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64 Cards in this Set
- Front
- Back
Dimensions of a typical mammalian muscle fiber
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Diameter 50-100 microns
Length 2-6 cm |
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Where are motor neurons located in general?
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Cranial nerves or ventral roots
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A typical muscle is controlled by about how many motor neurons?
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100
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Define muscle unit
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Muscle fibers innervated by a single neuron
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Define motor unit
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A motor neuron and the muscle fibers it innervates
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General organization for innervation of single muscle fiber
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Usually innervated by only one motor neuron and towards the middle of the fiber (some exceptions in development)
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Main neurotransmitter at the motor end plate
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Acetylcholine
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How does activity at the motor end plate help muscle fibers "reset" quickly
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Synaptic cleft is full of acetylcholinesterase which works rapidly
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The membrane of the muscle fiber is called the
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sarcolemma
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Speed of propagation in a muscle (general) out from the end plate region
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3-5 meters per second
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Does the propogation of action potential in muscle fiber require high or low transmembrane currents?
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High
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Why is the activity of one motor neuron recordable on the surface of the body?
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One motor neuron often activates hundreds of motor fibers
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Muscle fibers are made of _______ which are made of _______
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myofibrils, sarcomeres
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What is the functional unit of the muscle?
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The sarcomere
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What are the "myofilaments" in muscle?
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The contractile proteins
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The physiologic range of length of a sarcomere
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1.5-3.5 microns
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Give the general setup of the sarcomere in terms of z-disks, thick filaments and thin filaments
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1. Z-disk is the reference point
2. Thin filaments project in both directions from the Z-disk 3. Thick filaments float in the middle |
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General components of the thin filament
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* 1. F actin (pair of polymerized actin monomers)
2. tropomyosin 3. troponin |
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General components of the thick filament
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Myosin (about 250 molecules per filament)
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Which muscle element has the "heads" and how are they arranged?
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The thick filament has the myosin with it's heads; these are arranged towards the edges and the middle is more smooth
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What is the "sliding filament hypothesis" and who developed it?
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This is actin and myosin sliding past each other; Huxley in the 1950s.
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How does myosin become "cocked"
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Each head has an ATPase that uses ATP for conformation change
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About how much (distance) does the myosin "head" move
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About 0.06 microns
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How can the myosin "head" detach?
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The movement takes the "cocked" position away and the head of the myosin can release from the actin
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Is myosin "head" detachment an active or passive process
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active
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What two molecules/fuel are required for the release of the myosin "head"?
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ATP and calcium (the ATPase only works in the presense of Ca)
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What is the physiologic cause of rigor mortis?
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Myosin heads are unable to detach because the cell no longer can regenerate the ATP
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In muscle, what are conectins and what do they do?
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1. Very thin elastic structural elements
2. Help suspend the thick filament between the Z-disks 3. Accounts for some element of passive stretch 4. Keeps thin and thick filaments aligned when they are stretched and no longer overlap |
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Endomysial connective tissue is mostly what?
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collagen (parallel)
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When the myosin "head" is cocked is it attached or detached in relation to the thin filament?
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detached
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What contributes to the total force output of a muscle at the muscle spindle level?
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1. passive tension from connective tissue
2. active tension from cross bridges |
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What are the three physiologic elements of active muscle tension?
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1. the number of cross bridges
2. the force from each cross bridge 3. the velocity of the cross bridge motion |
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Actin-binding sites at rest are covered by
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troponin-tropomyosin complexes
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What part of the thin filament binds to calcium?
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the troponin
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Describe the sarcoplasmic reticulum
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longitudinal tubules and chambers that sequester and release calcium
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What is the state of calcium in the resting muscle cell?
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Intracellular calcium is low since there is active pumping into the sarcoplasmic reticulum
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Where is calcium stored in the resting muscle cell?
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In the terminal cisternae of the sarcoplasmic reticulum, which are bound tightly to the transverse tubules
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Describe the general release of calcium into the muscle cell
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1. Action potential causes depolarization of the muscle fiber, including the transverse tubules.
2. This is coupled to the cisternae 3. calcium is release through channels |
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General time frame from calcium release to full formation of cross-bridges
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25-50ms
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Calcium reuptake is fast. How long does it take to decrease cross-bridging and stop the contractile force?
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80-200 ms
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General time difference betwen onset and decay of muscle contraction
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Onset is much faster and relaxation from decreased cross bridging takes more time
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How are summation contractions and tetnus possible?
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1.One action potential does not cause enough calcium release to support all cross bridges
2. Repeat depolarization before calcium reuptake is complete allows for further cross bridging |
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Why is maximal contraction of muscle at the middle length (in terms of actin and myosin)?
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1. When muscle is too stretched the filaments don't overlap enough to cross
2. When muscle is too compact, it has passed the area of myosin heads 3. At the end of shortening the structures start to run into each other and push against the contraction |
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Which produces more force at the same level of activation: lengthening muscle or isometric contraction?
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Lengthening
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What determines the rate of energy consumption for cross bridging?
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The velocity
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Does a fastly contracting, fastly cycling sarcomere produce more or less force than its slow neighbor?
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Less
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Why do "fatigued" muscles take longer to relax?
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Because relaxation is an active process requiring ATP
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Type I muscle fibers are
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slow twitch
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Time and tension characteristics of type I muscle fibers
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Produce small amounts of tension for long periods of time
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Why are slow twitch muscles red?
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They rely on the glucose and oxygen of oxidative/aerobic metabolism and thus have lots of capillaries
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Why do type I muscle fibers have less contractile force than fast twitch?
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the fibers are smaller and have fewer contractile elements
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List the 3 types of muscle fibers
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Slow twitch (type I)
Fast fatigue resistant (Type IIA) Fast fatigable (Type IIB) |
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General characteristics of type IIA muscle fibers
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1. Fast fatigue resistant
2. Enough aerobic capacity to sustain action for several minutes |
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General characteristics of type IIB muscle fibers
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1. Fast fatigable
2. Glycogen stores for anaerobic metabolism into lactic acid 3. short lived 4. take hours to recover |
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General formula for the contractile force of a motor unit
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contractile force = (force of fiber type) * (number of muscle fibers innervated). Also dependent on firing rate
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How fast does a large fat neuron conduct compared to its skinny neighbor?
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Larger cell body and larger diameter equals faster conduction
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Relative force produced by slow-twitch fibers compared to fast-twitch fibers
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slow twitch often produce about 1% as much force
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What is the recruitment order of motor units?
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Weakest to strongest (meaning type-I, then IIA, then IIB since the velocity relates to force)
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Relationship between electrical resistance and cell surface area
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inversely related(so small cells have less electrical resistance and depolarize first)
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Maximum number of degrees of freedom in a joint (movement)
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6 (3 rotational and 3 translational)
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How does the nervous system allow muscles to make rapid changes in torque since the decay of cross bridging is slow?
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The agonist is activated vigorously and the antagonist is activated briefly after to counteract the excess torque
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The fastest reflex arc still takes about
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100ms
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Has does muscle co-contraction help maintain joint stability?
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With muscles contracted then force depends more on any length changes that might occurs from "perturbations" like standing on a rocking boat. These length tension changes adapt faster than even a reflex arc can.
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How is force over distance and force over time different?
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1. force over distance is work
2. force over time is momentum |