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Graded Potentials

Author: Sophia

what's covered

In this lesson, you will learn about cell-to-cell connections and how they transfer and regenerate an action potential. Specifically, this lesson will cover:

1. Graded Potentials
- 1a. Types of Graded Potentials
- 1b. Summation

1. Graded Potentials

recall

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.

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.

EXAMPLE

Testing the temperature of the shower, slightly warm water would only initiate a small change in a heat-sensitive receptor, whereas hot water would cause a large amount of change in the membrane potential.

Graded potentials can be of two sorts:

Depolarizing
Hyperpolarizing

For a neuronal 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 Ca²⁺ 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.

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 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 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 - 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.

1a. Types of Graded Potentials

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.

1b. Summation

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 the image below. If the total change in voltage in the membrane is a positive 15 mV, meaning that the membrane depolarizes from -70 mV to -55 mV, then the graded potentials will result in the membrane reaching threshold.

Graded potentials summate at a specific location at the beginning of the axon to initiate the action potential, namely the initial segment. This location has a high density of voltage-gated Na⁺ channels that initiate the depolarizing phase of the action potential.

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 the combination of various inputs to a neuron with each other. Temporal summation is the combination 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.

This graph has membrane potential, in millivolts, on the X axis, ranging from negative 90 to negative 40. Time is on the X-axis. The plot line is moving up and down between the resting membrane potential of minus 70 EV and the threshold potential of minus 55 EV. An EPSP causes the plot line to move higher, closer to the threshold potential. An IPSP causes the plot line to move lower, further away from the threshold potential. Toward the right side of the graph, the neuron receives an EPSP that pushes the membrane potential above the threshold, triggering an action potential that causes the plot line to quickly rise above positive 30 EV. The plot line then quickly drops back below minus 70 EV but then gradually increases back to minus 70. A picture of a neuron indicates that excitatory post-synaptic potentials are commonly provided by synapses on the neuron’s dendrites. Inhibitory post-synaptic potentials are commonly provided by synapses near the neuron’s axon hillock. — 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.

terms to know

Graded Potential

Local changes in the membrane potential.

Postsynaptic Potential (PSP)

The graded potential in the dendrites of a neuron that is receiving synapses from other cells.

Excitatory Postsynaptic Potential (EPSP)

A depolarizing graded potential in the postsynaptic membrane.

Inhibitory Postsynaptic Potential (IPSP)

A hyperpolarizing graded potential in the postsynaptic membrane.

Summate

Add together.

Spatial Summation

The combination of various inputs to a neuron with each other.

Temporal Summation

The combination of multiple action potentials from a single cell resulting in a significant change in the membrane potential.

summary

In this lesson, you learned how graded potentials are created, how the types of graded potentials affect membrane potential, and how summation functions to allow graded potentials to create an action potential.

Source: THIS TUTORIAL HAS BEEN ADAPTED FROM OPENSTAX “ANATOMY AND PHYSIOLOGY 2E.” ACCESS FOR FREE AT HTTPS://OPENSTAX.ORG/DETAILS/BOOKS/ANATOMY-AND-PHYSIOLOGY-2E. LICENSE: CC ATTRIBUTION 4.0 INTERNATIONAL.

Terms to Know

Excitatory Postsynaptic Potential (EPSP): A depolarizing graded potential in the postsynaptic membrane.
Graded Potential: Local changes in the membrane potential.
Inhibitory Postsynaptic Potential (IPSP): A hyperpolarizing graded potential in the postsynaptic membrane.
Postsynaptic Potential (PSP): The graded potential in the dendrites of a neuron that is receiving synapses from other cells.
Spatial Summation: The combination of various inputs to a neuron with each other.
Summate: Add together.
Temporal Summation: The combination of multiple action potentials from a single cell resulting in a significant change in the membrane potential.

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Graded Potentials

Table of Contents

1. Graded Potentials

1a. Types of Graded Potentials

1b. Summation