Muscle Fiber Types PDF

CONTRACTION OF MOTOR UNITS The contraction of skeletal muscle is the result of the activity of groups of muscle cells. The STRENGHT of muscle contraction is determined by SIZE and NUMBER of MOTOR UNITS being stimulated.  THE MOTOR UNIT IS THE FUNCTIONAL COMPONENT OF MUSCLE...

CONTRACTION OF MOTOR UNITS The contraction of skeletal muscle is the result of the activity of groups of muscle cells. The STRENGHT of muscle contraction is determined by SIZE and NUMBER of MOTOR UNITS being stimulated.  THE MOTOR UNIT IS THE FUNCTIONAL COMPONENT OF MUSCLE CONTRACTION WHAT IS MOTOR UNIT? A motor unit consists of one motor neuron and all of the muscle fibers it innervates. “A motor unit is made up of a motor neuron and the skeletal muscle fibers innervated by that motor neuron's axonal terminals.” When one neuron fires, all of the muscle cells which are stimulated by that one neuron will contract. Each motor unit acts independently of each other! When strong contraction is needed, the nervous system may cause more than one motor unit to be stimulated. The stimulation of additional motor units for increasing STRENGTH of contraction is called RECRUITMENT. A typical skeletal muscle receives innervation from ∼100 motor neurons. When the motor neuron generates an action potential, all the fibers in the motor unit fire simultaneously. Groups of motor units often work together to coordinate and increasing STRENGTH of the contractions of muscle (RECRUITMENT).  Motor Unit Size In addition to number of motor units firing, The number of cells per motor unit is an important factor for affecting STRENGTH and degree of muscle movement. The number of cells per motor unit = MOTOR UNIT SIZE Small motor unit Large motor unit “Precise movements are created by small motor units.”  EXTRAOCULAR MUSCLES THIGH MUSCLE Small motor unit Thigh muscles can have a thousand fibers in each unit, while extraocular muscles might have ten. Large motor unit “Gross movements are created by large motor units.” Large muscles exhibiting gross movements such as movements of thigh. A motor unit can be small or large, depending on the number of muscle fibers that it stimulates. The strength of a muscle contraction depends upon the intensity of the nervous system stimulus, the number and size of motor units activated, and the types of muscle fibers that are stimulated. For example, if you are getting ready to lift a heavy weight, your central nervous system will send a stronger signal. This will activate a greater number of large fast motor units, resulting in a more forceful muscle contraction than if you were merely picking up an apple. MUSCLE TONE Even a muscle is relaxed state, a random, asynchronous motor units contractions provide a nearly constant state of low tension and resistance to stretch called MUSCLE TONE. Muscle tone is due to random, asynchronous motor unit contractions. Muscle tone is due to contractions which are maintained by activities of the spinal cord. BTW; many muscles of the face and some muscles of the neck are controlled by cranial nerves, as opposed to spinal nerves. In this case, the impulses would come from the brain, not the spinal cord. Muscle tone: The constant, slightly contracted state of all muscles - does not produce active movements Keeps the muscles firm and ready to respond to stimulus Helps stabilize joints and maintain posture - due to spinal reflex activation of motor units in response to stretch receptors in muscles and tendons The strength of a muscle contraction depends upon; * the intensity of the nervous system stimulus, * the number and size of motor units activated, * the types of muscle fibers that are stimulated. What are the types of muscle fibers ? Every muscle fiber can be categorized as either slow or fast, depending on how quickly it can contract and their resistance to fatigue with repeated stimulation. Slow-twitch muscle fibers (Type I) are oxygen-dependent and contract relatively slowly, but can contract for longer periods of time without fatigue. Fast-twitch muscle fibers (Type II) are not oxygen-dependent and contract more rapidly than slow-twitch fibers, but tire relatively quickly (they also produce greater muscle power).  * White muscle (glycolytic fibers) * Red muscle (oxidative fibers) Fast-twitch muscle fibers (Type II) Slow-twitch muscle fibers (Type I) * Large in diameter * About half the diameter of white fibers * Light color due to reduced myoglobin * Dark-red color due to large myoglobin * Surrounded by few capillaries * Surrounded by many capillaries * Relatively few mithocondria * Numerous mithocondria * High glycogen content * LOW glycogen content    White muscle fiber Red muscle fiber * Which muscle fiber synthesis ATP mainly by KREBS CYCLE? * Which muscle fiber synthesis ATP mainly by GLYCOLYSIS? In fast-twitch fibers, the energy for In slow twitch fibers, the energy for contraction comes from phosphocreatine contraction comes from the breakdown of fat and glycogen reserves within the muscles, from the blood, muscle cells, and adipose glycogen stored within the liver, and tissue. Fat breakdown requires oxygen and glucose in the blood. minimal levels of glucose breakdown as well. All fiber types exist in skeletal muscles, but some muscles within the body— such as postural trunk muscles—have more slow-twitch fibers, while other muscles (such as those in the calves) have more fast-twitch fibers. The proportion of muscle fiber types varies from person to person based on both genetics and training. Athletes have muscle fiber compositions Power weight lifters, on the other hand, that complement their sport. Marathoners, have more fast twitch fibers that allow for instance, have higher levels of slow- feats of enormous muscular strength over twitch fibers that supply them with optimal short periods of time. Sedentary muscular endurance. individuals and people who do general resistance training typically have 50 percent slow-twitch and 50 percent fast- twitch fiber composition.  SUMMARY OF MUSCLE FIBER TYPES Slow twitch muscle fibres are good for endurance activities like long distance running or cycling. They can work for a long time without getting tired. Fast twitch muscles are good for rapid movements like jumping to catch a ball or sprinting for the bus. They contract quickly, but get tired fast, as they consume lots of energy. Most of your muscles are made up of a mixture of both slow and fast twitch muscle fibers. But, your soleus muscle in your lower leg and muscles in your back involved in maintaining posture contain mainly slow twitch muscle fibers and muscles that move your eyes are made up of fast twitch muscle fibers.  The force produced by a single fiber depends on the fiber diameter—the greater the diameter, the greater the force. Fast-glycolytic fibers have the largest diameters. Thus, a motor unit composed of 100 fast-glycolytic fibers produces more force than a motor unit composed of 100 slow-oxidative fibers.  In addition, fast-glycolytic motor units tend to have more muscle fibers. For both of these reasons, activating a fast- glycolytic motor unit will produce more force than activating a slow-oxidative motor unit. Thank you