Neurophysiology Chapter: Action Potential

Sodium (Na+) and potassium (K+)

This axon is large enough to allow for insertion of electrodes to measure electrical activity and study ion movement.

The membrane potential at rest is negative (-65mV). Potassium permeability is much higher than sodium permeability, contributing to the negative resting potential.

<p>Sodium permeability increases rapidly during depolarization. The influx of sodium ions makes the membrane potential more positive, leading to the characteristic spike of the action potential.</p> Signup and view all the answers

<p>The membrane potential returns to resting value due to the opening of potassium channels, allowing potassium ions to flow out of the cell, restoring the negative charge.</p> Signup and view all the answers

<p>The simple model fails to explain how sodium channels stay closed long enough for potassium channels to repolarize the membrane. This relates to the concept of inactivation of sodium channels, which prevents them from reopening immediately during repolarization.</p> Signup and view all the answers

<p>Sodium channel inactivation ensures that the membrane can repolarize properly, preventing continuous depolarization and allowing for the next action potential to occur.</p> Signup and view all the answers

<p>Their research revolutionized our understanding of the action potential by demonstrating the crucial role of sodium and potassium ions in generating nerve impulses. This laid the foundation for understanding how neurons communicate in the nervous system.</p> Signup and view all the answers

<p>By using an electrode, we can either impose a current across the membrane and measure the resulting membrane potential (Vm), or we can measure the current (Im) flowing across the membrane while holding the membrane potential at a specific value.</p> Signup and view all the answers

<p>Ohm's law states that Vm = Rm x Im, meaning the membrane potential (Vm) is equal to the product of the membrane resistance (Rm) and the current (Im) flowing across the membrane. This describes the relationship between these three variables and provides a framework for understanding the electrical activity of a neuron.</p> Signup and view all the answers

<p>The two approaches are: 1) Imposing a current (Im) and measuring the resulting membrane potential (Vm) This allows us to determine the membrane resistance (Rm) as well as the relationship between current and voltage. 2) Measuring the current (Im) while controlling the membrane potential (Vm) This allows us to determine the specific ion channels that are active at a given membrane potential.</p> Signup and view all the answers

<p>Separating the measurement of Vm and Im allows us to better understand the relationship between these two variables and how they contribute to the overall electrical activity of a neuron. It allows us to study the properties of specific ion channels and how they contribute to the generation and propagation of action potentials.</p> Signup and view all the answers

<p>Measuring the electrical activity of a neuron allows us to understand how neurons communicate with each other, how they process information, and how they contribute to the function of the nervous system. This knowledge is essential for understanding brain function and for developing treatments for neurological disorders.</p> Signup and view all the answers

<p>Tetrodotoxin</p> Signup and view all the answers

<p>The Na+ current (INa) inactivates.</p> Signup and view all the answers

<p>Patch-clamping</p> Signup and view all the answers

<p>Leak channels are always open, while voltage-gated channels open and close in response to changes in membrane potential.</p> Signup and view all the answers

<p>Closed</p> Signup and view all the answers

<p>All gates must be open.</p> Signup and view all the answers

<p>Activation</p> Signup and view all the answers

<p>Deactivation</p> Signup and view all the answers

<p>Inactivation</p> Signup and view all the answers

<p>Recovery from inactivation</p> Signup and view all the answers

<p>Na+ channel</p> Signup and view all the answers

<p>They are closed, i.e. deactivated.</p> Signup and view all the answers

<p>It prevents the continuous flow of Na+ ions, ensuring that the action potential is brief and does not last indefinitely.</p> Signup and view all the answers

<p>The key difference lies in the controlled variable. 'Voltage-clamp' experiments fix the membrane potential (Vm) and measure the resulting current (Im), while 'current-clamp' experiments fix the injected current (Ielec) and measure the resulting membrane potential (Vm).</p> Signup and view all the answers

<p>A voltage-clamp circuit continuously monitors the membrane potential (Vm) and adjusts the injected current (Ielectrode) to maintain Vm at a predetermined value (Vclamp). This ensures that the membrane potential is kept constant, allowing the researcher to study the underlying ion channels and their behavior.</p> Signup and view all the answers

<p>Voltage-clamp provides precise control over membrane potential (Vm), enabling the direct measurement of ion current (Im) at specific voltage values. This allows researchers to analyze the voltage dependence of ion channel activity and identify the specific currents carried by different ions.</p> Signup and view all the answers

<p>A voltage-clamp experiment allows researchers to mimic the depolarization and repolarization phases of an action potential by taking the neuron's membrane potential through different stages. By measuring the resulting current flow, the voltage-clamp technique reveals the underlying ionic basis of the action potential, including the activation and deactivation of sodium and potassium channels.</p> Signup and view all the answers

<p>Voltage-clamp experiments allow researchers to directly measure how ion channels respond to changes in membrane potential (Vm). By isolating the effect of specific voltage changes on ion channel activity, researchers can determine the voltage-dependence of ion channel opening and closing, and identify the specific ions that each channel conducts. This provides valuable information about the role of specific ion channels in neuronal signaling.</p> Signup and view all the answers

<p>Voltage-clamp experiments allow researchers to study the behavior of Na+ and K+ channels by fixing the membrane potential at specific values, such as resting and depolarized states. This allows for the isolation and measurement of the individual currents generated by Na+ and K+ channels, providing insights into their voltage-dependent activation and inactivation during an action potential.</p> Signup and view all the answers

<p>A voltage-clamp experiment begins by injecting a current (Ielectrode) and measuring the resulting membrane potential (Vm). If Vm is different from the desired clamp voltage (Vclamp), the injected current is adjusted until Vm equals Vclamp. This iterative process of measuring and adjusting the current ensures that the membrane potential remains fixed at the targeted value.</p> Signup and view all the answers

<p>The 'clamp' electrode provides the injected current (Ielectrode) to control the membrane potential, while the 'recording' electrode measures the resulting membrane potential (Vm). The clamp electrode is used to maintain the membrane potential at the desired value, while the recording electrode provides information about the actual voltage across the membrane.</p> Signup and view all the answers

<p>Sodium channels open quickly during depolarization, causing the rapid rise of the action potential. Potassium channels open more slowly and allow potassium ions to flow out, leading to the repolarization phase.</p> Signup and view all the answers

<p>The refractory period is a time after an action potential when the cell is less likely to generate another one. This is due to the inactivation of Na+ channels and the continued outward flow of K+ ions.</p> Signup and view all the answers

<p>The axon hillock is the region of the neuron where the axon emerges from the cell body. It has a high density of voltage-gated sodium channels, making it the site where action potentials are typically initiated.</p> Signup and view all the answers

<p>Myelin acts as an insulator, preventing the leakage of ions across the axon membrane. This allows for faster conduction of action potentials, as the signal jumps between the unmyelinated regions called nodes of Ranvier.</p> Signup and view all the answers

<p>Orthodromic conduction is the normal direction of action potential travel, from the cell body towards the nerve terminals. Antidromic conduction is the reverse direction, from the nerve terminals towards the cell body.</p> Signup and view all the answers

<p>Delayed rectification refers to the role of potassium channels in restoring the membrane potential to its resting value after depolarization. They open slowly and allow potassium ions to flow out, counteracting the influx of sodium ions during the rising phase.</p> Signup and view all the answers

<p>Electrotonic conduction refers to the passive spread of electrical current along the axon. In myelinated axons, the current flows mainly under the myelin sheath, allowing for rapid conduction without the need for opening of ion channels in those regions.</p> Signup and view all the answers

<p>Potassium channels are composed of four subunits, while sodium channels are made of four domains. This structural difference contributes to their distinct roles during the action potential.</p> Signup and view all the answers

<p>The inactivation of sodium channels prevents the membrane potential from being depolarized further and ensures that the action potential moves in one direction. It also limits the frequency of action potentials that can be generated.</p> Signup and view all the answers

<p>At rest, the concentration of sodium ions is higher outside the cell than inside. This concentration gradient, along with the membrane's permeability, helps maintain the negative resting potential of the cell.</p> Signup and view all the answers

<p>The membrane potential rapidly repolarizes during the falling phase as potassium channels open, allowing potassium ions to flow out of the cell. This counteracts the influx of sodium ions during the rising phase and brings the membrane potential back to its resting value.</p> Signup and view all the answers

<p>Voltage-gated ion channels serve as molecular switches that control the flow of ions across the cell membrane in response to changes in membrane potential, allowing for the generation and propagation of action potentials.</p> Signup and view all the answers

<p>The threshold is the minimum level of depolarization required to trigger the opening of a significant number of voltage-gated sodium channels, leading to the rapid depolarization that characterizes the rising phase of an action potential.</p> Signup and view all the answers

<p>Action potentials are the primary mechanism for transmitting information along neurons and between neurons. They act like electrical signals that travel along the neuron and can be passed on to other neurons at synapses, allowing for the complex communication that underlies all nervous system functions.</p> Signup and view all the answers

<p>The sodium-potassium pump actively transports sodium ions out of the cell and potassium ions into the cell, maintaining the concentration gradients of these ions that are essential for establishing the resting membrane potential.</p> Signup and view all the answers

<p>In myelinated axons, action potentials jump between the nodes of Ranvier, leading to saltatory conduction, which is much faster than the continuous conduction observed in unmyelinated axons.</p> Signup and view all the answers

<p>The typical sodium channel consists of two gates: an activation gate and an inactivation gate. When the membrane potential is at rest, the activation gate is closed, preventing sodium ions from passing through. During depolarization, the activation gate opens, allowing sodium ions to flow into the cell. However, after a brief period of time, the inactivation gate closes, blocking further sodium influx. This inactivation prevents the membrane potential from becoming too positive and is vital for the repolarization phase of the action potential.</p> Signup and view all the answers

<p>The delay in sodium channel opening contributes to the duration of the action potential. This delay occurs because it takes time for the activation gate to fully open. The longer the delay, the slower the rate of depolarization and therefore the slower the action potential propagation. Having a short delay ensures faster transmission of the action potential, which is crucial for rapid communication in the nervous system.</p> Signup and view all the answers

<p>Both sodium and potassium channels respond to changes in membrane potential, but their behaviors differ. Sodium channels open quickly in response to depolarization, allowing a rapid influx of sodium ions. They then quickly inactivate, preventing further sodium entry. Potassium channels, on the other hand, open more slowly in response to depolarization, allowing potassium ions to flow out of the cell, contributing to repolarization. Potassium channels typically remain open until the membrane potential returns to its resting state.</p> Signup and view all the answers

<p>The absolute refractory period is a time interval during which another action potential cannot be generated, regardless of the strength of the stimulus. This period coincides with the inactivation of sodium channels. Because the sodium channels are inactivated, they cannot respond to depolarization, preventing the generation of another action potential until they re-activate.</p> Signup and view all the answers

<p>Patch clamping allows us to measure the current flowing through individual ion channels during an action potential. By measuring the sodium current, we observe that it only flows during the Rising phase of the action potential. This demonstrates that the opening of sodium channels, and the subsequent influx of sodium ions, is directly responsible for the rapid depolarization that characterizes the Rising phase.</p> Signup and view all the answers

<p>The driving force is the combination of electrical and chemical forces that cause ions to move across the membrane. The concentration gradient is the chemical force, pushing ions from a region of high concentration to a region of low concentration. The membrane potential is the electrical force, driving positively charged ions towards a region of negative charge and vice versa. Together, these forces determine the direction and magnitude of ion movement through open channels.</p> Signup and view all the answers

<p>The non-inactivating potassium channel contributes to the repolarization phase of the action potential. As the membrane potential rises during depolarization, these channels open slowly and allow potassium ions to flow out of the cell. This efflux of potassium ions helps restore the negative membrane potential, returning the membrane to its resting state. The fact that these channels are non-inactivating ensures that the repolarization process continues until the resting potential is reached.</p> Signup and view all the answers

<p>Hodgkin and Huxley pioneered the understanding of the action potential by using the giant axon of squid, which provided a large and easily accessible system for studying electrical activity in neurons. Their research revealed the fundamental roles of sodium and potassium ions in generating the action potential and the importance of voltage-gated ion channels. Their model, known as the Hodgkin-Huxley cycle, remains a cornerstone of our current understanding of neuronal signaling.</p> Signup and view all the answers