A graded potential is a small, localized change in the electrical charge across a nerve cell’s membrane. Unlike the signals that travel long distances down nerve fibers, graded potentials vary in size depending on how strong the stimulus is, and they fade out over short distances. They are the nervous system’s first step in processing incoming information, whether that information comes from your senses, from other neurons, or from both at once.
How Graded Potentials Are Generated
Every nerve cell maintains a resting electrical charge, with the inside of the cell sitting at roughly -60 to -70 millivolts compared to the outside. A graded potential happens when a stimulus opens gated ion channels in the cell membrane, allowing charged particles (ions) to flow in or out. Any stimulus that opens a gated channel can create one.
Most graded potentials occur at the dendrites, the branch-like extensions where a neuron receives signals from other cells. A smaller number occur on the cell body itself. In both locations, the channels responsible are primarily ligand-gated, meaning they open when a chemical messenger (a neurotransmitter) released from a neighboring neuron binds to them. Sensory receptor cells generate graded potentials differently: a physical or chemical stimulus, such as pressure on the skin or light hitting the retina, opens channels directly or triggers a chain of molecular events that opens them.
Why “Graded” Matters
The word “graded” refers to the fact that these potentials come in varying sizes. A stronger stimulus opens more ion channels, which lets more ions flow, which produces a larger voltage change. A weak touch on your fingertip might shift the local membrane voltage by just a few millivolts, while a firm press shifts it by considerably more. This proportional relationship makes graded potentials analog signals, encoding stimulus intensity as a continuously variable voltage rather than as a simple on/off switch.
Graded potentials can also be positive or negative. An excitatory signal pushes the membrane voltage in the positive direction (depolarization), while an inhibitory signal pushes it in the negative direction (hyperpolarization). This flexibility lets neurons do something critical: weigh competing inputs before deciding whether to fire.
Excitatory vs. Inhibitory Potentials
When excitatory neurotransmitters like glutamate bind to receptors on a dendrite, they open channels that allow positively charged sodium ions to rush into the cell. This influx of positive charge makes the inside of the membrane less negative, a shift called depolarization. The result is an excitatory postsynaptic potential, or EPSP, which nudges the neuron closer to firing.
Inhibitory neurotransmitters work in the opposite direction. When GABA binds to its receptors, it opens channels that allow negatively charged chloride ions to flow into the cell. This extra negative charge makes the inside of the membrane more negative than it was at rest, a shift called hyperpolarization. The resulting inhibitory postsynaptic potential, or IPSP, pushes the neuron further from its firing threshold.
The distinction between excitation and inhibition comes down to a simple rule: if the voltage change drives the membrane toward the firing threshold, it’s excitatory; if it drives the membrane away from threshold, it’s inhibitory. Interestingly, an IPSP can sometimes be slightly depolarizing and still count as inhibitory, as long as it doesn’t push the membrane positive enough to reach the threshold for firing.
How Graded Potentials Fade With Distance
One defining feature of graded potentials is that they weaken as they spread from the point of origin. When sodium ions enter the cell at a particular spot, they are attracted toward negative charges on the inner surface of the membrane and begin to spread. But as they move, the voltage change dissipates. Think of it like dropping a stone into water: the ripple is strongest at the center and fades as it radiates outward. This is why graded potentials are sometimes called local potentials. They cannot carry a signal from your toe to your spinal cord on their own.
For a signal to travel long distances, it needs to be converted into an action potential, the all-or-nothing electrical spike that travels down the long fiber (axon) of a neuron without losing strength. Graded potentials set the stage for this conversion.
Summation: How Small Signals Add Up
A single graded potential is usually too small to trigger an action potential on its own. The neuron solves this problem through summation, the process of combining multiple graded potentials together. There are two forms.
Spatial summation occurs when graded potentials arrive at the same time from different locations on the neuron. Imagine three separate synapses on different dendrites all receiving excitatory signals simultaneously. Each one produces a small EPSP, and those voltage changes spread toward the cell body, where they overlap and combine. If the combined voltage is large enough, it reaches threshold and triggers an action potential.
Temporal summation occurs when a single synapse fires repeatedly in quick succession. The cell membrane can briefly store charge, so if a second EPSP arrives before the first one has fully faded, their voltages stack on top of each other. A rapid series of small signals at one synapse can build up enough cumulative depolarization to reach threshold.
Both types of summation work for inhibitory signals too. A barrage of IPSPs can make it much harder for the neuron to fire, even if excitatory signals are arriving at the same time. The neuron essentially tallies all incoming excitatory and inhibitory graded potentials, and the net result determines whether an action potential is launched. This integration is how your nervous system makes decisions at the cellular level.
Graded Potentials in Sensory Receptors
Your senses depend on graded potentials as the very first step of perception. Sensory receptor cells convert physical or chemical stimuli into electrical signals through a process called transduction. When light hits a photoreceptor in your retina, when pressure deforms a touch receptor in your skin, or when an odor molecule binds to a receptor in your nose, the end result is the opening or closing of ion channels that produce a graded potential.
Sensory receptors can be broadly grouped by what activates them: mechanoreceptors respond to physical forces like pressure, vibration, and sound waves; thermoreceptors respond to temperature; chemoreceptors respond to molecules, as in taste and smell; and photoreceptors respond to light. Some receptors are polymodal, responding to more than one type of stimulus. The capsaicin receptor, for example, responds to both heat and the chemical compounds that make chili peppers feel hot.
In these sensory cells, the size of the graded potential reflects the intensity of the stimulus. A louder sound produces a larger voltage change in the hair cells of the inner ear. A brighter light produces a larger change in retinal cells. This graded, proportional response is what allows your nervous system to distinguish a whisper from a shout or a dim glow from a spotlight, all before the signal is ever converted into the digital, all-or-nothing language of action potentials.
Graded Potentials vs. Action Potentials
- Size: Graded potentials vary in amplitude depending on stimulus strength. Action potentials are all-or-nothing, always the same size once triggered.
- Direction: Graded potentials can be depolarizing (positive shift) or hyperpolarizing (negative shift). Action potentials are always depolarizing.
- Location: Graded potentials occur mainly in dendrites, cell bodies, and sensory receptors. Action potentials occur along axons.
- Travel distance: Graded potentials decay over short distances. Action potentials regenerate themselves along the full length of the axon, traveling without losing strength.
- Signal type: Graded potentials are analog, encoding information as a range of voltages. Action potentials are digital, encoding information as frequency (how many fire per second) rather than size.
The relationship between the two is sequential. Graded potentials gather and integrate information at the input end of the neuron. If summation pushes the voltage past threshold at the junction between the cell body and the axon, an action potential fires and carries the message to the next cell in the chain.