Module 3, Lecture 17 - Kinesiol 2C03.docx

Motor Unit consists of a somatic motoneuron (motor neuron) and all the muscle fibers it innervates motoneuron = cell body (soma) + axon (and all axon collaterals) each axon collateral serves one muscle fiber the number of fibers per motor unit differs depending on the muscle in question...

Motor Unit consists of a somatic motoneuron (motor neuron) and all the muscle fibers it innervates motoneuron = cell body (soma) + axon (and all axon collaterals) each axon collateral serves one muscle fiber the number of fibers per motor unit differs depending on the muscle in question muscles responsible for gross movement have many fibers per motor unit (1000s) muscles responsible for fine/complex movement have few fibers per motor unit (10s) each motor unit will only innervate/activate a single muscle fiber type Motor Unit Territory fibers of different motor units will intermingle with each other (i.e. fibers of specific motor units are spread out across a larger area/across the muscle) this distribution allows the force of the motor units to be distributed over a larger area results in a smoother contraction and more equal force on the tendon this distribution also may also help delay muscle fatigue the active and inactive fibers can share metabolites and capillaries Motor Unit Types type I (slow twitch) type II (fast twitch): type IIA (fast oxidative glycolytic) and type IIX (fast glycolytic) other animals have type IIb motor units in place of type IIX motor units note: the motor units types are named after the fiber type they innervate Motor Unit Morphological Characteristics motoneuron soma size → type IIX/b > type IIA > type I axon diameter → type IIX/b > type IIA > type I the larger the diameter, the faster the conduction velocity of a nerve signal fiber number → type IIX/b > type IIA > type I fiber size (other animals) → type IIb > type IIA > type I humans have larger type II A fibers than type IIX additionally, the type I fibers in females are more or less the same size as their type IIA fibers Fiber Types: Type I (Slow Oxidative) are slow twitch, high oxidative muscle fibers contain slow myosin ATPase; will slowly breakdown ATP, resulting in slower cross-bridge cycling have more mitochondria and a better/larger blood supply make a lot of ATP using oxidative metabolism are fatigue resistant (i.e. they can sustain contractions over long periods of time) Fiber Types: Type II contain fast myosin ATPase; will quickly breakdown ATP, resulting in faster cross-bridge cycling there are two subtypes type IIA (fast oxidative-glycolytic) utilize both oxidative metabolism (are low oxidative fibers) and non-oxidative metabolism (i.e. the glycolytic system) have less mitochondria, a smaller blood supply, and less aerobic capacity than type I is the ‘middle’ fiber type; are not the strongest or most fatigue resistant but are good at a bit of everything type IIX (fast glycolytic) utilize mostly the glycolytic system (are not able to utilize oxidative metabolism well) have the fewest mitochondria, the least blood supply, and the lowest aerobic capacity are the most fatigable fiber type fuel for non-oxidative metabolism (i.e. carbs) is in limited supply in the muscle non-oxidative metabolism also builds up metabolic byproducts that eventually interfere with the ability to continue to make ATP Muscle FIbers and Training within a muscle, there is a mosaic/checkerboard of muscle fiber types hypertrophic or aerobic training can increase the size of all fiber types specific training however (ex. distance running vs. jumping), can result in further relative fiber type growth (i.e. can increase the size of type I, IIA, or IIX even more) additionally, although one is born with a certain fiber distribution, one can also train a certain fiber distribution as well can convert/transition fibers from a type IIA into a type IIX and vice versa Motor Unit Contraction Force fast twitch (i.e. type II) fibers/motor units have both a greater absolute force and specific force than slow twitch (i.e. type I) fibers/motor units specific force → force per unit muscle cross-sectional area (i.e. N/cm2) specific force is greater in type II fibers due to myosin heavy chain type II isoforms (fast twitch isoforms) produce more force per cross-bridge and have more attached cross-bridges (i.e. more CBs bound to the filament assuming maximal activation/saturating calcium concentration) Motor Unit Contraction Speed (Velocity) fast twitch fibers produce a more rapid contraction specifically, type IIX are the fastest followed by type IIA and then type I fibers fast twitch fibers are also able to generate more peak force at a faster rate the force velocity relationship amongst the isolated fibers is ‘stacked’ in appearance shows type IIX fibers producing more force at fast and slow velocities, followed by type IIA and ending with type I at the bottom the Isomax and Vmax are thus highest in type IIX fibers and smallest in type I fibers Determinants of Contraction Speed myosin ATpase activity myosin ATPase determines cross bridge cycle speed (is the rate limiting step); the greater/faster the activity, the greater the CB cycle speed myosin ATPase activity → type IIx > type IIA > type I type IIX fibers are more likely able to break down ATP? cross-bridge power stroke speed myosin heavy chain type II moves quicker than myosin heavy chain type I a possible cause is that the gliding velocity of myosin heavy chain type II is higher (i.e. has a better gliding capability) Ca 2+ release and reuptake type II fibers have a larger & more extensive SR; are thus able to undergo a more rapid and large release & uptake of Ca 2+ results in faster contraction & relaxation of the muscle fiber Motor Unit Force-Velocity Relationship as mentioned prior, type II fibers have a higher Vmax and ISOmax (are overall stronger and faster); the difference between the fiber types however is dynamic as concentric velocity increases, the difference between type I and type II fibers increases as eccentric velocity increases, the difference between type I and type II fibers decreases occurs because eccentric contractions require less ATP breakdown type II fibers are also able to maintain a higher degree of force over a given shortening velocity Motor Unit/Fiber Contraction and Power peak power, and the velocity at which it occurs, increases between type IIX fibers to type I fibers type IIX fast twitch fibers are also able to maintain this power over an extend range of velocities, with the range of velocities decreasing between the fibers type II fibers are also able to maintain a higher degree of force over a given shortening velocity Assorted Notes non-oxidative metabolism can make ATP at a much faster rate fast twitch fibers a have similar myofilament and myofibrillar density to slow twitch fibers immunofluorescent stain → involves attaching an antibody to a specific protein and then fluorescing that antibody so that it gives off a specific color there are hybrid fiber types (ex. type IIAX) windgate test → is a test whereby one pedals as hard as possible against a given or a set force in real life, power (watt generation) will translate to cycling speed that is, the greater the power, the faster the cycling speed