172 Anatomy & Physiology Essentials Copyright Goodheart-Willcox Co., Inc. many fibers. Motor units are typically contained within a portion of a muscle, but may also be interspersed with the muscle cells of other motor units. Action Potentials How does a motor neuron communicate with the muscle cells to stimulate them? As Figure 6.5 shows, a long, thin fiber called an axon connects the motor neuron cell body with the muscle fibers included in the motor unit. Close to the fibers, the axon branches into axon terminals, which in turn branch into individual muscle fibers. Each link between an axon terminal and a muscle fiber is called a neuromuscular junction. The axon terminal and fiber are separated by a tiny gap known as the synaptic cleft, which is filled with interstitial fluid (Figure 6.6). When a nerve impulse reaches the end of an axon terminal, a chemical called a neurotransmitter discharges and diffuses across the synaptic cleft to attach to receptors on the muscle fiber’s sarcolemma. The neuro­ transmitter that stimulates muscle is acetylcholine. The effect of acetylcholine is to make the sarcolemma temporarily permeable. This opens channels that allow positive sodium ions (Na+) to rapidly invade the fiber at the same time that positive potassium ions (K+) rush out of the fiber. Because more Na+ enters than K+ exits, the net effect is the creation of a relatively positive charge inside the muscle fiber. T he development of tension in a skeletal muscle is influenced by a number of variables, including signals from the nervous system, the properties of the muscle fibers, and the arrangement of fibers within the muscle. This section describes how muscle actions contribute to muscular strength, power, and endurance. Motor Units Muscle tissue cannot develop tension unless it is stimulated by one or more nerves. Because of the dependent relationship between the muscular system and the nervous system, the two are often referred to collectively as the neuromuscular system. A nerve that stimulates skeletal muscle, which is under voluntary control, is known as a motor neuron. A single motor neuron and all of the muscle cells that it stimulates is known as a motor unit (Figure 6.5). The motor unit is the functional unit of the neuro- muscular system. One motor neuron supplying impulses to a muscle may connect to anywhere between 100 and nearly 2,000 skeletal muscle fibers, depending on the size and function of the muscle. The small muscles responsible for finely tuned movements, such as those in the eyes and fingers, have small motor units with few fibers per motor unit. Large, powerful muscles, such as those surrounding the hips, have large motor units with SECTION 6.2 Skeletal Muscle Physiology Objectives • Describe a motor unit and explain the functional differences between motor units that contain large and small numbers of muscle fibers. • Describe the differences between slow-twitch and fast-twitch skeletal muscle fibers. • Discuss the concepts of muscular strength, power, and endurance. • Describe and give examples of the types of body motions that occur in the sagittal, frontal, and transverse planes. Key Terms abduction acetylcholine action potential adduction all-or-none law axon axon terminals circumduction cross bridges dorsiflexion eversion extension fast-twitch flexion hyperextension insertion inversion lateral rotation medial rotation motor neuron motor unit neuromuscular junction opposition origin parallel fiber architecture pennate fiber architecture plantar flexion pronation sarcomeres slow-twitch supination synaptic cleft tetanus