Dominic Stone
Membrane potential, also known as transmembrane potential or membrane voltage, is the difference in electric potential between the inside and outside of a biological cell. This difference in electric potential is what gives rise to the resting membrane potential, which is the state of a cell when it is not stimulated or excited. The resting membrane potential is important for the proper functioning of the nervous and muscular systems, and changes in this potential enable communication with other cells or initiate changes within the cell. In muscle cells, for example, an action potential causes the muscle to contract. The membrane potential is influenced by the concentration of ions inside and outside the cell, with sodium, potassium, calcium, and chloride ions playing a dominant role. These ions move through ion channels in the cell membrane, which can be passive or active. Active channels, for instance, can open or close depending on changes in the membrane potential.
| Characteristics | Values |
|---|---|
| Definition | Membrane potential is the difference in electric potential between the interior and exterior of a biological cell. |
| Formula | Membrane potential equals the interior potential minus the exterior potential. |
| Unit | Typical values of membrane potential are given in units of millivolts (mV). |
| Range | Typical membrane potential values range from -80 mV to -40 mV. |
| Resting Potential | The resting membrane potential is the result of the movement of different ion species through ion channels and transporters in the plasma membrane. |
| Resting Potential Range | The resting membrane potential typically ranges from -10 mV to -100 mV. |
| Resting Potential of Cells | All cells within the body have a characteristic resting membrane potential depending on their cell type. Neurons and the three types of muscle cells (smooth, skeletal, and cardiac) are of primary importance. |
| Resting Potential Value of Cells | The resting potential of a cell is approximately -73 mV. |
| Excitable Cells | Excitable cells include neurons, muscle cells, and some secretory cells in glands. |
| Non-excitable Cells | The term resting potential is also used for the membrane potential of non-excitable cells. |
| Excitability Regulators | The most important regulators of cell excitability are extracellular electrolyte concentrations (e.g., Na+, K+, Ca2+, Cl-) and associated proteins. |
| Crucial Ions | Sodium (Na+) and potassium (K+) ions have a dominant influence on the resting potential. |
| Ions Contributing to Membrane Potential | Sodium, potassium, calcium, and chloride ions contribute the most to the membrane potential. |
| Permeability | Permeability refers to the ability of ions to cross the membrane and is proportional to the number of open channels for a given ion. |
| Action Potential | During an action potential, a redistribution of ions occurs, with a large influx of sodium (+) into the cell, making the membrane potential less negative. |
| Sodium-Potassium Pump | The Na+/K+ pump maintains ionic concentration gradients by exchanging 3 Na+ ions from inside the cell for 2 K+ ions brought into the cell. |
| Repolarization | Repolarization is the restoration of the normal resting membrane potential after depolarization, typically involving the movement of potassium ions through membrane channels. |
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What You'll Learn
Resting membrane potential
The resting membrane potential (RMP) is the electrical potential difference across the plasma membrane of a cell when it is in a non-excited state. It is the result of the movement of various ion species through ion channels and transporters in the plasma membrane, which creates different electrostatic charges across the cell membrane. The resting membrane potential of a cell is typically negative, with the cytosol (intracellular fluid) being more electronegative than the extracellular fluid.
The RMP is important for the proper functioning of the nervous and muscular systems. Neurons and muscle cells are excitable cells, meaning they can transition from a resting state to an excited state. Upon excitation, these cells deviate from their resting membrane potential to undergo a rapid action potential before returning to rest. In neurons, the firing of an action potential allows the cell to communicate with other cells by releasing neurotransmitters. In muscle cells, the generation of an action potential causes the muscle to contract.
The RMP is influenced by the concentration of ions inside and outside the cell, including sodium, potassium, calcium, and chloride ions. The activity of the sodium-potassium pump, which maintains the ionic concentration gradient, also plays a role in determining the RMP. The permeability of the cell membrane to these ions is another factor that affects the RMP. Ion channels, such as passive and active channels, enable the migration of ions across the membrane.
The RMP of a cell can vary depending on its type. For example, in a typical neuron, the RMP is around -70 mV, while in a typical skeletal muscle cell, it is closer to −90 mV. The RMP is crucial for the generation and maintenance of action potentials in excitable cells, and deviations from the RMP can impact their physiological function.
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Action potential
The membrane potential of a cell is the difference in electric potential between the interior and the exterior of a biological cell. When the membrane potential of a cell remains unchanged for a long period, it is referred to as the resting potential. The resting membrane potential is the result of the movement of various ion species through ion channels and transporters in the plasma membrane, resulting in different electrostatic charges across the cell membrane.
Excitable cells, such as neurons and muscle cells, can transition from a resting state to an excited state. In the excited state, the cell undergoes a rapid action potential before returning to the resting state. An action potential is a large, all-or-nothing rise in membrane potential that usually follows a fixed time course.
In neuronal cells, an action potential begins with a rush of sodium ions into the cell through sodium channels, resulting in depolarization. This is followed by an outward rush of potassium through potassium channels, with both fluxes occurring by passive diffusion. In muscle cells, an action potential is initiated when an action potential travels along a motor nerve to its endings on muscle fibres. At the nerve endings, the nerve secretes acetylcholine (ACh), which acts on the muscle fibre membrane to open ACh-gated cation channels. This allows large quantities of sodium ions to enter the muscle fibre, causing local depolarization and the opening of voltage-gated sodium channels, thus initiating an action potential at the membrane.
The action potential causes the sarcoplasmic reticulum to release large quantities of calcium ions, which produce attractive forces between actin and myosin filaments, leading to muscle contraction. The T-tubular system in muscle cells also contributes to the action potential by enabling the conduction of excitation into the depths of the muscle fibre, ensuring that contractile activation is synchronized throughout the muscle fibre.
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Ion channels
There are two types of ion channels: passive channels and active channels. Passive channels are pores within the cell membrane through which molecules pass depending on their concentration gradient. Active channels, on the other hand, open and allow ion transport in response to changes in the membrane potential (potential-gated channels), the binding of specific proteins (ligand-gated channels), or mechanical stimuli.
The opening and closing of ion channels alter ion conductance, thereby determining resting potentials and generating action potentials. For example, in cardiomyocytes, an action potential is initiated when sodium channels open transiently and potassium channels close, leading to depolarization. Subsequently, the sodium channels close, calcium channels open to maintain depolarization, and then calcium channels become inactivated while potassium channels reopen, leading to membrane repolarization.
The most important ion channels for cell excitability are voltage-gated ion channels, which can be sensitive to sodium (Na+) ions, potassium (K+) ions, or calcium (Ca2+) ions. These channels open in response to changes in the membrane potential, allowing a rush of ions into the cell and resulting in depolarization. This process is essential for cell-to-cell communication and the initiation of intracellular processes. Mutations in genes encoding voltage-gated ion channels can lead to various diseases, including myotonia, epilepsy, and cardiac arrhythmias.
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Cell excitability
Muscle cells are excitable cells, meaning they can transition from a resting state to an excited state. This is due to their ability to generate action potentials, which cause the muscle to contract. The resting membrane potential of a cell refers to the electrical potential difference across the plasma membrane when the cell is in a non-excited state.
The cell membrane's permeability to ions, particularly sodium (Na+) and potassium (K+), is crucial in maintaining the resting membrane potential. In a normal cell, the membrane is more permeable to K+ because many K+ channels are open, while Na+ channels are mostly closed. This creates a concentration gradient with more Na+ in the extracellular space and more K+ inside the cell. The Na+/K+ pump plays a vital role in maintaining this gradient by exchanging three Na+ ions from inside the cell for every two K+ ions brought into the cell.
During an action potential, there is a redistribution of ions, with a rapid influx of Na+ ions into the cell, resulting in depolarization. This is followed by an outward rush of K+ ions, leading to repolarization. The movement of these ions creates an electrical current that propagates the action potential along the cell membrane.
The excitability of muscle cells is regulated by various factors, including extracellular electrolyte concentrations of Na+, K+, Ca2+, Cl-, and Mg2+, as well as associated proteins such as voltage-gated ion channels, ion transporters, and membrane receptors. Mutations in voltage-gated ion channels can lead to neuromuscular disorders, while hormones like progesterone and estrogen can modulate muscle cell excitability. Additionally, techniques like DMS and SFEMG have been used to study muscle fiber excitability and conduction along muscle fibers.
Overall, the ability of muscle cells to transition from a resting to an excited state through the generation of action potentials is fundamental to their excitability and proper functioning in the muscular system.
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Cell function
The membrane potential is a fundamental concept in biology, referring to the electrical potential difference across a cell's plasma membrane. This electrical potential is crucial for cell function, allowing cells to act as batteries and power various "molecular devices" embedded in the membrane. The membrane potential also plays a key role in transmitting signals within electrically excitable cells, such as neurons and muscle cells. These excitable cells can transition from a resting state to an excited state, with the resting membrane potential referring to the electrical potential when the cell is in a non-excited state.
The resting membrane potential is established by the movement of ions, such as sodium (Na+), potassium (K+),
The cell membrane's permeability to specific ions, such as sodium and potassium, also affects the membrane potential. Ion channels, which are specialised proteins, facilitate the migration of ions through the membrane. There are two types of ion channels: passive channels, which are pores that allow molecules to pass based on their concentration gradient, and active channels, which open in response to changes in membrane potential, ligand binding, or mechanical stimulation. In excitable cells, the opening and closing of these ion channels lead to local changes in membrane potential, generating signals that can be transmitted within the cell.
While the membrane potential is often associated with excitability, it has broader roles in both excitable and non-excitable tissues. For example, it is involved in critical processes such as cell cycle control, cell-volume regulation, proliferation, and wound healing. Additionally, in smooth muscle cells, the membrane potential plays a crucial role in controlling contraction. The modulation of the membrane potential is being explored as a potential therapeutic target for various diseases, including cancer and wound healing, as well as in the development of successful stem cell therapies.
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Frequently asked questions
Membrane potential, also known as transmembrane potential or membrane voltage, is the difference in electric potential between the inside and outside of a biological cell.
The resting membrane potential is the state of the cell when it is not stimulated or \"excited". The action potential is the rapid change in membrane potential that occurs when the cell is stimulated.
The resting membrane potential is caused by the movement of ions through various channels and transporters in the cell membrane. The most influential ions are sodium (Na+) and potassium (K+).
The generation of an action potential in muscle cells causes the muscle to contract. The membrane potential is also critical for muscle cell proliferation and wound healing.
Graded potentials occur in the membranes of nerve and muscle cells, epithelial cells, fat cells, gland cells, and sensory receptors. They often initiate cell functions, such as triggering the release of secretory vesicles from gland cells.
