Molly Ayala
The passive electrical signals refer to the passive flow of current in neurons, which are relatively poor conductors of electricity. The passive electrical properties of a nerve cell axon can be determined by measuring the voltage change resulting from a current pulse passed across the axonal membrane. If the current pulse is not sufficiently large to generate action potentials, the magnitude of the resulting potential change decays exponentially with increasing distance from the site of current injection. This is due to the injected current leaking out across the axonal membrane, resulting in less current available to change the membrane potential farther along the axon. The leakiness of the axonal membrane hinders the effective passive transmission of electrical signals, necessitating the myelination of axons to prevent significant signal decay and ensure faster signal speed.
| Characteristics | Values |
|---|---|
| Passive electrical signals decay with time due to the passive electrical properties of a nerve cell axon | The voltage change resulting from a current pulse passed across the axonal membrane decays exponentially with increasing distance from the site of current injection |
| The potential falls to a small fraction of its initial value at a distance of a couple of millimeters away from the site of injection | |
| The injected current leaks out across the axonal membrane, causing less current to be available to change the membrane potential farther along the axon | |
| The leakiness of the membrane slows the time course of the responses measured at increasing distances from the site where the current was injected | |
| The passive spread of electric potentials (electrotonic potential) is in contrast to action potentials, which are generated anew along excitable stretches of membrane and propagate without decay | |
| The myelinated sections of axons are not excitable and do not produce action potentials, and the signal is propagated passively as electrotonic potential | |
| The time constant is an index of how rapidly a membrane responds to a stimulus in time | |
| The space constant (or length constant) is an indication of how far a potential will spread along an axon in response to a subthreshold stimulus at another point |
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What You'll Learn
The leakiness of the axonal membrane
The axon is a thin tubular protrusion that extends from the soma, or cell body, of a neuron. It is responsible for transmitting electrical signals away from the soma to other neurons or target cells. The axonal membrane, which surrounds and insulates the axon, plays a crucial role in this transmission. However, the membrane is not a perfect insulator, and it allows some current to leak out.
When a current pulse is injected into the axon, it causes a change in the membrane potential, which then triggers an action potential. This action potential is a brief electrical signal that travels along the axon and allows neurons to communicate with each other. However, if the current pulse is not strong enough to generate an action potential, the resulting potential change will decay exponentially with distance from the injection site due to the leakiness of the axonal membrane. This is known as passive current flow or passive spread of electric potentials (electrotonic potential).
The passive electrical decay is also influenced by the myelin sheath, a fatty substance that wraps around the axon and acts as an insulator. Myelin, composed of either Schwann cells or oligodendrocytes, prevents ions from entering or escaping the axon, reducing signal decay and increasing signal speed. Myelinated sections of axons do not produce action potentials, and the signal is instead propagated passively.
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The role of voltage-dependent mechanisms
The passive electrical properties of a nerve cell axon can be determined by measuring the voltage change resulting from a current pulse passed across the axonal membrane. If this current pulse is not large enough to generate action potentials, the magnitude of the resulting potential change decays exponentially with increasing distance from the site of current injection. This is due to the injected current leaking out across the axonal membrane, meaning that less current is available to change the membrane potential farther along the axon. The leakiness of the axonal membrane prevents effective passive transmission of electrical signals in all but the shortest axons.
The passive flow of current depolarizes the membrane potential in the adjacent region of the axon, opening the Na+ channels in the neighbouring membrane. This local depolarization triggers an action potential, which then spreads in a continuing cycle until the end of the axon is reached. The regenerative properties of Na+ channel opening allow action potentials to propagate in an all-or-none fashion, acting as a booster at each point along the axon and ensuring the long-distance transmission of electrical signals.
The time constant is an important factor in the voltage-dependent mechanisms of action potential generation. It is an indication of how rapidly a membrane will respond to a stimulus in time. The smaller the time constant, the faster the membrane will respond to a stimulus. The time constant is analogous to the 0 to 60 rating of a high-performance car; the lower the 0 to 60 rating, the faster the car.
In addition to the time constant, the space constant (or length constant) is another important index. The space constant indicates how far a potential will spread along an axon in response to a subthreshold stimulus. The larger the space constant, the greater the distance the potential will spread. The space constant is a passive property of membranes.
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The impact of axonal diameter and myelination
The passive electrical signals in neurons decay with time due to the leakiness of the axonal membrane. This leakiness is one of the passive electrical properties of a nerve cell axon. The passive current flow in neurons results in the depolarization of the membrane potential in the adjacent region of the axon, triggering an action potential.
The myelination of axons, a process where oligodendrocytes in the central nervous system wrap the axon in multiple layers of myelin, also improves passive current flow. Myelin acts as an electrical insulator, reducing the ability of current to leak out of the axon and increasing the distance along the axon that a given local current can flow passively. This insulation effect of myelin speeds up action potential conduction. While unmyelinated axon conduction velocities range from 0.5 to 10 m/s, myelinated axons can conduct at velocities up to 150 m/s.
Theoretical studies have shown that specific myelination parameters, such as the ratio of axonal diameter to fiber diameter (g-ratio), can maximize the space constant and conduction velocity of electrical signals in axons. Additionally, the presence of myelin adds compact membrane layers in series with the axonal membrane, increasing the effective radial resistance and decreasing the effective capacitance of the axon. This reduction in capacitance enables faster saltatory conduction between nodes of Ranvier, allowing electrical signals to jump from node to node, further increasing the speed of propagation.
In summary, the axonal diameter and myelination play crucial roles in enhancing the passive flow of electrical signals in neurons. By reducing internal resistance, increasing distance, and improving insulation, these factors contribute to faster and more efficient signal conduction in the nervous system.
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The influence of passive current flow
One significant influence of passive current flow is its impact on the decay of electrical signals over distance. When a current pulse is injected into a nerve cell axon, if it is not sufficient to generate action potentials, the resulting potential change decays exponentially as it moves away from the injection site. This decay is due to the "leakiness" of the axonal membrane, which allows the injected current to leak out, reducing the available current to change the membrane potential further along the axon. This phenomenon limits effective passive transmission of electrical signals to very short axons, typically 1 mm or less in length.
The passive flow of current also plays a crucial role in the propagation of action potentials. While passive current flow alone cannot account for the long-distance transmission of these signals, it works in conjunction with active currents flowing through voltage-dependent ion channels. The passive flow of current depolarizes the membrane potential, opening Na+ channels and triggering an action potential. This regenerative process ensures the continuation of the action potential until it reaches the end of the axon.
The rate of action potential conduction, or conduction velocity, is influenced by the passive flow of current. Increasing the diameter of an axon reduces the internal resistance to passive current flow, resulting in higher conduction velocities. This principle is observed in nature, with larger axons generally conducting signals faster than smaller ones. Additionally, insulating the axonal membrane with myelin can improve passive current flow by reducing current leakage, leading to significantly increased conduction velocities.
In summary, the influence of passive current flow is integral to understanding the transmission of electrical signals, particularly in the context of neuroscience. It impacts the decay of signals over distance, the propagation of action potentials, and the rate of conduction. By manipulating the passive flow of current, nature has evolved strategies to optimize the efficiency of signal transmission in the nervous system.
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The time constant and its significance
The time constant, denoted by the Greek letter tau (τ), is a fundamental parameter in the analysis of dynamic systems' behaviour across various fields, including electronics, physics, and engineering. It characterises the response of a first-order, linear time-invariant (LTI) system to a step input change. The time constant is particularly useful in understanding how fast a system reaches its ultimate value.
In the context of RC (resistor-capacitor) circuits, the time constant represents the rate at which the circuit transitions from one stable state to another. It is calculated by multiplying the resistance of the circuit in ohms by the capacitance of the circuit in farads. In such circuits, the time constant signifies the time required to charge the capacitor to approximately 63.2% of its full value through an applied DC voltage. Conversely, it also represents the time needed to discharge the capacitor to about 36.8% of its initial value. This is significant because it provides insight into the circuit's growth rate or decay over time.
In neuroscience, the time constant is used to describe the behaviour of nerve cells when they receive an instantaneous stimulus. It helps characterise how the membrane potential changes over time in response to a stimulus. A smaller time constant indicates a faster response to a stimulus.
Additionally, time constants are utilised in control systems, such as integral and derivative action controllers, which are often pneumatic rather than electrical. They are also applied in signal processing, timing circuits, and filters. In control systems engineering, time constants are valuable for assessing system stability and fine-tuning parameters.
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Frequently asked questions
Passive electrical signals decay with time due to the leakiness of the axonal membrane. This prevents the effective passive transmission of electrical signals.
An axonal membrane is the thin tubular protrusion that travels away from the soma. It is insulated by a myelin sheath.
A myelin sheath is made up of either Schwann cells or oligodendrocytes, both of which are types of glial cells. Myelin wraps multiple times around the axonal segment, forming a thick fatty layer that prevents ions from entering or escaping the axon.
The myelin sheath prevents significant signal decay and ensures faster signal speed.
Examples of passive electrical signals include the transmission of electrical signals in neurons and nerve cells.
