Final answer:
Recruitment is the process that involves activating larger motor units in muscles as more force is required. This process starts with the activation of smaller, lower-threshold motor units, gradually including larger, higher-threshold motor units for greater strength, with action potentials playing a crucial role in force modulation.
Step-by-step explanation:
Neuromuscular Recruitment
The process that activates larger motor units as more strength is needed is known as recruitment. Initially, smaller motor units with low-threshold motor neurons are activated, which produce a minimal force. As demand for force increases, larger motor units with higher-threshold motor neurons are recruited to engage larger muscle fibers, thereby producing a greater contractile strength. This is an efficient system, as the nervous system can precisely control muscle tension by varying which and how many motor units are active at a given time.
During a muscle contraction, the number and size of motor units recruited increase in proportion to the required force, starting from the smallest to the largest. Eventually, if necessary, all motor units can be recruited for maximal contraction. However, this is an energy-intensive state that cannot be sustained for long, which is why not all motor units are active simultaneously to prevent fatigue.
Further force can be generated by increasing the frequency of action potentials, which causes an increase in calcium concentration around the tropomyosin, promoting the formation of cross-bridges between actin and myosin filaments.