Graded potentials are localized shifts in a neuron’s membrane potential. These temporary changes are typically associated with the dendrites and cell body, serving as initial responses to stimuli. They are fundamental to how neurons process incoming information.
What Makes Them “Graded”?
The term “graded” refers to the characteristic that their amplitude, or strength, directly varies with the intensity of the stimulus that caused them. A weak stimulus might produce a small graded potential, while a stronger stimulus results in a larger one. This is in contrast to action potentials, which are “all-or-nothing” events with a consistent amplitude once a certain threshold is reached.
Graded potentials also exhibit decremental spread, meaning their strength diminishes as they travel away from their point of origin. Furthermore, graded potentials can combine through a process called summation, either spatially or temporally. Spatial summation occurs when multiple graded potentials from different locations on the neuron arrive simultaneously and add up. Temporal summation involves multiple graded potentials from the same source arriving in rapid succession. This summation allows a neuron to integrate various inputs and determine whether to generate a larger signal.
How Graded Potentials Arise
Graded potentials emerge from the opening or closing of specific ion channels in the neuron’s membrane in response to a stimulus. These stimuli can include neurotransmitters binding to receptors or various forms of sensory input. When these channels open, ions like sodium (Na+), potassium (K+), calcium (Ca2+), or chloride (Cl-) move across the membrane, altering the electrical charge inside the cell.
The movement of ions leads to a localized change in the membrane potential, making it either more positive (depolarization) or more negative (hyperpolarization). Depolarizing graded potentials often result from the influx of positive ions like Na+ or Ca2+, making the inside of the cell less negative. Conversely, hyperpolarizing graded potentials can occur due to the efflux of K+ or the influx of Cl-, making the inside of the cell more negative. The specific type and amount of ion movement, and how long the channels remain open, influence the characteristics of the graded potential.
Their Role in the Nervous System
Graded potentials serve as initial electrical signals that neurons use to process incoming information. They are key in deciding whether a neuron will generate an action potential, the nervous system’s long-distance signal. Through their ability to depolarize or hyperpolarize the membrane, graded potentials can either excite a neuron, pushing its membrane potential closer to the threshold for firing an action potential, or inhibit it, moving the potential further away from that threshold.
The integration of multiple inputs, facilitated by the summation of graded potentials, primarily occurs at the dendrites and cell body. This allows the neuron to weigh various signals received from other neurons or sensory receptors. The summed effect of these graded potentials at the axon hillock, a specialized region at the base of the axon, determines if the neuron reaches the threshold to fire an action potential.
Where Graded Potentials Occur
Graded potentials are found in various parts of the nervous system, typically in regions where neurons receive input. Two prominent types are postsynaptic potentials and receptor potentials. Postsynaptic potentials (PSPs) occur in the dendrites and cell body when a neuron receives signals from other neurons at a synapse. These can be either excitatory postsynaptic potentials (EPSPs), which are depolarizing and increase the likelihood of an action potential, or inhibitory postsynaptic potentials (IPSPs), which are hyperpolarizing and decrease that likelihood.
Receptor potentials are generated in sensory neurons in response to specific external stimuli. For instance, sensory receptors in the skin produce receptor potentials when stimulated by touch or temperature changes. Specialized sensory cells, like those in the retina for vision or taste buds for taste, also generate receptor potentials that then influence associated sensory neurons.