Trystan Casey
Electrical excitability is a key characteristic of neurons, allowing them to receive and respond to internal and external stimuli. This property enables the nervous system to coordinate and act on these stimuli in a swift and efficient manner. Neuronal excitability is dependent on the presence of voltage-gated ion channels, which respond to changes in membrane potential by opening or closing. These channels play a central role in membrane excitation, with the passive diffusion of ions like Na+, K+, Ca2+, and Cl- through them generating electrical signals. The opening and closing of specific channels shape the membrane potential changes, resulting in characteristic electrical messages. The electrical excitability of neurons is what distinguishes them from other cells, and understanding this property is crucial for comprehending neuronal function and developing targeted treatments for neurological disorders.
| Characteristics | Values |
|---|---|
| Neuronal excitability | Defined as the threshold for neuronal action potential generation |
| A function of various factors, including the location and function of post-synaptic receptors | |
| Ion channels play a central role in membrane excitation | |
| Over 100 genes coding for subunits of ion channels have been identified | |
| Ion channels shape the membrane potential changes and give rise to characteristic electrical messages | |
| The opening and closing of specific channels shape the membrane potential changes | |
| The action potential depends on the presence of voltage-gated ion channels that respond to changes in membrane potential by opening or closing | |
| The axon is electrically excitable everywhere | |
| The electroplaque of the eel, and probably also cells of the nervous system, have neurally and electrically excitable membrane components intermingled |
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What You'll Learn
Ion channels and their role in membrane excitation
Neuronal excitability is a function of various factors, such as the location and function of post-synaptic receptors that mediate changes in neuronal membrane potential, ion fluxes across diverse membrane-bound ion channels, and structural features such as the location of the axon initial segment.
Ion channels are pore-forming membrane proteins that allow ions to pass through the channel pore. They are located within the membrane of all excitable cells, including neurons, muscle cells, and touch receptor cells. Ion channels play a role in membrane excitation as central as the role of enzymes in metabolism. They are often described as narrow, water-filled tunnels that allow only ions of a certain size and/or charge to pass through. This characteristic is called selective permeability. The archetypal channel pore is just one or two atoms wide at its narrowest point and is selective for specific ions, such as sodium or potassium.
The opening of ion channels permits the ions on either side of the plasma membrane to flow down a dual gradient. The exact direction of flow varies by ion type and depends on both the concentration difference and the voltage difference for each variety of ion. This ion flow results in the production of an electrical signal. The actual number of ions required to change the voltage across the membrane is quite small. During the short time that an ion channel is open, the concentration of a particular ion in the cytoplasm as a whole does not change significantly, only the concentration in the immediate vicinity of the channel.
In excitable cells, the electrical signal initiated by ion channel receptor activity travels rapidly over the surface of the cell due to the opening of other ion channels that are sensitive to the voltage change caused by the initial channel opening. The opening of just a single ion channel alters the electrical charge on both sides of the membrane. The resulting charge differential then causes adjacent voltage-sensitive channels to open in a chain reaction, creating a self-propagating electrical signal that travels down the entire length of the cell.
Ion channels are involved in establishing a resting membrane potential, shaping action potentials, and other electrical signals by gating the flow of ions across the cell membrane. They also control the flow of ions across secretory and epithelial cells and help regulate cell volume. Voltage-gated cation channels are responsible for the generation of self-amplifying action potentials in electrically excitable cells, such as neurons and skeletal muscle cells.
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The nervous system and its ability to respond to stimuli
The nervous system is a network of nerve cells that enable animals to receive and act on internal and external stimuli in a coordinated and speedy manner. The nervous system's ability to respond to stimuli is made possible by the electrical excitability of neurons.
Neurons are electrically excitable due to the presence of voltage-gated ion channels in their membranes. These ion channels play a crucial role in membrane excitation, similar to the role of enzymes in metabolism. When a stimulus is received, the voltage-gated ion channels respond by opening or closing, allowing ions such as Na+, K+, Ca2+, and Cl- to pass through. This movement of ions creates electrical disturbances that propagate through the nervous system, leading to a response to the stimulus.
At the excitatory synapse, glutamate, a neurotransmitter, is released by the presynaptic neuron. Glutamate binds to receptors on the postsynaptic membrane, opening gated channels. This allows sodium ions (Na+) to enter the cell, causing a local transmembrane potential change, known as depolarization. This change in membrane potential is a critical step in activating the postsynaptic neuron, enabling it to send signals to other neurons.
Another neurotransmitter, GABA (gamma-aminobutyric acid), acts as an inhibitory neurotransmitter. When released by the presynaptic neuron, GABA binds to receptors on the postsynaptic membrane, opening a chloride (Cl-) ion channel. The concentration gradient of chloride ions inside the neuron is low, and as Cl- is a negative ion, its electrochemical drive to enter the cell is relatively weak compared to Na+.
The electrical excitability of neurons is a complex process influenced by factors such as the location and function of post-synaptic receptors, ion fluxes across diverse membrane-bound ion channels, and structural features like the position of the axon initial segment. The understanding of neuronal excitability is essential for comprehending the nervous system's ability to respond to stimuli and coordinate appropriate actions.
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The role of neurotransmitters in signalling
The nervous system enables animals to respond to internal and external stimuli in a coordinated and speedy manner. This is achieved through a combination of electrical and chemical signals that arise in receptor organs, nerve cells, and effector organs like muscles and secretory glands. Electrical excitability is essential for neurons to receive and act on these stimuli.
Neurotransmitters are the body's chemical messengers and are crucial in transmitting signals between neurons or from neurons to other cells like muscle cells or gland cells. They are synthesized and released from synaptic vesicles into the synaptic cleft, where they interact with receptors on the target cell. Neurotransmitters can be categorized as small-molecule neurotransmitters or neuropeptides. Small-molecule neurotransmitters like acetylcholine, glutamate, and gamma-aminobutyric acid (GABA) are synthesized locally within the axon terminal, while neuropeptides are larger molecules synthesized within the cell body.
The effect of a neurotransmitter on the target cell depends on the receptor it binds to. Neurotransmitters can influence the target cell in three ways: excitatory, inhibitory, or modulatory. Excitatory neurotransmitters promote the generation of an electrical signal or action potential in the receiving neuron, while inhibitory neurotransmitters prevent it. Modulatory neurotransmitters like dopamine and serotonin influence the release or reuptake of other neurotransmitters and play roles in mood, sleep, and movement control.
The release of neurotransmitters is triggered by electrical signals called action potentials, which are generated by the opening and closing of voltage-gated ion channels in the presynaptic neuron's membrane. These channels respond to changes in membrane potential, and their opening and closing shapes the electrical messages transmitted.
Overall, neurotransmitters play a vital role in signalling by converting electrical signals into chemical messages that can influence the receiving neuron, thereby propagating or preventing an action potential. This process is essential for the nervous system's function and our ability to respond to various stimuli.
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The importance of axons in neuronal excitability
Neuronal excitability is a function of various factors, including the location and function of post-synaptic receptors, ion fluxes across diverse membrane-bound ion channels, and structural features such as the location of the axon initial segment. Axons are an important component of neuronal excitability as they are the long cables that snake away from the main part of the neuron, transmitting electrical impulses from the neuron to be received by other neurons. These impulses travel over long distances through axons, which are often encased in a fatty substance called myelin, acting as insulation.
The length of axons varies depending on the type of neuron, with some being just a millimetre long, while others, such as those connecting the brain to the spinal cord, can be over a metre in length. Axons typically develop side branches called axon collaterals, allowing a single neuron to send information to multiple others. These collaterals further split into smaller extensions called terminal branches, each ending in a synaptic terminal. Neurons communicate through these synapses, which are contact points between the axon terminals and the dendrites or cell bodies of other neurons.
The maintenance of axons is crucial, as they are much longer than the rest of the cell, requiring the transport of essential molecules and organelles. A gene called mec-17 is involved in stabilising the internal neuronal structure to support proper transport within the axon. Mutations in this gene or others with similar functions can lead to damaged axons and neurological diseases. When axons are damaged, they send signals to the surrounding tissue to initiate repair, but if they are not repaired properly, it can lead to disrupted neuronal communication and contribute to neurodegenerative diseases.
Additionally, research has shown that dopamine neuron axons play a significant role in integrating information, modulating excitability, and generating outputs independently from somatic activity. This indicates that axons are not just passive conductors but actively participate in neuronal processes, further highlighting their importance in neuronal excitability.
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The relationship between excitability and plasticity
Plasticity of neuronal excitability can be observed in vivo, with repeated bursting of layer 5 pyramidal neurons inducing long-term potentiation of intrinsic excitability in rat cortical motor neurons. This demonstrates the potential for functional plasticity in the whole brain. In vitro studies on brain slices and cultures of dissociated neurons have also contributed to our understanding of the induction and expression mechanisms of intrinsic plasticity. Classical conditioning, for instance, has been shown to alter intrinsic excitability in neurons from the pericruciate cortex, hippocampus, and cerebellum.
Synaptic plasticity, which refers to the changes in synaptic strength that contribute to learning and memory, is influenced by excitability. Excitatory postsynaptic potentials (EPSPs) are the result of a tight interplay between synaptic and intrinsic voltage-gated conductances, which can amplify or attenuate the synaptic potentials. Any modifications in this equilibrium can impact the probability of a synaptic input triggering an action potential. For example, long-term synaptic potentiation (LTP) in hippocampal neurons can down-regulate A-type K+ channels, altering neuronal excitability.
Intrinsic plasticity, on the other hand, refers to changes in the intrinsic electrical properties of a neuron, such as modifications in the threshold for action potential generation. This can be induced by neuronal firing or synaptic activity. Cholinergic modulation, for instance, promotes intrinsic plasticity in cortical pyramidal neurons, shifting the neuronal threshold potential. Neuronal hyperactivity can also be influenced by synaptic mechanisms, such as an increase in excitatory neurotransmission or a decrease in inhibitory neurotransmission.
The interplay between synaptic and intrinsic plasticity is crucial for understanding the relationship between excitability and plasticity. Synaptic plasticity defines the information content received by neurons through their connectivity network. However, the plasticity of cell-autonomous excitability can dynamically regulate the participation of individual neurons and the overall activity state of an ensemble. Synaptic long-term potentiation (LTP) translates experiences into enhanced synaptic efficacy, contributing to the establishment and updating of synaptic connectivity in an experience-dependent manner.
In conclusion, the relationship between excitability and plasticity is multifaceted. Excitability influences the induction of plasticity, with changes in neuronal firing and synaptic activity impacting the intrinsic electrical properties of neurons. Plasticity, in turn, alters the excitability of neurons by modifying their structure and function. Both synaptic and intrinsic plasticity play important roles in learning, memory, and the overall activity of neural ensembles.
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Frequently asked questions
Electrical excitability is important to neurons as it allows them to transmit signals to other neurons. This is done through the diffusion of ions through ion channels in the neuron's membrane.
Ion channels are molecular pores in the neuron's membrane that allow ions to pass through. These ions include Na+, K+, Ca2+, and Cl-. The movement of these ions creates electrical disturbances that carry signals from one neuron to another.
Neurons are a type of nerve cell that plays a crucial role in the nervous system. The nervous system enables animals to respond to internal and external stimuli in a coordinated and rapid manner. Electrical excitability in neurons facilitates this process by generating electrical signals that can be transmitted quickly to other parts of the body.
