The brain is a sophisticated organ that executes and regulates essential bodily processes. The vast network of nerves that carry signals to and from the brain helps the brain conduct its functions. These nerve impulses are conducted thanks to the membrane potential that has evolved across the nerve cell membrane. Hyperpolarization is a phenomenon that contributes to this neuronal communication.
Hyperpolarization is when cell membrane potential gains a negative charge because of increased efflux of cations, an influx of anions, or inhibition of resting ion flux. This process stops the absorption of stimuli to inhibit the formation of impulses after the conduction of the action potential.
The complexity of neuronal transmission continues to perplex and fascinate researchers. In this article, we explore the hyperpolarization phenomena, the significance of the process being highlighted, and the difficulties connected to this condition of the nerve cell membrane.
What Happens During Hyperpolarization?
Hyperpolarization describes a process wherein a cell's membrane potential gains a greater negative charge than its resting potential. To explain it in simple terms, hyperpolarization is the inverse of depolarization, in that the cell membrane becomes even more negatively charged.
This process is caused by an elevated efflux of potassium ions, or influx of calcium anions, through channels in the cell membrane. However, inhibiting this current flow can also lead to hyperpolarization if a cell has resting sodium ion or calcium ion flux.
Hyperpolarization aims to prevent the nerve cell from absorbing more stimuli or, at the very least, raise the threshold for any incoming stimuli. Therefore, this process is an inhibiting post-synaptic potential because it prevents the neuron from generating an impulse.
Hyperpolarization inhibits the further formation of electrical impulses after the action potential has been sent along the axon, which helps to prevent a stimulus from sending an action potential in the opposite direction. Therefore, hyperpolarization is a crucial stage in transmitting electrical impulses around the body.
How Is A Cell's Membrane Potential Established?
A cell's membrane potential is determined by the distribution of ions across the membrane, established by the channels present in the cell membrane. These ion channels govern the cell's permeability to different kinds of ions, thereby controlling the movement of ions across the membrane.
The sodium-potassium pump and the sodium ion and potassium ion leak channels are the two main types of membrane channels that contribute to creating the membrane potential. At resting membrane potential, the net flow of ions through these channels is zero, and the neuron's resting potential would typically be about -70 mV. This overall negative charge indicates that the inside of the neuron is negatively charged relative to the outside cell.
This unequal charge distribution occurs because of the formation of a potassium ion concentration gradient. Compared to the exterior of the cell, the intracellular concentration of potassium ions is greater, which creates a concentration gradient of potassium ions that promotes the diffusion of potassium ions through the leaky channels of the cell from inside to outside of the cell.
The quantity of anions trapped inside the cell rises as the potassium ions depart, which causes the negative charges to build up inside the cell while the positive charges build up outside the cell. The interior of the cell becomes somewhat negative in comparison to the exterior because more positively charged ions are being withdrawn from it than added.
Ultimately, it is deviations from the cell's resting membrane potential caused by changes in the movement of ions that contributes to action potentials and the occurrence of hyperpolarization.
What Is An Action Potential?
An action potential is a recognizable pattern of membrane potential changes occurring in response to a stimulus large enough to depolarize the membrane potential past its threshold level. Neural and muscle cells are two types of cells that communicate by way of an action potential.
However, you may be wondering what constitutes a stimulus? From an electrical perspective, a stimulus with a sufficient electrochemical value, measured in millivolts, triggers an action potential. However, action potentials cannot always be triggered by stimuli. A sufficient electrochemical value must be present in the stimulus for the cell's negativity to be reduced to the action potential threshold.
Stimuli are classified according to their distance from the action potential threshold. There are subthreshold, threshold, and suprathreshold stimuli. As mentioned, stimuli will not always induce action potentials since their energy is below the required threshold. Therefore, these stimuli are appropriately termed 'subthreshold stimuli.'
Threshold stimuli reach the action potential threshold of a cell, resulting in the depolarization of the cell and ultimately the transmission of the nervous impulse. However, suprathreshold stimuli may also generate action potentials, which are stronger than threshold stimuli.
Although these suprathreshold stimuli are greater than threshold stimuli, there is no difference in the size and duration of the action potential they induce. Instead, as the stimulus's intensity increases, so does the frequency of action potentials it causes.
For example, suppose your hand touches a hot stove. In that case, the intense sensation in your hand occurs because touching the hot stove caused an intense, suprathreshold stimulus, producing more action potentials that fired more frequently. However, when you pick up a warm cup of coffee, you don't experience the same sensation because a smaller threshold stimulus is induced.
Therefore, an action potential is produced when a stimulus raises the membrane potential to threshold levels. Typically, the threshold potential ranges from -50 to -55 mV. The action potential follows the all-or-nothing rule in its behavior. This rule implies that any stimulus below the threshold has no effect, but stimuli above the threshold cause the excitable cell to respond fully.
Depolarization, overshoot, and repolarization are the three stages of an action potential. The membrane potential has two more states connected to the action potential. The first is called hypopolarization and occurs before depolarization, whereas the second is called hyperpolarization and occurs after repolarization.
The first rise in membrane potential to the threshold potential is known as hypopolarization. The voltage-gated sodium channels are opened by the threshold potential, which results in a significant inflow of sodium ions. This stage is closely followed by depolarization.
As the cell depolarizes, the inside of the cell becomes increasingly electropositive, approaching the electrochemical equilibrium potential of sodium, which is +61 mV. The overshoot phase is this period of elevated positive charge.
The sodium permeability abruptly drops after the overshoot because of its channels shutting. The voltage-gated potassium channels are opened when the cell potential overshoots, which results in a significant potassium outflow and a reduction in the cell's electropositivity. This phase, known as the repolarization phase, aims to return the resting membrane potential.
Hyperpolarization invariably comes before repolarization, in which the membrane potential is lower than the initial membrane potential. However, the membrane quickly sets the values of membrane potential once more. When the membrane potential reaches the resting voltage that existed before the shock, the body is said to be in a resting condition.
What Is The Difference Between Hyperpolarization and Depolarization?
As we know, hyperpolarization is the increase in negative charge inside the nerve cell. Hyperpolarization is classified as an inhibitory post-synaptic potential because it stops the nerve cell from producing an impulse.
Similarly, depolarization is classified as an activating post-synaptic potential because it occurs when the nerve cell surpasses the action potential threshold, a decrease in negative charge, which results in developing a nerve impulse.
The post-synaptic potential is a graded potential, meaning that its hyperpolarization or depolarization level fluctuates with ion channel stimulation. Instead of following the all-or-nothing rule, graded potentials are variations in membrane potential that come in different sizes.
What Is The Difference Between Graded Potentials And Action Potentials?
A membrane potential with a range in amplitude is a graded potential. Graded potentials occur because of the opening of ligand-gated ion channels and can be uniformly transmitted in all directions. As the size of the input stimuli increases, so does the amplitude of the graded potential. Furthermore, the overall amplitude of a graded potential can be altered through summations.
The capacity of nerve cells to incorporate several polysynaptic potentials at numerous synapses is called summation. Summation could be spatial, meaning signals are acquired from multiple synapses in one go, or temporal, which means signals are obtained sequentially. Furthermore, spatial summations and temporal summations can coincide.
Receptor potentials, post-synaptic potentials, and end plate potentials are the three main types of graded potentials produced by different cells in the body. Specialized sensory receptor cells produce receptor potentials, nerve cells produce post-synaptic potentials, and muscle cells produce end plate potentials.
Graded potentials can be further classified as excitatory post-synaptic potentials and inhibitory post-synaptic potentials. While inhibiting post-synaptic potentials happen with hyperpolarization, excitatory post-synaptic potentials happen during depolarization.
An action potential is a modification in electrical potential that occurs as impulses travel through the membrane of a neural or muscle cell. Depolarization, repolarization, and refractory period are the three primary phases of an action potential. Depolarization is the term used to describe an abrupt shift in the membrane potential from resting membrane potential to a more positive charge.
The membrane depolarization is a result of the opening of the ion-gated channel. The migration of the positively charged sodium ions into the nerve cell increases the positive charge inside the cell when the sodium channels open.
Repolarization is the process of restoring the negative charge inside the nerve cell. The opening of the potassium channels is what causes this. Lowering the positive charge inside the nerve cell results from the entry of potassium ions into the cell's outer membrane. The interval between two action potentials is referred to as the refectory period.
Sodium-potassium channels are activated to replenish the resting potential during the refectory phase. In the resting potential, the potassium ions are highly concentrated inside the nerve cell, whereas the sodium ions are highly concentrated outside the nerve cell.
There are several essential differences between graded potentials and action potentials. Graded potential describes a membrane potential, which can differ in amplitude and occur due to depolarization or hyperpolarization. Furthermore, graded potentials may have variable signal strengths transmitted over short distances and generated by ligand-gated ion channels.
Action potential refers to a shift in the electrical potential, which is associated with the transmission of impulses along a neural or muscle cell membrane. However, action potentials can only occur in response to depolarization. An action potential requires a larger threshold depolarization generated by voltage-gated ion channels and is transmitted over longer distances.
The most significant difference between graded potentials and action potentials is the difference in their amplitude. Graded potentials vary in amplitude, and may lose their strength during transmission, while action potentials have greater amplitudes and do not lose their strength during transmission. However, the overall effect of graded potentials may be greater than that of an action potential because graded potentials can undergo summation.
How Is Hyperpolarization Related To Hypokalaemia?
Hypokalemia, a dangerously low potassium level, is a common side effect associated with thiazide diuretics used to treat conditions such as hypertension, heart disease, and renal disease. The loss of potassium ions occurs in the renal tubules of the kidneys in response to the diuretic-induced stimulation of the aldosterone-sensitive sodium pump, which promotes the absorption of sodium ions and the excretion of potassium ions.
Altering the levels of ions responsible for establishing cellular membrane potential affects action potential transmission significantly. The immediate electrophysiological effects include hyperpolarization of the resting membrane, restriction of critical enzymes, and reduced potassium ion channel conductance.
These electrophysiological effects lead to an increased duration of the action potential and a decrease in repolarization potassium ion reserve. Furthermore, these effects cause significant delays in the repolarization phase of the action potential, which can lead to harmful abnormalities in cardiac function.
Although the immediate effects of hypokalemia have generally been attributed to a decrease in positively charged potassium ions channel conductance, more recent studies, show that indirect effects of hypokalemia also significantly contribute to the activation of late sodium ion and calcium ion streams.
Altering the sodium ion and calcium ion conductance significantly lowers the repolarization capacity. Reducing the cell's ability to enter the repolarization phase, as we know, increases the risk of cardiac function abnormalities, including arrhythmias such as torsade's de pointes, polymorphic ventricular tachycardia, and ventricular foci.
Conclusion
A cell's membrane potential becomes hyperpolarized when there is an increase in cation outflow, an anion inflow, or suppression of resting ion flux. This procedure halts the absorption of stimuli that might otherwise impede the generation of impulses after the action potential has been conducted.
References
- https://www.khanacademy.org/science/biology/human-biology/neuron-nervous-system/a/the-membrane-potential
- https://teaching.ncl.ac.uk/bms/wiki/index.php/Resting_membrane_potential#:~:text=Resting%20membrane%20potentials%20are%20maintained,the%20outside%20of%20the%20cell.
- https://human-memory.net/hyperpolarization/
- https://pediaa.com/difference-between-graded-potential-and-action-potential/#:~:text=The%20main%20difference%20between%20graded,be%20transmitted%20over%20long%20distances.
