Otis Singleton
Electrically excitable tissues are a group of cells that can generate and transmit electrochemical impulses along cell membranes in response to external stimuli. They include nervous, muscle, and glandular epithelial tissues. These tissues have the ability to generate action potentials, which are electrical signals created by the movement of ions across cell membranes. The opening and closing of ion channels drive the production of these electrical signals, which can be quickly transmitted to other cells. The goal of electrically stimulating excitable tissues is to control the initiation and propagation of these action potentials and, consequently, the release of neurotransmitters and nervous system signaling.
| Characteristics | Values |
|---|---|
| Type | Nervous, muscle, and glandular epithelial tissues |
| Response to Stimuli | Ability to generate action potentials |
| Resting Membrane Potential | -70 mV |
| Function | Control of initiation and propagation of action potentials |
| Cell Types | Neurons, muscle cells, nerve cells, endocrine cells |
| Mechanism | Opening and closing of ion channels |
| Channels | Voltage-gated Na+ and K+ channels |
| Signal Transmission | Electrochemical impulses along cell membranes |
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What You'll Learn
Action potentials
All cells in animal body tissues are electrically polarized, meaning they maintain a voltage difference across the cell's plasma membrane, known as the membrane potential. This electrical polarization is due to the interplay between protein structures in the membrane, called ion pumps and ion channels. In neurons, the types of ion channels in the membrane usually vary across different parts of the cell, giving them different electrical properties. As a result, some parts of the neuron membrane may be excitable (capable of generating action potentials), while others are not.
The initiation of a neuronal action potential usually occurs at the axon hillock, the most proximal segment of an axon. However, in sensory neurons, the action potential is initiated at the distal terminal of the axon and propagates toward the central nervous system. In these spike initiation zones, a 50-fold increase in Nav receptor density decreases input resistance, requiring less excitation to induce an action potential.
When a stimulus causes the neuron membrane to depolarize past a threshold, an all-or-none action potential is triggered, involving changes in sodium and potassium permeability. The action potential then propagates along axons without diminishing in amplitude due to its distinct phases of rapid depolarization and repolarization. The amplitude, duration, and shape of the action potential are determined largely by the properties of the excitable membrane and not the amplitude or duration of the stimulus.
Regularly spaced unmyelinated patches, called the nodes of Ranvier, generate action potentials to boost the signal. This type of signal propagation is known as saltatory conduction, providing a favorable trade-off between signal velocity and axon diameter.
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Electrochemical impulses
Electrically excitable tissues, such as nervous, muscle, and glandular epithelial tissues, have the unique ability to generate and transmit electrochemical impulses. These impulses, known as action potentials, are essential for the initiation and propagation of signals within the body, facilitating communication between cells and coordinating various physiological processes.
At rest, these excitable cells maintain a negative interior membrane potential of around -70 mV due to ion pumps and selective ion permeability. This electrical polarization is a result of the intricate interplay between protein structures, ion pumps, and ion channels embedded in the cell membrane. The concentration gradients of ions like potassium and sodium across the membrane contribute to this resting potential.
When stimulated, the membrane undergoes depolarization, leading to a rapid rise in voltage. This triggers the opening of voltage-gated ion channels, allowing the influx of sodium ions and the efflux of potassium ions. The change in the electrochemical gradient further amplifies the membrane potential, causing more channels to open and generating a nerve impulse or "spike."
This nerve impulse then propagates along the cell membrane, transmitting the electrochemical signal to adjacent cells. The distinct phases of rapid depolarization and repolarization ensure the nerve impulse's amplitude remains consistent as it travels along axons. The propagation of these action potentials through excitable tissues enables the release of neurotransmitters and effective nervous system signaling.
Functional electrical stimulation (FES) is a technique that utilizes electrical currents to stimulate excitable tissues, supplementing or restoring function in neurologically impaired individuals. By controlling the initiation and propagation of action potentials, FES can modulate nervous system signaling and impact overall organism responses to stimuli.
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Ion channels
Electrically excitable tissues, such as nerves and muscles, have the ability to generate signals that can be transmitted to other cells. The movement of ions across cell membranes in living tissues generates electrical potentials.
In electrically excitable cells, such as neurons and muscle cells, the membrane potential is utilised for transmitting signals within the cell. The opening or closing of ion channels at specific points on the membrane leads to a local change in membrane potential, which is then sensed by adjacent or distant ion channels. This triggers a response, with ion channels opening or closing in a chain reaction, propagating the signal.
The specific ions involved in this process include sodium and potassium. When a threshold stimulus is reached, voltage-gated sodium channels open, allowing a rapid influx of sodium ions, while potassium ions efflux, causing a temporary reversal of the polarity. This process is essential for the generation of action potentials, which are electrical signals that propagate along the cell membrane.
The behaviour of these ion channels can be studied using techniques such as the patch-clamp method, which has provided detailed knowledge of the function of individual channels. The excitability of a cell is influenced by the availability and functionality of these ion channels. For example, genetic mutations or specific drugs can cause a loss of sodium channel function, reducing membrane excitability.
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Resting potential
The resting membrane potential is a crucial concept in the functioning of the nervous and muscular systems. It is defined as the electrical potential difference across the plasma membrane of a cell when it is in a non-excited state. The resting potential is a result of the movement of various ion species, including sodium (Na+) and potassium (K+), through ion channels and transporters in the plasma membrane. The cell membrane is selectively permeable to these ions, allowing them to move in and out of the cell and creating an uneven distribution of ions on either side of the membrane.
In most neurons, the resting potential has a value of approximately −70 mV or -0.07 V. This negative value indicates that the interior of the cell has a negative charge relative to the exterior. The difference in voltage is due to the concentration gradient of ions across the membrane. Specifically, the sodium-potassium pump, powered by ATP, transports two potassium ions inside the cell and three sodium ions outside, creating a higher concentration of potassium ions inside and sodium ions outside the cell.
The movement of ions across the membrane is influenced by the concentration gradient and the electrical gradient. The concentration gradient drives the movement of ions from an area of higher concentration to an area of lower concentration. Meanwhile, the electrical gradient, or membrane potential, exerts a force that opposes the movement of ions down the concentration gradient. At equilibrium, these two forces balance each other out, resulting in a stable resting membrane potential.
The resting potential is important because it allows cells to transition from a resting state to an excited state. When a stimulus causes the membrane to depolarize past a threshold, an action potential is triggered. This involves a rapid change in the permeability of the membrane to sodium and potassium ions, leading to a brief reversal of the potential. The action potential then propagates along axons, allowing neurons to communicate with other cells and muscle cells to contract.
Overall, the resting membrane potential is a fundamental property of electrically excitable tissues, such as nervous and muscle tissues, and plays a crucial role in their ability to generate and transmit electrochemical impulses.
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Voltage-gated channels
Electrically excitable tissues, such as nerves and muscles, have the ability to generate signals that can be transmitted to other cells. This is achieved through the opening and closing of ion channels, which are driven by electrical field changes or ligand binding. Voltage-gated ion channels play a crucial role in this process.
The movement of these ions through voltage-gated channels creates a local change in the membrane potential, which is the difference in electric potential between the interior and exterior of a cell. This change in electric potential is measured in volts and can range from −80 mV to −40 mV in typical biological systems. The membrane potential allows cells to function as batteries, providing the power needed to operate various "molecular devices" embedded in the membrane.
In electrically excitable cells, the opening and closing of voltage-gated channels lead to the generation of action potentials. Action potentials are rapid and dramatic openings of ion channels, specifically Na+ and K+ channels, that occur in sequence. The sequential opening of these channels creates a wave of excitation, or an action potential, that propagates along the cell membrane.
The specific properties of voltage-gated channels can vary depending on the type of tissue. For example, different types of voltage-gated Na+ channels are involved in action potential generation, with unique properties depending on the tissue of origin, such as nerve, muscle, or other excitable tissues.
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Frequently asked questions
Electrically excitable tissues include nervous, muscle, and glandular epithelial tissues.
Electrically excitable tissues have the ability to generate and transmit electrochemical impulses along cell membranes.
The generation and transmission of electrochemical impulses in electrically excitable tissues are due to the movement of ions across cell membranes, which creates a membrane potential.
Membrane potential, or transmembrane potential, refers to the difference in electric potential between the interior and exterior of a biological cell. It is measured in milli volts (mV) and typically ranges from −80 mV to −40 mV, with the inside usually being negative relative to the outside.
Action potentials, also known as impulses, are electrical signals generated by excitable cells in response to certain stimuli (electrical, chemical, or mechanical). They involve the opening and closing of ion channels, leading to changes in ion permeability and the propagation of the signal along the cell membrane.
