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By the end of this section, you will be able to:
  • Explain the differences between the types of graded potentials
  • Categorize the major neurotransmitters by chemical type and effect

The electrical changes taking place within a neuron, as described in the previous section, are similar to a light switch being turned on. A stimulus starts the depolarization, but the action potential runs on its own once a threshold has been reached. The question is now, “What flips the light switch on?” Temporary changes to the cell membrane voltage can result from neurons receiving information from the environment, or from the action of one neuron on another. These special types of potentials influence a neuron and determine whether an action potential will occur or not. Many of these transient signals originate at the synapse.

Graded potentials

Local changes in the membrane potential are called graded potentials and are usually associated with the dendrites of a neuron. 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 ( [link] ). For a membrane at the resting potential, a graded potential represents a change in that voltage either above -70 mV or below -70 mV. Depolarizing graded potentials are often the result of Na + or Ca 2+ entering the cell. 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. Hyperpolarizing graded potentials can be caused by K + leaving the cell or Cl - entering the cell. 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.

Graded potentials

The graph has membrane potential, in millivolts, on the X axis, ranging from negative 90 to positive 30. Time is on the X axis. The left half of the plot line is labeled the depolarizing graded potential. The plot has four progressively larger peaks, with each starting at the resting membrane potential of negative 70. The lowest peak reaches to about negative 65 and is narrow in width, as this represents a small stimulus that causes a small depolarization of the cell membrane. The second peak reaches to about negative 60 but is still narrow. This represents a larger stimulus causing more depolarization. The third peak also reaches to negative 60, but is about twice as wide as the other two peaks. This represents a stimulus of longer duration, which causes a longer lasting depolarization. However, this stimulus is not greater in strength than the previous stimulus. The rightmost peak among the depolarizing graded potentials reaches above the threshold line to about negative 51. This represents a stimulus of sufficient strength to trigger an action potential. The right half of the plot is labeled the hyperpolarizing graded potential. The plot line in this half begins at the resting potential of negative 70, but then drops to more negative membrane potentials. The first peak drops to negative 75 EV, the second peak drops to negative 80 EV and the third peak drops to negative 88 EV. These peaks represent a stimulus that results in hyperpolarization, which is triggered by the activation of specific ion channels in the cell membrane.
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.

Types of graded potentials

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.

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Source:  OpenStax, Anatomy & Physiology. OpenStax CNX. Feb 04, 2016 Download for free at http://legacy.cnx.org/content/col11496/1.8
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