Final answer:
The efferent neurons responsible for causing contraction of skeletal muscles originate from the ventral horn of the spinal cord, as part of the lower motor neurons within the somatic nervous system.
Step-by-step explanation:
The efferent neurons that innervate skeletal muscle and induce them to contract originate from the ventral horn of the spinal cord. These neurons, which are part of the somatic nervous system, are specifically known as lower motor neurons and are imperative for muscle contraction. Each of these motor neurons may extend its axon to connect with multiple muscle fibers, forming what we know as nerves. The axon's length is determined by the distance it needs to cover to reach the muscles it controls.
The corticospinal tract plays a significant role in voluntary muscle movement and is composed of upper motor neurons and lower motor neurons. The former originates in the primary motor cortex and synapses with the latter in the ventral horn, which in turn project to skeletal muscles to induce contraction.