40 Questions
What is the primary reason for the unequal distribution of ions in the ECF and cytosol?
The plasma membrane has more K leak channels than Na leak channels.
What would be the effect on the resting membrane potential if the plasma membrane had more Na leak channels than K leak channels?
The inside of the membrane would become more positive.
Why do anions not follow potassium ions out of the cell?
Anions are attached to non-diffusible molecules such as ATP and large proteins.
What is the function of the Na/K ATPases in maintaining the resting membrane potential?
To pump out sodium ions as fast as they leak in and bring in potassium ions.
Why are the Na/K ATPases considered electrogenic?
Because they pump out more positive charges than they bring into the cell.
What is the net effect of the Na/K ATPases on the resting membrane potential?
It becomes more negative.
What is the ratio of sodium ions pumped out of the cell to potassium ions pumped into the cell by the Na/K ATPases?
3:2
What is the main factor that contributes to the inside-negative resting membrane potential of a cell?
The unequal distribution of ions in the ECF and cytosol.
What is the primary cause of the resting membrane potential?
Unequal distribution of ions in the ECF and cytosol
What is the typical value of the resting membrane potential in neurons?
70 mV
What is the purpose of the reference electrode in measuring the resting membrane potential?
To provide a point of reference for the recording microelectrode
What is the significance of the minus sign in the resting membrane potential?
It indicates that the inside of the cell is negative relative to the outside
What is the main cation in the cytosol?
Potassium (K)
What is the instrument used to detect the electrical difference across the plasma membrane?
Voltmeter
What is the term used to describe a cell that shows a membrane potential?
Polarized
What is the dominant anion in the cytosol?
All of the above
What is the main reason for the stimulus artefact in the action potential curve?
Leakage of current from the stimulating electrode to the recording electrode
What is the duration of the latent period in the action potential curve?
0.5-1 ms
What is the value of the resting membrane potential in the given action potential curve?
-70 mV
What is the charge at the anode in the given action potential curve?
-ve charge
What is the direction of ion movement during depolarization in the given action potential curve?
Na+ entry and K+ exit
What is the phase of the action potential curve that occurs between points c and d?
Repolarization
What is the value of the reversal of polarity (over shoot) in the given action potential curve?
+35 mV
What is the phase of the action potential curve that occurs between points e and f?
After hyperpolarization
What is the potential at which depolarization occurs very rapidly?
-55 mV
What is the duration of the spike potential in an axon?
1 ms
What is the term for the phase of rapid rise of potential in depolarization and a rapid fall in repolarization phase?
Spike potential
What is the primary reason why Na+ cannot enter the cell during the resting membrane potential?
The membrane is impermeable to Na+
What is the term for the phase of slow repolarization that follows the rapid fall in spike potential?
Phase of negative after potential
What is the resting membrane potential due to?
Distribution of more cations outside the cell membrane and more anions inside the cell membrane
What is the term for the phase of rapid fall in potential after the spike potential?
Repolarization phase
What is the reason for the rapid increase in permeability for Na+ ions during the depolarization phase?
Opening of voltage-gated Na+ channels
What is the primary mechanism by which the membrane potential changes rapidly during an action potential?
Rapid influx of Na+ ions into the cell
What is the role of calcium ions in the development of action potential?
Calcium ions have a dual role, where low extracellular Ca2+ concentration increases excitability and high concentration stabilizes the membrane
During the rapid repolarization phase, which of the following occurs?
K+ channels open up and Na+ channels remain closed
What is the role of the Na+-K+ ATPase pump in the restoration of the resting membrane potential?
The pump results in the efflux of three Na+ and influx of two K+ ions
What is the effect of low extracellular Ca2+ concentration on the excitability of nerve and muscle?
It increases the excitability of nerve and muscle
During the depolarization phase, which of the following occurs?
Na+ channels open up and K+ channels remain closed
What is the role of the slow calcium channels in the development of action potential?
They open up and admit Ca2+ ions during the development of action potential
What is the Hodgkin cycle?
A positive feedback spiral that results in rapid change in membrane potential
Study Notes
Resting Membrane Potential (RMP)
- RMP is an electrical potential difference (voltage) across the membrane in excitable cells
- Occurs due to the buildup of negative ions in the cytosol and an equal buildup of positive ions in the extracellular fluid (ECF) along the outside surface of the membrane
- Separation of positive and negative electrical charges creates potential energy, measured in volts or millivolts (1 mV = 0.001 V)
- Greater difference in charge across the membrane results in a larger membrane potential (voltage)
Measurement of RMP
- Measured using a recording microelectrode inserted inside the cell and a reference electrode placed outside the cell in the ECF
- Both electrodes are connected to a voltmeter, which detects the electrical difference (voltage) across the plasma membrane
RMP in Neurons
- Ranges from 40 to 90 mV, with a typical value of 70 mV
- Minus sign indicates that the inside of the cell is negative relative to the outside
- A cell with a membrane potential is said to be polarized
Factors Contributing to RMP
- Unequal distribution of ions in the ECF and cytosol (major factor)
- ECF is rich in Na and Cl- ions, while cytosol is rich in K ions and anions such as phosphates and amino acids
- Plasma membrane has more K leak channels than Na leak channels, resulting in a greater number of potassium ions diffusing out of the cell and more positive ions entering the cell
- Inability of most anions to leave the cell
- Most anions inside the cell are attached to non-diffusible molecules, preventing them from leaving the cell
- Electrogenic nature of Na/K ATPases
- These pumps help maintain the RMP by pumping out Na and bringing in K, contributing to the negativity of the RMP
Action Potential
- Recording of action potential of a large mammalian myelinated nerve fiber has several components:
- Stimulus artifact
- Latent period
- Firing level
- Overshoot
- Spike potential
- Repolarization
- After depolarization
- After hyperpolarization
Role of Voltage-Gated Na+ and K+ Channels
- Development of action potential was studied by Hodgkin and Huxley using the voltage clamp technique
- According to the Hodgkin-Huxley theory, the sequence of events is:
- Polarization phase
- Depolarization phase
- Repolarization phase
- After depolarization and hyperpolarization phases
- Ionic basis of action potential:
- Depolarization phase: increase in permeability of the membrane for Na+ ions, leading to rapid influx of Na+ ions into the cell
- Repolarization phase: rapid efflux of K+ ions from the cell
Role of Calcium Ions
- Ca2+ ions also play a role in the development of action potential
- Concentration of Ca2+ in ICF is very low compared to ECF
- When Na+ channels are open, some Ca2+ ions move inside the cell through these channels
- Separate class of slow calcium channels also exists in the cell membrane, which open up during the development of action potential
Important Note
- Low extracellular Ca2+ concentration increases the excitability of nerve and muscle by decreasing the amount of depolarization necessary for initiating changes in Na+ and K+ conductance required for action potential
- Increase in Ca2+ concentration stabilizes the membrane by decreasing excitability
Learn about the electrical potential difference across the membrane in excitable cells, caused by the buildup of ions on either side of the membrane.
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