Muscles - The Neuromuscular Junction Each skeletal muscle fiber is connected by an axon from a nerve cell called a motor neuron. This motor neuron extends outward from the brain or spinal cord, to the muscle fiber. The muscle fiber contracts only when a motor neuron stimulates it. The functional connection between the motor neuron and muscle fiber is called a neuromuscular junction. Here, the muscle fiber membrane is specialized to form a motor end plate. In this region of the muscle fiber, nuclei and mitochondria are abundant, and the cell membrane (sarcolemma) is extensively folded. The end of the motor neuron branches and projects to the motor end plate. The cytoplasm at the terminal ends of these motor neuron fibers is rich in mitochondria and contains many tiny vesicles (synaptic vesicles) that store chemicals called neurotransmitters.