《太空生物工程与生命支撑》（英文版）Muscle

X-bridges (a-m\ 3 Contributions WA Passive (yam) Resistance 3-5 Developed(I-P=D) Characteristics Tth⊥ Mechanisms Hills Eqn(pg7) Recruit >fibers Force (15 Limitations Sarcomeres 50-60 Fused tetanus, Building a muscle Fining Strategy Control CC. SE PE Speed: Fast Active: CC=T, nonlin dashpot Properties 0 Conceptual Model/ Size Principle thermal Posture: mG Coordination Extensors Space Flig untermeasures Mass Duration Questions/ Activation YUILe

Outline Review of muscle contraction From aP generation to contraction of fibers Muscle proprioceptors(spindles and golgi tendons) Afferent and efferent axons

Outline • Review of Muscle Contraction – From AP generation to contraction of fibers – Muscle proprioceptors (spindles and Golgi tendons) – Afferent and efferent axons

Muscle Strength Loss in Spaceflight Start with Earth-normal muscle magnitudes and directions for mid-stance (Cheal et al., 19921 Mag(BW X(med-lat) Y(post-ant)Z (dist-prox) Gluteus medius 0.80 -0.67 0.18 0.72 Gluteus minimus 0.30 -0.78 0.21 0.59 lliopsoas 1.30 -0.10 0.73 0.68 Reduce muscle strength with duration of weightlessness 40% lower at 6 months. 60% lower at 12 months based on lit 21%o lower peak activated force 17 day flight [Widrick et al., 1999 120 days of HDT bed rest (Koryak, 1999: 44%/33%(M/F)decline in isometric max voluntary contraction(MVC) 36%/11%(M/F) decline in isometric twitch contraction(Pt) 34%/24%(M/F)decline in tetanic contraction force(Po) Maximal explosive power(MEP)reduced to 67% after 31 days, and to 45%after 180 days of space flight (Antonutto et al., 1999]

Muscle Strength Loss in Spaceflight • Start with Earth-normal muscle magnitudes and directions: for mid-stance [Cheal et al.,1992] : Mag (BW) X (med-lat) Y (post-ant) Z (dist-prox) Gluteus medi us 0.80 -0.67 0.18 0.72 Gluteus minim us 0.30 -0.78 0.21 0.59 Iliopso as 1.30 -0.10 0.73 0.68 • Reduce muscle strength with duration of weightlessness: – 40% lower at 6 month s, 6 0 % lower at 1 2 months, based on lit. • 21% lo w er pe a k activated force 17 day flight [Widrick et al., 1999] • 120 days of H D T bed rest [Kory ak, 1999] : – 44% / 33% (M/F) decline i n isometric max. voluntary contraction (MVC) – 36% / 11% (M/F) decline i n isometric t witc h contraction (Pt) – 34% / 24% (M/F) decline in tetanic contraction force (Po) • Maximal e xplo sive po w er (MEP) reduced to 67% after 31 d a ys, and to 45% after 180 days of space flight [Anto n utto et al., 1999]

Muscles: Effectors of the Motor System The major output of the elaborate information processing that takes place in our brain is the generation of a contractile force in our skeletal muscles · Muscle fasciculus Muscle fiber · Myofibril Sarcomere each motor neuron innervates a number of muscle fibers s although Each muscle fiber is innervated by only one motor neuron The motor neuron and all the fibers it innervates is called a motor unit ( the smallest functional unit controlled by the motor system)

Muscles: Effectors of the Motor System • The major output of the elaborate information processing that takes place in our brain is the generation of a contractile force in our skeletal muscles. • Muscle fasciculus – Muscle fiber • Myofibril – Sarcomere • Each muscle fiber is innervated by only one motor neuron, although each motor neuron innervates a number of muscle fibers • The motor neuron and all the fibers it innervates is called a motor unit (the smallest functional unit controlled by the motor system)

Muscles: Effectors of the Motor System The number of muscle fibers innervated by one motor neuron is called the innervation ratio. The innervation ratio can vary between 10 and 2000 a low innervation ratio indicates a greater capacity for finely grading the muscle total force

Muscles: Effectors of the Motor System • The number of muscle fibers innervated by one motor neuron is called the innervation ratio. The innervation ratio can vary between 10 and 2000 • A low innervation ratio indicates a greater capacity for finely grading the muscle total force

Muscles: Effectors of the Motor System A simplified sequence from AP generation to muscular contraction Motor neuron fires an action potentia It propagates down the motor axon until it reaches the neuro-muscular junction It triggers an ap in the muscle fiber This aP is propagated rapidly over the surface of the fiber and conducted into the myofibril by mean of the T-tubule system This in turn releases Ca* from the Sarcoplasmic Reticulum(Sr)-the sr serves as a store of catt This in turn triggers the cyclic motion of Myosin heads, attaching and detaching on the Actin filaments, thus forming cross-bridges and generating the pulling force Ca*t are pumped back to the sr

Muscles: Effectors of the Motor System A simplified sequence from AP generation to muscular contraction • Motor neuron fires an action potential • It propagates down the motor axon until it reaches the neuro-muscular junction • It triggers an AP in the muscle fiber • This AP is propagated rapidly over the surface of the fiber and conducted into the myofibril by mean of the T-tubule system • This in turn releases Ca++ from the Sarcoplasmic Reticulum (SR)-the SR serves as a store of Ca++ • This in turn triggers the cyclic motion of Myosin heads, attaching and detaching on the Actin filaments, thus forming cross-bridges and generating the pulling force • C a++ are pumped back to the SR

Muscles: Effectors of the Motor System The force of contraction depends on the length of the muscle (ength-tension relationship The force of contraction also depends on the relative rates of movement of the Actin and Myosin filaments(tension-velocity relationship, Hills curve) Motor units are recruited in a fixed order from the weakest to the strongest( Henneman size principle): The weakest inputs recruit the slow units which generate the smallest force and are most resistant to fatigue. The fast fatigue-resistant are recruited next followed by the fast fatigable units which generate the strongest orce

Muscles: Effectors of the Motor System • The force of contraction depends on the length of the muscle (length-tension relationship) • The force of contraction also depends on the relative rates of movement of the Actin and Myosin filaments (tension-velocity relationship, Hill’s curve) • Motor units are recruited in a fixed order from the weakest to the strongest (Henneman size principle): The weakest inputs recruit the slow units which generate the smallest force and are most resistant to fatigue. The fast fatigue-resistant are recruited next, followed by the fast fatigable units which generate the strongest force

Muscles: Effectors of the Motor System Muscle proprioceptors(spindles and golgi tendons) There are different types of receptors which respond to light, sound, odor heat, touch, pain, etc The receptors which lead to conscious sensations are called exteroceptors those which are not responsible for conscious sensation are called-primary in motor functions-are called proprioceptors Spindle organs Those are stretch receptors scattered deep within all muscles. They are usually attached in parallel with a muscle fiber, and therefore experience the same relative length change. Spindles give information about its length and rate of change of its length Golgi tendon They are found very close to the junction between tendon and muscle fibers They are placed in series with the muscle fibers and respond to the tendon stretch which accompanies a muscle tension. Thus they are force transducers for the muscle

Muscles: Effectors of the Motor System • Muscle Proprioceptors (spindles and Golgi tendons) There are different types of receptors which respond to light, sound, odor, heat, touch, pain, etc. The receptors which lead to conscious sensations are called exteroceptors, those which are not responsible for conscious sensation are called-primary in motor functions- are called proprioceptors – Spindle organs Those are stretch receptors scattered deep within all muscles. They are usually attached in parallel with a muscle fiber, and therefore experience the same relative length change. Spindles give information about its length and rate of change of its length – Golgi tendon They are found very close to the junction between tendon and muscle fibers. They are placed in series with the muscle fibers and respond to the tendon stretch which accompanies a muscle tension. Thus they are force transducers for the muscle

Muscles: Effectors of the motor System The nerve axons which run out of the spinal cord are called efferent. the ones that carry information to the cord are afferent Group i afferent fibers have large diameters therefore relatively high conduction velocities. They bring information from the spindle (la) and the golgi(ib )to the cord The efferent which innervate the main muscle mass are the a. and those that serve the intrafusal fibers within the spindles are called The stretch reflex, co-activation of a-mn and y-mn

Muscles: Effectors of the Motor System • The nerve axons which run out of the spinal cord are called efferent, the ones that carry information to the cord are afferent • Group I afferent fibers have large diameters therefore relatively high conduction velocities. They bring information from the spindle ( I a ) and the golgi (Ib) to the cord • The efferent which innervate the main muscle mass are the α, and those that serve the intrafusal fibers within the spindles are called γ • The stretch reflex, co-activation of α -mn and γ-mn

Muscles: Effectors of the Motor System Stretch reflex stiffness Until recently, it was supposed that the tendon organ served as a sensor which turned off muscle activity (inhibited a-mn) when muscle force rose beyond safe levels Afferent activity from both spindles and golgi tendons balance in such a way that neither muscle force nor muscle length should be considered as controlled quantity rather their ratio( the stiffness or change in force per change in length ) appears to be fixed by the stretch reflex

Muscles: Effectors of the Motor System Stretch reflex stiffness • Until recently, it was supposed that the tendon organ served as a sensor which turned off muscle activity (inhibited α-mn) when muscle force rose beyond safe levels • Afferent activity from both spindles and Golgi tendons balance in such a way that neither muscle force nor muscle length should be considered as controlled quantity, rather their ratio (the stiffness or change in force per change in length) appears to be fixed by the stretch reflex