Cell membranes have an electric potential. Neurons can vary their membrane potential in response to stimuli. When the membrane potential varies with the strength of the stimulus, it is called a graded potential. It is caused by the action of ligand gated ion channels which control the flow of ions across the membrane, thereby varying the membrane potential.
Neurotransmitters tend to cause a graded potential in the post synaptic neuron. Graded potentials can be either inhibitory or excitatory depending on whether the membrane is more polarized or less polarized. An inhibitory potential makes the neuron less likely to have an action potential by making the membrane more negative. An excitatory potential makes the neuron more likely to have an action potential by making the membrane less negative. Either way, the magnitude of the graded potential depends on the size of the stimulus. Graded potentials have a much smaller amplitude than an action potential and are more easily reversed. Ions such as sodium, potassium or chlorine ions are transported through ligand gated ion channels present in the membrane of the neuron. Action potential can be reached through summation of graded potential over time and space. Temporal summation occurs when the presynaptic neuron fires local potentials so rapidly that they overlap on each other to trigger an action potential. Spatial summation occurs when potentials from different synapses add up locally to trigger an action potential.