Transcript motor unit
17.5.12 1. (a) (b) 2. (a) (b) (c) (d) A change in resting membrane corresponds to a change in Charge flowing across the membrane Charge stored on membrane capacitor Potential difference exists Across the two ends of axon In the intracellular space of neurons Across the neuron membrane In the extracellular space of neurons potential The bright regions in the pictures above indicate unidirectional (a) Flow of ions (b) Propagation of action potential Voluntary Muscles: Voluntary muscles are under the control of central nervous system Skeletal muscles Involuntary Muscles: Not under the control of brain Cardiac muscles Smooth muscles 1. 2. The magnitude of the tension developed by a muscle fiber when it contracts (active tension) depends on the Initial resting length of the muscle (muscle fiber) Frequency of the stimulation No stimulation No cross bridge formation because cross bridge binding site is physically covered by troponin-tropomyyosin complex No tension produced The action potential moves across the surface membrane and into muscle fibers interior through T tubule. An action potential in the T tubule triggers release of Ca2+ from the sarcoplasmic reticulum into cytosol An action potential at neuron terminal stimulates release of acetylcholine which diffuses across the cleft and triggers an action potential in the muscle fiber Released Ca2+ binds to troponin on thin filaments Binding trggers the cross bridge to bend pulling the thin filament over thee thick filament. This power stroke is powered by energy provided by ATP Ca2+ binding to troponin causes tropomyosin to change shape, physically moving it away from its blocking position which uncovers the binding sites on actin for the myosin cross bridges At resting length (2.0-2.2µm), a skeletal muscle fiber develops maximum tension if stimulated because this is the length where optimal overlap of thick filament cross bridges and thin filament cross bridge binding site occurs If the muscle length is increased or decreased from the plateau region tension developed on stimulating is lesser A single action potential in a muscle fiber produces a brief, weak contraction called a “twitch” Greater tension can be produced by repeated stimulation of the muscle fiber before it relaxes. Two twitches from two action potentials add together or sum to produce greater tension. This twitch summation is similar to temporal summation of EPSPs at the postsynaptic neuron Twitch summation is possible only because the duration of the action potential (1-2msec) is much shorter than the duration of the resulting twitch (100ms) If the muscle fiber is stimulated so rapidly that it does not have a chance to relax at all between stimuli, a smooth sustained contraction of maximal strength known as “tetanus” occurs. A tetanic contraction is usually four times stronger than a single twitch As soon as Ca2+ is released in response to an action potential, the sarcoplasmic reticulum starts pumping Ca2+ back into lateral sacs As less cytosolic Ca2+ concentration subsequently declines, less Ca2+ is present to bind with troponin, so some of the troponin-tropomyosin complexes slip back into their blocking position Consequently, not all cross bridge binding sites remain available to participate in the cycling process during a single twitch induced by a single action potential On repeated stimulation by successive action potentials, the cytosolic concentration of Ca2+ remains high The prolonged availability of Ca2+ in the cytosol permits more of the cross bridges to continue participating in the cycling process for a longer time With an increase in the frequency of action potentials, duration of elevated cytosolic Ca2+ concentration increases and contractile activity likewise increases until a maximum tetanic contraction is achieved With the tetanus, the maximum number of cross bridge binding sites remain uncovered so that cross bridge cycling and consequently tension development is at its peak When a motor neuron enters a muscle, it branches with each axon terminal supplying a single muscle fiber One motor neuron supplies a number of muscle fibers but each muscle fiber is supplied by only one motor neuron When a motor neuron is activated, all the muscle fibers it supplies are stimulated to contract simultaneously This team of concurrently activated components : one motor neuron plus all the muscle fibers it innervates –is called a motor unit Total tension developed by a whole muscle depends on Number of muscle fibers in a motor unit that are activated simultaneously Number of motor units in the whole muscle that are activated simultaneously Because the muscle fibers are connected in parallel and exert forces on the same tendon, the force produced in all the fibers will be summed at the tendon. This is the spatial summation of tension Total tension developed by a muscle is due to both spatial and temporal summation of tension The minimum tension is produced by a single twitch in the smallest motor unit (with least number of muscle fibers) The maximum tension is produced by tetanic contractions in all motor units of a muscle