Final answer:
The brain increases neural firing during muscle contraction to code stimulus rate. The somatic nervous system adjusts muscle contraction speed in response to conditions, and increased action potential frequency can lead to complete tetanus with continuous contractions.
Step-by-step explanation:
The brain increases the frequency of neural firing to code stimulus rate during muscle contraction. The somatic nervous system responds to local conditions by adjusting the speed of muscle contraction. If the frequency of motor neuron signaling increases, it leads to summation and an increase in muscle tension. This process can range from incomplete tetanus, which is characterized by quick cycles of contraction with short relaxation phases, to complete tetanus, where contractions become continuous due to very high stimulus frequencies that eliminate relaxation phases.
Normal muscle activity is sustained and generates force through a graded muscle response. The action potentials from motor neurons are essential, and their frequency along with the number of neurons transmitting these impulses determine the tension produced in skeletal muscle. For greater force, more myofibers within the muscle respond to action potentials, as seen when a significant load is lifted.