these channels helps stabilize the charge difference between the inside and outside of the cell.

Question 32

Potassium Leaky Channels

Answer 32

are the most common type of leaky channels in neurons. They allow potassium ions to move out of the cell, which is crucial for maintaining the negative charge inside the neuron.

Since potassium is at a higher concentration inside the cell compared to outside, K+ ions naturally move out through these leaky channels, following their concentration gradient. This outflow of positive ions contributes significantly to the negative resting membrane potential of about -70 mV in neurons.

Question 33

Role of Leaky Ion Channels in Neurons:

Answer 33

Resting Membrane Potential: Leaky channels, particularly for potassium, are primarily responsible for establishing the resting membrane potential. While potassium moves out of the cell, there are fewer leaky sodium (Na+^++) channels, which leads to a greater exit of positive charges than entry, maintaining a negative intracellular charge.

Ion Balance: Even though leaky channels allow ions to flow, they don’t disrupt the long-term ionic balance. Na+ /K+ ATPase pump works continuously to restore the concentration gradients by pumping sodium out of the cell and potassium back into the cell.

Electrical Excitability: While leaky channels contribute to the resting membrane potential, they also help set the conditions necessary for action potentials. Without a stable resting membrane potential created by these channels, neurons couldn’t respond to stimuli by firing electrical signals.

Question 34

Diffusion and Electrostatics

Answer 34

Voltage Source: On the left side, the symbol a time-varying voltage source. The “+” and “−” signs indicate the polarity of this source.

The capacitor is connected in series with the voltage source. It stores electrical energy and creates a current flowing through it. Capacitors resist changes in voltage and allow AC (alternating current) to pass through more easily than DC (direct current).

Resistor: is connected in series with the capacitor. It creates a current flowing through it. Resistors oppose the flow of electric current, causing a voltage drop proportional to the current (Ohm’s Law)

Current: The overall current flowing through the circuit is labeled i(t)i(t)i(t), which splits into two branches as it moves through the capacitor and resistor.

Voltage Across the Resistor (VNV_NVN): The voltage drop across the resistor RmR_mRm is labelled VNV_NVN, with the positive end on the bottom and the negative end on the top. This indicates that the current is flowing downwards through RmR_mRm, from a higher to a lower potential.

Currents in Components:
1. ic) represents the current flowing through the capacitor.
2. iR(t)i_R(t)iR(t) represents the current flowing through the resistor.

Question 35

Cell Membrane as an RC Circuit:

Answer 35

Voltage Source: Represents the membrane potential (voltage across the cell membrane).

Capacitor: Represents the cell membrane (lipid bilayer). Stores charge, like the difference in charge inside and outside the cell.

Resistor: Represents ion channels in the membrane. Allows ions to flow in and out, controlling the current.

Current: Represents the total flow of ions through the membrane.
How it works: Capacitor stores charge when ions move across the membrane. Resistor controls ion flow, similar to ion channels. The model shows how the cell membrane charges and discharges, like when a neuron fires.

Question 36

Kirchhoff’s Second Law (Voltage Law):

Answer 36

The sum of all voltages around a closed loop or mesh is equal to zero.
This principle is based on the conservation of energy. In a closed loop, the total energy gained by charges (due to sources like batteries) must be equal to the total energy lost (due to resistive elements or drops in potential).
Mathematically, for a given loop: V=0

Question 37

Mesh Analysis (Mesh Current Method)

Answer 37

The mesh current method calculates the current in each loop (or mesh) of a circuit. In this approach, Kirchhoff’s Voltage Law (KVL) is applied to each loop, forming a system of equations that can be solved to find the unknown currents.
Steps to use mesh analysis:
1. Assign mesh currents to each independent loop.
2. Apply Kirchhoff’s Voltage Law (KVL) to each loop, summing the voltage drops and rises.
3. Solve the resulting system of equations for the unknown mesh currents.
Example:
Consider a simple resistive circuit with two loops and a shared resistor. Kirchhoff’s Voltage Law is applied to each loop, resulting in two equations:

Question 38

Applying to Biological Membranes (Electrical circuit analysis)

Answer 38

In the context of biophysics or cell membrane models, the analogy can be extended to represent the flow of ionic currents through ion channels as the equivalent of electrical circuits. Here, membrane potential would correspond to the voltage, and ion channels would act as resistive elements, with the ionic currents being analogous to mesh currents in the circuit.
By applying Kirchhoff’s Voltage Law to membrane potential models, you can analyze how different ionic currents contribute to the overall voltage across the membrane.

Question 39

Movement of particles
Brownian motion

Answer 39

The random movement of the particles or molecules in a medium governed by temperature (thermal agitation).

Question 40

Ion movement through membrane channels is regulated by:

Answer 40

diffusion flux, electrostatic flux and active pumps

Question 41

Electro diffusion flux

Answer 41

flux refers to the movement of charged particles (ions) in response to an electric field, combined with the effects of concentration gradients. It’s a key concept in electrochemistry and is often described by the Nernst-Planck equation, which accounts for both diffusion (movement due to concentration gradients) and electromigration (movement due to an electric field).

Question 42

Key Components of Electro diffusion Flux:

Answer 42

Diffusion: Movement of particles from regions of higher concentration to regions of lower concentration. This is described by Fick’s laws of diffusion.
Electromigration: Movement of charged particles due to the influence of an electric field. Charged species will migrate towards the electrode of opposite charge.
Concentration Gradient: The change in concentration of ions in a solution. A steep gradient can lead to a significant flux of ions.
Electric Field: The force exerted on charged particles due to an external electric field, influencing their movement.

Question 43

The process of diffusion causes particles or molecules to move from …

Answer 43

from regions of high concentration to regions with low concentration.

Question 44

The electric gradient causes particles or molecules to move from …

Answer 44

regions of high charge to regions whith low charge.

Question 45

The equilibrium potential is reached when the diffusion flux …

Answer 45

equals the electrostatic flux. This means there is no net flow in either direction.