University:
Kentucky State University
Course:
BIO 303 | Human Physiology
Academic year:

2023
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435

Pages:

44
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palangresq3v

Physiology 1 for pharmacy students Graded potential and action potential 8-11-2021 Polarization • In neurons, the resting membrane potential ranges from −40 to −90 mV. A typical value is −70 mV. The minus sign indicates that the inside of the cell is negative relative to the outside. • A cell that exhibits a membrane potential is said to be polarized. Most body cells are polarized; the membrane potential varies from +5 mV to −100 mV in different types of cells. Graded potential • A small deviation from the resting membrane potential that makes the membrane either more polarized (inside more negative) or less polarized (inside less negative). • When the response makes the membrane more polarized (inside more negative), it is termed a hyperpolarizing graded potential. • When the response makes the membrane less polarized (inside less negative), it is termed a depolarizing graded potential. Hyperpolarization Depolarization Graded potential • A graded potential occurs when a stimulus causes mechanically-gated or ligand-gated channels to open or close in an excitable cell’s plasma membrane. • To say that these electrical signals are graded means that they vary in amplitude (size), depending on the strength of the stimulus. Graded potential • They are larger or smaller depending on how many ligandgated or mechanically-gated channels have opened (or closed) and how long each remains open. Graded potential • The opening or closing of these ion channels alters the flow of specific ions across the membrane producing a flow of current that is localized, which means that it spreads to adjacent regions along the plasma membrane in either direction from the stimulus source for a short distance and then gradually dies out as the charges are lost across the membrane through leak channels. Graded potential • This mode of travel by which graded potentials die out as they spread along the membrane is known as decremental conduction. Because they die out within a few millimeters of their point of origin, graded potentials are useful for shortdistance communication only. Graded potential • Although an individual graded potential undergoes decremental conduction, it can become stronger and last longer by summating with other graded potentials. • Summation is the process by which graded potentials add together. Graded potential • If two depolarizing graded potentials summate, the net result is a larger depolarizing graded potential. • If two hyperpolarizing graded potentials summate, the net result is a larger hyperpolarizing graded potential. • If two equal but opposite graded potentials summate (one depolarizing and the other hyperpolarizing), then they cancel each other out and the overall graded potential disappears. Graded potential • Graded potentials have different names depending on which type of stimulus causes them and where they occur. • For example, when a graded potential occurs in sensory receptors are called receptor potential. Action potential • A sequence of rapidly occurring events that decrease and reverse the membrane potential and then eventually restore it to the resting state. • An action potential has two main phases: a depolarizing phase and a repolarizing phase. Phases of action potential • During the depolarizing phase, the negative membrane potential becomes less negative, reaches zero, and then becomes positive. Phases of action potential • During the repolarizing phase, the membrane potential is restored to the resting state. Phases of action potential • Following the repolarizing phase there may be an afterhyperpolarizing phase, during which the membrane potential temporarily becomes more negative than the resting level. Voltage-gated channels • Two types of voltage-gated channels open and then close during an action potential. • These channels are present mainly in the axon plasma membrane and axon terminals. Voltage-gated channels • The first channels that open, the voltage-gated Na+ channels, allow Na+ to rush into the cell, which causes the depolarizing phase. Voltage-gated channels • Then voltage-gated K+ channels open, allowing K+ to flow out, which produces the repolarizing phase. • The after-hyperpolarizing phase occurs when the voltage-gated K+ channels remain open after the repolarizing phase ends. Threshold • An action potential occurs in the membrane when depolarization reaches a certain level termed the threshold. • Different excitable cells have different thresholds for generation of an action potential, but the threshold in a particular cell usually is constant. Threshold • The generation of an action potential depends on whether a particular stimulus is able to bring the membrane potential to threshold. • An action potential will not occur in response to a subthreshold stimulus, a weak depolarization that cannot bring the membrane potential to threshold. Threshold • An action potential either occurs completely or it does not occur at all. This characteristic of an action potential is known as the all-or-none principle. Threshold Threshold • Several action potentials will form in response to a suprathreshold stimulus, a stimulus that is strong enough to depolarize the membrane above threshold. • Each of the action potentials caused by a suprathreshold stimulus has the same amplitude (size) as an action potential caused by a threshold stimulus. Threshold • Once an action potential is generated, the amplitude of an action potential is always the same and does not depend on stimulus intensity. • The greater the stimulus strength above threshold, the greater the frequency of the action potentials until a maximum frequency is reached. Resting state Depolarization phase Repolarization phase-1 Repolarization phase-2 Refractory period • The period of time after an action potential begins during which an excitable cell cannot generate another action potential in response to a normal threshold stimulus is called the refractory period. • In contrast to action potentials, graded potentials do not exhibit a refractory period. Absolute refractory period • During the absolute refractory period, even a very strong stimulus cannot initiate a second action potential. • This period coincides with the period of Na+ channel activation and inactivation. • Inactivated Na+ channels cannot reopen; they first must return to the resting state. Relative refractory period • The relative refractory period is the period of time during which a second action potential can be initiated, but only by a larger-than normal stimulus. • It coincides with the period when the voltage-gated K+ channels are still open after inactivated Na+ channels have returned to their resting state. Propagation of action potential • In contrast to the graded potential, an action potential is not decremental (it does not die out). • Instead, an action potential keeps its strength as it spreads along the membrane. This mode of conduction is called propagation. Types of propagation 1- Continuous conduction, which involves step-by-step depolarization and repolarization of each adjacent segment of the plasma membrane. The action potential propagates only a relatively short distance in a few milliseconds. Continuous conduction occurs in unmyelinated axons and in muscle fibers. Types of propagation 2- Saltatory conduction, a special mode of action potential propagation that occurs along myelinated axons. It occurs because of the uneven distribution of voltage-gated channels. Few voltage-gated channels are present in regions where a myelin sheath covers the axolemma. By contrast, at the nodes of Ranvier, the axolemma has many voltage-gated channels. Hence, current carried by Na+ and K+ flows across the membrane mainly at the nodes. Factors affecting the speed of propagation • 1. Amount of myelination. Action potentials propagate faster along myelinated axons than along unmyelinated axons. • 2. Axon diameter. Larger diameter axons propagate action potentials faster due to their larger surface areas. • 3. Temperature. Axons propagate action potentials at lower speeds when cooled. Comparison between graded potential and action potential

Graded Potential and Action Potential

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