Flashcards in this deck (137)

• An action potential is a transient change in the membrane potential of a cell.
  physiology action_potential
• The nodes of Ranvier are gaps in the myelin sheath that facilitate saltatory conduction.
  physiology neurons myelination
• The refractory period is the time during which a neuron is unable to fire another action potential.
  physiology neurons
• The basic properties of voltage-gated Na+ and K+ channels underlie the action potential in nerve and skeletal muscle cells.
  physiology channels action_potential
• The activation of both voltage-gated Na+ and K+ channels determines the shape, amplitude, and duration of the action potentials.
  physiology action_potential channels
• The absolute refractory period is a phase where no amount of stimulation can elicit an action potential.
  physiology refractory_period
• Threshold potential is the level of depolarization needed to trigger an action potential.
  physiology action_potential threshold
• Saltatory conduction occurs in myelinated axons, allowing for faster action potential propagation.
  physiology saltatory_conduction myelination
• Electrotonic potentials are local changes in membrane potential that can decrement in amplitude as they spread from their origin.
  physiology potentials
• The length constant describes how far an action potential can propagate along a nerve fiber.
  physiology length_constant action_potential
• The hyperpolarizing after-potential in nerve cells and the depolarizing after-potential in skeletal muscle cells are important for their function.
  physiology after_potential
• Action potentials convey information in the nervous system and initiate contraction in muscle.
  physiology action_potentials nervous_system
• The characteristics of action potentials vary from tissue to tissue.
  physiology action_potentials tissues
• Action potentials are initiated by membrane depolarization which opens voltage-dependent ionic channels.
  physiology action_potentials depolarization
• In nerve and skeletal muscle cells, the rapid upstroke of the action potential is produced by the opening of voltage-gated Na+ channels.
  physiology action_potentials sodium_channels
• For SA node, AV node, and most smooth muscle cells, the upstroke of the action potential is produced by the opening of voltage-gated Ca2+ channels.
  physiology action_potentials calcium_channels
• The falling phase of action potentials in nerve and skeletal muscle is caused by the inactivation of voltage-gated Na+ channels and the opening of voltage-gated K+ channels.
  physiology action_potentials potassium_channels
• Na+ channel inactivation involves a different gate than does activation of the channel.
  physiology action_potentials sodium_channels
• Action potentials are often referred to as all-or-none, meaning they are stereotypical in size and duration.
  physiology action_potentials all_or_none
• Action potentials are normally non-decremental and propagate in one direction toward the nerve terminals.
  physiology action_potentials propagation
• The action potential in the heart is initiated in the SA node and spreads throughout the heart, but the action potential in the atria is of different size, shape, and duration than that of the SA node.
  physiology action_potential heart
• Each region of the heart, including the AV node, Purkinje fibers, and ventricles, has different underlying ionic channels and morphology.
  physiology action_potential heart
• During and immediately following an action potential, excitable cells are refractory, meaning they cannot produce a second action potential for some time.
  physiology action_potential refractory
• The major reason for the unidirectional property of action potential propagation is that a region that has just undergone an action potential is refractory and cannot be re-excited.
  physiology action_potential unidirectional
• The threshold potential is the potential at which an action potential will be initiated; the membrane must be depolarized to reach this threshold.
  physiology action_potential threshold
• An action potential will propagate away from its site of origin due to local current flow when one region of an excitable cell becomes positive relative to neighboring regions.
  physiology action_potential propagation
• The overshoot and hyperpolarizing afterpotential are characteristic of most neuronal action potentials, while skeletal muscle action potentials show a depolarizing afterpotential.
  physiology action_potential neurons
• Positive charge moving into a cell or negative charge moving out causes a negative or inward current.
  neurobiology current axon
• The local current causes neighboring regions to depolarize, activating Na+ channels, leading to an action potential.
  neurobiology action_potential depolarization
• Regions that have undergone an action potential are temporarily refractory, preventing another action potential.
  neurobiology refractory action_potential
• Factors influencing the speed of action potential propagation include axon diameter, resting membrane resistance, and the number of Na+ channels.
  neurobiology action_potential propagation
• Voltage-gated ionic channels include Na+, K+, and Ca2+ channels, crucial for understanding action potentials.
  neurobiology voltage_gated_channels action_potential
• There are at least 8 different types of voltage-gated Na+ channels, each encoded by different genes.
  neurobiology na_channels genes
• Voltage-gated K+ channels vary in type but share certain features described in this lesson, differing from tissue to tissue.
  neurobiology k_channels tissue
• The voltage-gated sodium channel has two gates: an activation gate and an inactivation gate.
  neurobiology sodium_channel gates
• The voltage-gated Na+ channel has two gates: the activation gate and the inactivation gate. Both must be open for Na+ ions to cross the membrane.
  biology channels sodium
• At resting potential (about -90 mV for skeletal muscle cells), the activation gate is closed and the inactivation gate is open.
  biology membrane potential
• When the membrane is depolarized, the activation gate quickly opens and the inactivation gate closes after a few msec.
  biology depolarization gates
• The probability of a gate moving depends on membrane potential. The fraction of open gates increases as voltage increases.
  biology probability membrane
• The inactivation gate takes more time to move than the activation gate, which is critical to remember.
  biology timing gates
• In a single channel, gates are either open or closed, but the average behavior of many channels is important for cellular function.
  biology channels function
• A skeletal muscle fiber can have roughly 2-3 billion voltage-gated Na+ channels, while a ventricular myocyte may have as many as 1 million.
  biology channels muscle
• The average behavior of channels can be reproduced by repeated trials of membrane depolarization, showing stochastic behavior.
  biology depolarization trials
• In trials of membrane depolarization, the activation gate opens in 5-10% of trials, while the inactivation gate closes in 95% or more.
  biology trials gates
• The movement of Na+ channels is a stochastic process, differing from trial to trial, but a well-defined average behavior emerges with enough trials.
  biology channels na+
• Resting potentials for skeletal muscle and cardiac muscle Na+ channels are typically around –85 to –90 mV, while nerve cells have resting potentials of about –70 mV.
  biology potentials na+
• Na+ channels in different tissues are usually encoded by different genes and have somewhat different properties.
  biology genes na+
• There is only one way to open a Na+ channel: both the activation gate and inactivation gate must be open simultaneously.
  biology channels na+
• The probability that a Na+ channel will be open in steady state is very small, with a maximum of only roughly 0.003 near –60 mV.
  biology probability na+
• The activation gate of the Na+ channel opens more rapidly than the inactivation gate closes, allowing for a transient current when the membrane is abruptly depolarized.
  biology current na+
• The voltage-gated K+ channel is often referred to as the delayed rectifier K+ channel, but its characteristics can differ in different tissues.
  biology channels k+
• The K+ channel has only a single activation gate, which opens when the membrane is depolarized but opens more slowly than the Na+ channel's activation gate.
  biology channels k+
• The K+ channel has only one gate, which is the activation gate.
  biology channels k+
• More depolarization is needed for K+ channel activation compared to Na+ channels.
  biology channels k+
• Voltage-gated K+ channels activate more slowly than voltage-gated Na+ channels, which activate in less than 1 msec.
  biology channels action_potential
• The activation gates of K+ channels close more gradually than those of Na+ channels upon repolarization.
  biology channels k+
• It takes roughly 5 msec for K+ channels to close after an action potential, while Na+ channels close in less than 1 msec.
  biology channels action_potential
• The K+ channel does not have an inactivation gate because it tends to turn itself off due to repolarization toward EK.
  biology channels k+
• Voltage-gated Na+ channels rapidly activate in response to membrane depolarization (in less than 1 msec) and then inactivate more slowly.
  biology channels na+
• Na+ channels have two gates: the activation gate and the inactivation gate.
  biology channels na+
• Open Na+ channels move the membrane potential toward ENa, which is about +60 mV.
  biology channels na+
• Voltage-gated K+ channels open in response to membrane depolarization more gradually than Na+ channels, requiring about 1 msec.
  biology channels k+
• K+ channels help restore the membrane potential back toward the K+ equilibrium potential, usually about –90 to –100 mV.
  biology channels k+
• To elicit an action potential, the membrane must first be depolarized by a sufficient amount, known as the threshold.
  neuroscience action_potential membrane
• As the membrane is depolarized, the first thing that begins to happen is the opening of Na+ channel activation gates, which provides a small inward current.
  neuroscience na+_channels depolarization
• Each Na+ channel that opens helps drive the membrane potential toward ENa, which is about +60 mV.
  neuroscience membrane_potential ena
• At a critical point of membrane potential, enough Na+ channels will have opened so that the process becomes self-sustaining, an example of positive feedback.
  neuroscience positive_feedback self_sustaining
• As Na+ channels inactivate, the drive to bring the membrane potential towards ENa declines, while the activation of K+ channels increases the drive towards EK, about –90 to –100 mV.
  neuroscience k+_channels repolarization
• Following the action potential, nerve cells experience a hyperpolarizing afterpotential, where the membrane potential is more negative than the resting potential due to the slow closing of K+ channels.
  neuroscience afterpotential hyperpolarization
• In skeletal muscle cells, a depolarizing afterpotential occurs, where the membrane potential is less negative than the resting potential, primarily due to the transverse tubular system.
  neuroscience afterpotential skeletal_muscle
• During the falling phase of the action potential, all voltage-gated Na+ channels become inactivated, leading to an absolute refractory period where another action potential cannot be produced.
  neuroscience refractory_period na+_channels
• The absolute refractory period lasts until the membrane potential returns to approximately its resting level, requiring roughly 1 msec for channels to recover from inactivation.
  neuroscience refractory_period membrane_potential
• During the hyperpolarizing afterpotential, the membrane permeability to K+ remains elevated, leading to the relative refractory period.
  neuroscience action_potential refractory_period
• The threshold potential is defined as the membrane potential that is just sufficient to produce an action potential.
  neuroscience threshold action_potential
• Threshold is reached when enough Na+ channels have been opened, allowing membrane depolarization to become self-sustaining.
  neuroscience threshold na+_channels
• Factors influencing threshold potential include the permeability of the membrane to K+ and/or Cl- and the number of voltage-gated Na+ channels.
  neuroscience threshold permeability
• The rate at which the membrane depolarizes can also influence the threshold potential needed to initiate an action potential.
  neuroscience threshold depolarization
• To initiate an action potential, the membrane must be rapidly depolarized over a period of a few msec.
  neuroscience action_potential membrane
• If the membrane depolarizes very gradually, it may never produce an action potential due to the inactivation of Na+ channels.
  neuroscience action_potential na_channels
• About 5% of all Na+ channels will activate if the membrane is abruptly depolarized from –90 mV to –60 mV.
  neuroscience action_potential na_channels
• At –60 mV, it requires about 1-2 msec for an activation gate to open and about 10 msec for an inactivation gate to close.
  neuroscience action_potential timing
• Na+ channels begin to open their activation gates when the membrane potential reaches about –70 mV.
  neuroscience na_channels membrane_potential
• When the membrane potential reaches –69 mV one second later, very few of the channels activated at –70 mV will still be open.
  neuroscience na_channels membrane_potential
• At –60 mV, roughly very few of the channels that activated during gradual depolarization will still be open.
  neuroscience na_channels threshold
• At –60 mV, about 5-10% of Na+ channels are still available to open (not inactivated).
  neuroscience na_channels threshold
• With a slow rate of depolarization, it is never likely that enough channels will open simultaneously to produce an action potential.
  neuroscience action_potential depolarization
• Threshold is not a constant value of membrane potential; gradual depolarization affects the number of Na+ channels available to open.
  neuroscience threshold na_channels
• Changes in membrane potential that do not reach threshold are called subthreshold or local potentials.
  neuroscience subthreshold membrane_potential
• Subthreshold potentials spread decrementally from their site of origin, meaning the potential decreases progressively with distance away from its origin.
  physiology potentials
• The characteristics of voltage-gated Na+ and K+ channels are crucial for understanding the initiation of the action potential in nerve and skeletal muscle.
  physiology action_potential
• When the membrane is rapidly depolarized, Na+ channels will begin to open (activate), leading to further depolarization through positive feedback.
  physiology depolarization
• The inactivation of Na+ channels and the activation of K+ channels terminate the action potential.
  physiology action_potential
• Following the action potential, there is a hyperpolarizing afterpotential caused by the slow closing of voltage-gated K+ channels.
  physiology afterpotential
• Threshold is the membrane potential at which enough Na+ channels have opened to overcome background currents, leading to a self-sustaining process.
  physiology threshold
• Factors influencing the threshold potential include the resting membrane permeability to K+ and Cl–, the number of voltage-gated Na+ channels, and the rate of membrane depolarization.
  physiology threshold
• Action potentials propagate along a cell from the site of origin, with the basic mechanism involving an injection of positive charge due to open voltage-gated Na+ channels.
  physiology propagation
• When an action potential occurs, the interior of the excitable cell becomes positive at the site relative to neighboring regions, leading to current flow and further depolarization.
  physiology current_flow
• The refractory period after an action potential prevents the immediate generation of another action potential, contributing to the unidirectional nature of action potential propagation.
  physiology refractory
• After an action potential, a neuron is in a refractory period, unable to produce another action potential for a brief time. This accounts for the unidirectional nature of action potential propagation.
  neuroscience action_potential refractory
• The speed of action potential propagation varies from about 0.1 m/s to more than 100 m/s depending on several factors.
  neuroscience action_potential speed
• The larger the diameter of a fiber, the more rapid the action potential propagation, all else being equal.
  neuroscience action_potential fiber_diameter
• Higher resting membrane resistance (Rm) leads to faster action potential propagation due to fewer open channels at rest.
  neuroscience action_potential membrane_resistance
• Intracellular longitudinal resistance (Ri) measures how easily the cytoplasm conducts electrical current; lower Ri leads to faster propagation velocity.
  neuroscience action_potential longitudinal_resistance
• The density of voltage-gated Na+ channels affects action potential propagation speed; more channels result in faster conduction.
  neuroscience action_potential sodium_channels
• Membrane capacitance (Cm) affects conduction velocity; lower capacitance results in faster action potential conduction.
  neuroscience action_potential membrane_capacitance
• The length constant quantifies the decrement of subthreshold potentials as they spread from their site of origin.
  neuroscience action_potential length_constant
• Subthreshold potentials decrease in amplitude as they spread away from their site of origin, which is quantified by the length constant.
  neuroscience action_potential subthreshold
• The change in voltage decreases with distance away from the site of the stimulus in axons.
  biology neuroscience voltage
• The length constant is usually denoted by λ, and at a distance x from the site of origin, the potential is given by: Vx = Vmax[exp(−x/λ)].
  biology neuroscience length_constant
• At a distance λ from the site of origin, the membrane potential will have fallen to 37% of its maximum value.
  biology neuroscience membrane_potential
• The formula for the length constant is: λ = (𝑎𝑅𝑚/2𝑅𝑖)^{1/2}.
  biology neuroscience length_constant
• The length constant becomes larger as the radius increases, the resting membrane resistance increases, and the cytoplasmic resistance decreases.
  biology neuroscience length_constant
• In non-myelinated axons, action potentials propagate as membrane voltage spreads and voltage-dependent Na channels open.
  biology neuroscience action_potentials
• Once a Na channel opens, ions enter the cell by facilitated diffusion through the channels down the concentration gradient.
  biology neuroscience na_channels
• Positively charged Na ions nudge adjacent ions down the axon via electrostatic repulsion.
  biology neuroscience ionic_current
• A basic characteristic of action potentials is that they propagate from their point of origin without significant changes in amplitude, shape, or velocity.
  biology neuroscience action_potentials
• The basic mechanism for action potential propagation is that an area of membrane presently undergoing an action potential acts as a stimulus to produce action potentials in neighboring regions.
  biology neuroscience action_potentials
• Action potentials propagate in a unidirectional manner due to the refractory nature of areas that have already undergone an action potential.
  neuroscience action_potential propagation
• Factors affecting the speed of action potential propagation include: fiber diameter, resting membrane resistance, intracellular longitudinal resistance, voltage-gated Na+ channels, voltage-gated Ca2+ channels, membrane capacitance, and myelin presence.
  neuroscience action_potential propagation
• A basic characteristic of action potentials is that they propagate from their point of origin without significant changes in amplitude, shape, or velocity.
  neuroscience action_potential characteristics
• The fastest conducting nerve fibers are myelinated fibers, formed by oligodendrocytes in the CNS and Schwann cells in the PNS.
  neuroscience myelination nerve_fibers
• Regions covered by myelin are called internodes, while the exposed regions are known as nodes of Ranvier.
  neuroscience myelination anatomy
• Only the axonal membrane at the nodes of Ranvier contains high densities of voltage-gated Na+ and K+ channels.
  neuroscience action_potential channels
• Myelin increases axonal membrane resistance and decreases its effective capacitance, allowing charge to travel further.
  neuroscience myelination properties
• Significant changes in membrane permeability and ionic current injection occur only at the nodes of Ranvier, making conduction in myelinated nerves discontinuous or saltatory.
  neuroscience action_potential conduction
• Action potential propagation is much faster in myelinated than in non-myelinated nerves, reaching speeds over 120 meters/second.
  neuroscience action_potential speed
• Electromyography (EMG) is a technique for recording the electrical activity of skeletal muscle.
  neuroscience emg muscle
• EMGs measure potential outside of the muscle fiber, where depolarization causes the inside of the membrane to become more positive and the outside to become more negative.
  neuroscience emg depolarization
• Muscle tissue at rest is normally inactive, resulting in a flat EMG reading as both electrodes see the same charge outside the cell.
  neuroscience emg muscle_activity
• As the muscle contracts, action potentials generate deflections in the EMG signal.
  neuroscience emg muscle_contraction
• An EMG should give a flat reading under certain conditions.
  emg physiology muscle
• As the muscle is contracted, action potentials are generated, causing deflections in the EMG signal.
  emg action_potentials muscle_contraction
• The depolarization occurs on the left side, causing the anode to detect relatively negative charge outside the cell.
  emg depolarization anode
• The cathode becomes positive relative to the anode, resulting in an upward deflection in the EMG trace.
  emg cathode deflection
• As the strength of contraction increases, more fibers are recruited and the signal becomes stronger.
  signal_strength muscle contraction
• EMGs allow measurement of amplitude and number of muscle depolarizations for diagnostic clues.
  emg diagnostics neuropathies
• The study of EMGs will provide insights into neuropathies and myopathies later in the year.
  emg neuropathies myopathies