Chapter 9.4 Cheat Sheet (DRAFT) by jjovann

•Recru­itment works on size principle
–Motor units with smallest muscle fibers recruited first
–Motor units with more, and larger, fibers recruited if more force is required
–Largest motor units activated only for most powerful contra­ctions where maximal force is needed

•Muscle changes in length and moves load
–Thin filaments slide
•Isotonic contra­ctions either concentric or eccentric:
–Conce­ntric contra­ctions
—muscle shortens and does work
–Eccentric contra­ctions
—muscle generates force as it lengthens
•The negative rep

•Neuron
–Funct­ional unit of the nervous systems
•Cell that commun­icates to others (neurons or targets) via the release of neurot­ran­smi­tters
•Nerve
–Bundles of axons (elect­rical signal relaying portions of neurons)
•Ex: Sciatic nerve

•Why continued:
•During tetanic contra­ction, the successive APs each release Ca from the SR before much of the Ca from the previous AP can be pumped back into the SR.
–This results in persistent elevation of sarcop­lasmic Ca concen­tration
»This prevents a decline in the number of available binding sites on the thin filaments by keeping them unblocked
–Results in many more cross-­bridges formed and power strokes which equals more tension
–Another cause
•Lower tension in single twitches are also a result of the elasticity of muscle tendons and the protein Titan
–These must stretc­h/c­ompress before tension produced from the contra­ctile units is transf­erred
»Like a bungee cord or spring
–Because a single twitch is so brief, the cross-­bridge activity is already declining before force has been fully transf­erred through the elastic structures
–This is less of a factor during tetanic stimul­ation because of the long duration of cross-­bridge activity and force generation

•Wave (temporal) summation
–Increased stimulus frequency (muscle does not completely relax between AP stimuli)  second contra­ction of greater force
•Addit­ional Ca2+ release from SR with second stimulus stimulates re-unb­locking of myosin binding sites and more shortening before full relaxation length is obtained
•Continued temporal stimulus frequency unfused (incom­plete) tetanus
–Produces sustained, but quivering, contra­ction that increases in successive twitch maximum tension
•Once again, to a point

•Muscle fibers that are part of a motor unit are found spread throughout a muscle
–A single motor unit activation typically causes weak contra­ction of that muscle
•However, when activated, only the muscle fibers of that motor unit in that muscle are contra­cting
•Motor units in a muscle usually contract asynch­ron­ously (not at the same time)
–helps prevent, or decrease, overall muscle fatigue

•Muscle contra­ctions exhibit graded responses
–Varying strength of contra­ction for different demands
•Required for proper control of skeletal movement
•Responses graded by
1.Changing frequency of stimul­ation
2.Changing strength of stimul­ation
•More so, applies to activation of additional motor units activating more muscle fibers in a muscle
•APs in motor neurons and muscle fibers do not increase in strength

•If successive stimuli are given quickly enough, muscle reaches maximal tension fused­(co­mplete) tetany results
–Smooth, sustained contra­ction
–No muscle relaxation due to consis­tently high sarcop­lasmic Ca from continued release from SR
–Muscle fiber reaches sustained maximum tension
–Will eventually lead to muscle fatigue as metabo­lites accumulate and ionic imbalances form
•During fatigue, muscle cannot contract and tension returns toward zero

•Each muscle connected to at least one motor nerve
–Motor nerve contains axons of a few to hundreds of motor neurons
–Indiv­idual axon of a motor neuron can branch many times before ending at nerve terminals, each nerve terminal making a NMJ with a single muscle fiber
•Motor unit = motor neuron and all (four to several hundred) muscle fibers it innervates (controls)

•Motor unit's response to single action potential of its motor neuron
–Results in an AP in the muscle fibers connected through NMJs with that neuron
–Produces short period of motor unit’s muscle fiber contra­ction that generates tension
•Different strength and duration of twitches in whole muscle can be observed
–Due to variations in metabolic properties and enzymes between muscle fiber types
–Different whole muscles have differing ratios of muscle fiber types•­Sim­plest contra­ction (twitch) observable in lab
–The tension (force) generated can be recorded by a sensor and graphed as a myogram

•Load greater than tension muscle can develop
•Tension increases to muscle's capacity, but muscle neither shortens nor lengthens
–Cross bridges generate force but do not move actin filaments

•Same principles apply to contra­ction of a single fiber as well as the whole muscle
•Contr­action produces muscle tension
–Force exerted on load or object to be moved
•Force and duration of contra­ction vary in response to stimuli of different freque­ncies and intens­ities
•Contr­action may, or may not, shorten muscle
–Isometric contra­ction
–Isotonic contra­ction

•Recru­itment (multiple motor unit summation) controls force of contra­ction of a whole muscle
•Subth­reshold stimuli
– a stimulus that is too weak to illicit activation of any motor units in a muscle
–no observable contra­ctions or tension
•Threshold stimulus: the stimulus strength that causes first observable muscle contra­ction (activ­ation of a motor unit) and generation of tension
•Maximal stimulus
– strongest stimulus that increases contra­ctile force of whole muscle to its maximum

•Threshold stimuli produces twitch
•“Muscle fiber obeys an all-or­-none law” contra­cting to its maximum or not at all
–not a necess­arily a true statement since twitches can vary in strength
•depends upon Ca2+ concen­tra­tion, previous stretch of the muscle, temper­ature, pH, and hydration
•AP (action potential) in muscle cell occurs much faster than actual contra­ction
–2 millis­econds vs. up to 100 millis­econds respec­tively
–Thus a second AP may be initiated during the period of mechanical activity of the muscle fiber

•Three phases of muscle twitch
–Latent period: events of excita­tio­n-c­ont­raction coupling
•Action potential must cause all of the events we discussed in the previous lectures for muscle fibers to shorten
•no “visible” shortening or tension of the muscle
–Period of contra­ction: sarcomere shortening from cross bridge formation and power stroke produce visible tension (External tension)
•Visible shortening and tension of muscle
–Period of relaxa­tion: Ca2+ reentry into SR; tension declines to zero
•Muscle contracts faster than it relaxes

•Contr­action may/may not shorten muscle
–Isometric contra­ction: no shorte­ning; muscle tension increases but does not exceed load
•Same length
–Isotonic contra­ction: muscle shortens because muscle tension exceeds load
•Same tension, or tone, once load exceeded

•Constant, slightly contracted state of all muscles
•Due to spinal reflexes
–Groups of motor units are altern­ately activated in response to input from stretch receptors in muscles
•Keeps muscles firm, healthy, and ready to respond

•During recrui­tment, muscle contracts more vigorously as stimulus strength increases above threshold
–This applies to stimul­ation of the motor nerve with an electrode that applies voltage
•Shocking the motor nerve
•Contr­action force of whole muscle precisely controlled by recrui­tment
–Recru­itment (multiple motor unit summation)
–Used to increase the force of a muscle contra­ction by activating additional motor units of that muscle
•Each muscle fiber capable of generating a certain amount of force
•More active motor units = more muscle fibers shortening and pulling = more force generation
•Beyond maximal stimulus no increase in force of contra­ction
–Why?

•Fine control
–Produce small, finely controlled movements
–small motor units can contain as few as 10-20 muscle fibers
–eye muscles
–Larynx muscles
•Strength control
–Produce large, strong movements
–gastr­ocn­emius muscle can have as many as 2000 muscle fibers in a motor unit

•Why is tetanic tension so much greater than single twitch tension?
–Isometric tension produced by muscle fiber at any moment depends mainly on the total number of cross bridges undergoing power stroke.
•A single AP in a skeletal muscle fiber briefly releases enough calcium to saturate troponin C making all of the myosin sites on the thin filament initially available.
•However, the binding of the energized myosin head to the myosin binding site takes time
–During this time, the Ca that was released by the SR is being pumped back into it by the SR calcium ATPases (SERCA pumps)
•Thus, after a single AP and Ca release, the sarcop­lasmic Ca concen­tration begins to decrease and the tropon­in/­tro­pom­yosin complex starts to re-block a lot of the binding sites before many of the actin/­myosin cross-­bridges can be formed

•Muscle stimul­ation at variable freque­ncies of APs–low frequency (up to 10 stimul­i(A­P)/sec)
•each stimulus produces an identical twitch response
•Max tension produced from each twitch remains equal
–moderate frequency (between 10-20 stimul­i/sec)
•each twitch has time to recover but develops more tension than the one before (treppe phenom­enon)
–Not enough time between stimuli for sarcop­lasmic calcium to return to full resting levels
»thus calcium not completely put back into SR
–Increase in sarcop­lasmic Ca2+ concen­tration with each successive AP stimul­ation = longer unblocking = more cross bridge formation and power strokes= increase in tension produced
»...To a certain point