Systems Biology
An action potential is an electric signal used by neurons to transmit messages between the brain and other cells. It goes through set phases, controlled by the opening and closing of two kinds of voltage-gated channels: sodium and potassium channels. Voltage-gated sodium channels have activation and inactivation gates, while voltage-gated potassium channels have a single activation gate. These channels drive the different phases of an action potential, such as the resting phase, depolarization, and repolarization. At rest, the membrane potential is about -70 millivolts, with higher concentration of sodium ions outside the neuron and higher concentration of potassium ions inside.
A stimulus that raises the membrane potential to a threshold value of -55 millivolts initiates an action potential. Sodium activation gates open, allowing sodium ions into the cell and causing depolarization. As the membrane potential nears +35 millivolts, sodium inactivation gates close, and potassium channels slowly open, leading to repolarization. Hyperpolarization occurs when potassium ions continue to flow out of the cell, temporarily making the membrane more negative. To restore the resting membrane potential, the sodium-potassium ATPase pump exchanges three sodium ions out of the cell for every two potassium ions it brings in. Myelination, fiber diameter, and nodes of Ranvier impact the speed and strength of signal conductance.
Lesson Outline
<ul> <li>Introducing voltage-gated channels: sodium and potassium channels</li> <li>The beginning of action potentials at axon hillock</li> <li>Resting phase</li> <ul> <li>The state of the cell membrane at rest (-70 mV)</li> <li>Sodium and potassium activation gates are closed</li> </ul> <li>Sequence of events in an action potential</li> <ul> <li>The threshold value to generate an action potential (-55 mV)</li> <li>Sodium activation gate opens, sodium rushes into the neuron and depolarization phase starts</li> <li>Peak of membrane potential (+35 mV) and sodium inactivation gate closes</li> <li>Opening of potassium channels and start of repolarization</li> <li>Hyperpolarization occurs (membrane potential less than -70 mV), due to the slow closure of the potassium activation gates</li> <li>Refractory periods: Absolute and relative</li> <li>Regaining of resting membrane potential by sodium-potassium ATPase pump action</li> </ul> <li>Action potential propagate in one direction down an axon</li> <li>Roles of myelination (insulation around membrane), nodes of Ranvier (breaks in myelination where action potentials occur), and fiber diameter (wider axon fibers transmit signals faster than thinner fibers)</li> <li>Effects of excitatory stimuli (depolarize, push towards action potential) and inhibitory stimuli (hyperpolarize, push away from action potential)</li> </ul>
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FAQs
Sodium and potassium channels play crucial roles in generating action potentials. During the resting phase, both channels are closed. When the neuron receives a stimulus, sodium channels open, allowing sodium ions to enter the cell. This influx of sodium ions causes depolarization, raising the membrane potential above the threshold. To restore the resting membrane potential, potassium channels open, and potassium ions exit the cell, initiating repolarization. The brief hyperpolarization period prevents continuous firing of the neuron, ensuring signal fidelity.
The resting membrane potential in a neuron is maintained by the unequal distribution of ions, primarily potassium (K+) and sodium (Na+) ions, across the cell membrane. This distribution is achieved through the action of ion pumps and channels, specifically the sodium-potassium pump (Na+/K+ ATPase). This pump actively transports three Na+ ions out of the neuron and two K+ ions in, creating a charge difference that stabilizes the resting membrane potential at around -70 mV.
An action potential consists of several phases: resting phase, depolarization, repolarization, and hyperpolarization. In the resting phase, the neuron's resting membrane potential is maintained. Upon receiving a stimulus, voltage-gated sodium channels open, leading to depolarization, as the influx of Na+ ions raises the membrane potential. At around +35 mV, voltage-gated sodium channels close, and voltage-gated potassium channels open, causing repolarization, as K+ ions leave the cell. The potassium channels close slightly later, resulting in a brief hyperpolarization phase before returning to the resting membrane potential.
The nodes of Ranvier, found in myelinated neurons, are gaps in the myelin sheath where ion channels are concentrated. These nodes play a crucial role in the rapid conduction of action potentials by facilitating saltatory conduction. The action potential "jumps" from one node to the next, maintaining its strength and speed along the axon, rather than propagating continuously. This method of conduction is more energy-efficient and allows for faster transmission of signals compared to continuous propagation.
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