Physiology Web at www. Test Questions. Daily Quiz. Physiology Tutor. Depending on the stimulus, graded potentials can be depolarizing or hyperpolarizing.
Action potentials always lead to depolarization of membrane and reversal of the membrane potential. Amplitude is proportional to the strength of the stimulus. Amplitude is all-or-none; strength of the stimulus is coded in the frequency of all-or-none action potentials generated. Amplitude is generally small a few mV to tens of mV. Duration of graded potentials may be a few milliseconds to seconds. Action potential duration is relatively short; ms.
Ion channels responsible for graded potentials may be ligand-gated extracellular ligands such as neurotransmitters , mechanosensitive, or temperature sensitive channels, or may be channels that are gated by cytoplasmic signaling molecules. No refractory period is associated with graded potentials. Absolute and relative refractory periods are important aspects of action potentials.
The amount of change in the membrane potential is determined by the size of the stimulus that causes it. In the example of testing the temperature of the shower, slightly warm water would only initiate a small change in a thermoreceptor, whereas hot water would cause a large amount of change in the membrane potential.
Graded potentials can be of two sorts, either they are depolarizing or hyperpolarizing Figure 1. Both of these ions have higher concentrations outside the cell than inside; because they have a positive charge, they will move into the cell causing it to become less negative relative to the outside. If a positive charge moves out of a cell, the cell becomes more negative; if a negative charge enters the cell, the same thing happens. Figure 1. Graded Potentials.
Graded potentials are temporary changes in the membrane voltage, the characteristics of which depend on the size of the stimulus. Some types of stimuli cause depolarization of the membrane, whereas others cause hyperpolarization. It depends on the specific ion channels that are activated in the cell membrane. For the unipolar cells of sensory neurons—both those with free nerve endings and those within encapsulations—graded potentials develop in the dendrites that influence the generation of an action potential in the axon of the same cell.
This is called a generator potential. For other sensory receptor cells, such as taste cells or photoreceptors of the retina, graded potentials in their membranes result in the release of neurotransmitters at synapses with sensory neurons.
This is called a receptor potential. A postsynaptic potential PSP is the graded potential in the dendrites of a neuron that is receiving synapses from other cells. Postsynaptic potentials can be depolarizing or hyperpolarizing. Depolarization in a postsynaptic potential is called an excitatory postsynaptic potential EPSP because it causes the membrane potential to move toward threshold. Hyperpolarization in a postsynaptic potential is an inhibitory postsynaptic potential IPSP because it causes the membrane potential to move away from threshold.
Figure 2. Postsynaptic Potential Summation The result of summation of postsynaptic potentials is the overall change in the membrane potential. At point A, several different excitatory postsynaptic potentials add up to a large depolarization. At point B, a mix of excitatory and inhibitory postsynaptic potentials result in a different end result for the membrane potential. All types of graded potentials will result in small changes of either depolarization or hyperpolarization in the voltage of a membrane.
These changes can lead to the neuron reaching threshold if the changes add together, or summate. The combined effects of different types of graded potentials are illustrated in Figure 2. For receptor potentials, threshold is not a factor because the change in membrane potential for receptor cells directly causes neurotransmitter release. However, generator potentials can initiate action potentials in the sensory neuron axon, and postsynaptic potentials can initiate an action potential in the axon of other neurons.
Graded potentials summate at a specific location at the beginning of the axon to initiate the action potential, namely the initial segment. For sensory neurons, which do not have a cell body between the dendrites and the axon, the initial segment is directly adjacent to the dendritic endings.
For all other neurons, the axon hillock is essentially the initial segment of the axon, and it is where summation takes place. Summation can be spatial or temporal, meaning it can be the result of multiple graded potentials at different locations on the neuron, or all at the same place but separated in time. Spatial summation is related to associating the activity of multiple inputs to a neuron with each other. Temporal summation is the relationship of multiple action potentials from a single cell resulting in a significant change in the membrane potential.
Spatial and temporal summation can act together, as well. The process of converting electrical signals to chemical signals and back requires subtle changes that can result in transient increases or decreases in membrane voltage. To cause a lasting change in the target cell, multiple signals are usually added together, or summated. Does spatial summation have to happen all at once, or can the separate signals arrive on the postsynaptic neuron at slightly different times?
Explain your answer. There are two types of connections between electrically active cells, chemical synapses and electrical synapses. In a chemical synapse , a chemical signal—namely, a neurotransmitter—is released from one cell and it affects the other cell. In an electrical synapse , there is a direct connection between the two cells so that ions can pass directly from one cell to the next.
If one cell is depolarized in an electrical synapse, the joined cell also depolarizes because the ions pass between the cells. Chemical synapses involve the transmission of chemical information from one cell to the next. This section will concentrate on the chemical type of synapse. An example of a chemical synapse is the neuromuscular junction NMJ described in the chapter on muscle tissue.
In the nervous system, there are many more synapses that are essentially the same as the NMJ. All synapses have common characteristics, which can be summarized in this list:.
Other synapses are similar to this, and the specifics are different, but they all contain the same characteristics. Once in the synaptic cleft, the neurotransmitter diffuses the short distance to the postsynaptic membrane and can interact with neurotransmitter receptors. Receptors are specific for the neurotransmitter, and the two fit together like a key and lock.
Receptor potentials are generated in the specialized sensory receptor cells. Postsynaptic potentials are generated in the nerve cells. The end plate potentials are generated in the muscle cells. Action potential refers to a change in the electrical potential, which is associated with the transmission of impulses along the membrane of a nerve cell or muscle cell.
The three main stages of an action potential are the depolarization, repolarization, and refractory period. A sudden change in the membrane potential is referred to as a depolarization. Here, the internal charge changes from negative to positive. The opening of the ion-gated channels causes the membrane depolarization. As the sodium channels are opened, the migration of the positively-charged sodium ions into the nerve cell causes more positive charge inside the cell.
The three stages of the action potential are shown in figure 2. Figure 2: Stages of Action Potential. The restoration of the negative charge inside the nerve cell is known as the repolarization. This is caused by the opening of the potassium channels.
The influx of potassium ions into the outside of the nerve cell causes the reduction of the positive charge inside the cell. Refectory period refers to the time period between two action potentials. During the refectory period, sodium-potassium channels are opened to restore the resting potential. In the resting potential, the concentration of the sodium ions is high outside of the nerve cell while the concentration of the potassium ions is high inside the nerve cell.
Graded Potential: Graded potential refers to a membrane potential, which can vary in amplitude.
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