Neurons communicate through electrical signals. Graded potentials are local changes in a neuron’s membrane potential, typically arising in the dendrites and cell body. These temporary shifts serve as input signals, indicating the neuron is receiving information and preparing for further processing.
Defining Characteristics
Graded potentials possess distinct characteristics. Their strength, or amplitude, is directly proportional to the stimulus intensity; a stronger stimulus produces a larger potential. They also exhibit decremental conduction, meaning their strength diminishes as they spread passively away from their point of origin. Another important characteristic is summation, where multiple graded potentials can combine to produce a larger, integrated response.
Temporal summation occurs when a single presynaptic neuron repeatedly stimulates the postsynaptic neuron in rapid succession, causing individual potentials to add up over time. Spatial summation involves multiple presynaptic neurons simultaneously stimulating different locations on the postsynaptic neuron, with their individual potentials converging and summing at a common point. This ability to summate allows neurons to integrate various incoming signals.
Origin of Graded Potentials
Graded potentials originate from two primary sources. The most common involves neurotransmitters released from the axon terminals of other neurons. When these neurotransmitters bind to specific receptors on the postsynaptic neuron’s membrane, they trigger the opening of ion channels, leading to a localized change in the membrane potential. This ion movement alters the electrical charge across the membrane, creating the graded potential. Sensory stimuli also generate graded potentials in specialized receptor cells, converting physical energy into electrical signals.
Graded potentials can be either excitatory postsynaptic potentials (EPSPs) or inhibitory postsynaptic potentials (IPSPs). EPSPs are depolarizations that make the neuron’s membrane potential less negative, bringing it closer to the threshold for firing an action potential. Conversely, IPSPs are hyperpolarizations that make the neuron’s membrane potential more negative, making it less likely to generate an action potential.
Impact on Neuron Firing
Graded potentials influence whether a neuron generates an action potential. As they spread across the dendrites and cell body, they converge at the axon hillock, often referred to as the neuron’s “trigger zone.” Here, the sum of all incoming EPSPs and IPSPs determines the net change in the membrane potential. If this combined potential reaches the threshold potential (typically around -55 mV), it triggers the rapid opening of voltage-gated ion channels, initiating an action potential. Graded potentials integrate excitatory and inhibitory inputs, allowing the neuron to decide whether to fire an an action potential and transmit information along its axon.
Graded Potentials and Action Potentials
Graded potentials and action potentials serve distinct but complementary roles. Graded potentials are variable in amplitude, with their strength directly reflecting the intensity of the stimulus. In contrast, action potentials are “all-or-none” events, meaning they fire at a uniform, fixed amplitude once the threshold is reached. Regarding propagation, graded potentials are decremental; their strength diminishes over distance from their origin. Action potentials, however, are non-decremental and propagate along the axon without losing strength.
Graded potentials typically occur in the dendrites and cell body, serving as local input signals. Action potentials are generated at the axon hillock and travel along the axon for long-distance communication. The duration of graded potentials can vary depending on the stimulus, while action potentials have a relatively fixed, brief duration. Their primary purpose is to integrate incoming information and determine if an action potential should be generated.