Final answer:
The brain recruits motor units based on the required muscle strength for a movement, starting with smaller units and progressing to larger ones as needed. Communication via the neuromuscular junction and a pathway from the motor cortex to the spinal cord motor neurons is essential for this process.
Step-by-step explanation:
The brain recruits motor neurons based on the muscle strength needed for a particular movement. This process of recruitment involves activating the appropriate motor units, which are comprised of a motor neuron and the muscle fibers it controls. Initially, the brain activates smaller, more excitable motor units that govern smaller muscle fibers for fine, precise movements. As more strength is required, larger motor units with bigger, higher-threshold motor neurons are recruited to activate larger muscle fibers, ultimately leading to stronger muscle contractions.
This pattern of recruitment allows for actions as delicate as picking up a feather with minimal force to lifting heavy weights by utilizing larger motor units. Additionally, the neuromuscular junction, where the motor neuron attaches to a muscle, is fundamental in this communication process. Voluntary movements engage a pathway starting from the primary motor cortex, traveling down through the corticospinal tract, to synapse on motor neurons in the ventral horn of the spinal cord, which then project to the skeletal muscles causing contraction.
Lastly, the nervous system efficiently maintains muscle activity by allowing some motor units to rest while others are active, thus preventing complete muscle fatigue and ensuring prolonged muscular contraction when necessary.