Molly Ayala
Neurons are electrically excitable cells that react to input by producing electrical impulses, known as action potentials. These impulses are generated and propagated by changes to the cationic gradient (mainly sodium and potassium) across their plasma membranes. The neuron's membrane potential is generated via a difference in the concentration of charged ions. The electrical potential of the membrane is influenced by the 1000 synapses that a neuron has on average. The electrotonic potential refers to the passive spread of charge inside a neuron, resulting from a local change in ionic conductance.
| Characteristics | Values |
|---|---|
| Definition | Electrical potential in a neuron is the production of electrical impulses, propagated as action potentials throughout the cell and its axon. |
| Action Potential | A propagated impulse. |
| Electrotonic Potential | A non-propagated local potential, resulting from a local change in ionic conductance. |
| Amplitude | 5-20 mV |
| Duration | 1 ms to several seconds |
| Constants | Membrane time constant τ, membrane length constant λ |
| Membrane Time Constant τ | 1-20 ms |
| Membrane Length Constant λ | Measures how far it takes for an electrotonic potential to fall to 1/e or 37% of its amplitude at the place where it began |
| Ribbon Synapses | Found in sensory neurons, they are of a unique structure that allows them to respond dynamically to inputs from electrotonic potentials. |
| Excitatory Postsynaptic Potentials (EPSPs) | Electrotonic potentials that increase the membrane potential by depolarizing the membrane, making it more likely for an action potential to occur. |
| Inhibitory Postsynaptic Potentials (IPSPs) | Electrotonic potentials that decrease the membrane potential by hyperpolarizing the membrane, making it harder for an action potential to occur. |
| Resting Membrane Potential | −60mV to −70mV |
| Threshold Potential | −55mV |
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What You'll Learn
Electrotonic potential
In physiology, electrotonus refers to the passive spread of charge inside a neuron and between cardiac muscle cells or smooth muscle cells. This passivity means that voltage-dependent changes in membrane conductance do not contribute. Neurons and other excitable cells produce two types of electrical potential, one of which is the electrotonic potential.
On the other hand, electrotonic potentials that decrease the membrane potential are called inhibitory postsynaptic potentials (IPSPs). They hyperpolarize the membrane and make it harder for a cell to have an action potential. IPSPs are associated with Cl− entering the cell or K+ leaving the cell. IPSPs can interact with EPSPs to "cancel out" their effect.
The electrotonic potential travels via electrotonic spread, which involves the attraction of opposite and repulsion of like-charged ions within the cell. Electrotonic potentials can sum spatially or temporally. Spatial summation is the combination of multiple sources of ion influx, whereas temporal summation is a gradual increase in overall charge due to repeated influxes in the same location.
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Action potential
Neurons are electrically excitable cells that can react to input by producing electrical impulses, which are propagated as action potentials throughout the cell and its axon. Action potentials are generated by changes to the cationic gradient (mainly sodium and potassium ions) across the plasma membrane. These electrical impulses allow neurons to interact with each other at synapses via synaptic transmission.
The resting state of a neuron is when it is not sending an electrical signal. During this time, the inside of the neuron is negatively charged relative to the outside. This state is called the resting membrane potential and is typically about −60 mV. The resting potential is maintained by an unequal distribution of ions across the cellular membrane, established by ATP-dependent pumps, particularly sodium-potassium antiporters. These exchangers pump sodium ions out of the cell and potassium ions into the cell.
An action potential occurs when a stimulus causes the resting potential to move towards 0 mV. This change in membrane potential opens voltage-gated cationic channels, allowing sodium ions to rush into the neuron. This causes the neuron to become more positively charged and depolarized. As more channels open, a greater electric current is produced across the cell membrane. This process continues until all available ion channels are open, resulting in a large upswing in the membrane potential.
Once the threshold level is reached, an action potential of a fixed size is fired. The rapid influx of sodium ions causes the polarity of the plasma membrane to reverse, and the ion channels then rapidly inactivate. As the sodium channels close, sodium ions can no longer enter, and they are actively transported out of the plasma membrane. Potassium channels are then activated, and there is an outward current of potassium ions, returning the electrochemical gradient to the resting state.
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Excitatory postsynaptic potentials (EPSPs)
In neuroscience, an excitatory postsynaptic potential (EPSP) is a temporary depolarization of the postsynaptic membrane potential, which makes the postsynaptic neuron more likely to fire an action potential. EPSPs are caused by the flow of positively charged ions, such as Na+ or Ca2+, into the postsynaptic cell, resulting in an increase in membrane potential. This flow of ions is known as an excitatory postsynaptic current (EPSC).
EPSPs are graded potentials, meaning they have an additive effect. When multiple EPSPs occur on a single patch of the postsynaptic membrane, their combined effect is the sum of the individual EPSPs. This is in contrast to action potentials, which follow an all-or-none law of propagation. The amplitude of EPSPs can vary, with larger EPSPs resulting in greater membrane depolarization and increasing the likelihood of the postsynaptic cell reaching the threshold for firing an action potential.
EPSPs are often caused by the release of neurotransmitters, such as glutamate, from a presynaptic neuron. These neurotransmitters bind to receptors on the postsynaptic cell, which contain ion channels capable of passing positively charged ions. At excitatory synapses, the ion channel typically allows sodium ions into the cell, generating an excitatory postsynaptic current and causing an increase in membrane potential.
The study of EPSPs, particularly in the hippocampus region of the brain, has revealed that neurons arranged in certain ways can result in extracellular signals from synaptic excitation adding up to give a signal that can be easily recorded with a field electrode. This is known as the field potential and has been studied in the context of hippocampal long-term potentiation (LTP).
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Inhibitory postsynaptic potentials (IPSPs)
IPSPs are associated with Cl- entering the cell or K+ leaving the cell. This movement of ions leads to an increase in the negative charge on the inner surface of the postsynaptic membrane, a process known as hyperpolarization. This hyperpolarization acts to make the membrane potential more negative than the threshold potential required for an action potential, thus reducing the likelihood of an action potential being generated.
The size of the neuron can impact the inhibitory postsynaptic potential, with smaller neurons exhibiting simpler temporal summation of postsynaptic potentials, while larger neurons have more prolonged interactions between neurons due to the greater number of synapses and ionotropic receptors.
IPSPs and EPSPs compete with each other at numerous synapses of a neuron, influencing whether an action potential is generated. This balance between excitatory and inhibitory inputs is crucial in the integration of electrical information in the nervous system.
Research into IPSPs has provided insights into various areas, including the study of learned behaviour and the effects of opioids and morphine on dopamine neurons.
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Resting membrane potential
Neurons are excitable cells that can transition from a resting state to an excited state. The resting membrane potential is the electrical potential difference across the plasma membrane when the cell is in a non-excited state. It is the result of the movement of several different ion species through various ion channels and transporters (uniporters, cotransporters, and pumps) in the plasma membrane. These movements result in different electrostatic charges across the cell membrane.
The resting membrane potential is crucial for the proper functioning of the nervous and muscular systems. When excited, these cells deviate from their resting membrane potential to undergo a rapid action potential before returning to rest. In neurons, the firing of an action potential allows the cell to communicate with other cells by releasing various neurotransmitters.
The resting membrane potential is influenced by the concentrations of ions in the fluids on both sides of the cell membrane and the ion transport proteins present in the cell membrane. The membrane potential is stable or "resting" when the rate at which ions enter and leave the cell is the same, resulting in no net ion current. This equilibrium is achieved when the tendency of ions to leave the cell by running down their concentration gradient is balanced by the tendency of the membrane voltage to pull ions back into the cell.
The resting membrane potential in most neurons has a value of approximately −70 mV or −0.07 V. This slight deviation from −90 mV, the equilibrium potential for potassium ions, is due to the leakage of sodium and other ions out of the cell at rest. The membrane is permeable to potassium ions at rest due to the presence of open channels, and the high relative permeability of potassium results in the resting membrane potential being close to the potassium equilibrium potential.
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Frequently asked questions
Electrical potential is the difference in voltage between the inside and outside of a cell, also known as the membrane potential.
The membrane potential of a neuron is typically around −60mV to −70mV. This means that the inside of the neuron has a negative voltage relative to the outside.
An action potential is a rapid change in voltage across a cell's membrane, often triggered by a stimulus. It is sometimes referred to as a nerve impulse or a spike.
An action potential occurs in a neuron when its membrane potential reaches a certain threshold, causing voltage-gated ion channels to open. This allows an influx of positively charged sodium ions, changing the electrochemical gradient and resulting in a further rise in membrane potential.
An electrotonic potential is a type of electrical potential specific to neurons and other excitable cells. It is a local change in ionic conductance that can spread along the cell membrane, resulting in a graded potential. Electrotonic potentials can trigger action potentials by depolarizing the membrane above the threshold potential.
