# Graded Potential --- A **graded potential** is chemical signal that one [[neurons|neuron]] can send to another that can cause a change the [[membrane potential]] of that neuron. If enough excitatory graded potentials happen in a short enough time span, then the membrane potential can reach the threshold at which an [[action potential]] can occur. Neurons that send the graded potential are called the **presynaptic neurons** and the receiving neuron is the **postsynaptic neuron**. Graded potentials can either be *excitatory* (if it brings the [[membrane potential]] less negative) or *inhibitory* (if it brings the [[membrane potential]] more negative). Excitatory graded potentials are called **EPSPs** (excitatory postsynaptic potentials) and inhibitory graded potentials are called **IPSPs** (inhibitory postsynaptic potentials). No single graded potential triggers an [[action potential]]. Graded potentials can add on to each other and EPSPs and IPSPs effectively cancel each other out. There must be enough EPSPs either in rapid succession from the same source (temporal summation) or simultaneously from multiple (temporal summation) for there to be a great enough change in the membrane potential to trigger ## EPSPs **EPSPs** are *excitatory* graded potentials. They occur when the presynaptic neuron releases the [[neurotransmitter]] that binds to and opens [[channel proteins#Gated Channels|chemically-gated]] [[ion|cation]] channels. Both Na$^+$ and K$^+$ are free to pass through these, but Na$^+$ flows in faster than K$^+$ flows out, so a net positive results. ## IPSPs **IPSPs** are *inhibitory* graded potentials. They occur when the presynaptic neuron releases the [[neurotransmitter]] that binds to and opens [[channel proteins#Gated Channels|chemically-gated]] K$^+$ or Cl$^-$ channels. This causes K$^+$ to move out and Cl$^-$ moves in—either way there is a net negative charge change as a result. ___