Where do motor neurons extend to connect with skeletal muscle fibers?

The correct answer highlights the pathway of motor neurons as they connect with skeletal muscle fibers. Motor neurons originate in the brain or spinal cord and extend their axons to the sarcolemma, which is the cell membrane of a skeletal muscle fiber. This connection is critical for muscle contraction as it allows for the transmission of signals that initiate the contraction process.

When a motor neuron stimulates a muscle fiber, it releases neurotransmitters at the neuromuscular junction, leading to a cascade of events that result in muscle contraction. The direct link from the central nervous system to the muscle fibers ensures that precise signals can be delivered, allowing for coordinated muscle actions.

In contrast, the other options distract from this direct pathway: the heart's connections to organ systems are related to cardiac muscle, not skeletal muscle; the lungs' connection to the diaphragm pertains to respiratory function rather than voluntary muscle movement; and the digestive system's influence on abdominal wall muscles is not relevant to the focus on motor neuron pathways to skeletal muscles. Therefore, the connection between the brain or spinal cord and the sarcolemma is the appropriate and essential description of motor neuron pathways to skeletal muscles.