change in cellular excitability. Moderate levels of synaptic plasticity produce nonsynaptic changes that will synergistically act with the synaptic mechanisms to strengthen a response. Conversely, more robust levels of synaptic plasticity will produce nonsynaptic responses that will act as a negative feedback mechanism. The negative feedback mechanisms work to protect against saturation or suppression of the circuit activity as a whole. Axonal modulation Axonal modulation is a type of plasticity in which the number, activity, or location of ion channels in the axon changes. This causes the neuron to behave differently when stimulated. The modulation of ion channels is a response to a change in the stimulation frequencies of a neuron. Propagation plasticity Because it is the summation of the action potentials that eventually results in the threshold polarization being crossed, the temporal relationship of different input signals is very important in determining if and when a post-synaptic neuron will fire. Over time, the time it takes an action potential to propagate down the length of a particular axon can change. In one experiment multielectrode arrays were used to measure the time it took for action potentials to travel from one electrode to another, called latency. The neurons were then stimulated and the value of the latency was recorded over time. The latency values changed over time, suggesting that axonal plasticity influenced the propagation of action potentials. Shunting Shunting is a process in which axonal ion channels open during the passive flow (not requiring an ion pump) of a subthreshold depolarization down the axon. Usually occurring at axonal branch points, the timing of these channels opening as the subthreshold signal arrives in the area causes a hyperpolarization to be introduced to the passively flowing depolarization. Therefore, the cell is able to control which branches of the axon the subthreshold depolarization current flows through, resulting in some branches of the axon being more hyperpolarized than others. These differing membrane potentials cause certain areas of the neuron to be more excitable than others, based on the specific location and occurrence of shunting. High frequency stimulation Short-term effects: High frequency stimulation of a neuron for a short period of time increases the excitability of the neuron by lowering the amount of voltage required to fire an action potential. High frequency stimulation leads to an increase in the intracellular concentration of sodium and calcium ions due to the repeated opening of voltage-gated sodium and