Biological neurons use voltage changes across their cell membranes to create signals. Mechanical (e.g., touch) and chemical (e.g., odors) input received by the sensory division of the peripheral nervous system triggers the initial positive voltage change. If the voltage rises above a threshold level, an action potential results, which is a self-regenerating voltage spike that travels all the way to the axon terminal, which is the end of the neuron. At the axon terminal, neurotransmitter chemicals are released that can bind to receptors on the next neuron in the circuit, called the postsynaptic neuron. These neurotransmitter chemicals cause another change (negative or positive, depending upon the type of chemical and receptor) in voltage in the postsynaptic neuron, repeating the cycle described above. In this way, the initial action potential in the sensory part of the peripheral nervous system can signal neurons in the central nervous system (made up of the brain and spinal cord), and then back out to motor neurons in the peripheral nervous system, and finally to muscles to produce some action.
NeuroBytes neuron simulators process signals in similar ways as biological neurons. Each NeuroBytes has an LED embedded into it that makes it easy to understand how excited or inhibited the simulator is. At rest, the neuron is green. As it gets excited, the LED changes to red. If the excitation exceeds the threshold level, the LED flashes white, indicating an action potential signal. When a neuron is inhibited, the LED turns blue. Whether a neuron is excited or inhibited by an incoming signal depends upon the type of neurotransmitter cable used.