Neurons within the body communicate by triggering each other, and this triggering sends signals to the brain. The brain uses these signals to decide what action needs to be taken. Depending on the intensity of the signal, the signaling among neurons can be slow or fast. Temporal summation refers to the constant triggering of the neuron by another within the same area in the axon. The five phases of temporal summation include relaxation, incomplete and complete tetanus, treppe and fatigue.
Phase 1: Relaxation
The relaxation phase is the first and last phase a neuron goes through during temporal summation. At this stage the neuron is not contracting or sending signals to neighboring neurons. No ion channels are open within the membrane and neurotransmitters that signal a contraction or signal another neuron are not released. When a neuron completes a temporal summation cycle, it will go back into the relaxation phase.
Phase 2: Treppe
When a neuron is triggered, it will contract or carry out a motor function. Only one trigger, above threshold, is needed for the neuron to fire. The neuron will move into the treppe phase, when it receives constant stimulation from neighboring axons. The triggering is constant and the target neuron, although simultaneously keeps contracting, will have ample time for full relaxation between contractions.
Phase 3: Incomplete Tetanus
The neuron enters the incomplete phase of temporal summation when the relaxation times are shortened due to the high frequency signaling. In temporal summation, the neuron, even though has already been activated, will continue to receive signaling by the neighboring neuron. This constant signaling result in shorter contractions at much higher frequencies than those experienced during the treppe phase. The neuron is still able to relax during contractions but the relaxation time is reduced and relaxation cannot be completed before another contraction occurs.
Phase 4: Complete Tetanus
The neuron moves to the complete tetanus phase when it begins to receive constant triggering by surrounding neurons. Similar to incomplete tetanus phase, a neuron during this phase will continue to contract but will not have time to relax. This results in short contracts at higher frequency. The neuron is being stimulated without having time to rest. The lack of relaxation will eventually cause the neuron to stop contracting and enter the fatigue phase.
Phase 5: Fatigue
A neuron has a stimulation threshold and once at this stage the neuron cannot contract any further. During the fatigue phase, the muscle is not able to continue with rapid contractions and has already used up its energy stores to keep up with the stimulation. There is an imbalance of membrane ions and acidic buildup within the intracellular compartments of the cell. Neuron contraction at this phase does not occur and the tensile strength of the cell is altered.