 In this video I will define motor unit, muscle tension, and twitch, and describe the factors that contribute to regulation of muscle tension. Contraction of skeletal muscles is stimulated at the neuromuscular junction by the neurotransmitter acetylcholine released from the axon terminal of a somatic motor neuron. The axon of a single somatic motor neuron can branch to innervate multiple skeletal muscle fibers. A single motor neuron and all the muscle fibers it supplies is known as a motor unit. When an action potential travels down the axon of a somatic motor neuron, this will lead to excitation of all of the skeletal muscle fibers in that motor unit. A twitch is a single contraction in a skeletal muscle resulting from a single action potential spreading through the skeletal muscle resulting from the excitation of that muscle by a somatic motor neuron. In this graph we see a myogram of a muscle twitch. There is a latent period of the twitch when the action potential is spreading through the skeletal muscle before contraction begins. Then during the contraction period we see there is an increase in muscle tension. Muscle tension is the force a muscle exerts on an object when contracted so we could measure the muscle tension by attaching a device known as a force transducer to the tendon of a muscle and when the muscle contracts pulling on the force transducer we are able to measure the amount of muscle tension that is produced by that muscle. You can see here the duration of the contraction period is about 20 milliseconds then the relaxation period begins as calcium is being pumped back into the sarcoplasmic reticulum. The sliding filament mechanism will be inactivated as triple myosin will cover the binding sites of actin preventing cross bridge formation and so the twitch of a skeletal muscle is an all or nothing response stimulated by a single action potential. Although a twitch is an all or nothing response there are a variety of factors that can affect the amount of muscle tension that is produced during contraction. One of those factors is the length tension relationship. If muscles are stretched to a longer than optimal length this will lead to a decrease in the amount of cross bridges that form between myosin and actin. Similarly if the length of the muscle is less than optimal there will be a smaller number of cross bridges that can form between myosin and actin so the optimal length is where a muscle can produce the greatest amount of muscle tension and this results from having the highest number of cross bridges forming between myosin and actin. Another factor that influences muscle tension is motor unit recruitment. This refers to the activation of more motor units within a skeletal muscle organ. Motor unit recruitment follows an order from the smallest motor units to the largest motor units so we can see in the graph here when motor unit 1 is activated there's a small amount of tension developed and then the nervous system can activate motor unit 2 in addition to motor unit 1 and you'll notice that the skeletal muscle fibers in motor unit 2 are larger than the fibers in motor unit 1 and so motor unit 2 will produce an even larger amount of force than motor unit 1 and this will be added to the amount of force produced by motor unit 1 as both motor unit 1 and motor unit 2 are activated simultaneously and then to produce an even greater amount of force we can activate a third motor unit with even larger muscle fibers and so the motor unit recruitment following from the smallest motor units to the largest motor units leads to increasing muscle tension. Another major factor that affects muscle tension is the frequency of stimulation. If a somatic motor neuron is firing action potentials at a high frequency that motor neuron can stimulate a high frequency of action potentials within the skeletal muscle fibers of that motor unit we can see in the graph on the left a moderate frequency produces wave summation where the first twitch is produced and relaxation begins then another twitch occurs and adds to the first twitch producing a greater muscle tension. At high frequency stimulation muscle contraction will produce tetanus or tetanic contraction shown in the graph on the right. In tetanus the skeletal muscle fibers are firing action potentials at a very high frequency so that there is no time for relaxation between successive twitches this leads to a sustained smooth contraction producing a very high amount of muscle tension and tetanus is the way that we normally stimulate our muscles to produce contraction enabling the movement of our body as individual muscle twitches produce too small amount of muscle tension and are too short in duration to really be functionally useful instead the primary way that our skeletal muscles are excited for producing movement is tetanus using relatively high frequency stimulation