Question

What is the resting potential, and how is it created and maintained? (page 278 )

Short answer

The resting potential, typically around -70mV, is the electrical potential difference across a neuron's plasma membrane when it is unstimulated and inactive. It is created and maintained through the activity of ion channels and the sodium-potassium pump. Ion channels allow specific ions, like potassium (K+) and sodium (Na+), to pass through, while the sodium-potassium pump actively transports three Na+ ions out of the cell and two K+ ions into the cell, creating an ion imbalance. The resting membrane potential is maintained by a balance between the concentration gradient (created by the different ion concentrations) and the electrostatic gradient (created by the charge difference across the membrane).

Step-by-step solution

Definition of Resting Potential

The resting potential is the electrical potential difference across the neuronal plasma membrane when the neuron is in an unstimulated, inactive state. It is typically around -70 millivolts (mV). The resting potential is crucial for the proper functioning of neurons, as it sets the stage for electrical signals (action potentials) to travel along the axon and communicate with other cells.

Ion Channels and the Sodium-Potassium Pump

The resting potential is created and maintained primarily through the activity of ion channels in the neuronal membrane and the sodium-potassium pump. Ion channels are protein structures embedded in the membrane that allow specific ions, such as sodium (Na+) and potassium (K+), to pass through. At rest, some of these channels are open, leading to the diffusion of potassium ions (K+) out of the cell, while most sodium channels are closed, keeping a high concentration of sodium ions (Na+) outside the cell. The sodium-potassium pump plays a crucial role in maintaining this ion balance. This pump uses energy from ATP to actively transport three sodium ions (Na+) out of the cell and two potassium ions (K+) into the cell, creating an imbalance in the distribution and concentration of ions across the membrane.

Maintaining the Resting Membrane Potential

The resting membrane potential is maintained through a balance of forces - the concentration gradient and the electrostatic gradient. As potassium ions (K+) diffuse out of the cell through open potassium channels, the inside of the cell becomes more negatively charged compared to the outside, creating an electrostatic gradient. There is also a difference in the concentration of ions inside and outside the cell, creating a concentration gradient. Eventually, the electrostatic gradient becomes strong enough to counteract the concentration gradient, preventing more potassium ions (K+) from leaving the cell. The resting potential is maintained by the balance between these opposing gradients and the ongoing activity of the sodium-potassium pump, which continuously works to restore the ion balance across the membrane.

Key concepts

Neuronal Plasma Membrane

The neuronal plasma membrane is a critical structure that envelops the cell, giving it shape and allowing it to communicate with its environment. But more than just a simple barrier, it's a dynamic gateway, selectively controlling the movement of ions and molecules in and out of the neuron.

Composed of a double layer of phospholipids interspersed with proteins, the neuronal plasma membrane maintains a distinct internal environment. This semipermeable membrane is key to establishing the neuron's resting potential. One might compare it to a vigilant guard, ensuring that only the appropriate substances pass through, much like security at a selective venue.

Understanding the plasma membrane's structure and functions is essential for grasping how neurons process information. The intricate dance of ions across this membrane generates electrical signals that are the foundation of nervous system activity. Hence, students should pay close attention to the membrane's role in enabling neurons to act as the body's communication network.

Ion Channels

Ion channels are the portals that stud the neuronal plasma membrane, each serving as a passage for specific ions, like tiny molecular tunnels. Think of them as specialized doors with a 'VIP list', opening only for certain guests—in this case, ions like sodium (Na+) and potassium (K+).

During the neuron's resting state, potassium channels are slightly ajar, granting exit to K+ ions. This creates a negative charge inside the neuron since K+ ions are positively charged. Conversely, sodium channels are typically shut tight, preventing Na+ ions from flooding in and disturbing the delicate balance. This mechanism of open and closed channels is pivotal in establishing and maintaining the neuron's resting potential.

Using analogies or real-life examples, like water flowing through a pipe with valves, can help students visualize the concept of ion flow and its regulation. It's crucial for learners to realize how the selective permeability of these channels is directly linked to neuronal function, affecting everything from thought to muscular movement.

Sodium-Potassium Pump

The sodium-potassium pump is the diligent workhorse of the neuron, tirelessly consuming energy to maintain the ion balance that dictates the resting potential. Picture a dedicated worker who uses their own energy to move objects to their correct place, ensuring the environment remains orderly.

For every three Na+ ions shown the door out of the cell, two K+ ions are welcomed in. This disproportionate exchange is critical—it's what establishes the resting potential's characteristic charge difference across the membrane. The activity of this pump is ceaseless, as it keeps the neuron primed and ready for action.

Students often benefit from learning by doing. It's helpful to compare the sodium-potassium pump to a battery that charges the neuron, hinting at its preparedness for transmitting signals. By consistently returning to this energy-dependent process, students can appreciate the dynamic nature of cell physiology and how it underpins the function of the entire nervous system.