Question

Suppose that gated ion channels for \(\mathrm{Na}^{+}\)or \(\mathrm{Ca}^{2+}\) opened in the plasma membrane of a muscle cell. The membrane potential of that cell would a. move toward the equilibrium potential for that ion. b. become less negative than the resting membrane potential. c. move farther away from the potassium equilibrium potential. d. all of these.

Short answer

When the gated ion channels for Na+ or Ca2+ open in the plasma membrane of a muscle cell, the membrane potential will move toward the equilibrium potential for that ion (a), become less negative than the resting membrane potential (b), and move farther away from the potassium equilibrium potential (c). Consequently, the correct answer is (d) all of these.

Step-by-step solution

Review resting membrane potential & equilibrium potentials

To solve this problem, recall that the resting membrane potential is a stable voltage across the cell membrane when a cell is at rest. This potential is typically negative, ranging from -55 mV to −90 mV. Additionally, each ion has an equilibrium potential, which is the membrane potential at which the net flow of that specific ion across the membrane is zero. For example, the equilibrium potential for Na+ is typically between +50 and +60 mV, and the equilibrium potential for Ca2+ is between +100 and +135 mV.

Analyze choice a: movement toward equilibrium potential

When the gated ion channels for Na+ or Ca2+ open, these ions can rush into the cell down their electrochemical gradient. As the concentration of positive ions inside the cell increases, the membrane potential moves toward the equilibrium potential for these ions. Choice a is accurate.

Analyze choice b: becoming less negative

Since the equilibrium potentials for Na+ and Ca2+ are both positive values, as the membrane potential moves towards these values, it becomes less negative. Thus, choice b is also accurate.

Analyze choice c: moving away from the potassium equilibrium potential

When we analyze the effect of ion channels opening for Na+ or Ca2+, this does not relate to the potassium equilibrium potential directly. However, the overall membrane potential moving toward the Na+ or Ca2+ equilibrium potential would cause the membrane potential to move farther away from the potassium equilibrium potential, which is also negative in value (around -90 mV). Therefore, choice c is accurate as well.

Conclude which choices are accurate

All three choices (a, b, and c) are accurate descriptions of what happens when gated ion channels for Na+ or Ca2+ open in the plasma membrane of a muscle cell. Therefore, the correct answer is (d) all of these.

Key concepts

Equilibrium Potential

The concept of equilibrium potential is vital to understand how cells maintain specific voltages across their membranes. When looking at a single ion type, the equilibrium potential is the membrane potential where the net movement of that ion across the cell membrane ceases. In technical terms, it's the electrical potential difference that precisely balances an ion's concentration gradient, preventing further passive movement of the ion into or out of the cell.

To calculate the equilibrium potential for an ion, the Nernst equation is used, which includes the ion's concentrations inside and outside the cell, the temperature, and the ion's charge. For sodium (Na+), the equilibrium potential is usually positive, around +60 mV, meaning that when a sodium channel opens, sodium ions will naturally move into the cell until the inner membrane potential reaches around this value.

Resting Membrane Potential

The resting membrane potential is akin to the baseline electrical charge across a cell's membrane when it is not excited or transmitting impulses. It is mainly established by the relative concentrations of potassium (K+) ions inside and outside of the cell, and the cell membrane's permeability to different ions. Potassium ions have a major influence because the membrane is more permeable to K+ than to other ions like Na+ and Ca2+.

Most cells maintain a resting membrane potential between -55 and -90 millivolts (mV), which sets the stage for communication and excitability in nerve and muscle cells. It's the state of 'readiness' from which all changes in membrane potential start. The inside of the cell is typically more negative compared to the outside due to this resting state.

Gated Ion Channels

Gated ion channels are like specialized doors that control the flow of ions across the cell's membrane. These channels can be gated, or opened and closed, in response to a variety of signals including electrical changes, mechanical stress, or chemical ligands.

These gates are fundamentally important for cell signaling and function as they regulate the movement of ions such as Na+, K+, Ca2+, and Cl-, among others. Voltage-gated channels, for example, open in response to changes in membrane potential, playing a key role in the generation and propagation of electrical signals in nerve and muscle cells. In contrast, ligand-gated channels open in response to the binding of a specific neurotransmitter or other signaling molecule.

Electrochemical Gradient

The electrochemical gradient is the overall driving force that dictates the direction an ion will move across the cell membrane. It encompasses the concentration gradient (difference in ion concentration between the inside and outside of the cell) and the electrical gradient (difference in charge across the membrane).

Ions will generally move from an area of high concentration to an area of low concentration, as well as from areas of like charges to areas with opposite charges. The combined effect of the ion's concentration gradient and the electrical gradient makes up the electrochemical gradient. Depending on the state of ion channels, ions move accordingly to achieve balance or react to cellular signals, directly influencing the cell's membrane potential.