Final answer:
A motor unit discharges are measured using electromyography (EMG). A myogram is used to track muscle contractions and the frequency of motor unit discharges. Patterns like twitches, wave summation, and tetanus indicate motor unit activity and function.
Step-by-step explanation:
To determine when a motor unit begins to discharge and its discharge frequency, we employ a method that records the electrical activity of muscles called electromyography (EMG). Experimentally, when a neuronal action potential reaches the neuromuscular junction (NMJ), it triggers the release of acetylcholine (ACh) which causes the muscle fiber to depolarize and initiate a contraction. This process can be visualized using a myogram, which is an instrument that measures muscle tension over time.
A single contraction from a motor neuron to its motor unit results in a muscle twitch, which has three phases: the latent period, the contraction phase, and the relaxation phase. If the frequency of motor neuron signaling increases, successive stimuli can add together in a process called wave summation. With high enough frequency, a continuous contraction called tetanus occurs.
During experimental observation, we record the pattern of these contractions through EMG. The change in muscle tension and the frequency of discharges can be calculated from the myogram trace. This combination of EMG and myography allows for precise measurement of motor unit activation and functions.