The nervous system controls all motor units and the associated muscles. The parts of the nervous system referred to here are the brain, spinal cord, and motor neurons. Although positive adaptations to those parts of your nervous system have positive flow-on effects to the autonomic nervous system (ANS). The ANS is mainly responsible for subconscious activity such as body temperature, breathing, heart rate, etc.
As a result of training, adaptations occur in both the nervous system and muscle fibers (neuromuscular system). A gain in strength is often thought to be due to an increase in size or quality of muscle. Whereas in reality, greater strength is mainly attributable to changes in the nervous system (neural engagement).
For greater neural engagement or strength, you have to train the nervous system for the particular activity you want to do. The nervous system coordinates the specific control of all motor units involved in those muscle actions. This training involves many agonist and antagonist interactions. Plus it is controlling the many other different muscle force interactions that suit the particular movement in the most efficient way to create the most amount of force.
Looking at the body as a whole and the biology involved, this is quite a complex system of interactions beyond any one muscle group.