>> The dendrites and cell body of a neuron may receive thousands of synaptic connections from other neurons. When these neurons are activated, they release neurotransmitters that open ligand-gated ion channels in the membrane of the receiving neuron. As ions flow through these channels, the membrane potential of the receiving neuron changes at the point of stimulus and passively spreads along the membrane. Some of these synapses shown here in green are considered excitatory because they make it more likely that the receiving neuron will fire an action potential. Excitatory signals depolarize the membrane, making the membrane potential less negative. This change is called an excitatory postsynaptic potential. Inhibitory signals, shown here coming from the red connections, hyperpolarize the membrane, making the membrane potential more negative and making the neuron less likely to fire an action potential. This change in the membrane potential is called an inhibitory postsynaptic potential. If the neuron receives enough excitatory stimulation, the membrane potential will depolarize sufficiently for the receiving neuron to fire an action potential, beginning at a region called the axon hillock. The changes in membrane potential at the axon hillock can be measured with a voltmeter. The voltmeter measures the difference between a recording electrode inside the cell, and a reference electrode outside the cell. This value is normally about negative 70 millivolts, which is the resting potential of the neuron. If enough excitatory signals arrive close enough together in time, they may depolarize the membrane above a certain voltage, called the threshold potential. At this point, an action potential is generated. The action potential propagates the signal along the entire length of the axon.