Chapter 6 The Muscular System 173 Copyright Goodheart-Willcox Co., Inc. Spinal cord Motor neuron cell bodies Motor unit A Motor unit B Nerve Muscle Axons Muscle fibers Axon terminals at neuromuscular junctions © Body Scientific International Figure 6.5 Each motor unit includes a motor neuron and all the muscle fibers it activates. Axon Axon terminal Vesicles containing acetylcholine Synaptic cleft Sarcolemma Muscle fiber Acetylcholine receptor sites Diffusing acetylcholine © Body Scientific International Figure 6.6 The neuromuscular junction is the site at which nerve impulses are transmitted to muscle.