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64 Cards in this Set

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Dimensions of a typical mammalian muscle fiber
Diameter 50-100 microns
Length 2-6 cm
Where are motor neurons located in general?
Cranial nerves or ventral roots
A typical muscle is controlled by about how many motor neurons?
100
Define muscle unit
Muscle fibers innervated by a single neuron
Define motor unit
A motor neuron and the muscle fibers it innervates
General organization for innervation of single muscle fiber
Usually innervated by only one motor neuron and towards the middle of the fiber (some exceptions in development)
Main neurotransmitter at the motor end plate
Acetylcholine
How does activity at the motor end plate help muscle fibers "reset" quickly
Synaptic cleft is full of acetylcholinesterase which works rapidly
The membrane of the muscle fiber is called the
sarcolemma
Speed of propagation in a muscle (general) out from the end plate region
3-5 meters per second
Does the propogation of action potential in muscle fiber require high or low transmembrane currents?
High
Why is the activity of one motor neuron recordable on the surface of the body?
One motor neuron often activates hundreds of motor fibers
Muscle fibers are made of _______ which are made of _______
myofibrils, sarcomeres
What is the functional unit of the muscle?
The sarcomere
What are the "myofilaments" in muscle?
The contractile proteins
The physiologic range of length of a sarcomere
1.5-3.5 microns
Give the general setup of the sarcomere in terms of z-disks, thick filaments and thin filaments
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
General components of the thin filament
* 1. F actin (pair of polymerized actin monomers)
2. tropomyosin
3. troponin
General components of the thick filament
Myosin (about 250 molecules per filament)
Which muscle element has the "heads" and how are they arranged?
The thick filament has the myosin with it's heads; these are arranged towards the edges and the middle is more smooth
What is the "sliding filament hypothesis" and who developed it?
This is actin and myosin sliding past each other; Huxley in the 1950s.
How does myosin become "cocked"
Each head has an ATPase that uses ATP for conformation change
About how much (distance) does the myosin "head" move
About 0.06 microns
How can the myosin "head" detach?
The movement takes the "cocked" position away and the head of the myosin can release from the actin
Is myosin "head" detachment an active or passive process
active
What two molecules/fuel are required for the release of the myosin "head"?
ATP and calcium (the ATPase only works in the presense of Ca)
What is the physiologic cause of rigor mortis?
Myosin heads are unable to detach because the cell no longer can regenerate the ATP
In muscle, what are conectins and what do they do?
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
Endomysial connective tissue is mostly what?
collagen (parallel)
When the myosin "head" is cocked is it attached or detached in relation to the thin filament?
detached
What contributes to the total force output of a muscle at the muscle spindle level?
1. passive tension from connective tissue
2. active tension from cross bridges
What are the three physiologic elements of active muscle tension?
1. the number of cross bridges
2. the force from each cross bridge
3. the velocity of the cross bridge motion
Actin-binding sites at rest are covered by
troponin-tropomyosin complexes
What part of the thin filament binds to calcium?
the troponin
Describe the sarcoplasmic reticulum
longitudinal tubules and chambers that sequester and release calcium
What is the state of calcium in the resting muscle cell?
Intracellular calcium is low since there is active pumping into the sarcoplasmic reticulum
Where is calcium stored in the resting muscle cell?
In the terminal cisternae of the sarcoplasmic reticulum, which are bound tightly to the transverse tubules
Describe the general release of calcium into the muscle cell
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
General time frame from calcium release to full formation of cross-bridges
25-50ms
Calcium reuptake is fast. How long does it take to decrease cross-bridging and stop the contractile force?
80-200 ms
General time difference betwen onset and decay of muscle contraction
Onset is much faster and relaxation from decreased cross bridging takes more time
How are summation contractions and tetnus possible?
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
Why is maximal contraction of muscle at the middle length (in terms of actin and myosin)?
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
Which produces more force at the same level of activation: lengthening muscle or isometric contraction?
Lengthening
What determines the rate of energy consumption for cross bridging?
The velocity
Does a fastly contracting, fastly cycling sarcomere produce more or less force than its slow neighbor?
Less
Why do "fatigued" muscles take longer to relax?
Because relaxation is an active process requiring ATP
Type I muscle fibers are
slow twitch
Time and tension characteristics of type I muscle fibers
Produce small amounts of tension for long periods of time
Why are slow twitch muscles red?
They rely on the glucose and oxygen of oxidative/aerobic metabolism and thus have lots of capillaries
Why do type I muscle fibers have less contractile force than fast twitch?
the fibers are smaller and have fewer contractile elements
List the 3 types of muscle fibers
Slow twitch (type I)
Fast fatigue resistant (Type IIA)
Fast fatigable (Type IIB)
General characteristics of type IIA muscle fibers
1. Fast fatigue resistant
2. Enough aerobic capacity to sustain action for several minutes
General characteristics of type IIB muscle fibers
1. Fast fatigable
2. Glycogen stores for anaerobic metabolism into lactic acid
3. short lived
4. take hours to recover
General formula for the contractile force of a motor unit
contractile force = (force of fiber type) * (number of muscle fibers innervated). Also dependent on firing rate
How fast does a large fat neuron conduct compared to its skinny neighbor?
Larger cell body and larger diameter equals faster conduction
Relative force produced by slow-twitch fibers compared to fast-twitch fibers
slow twitch often produce about 1% as much force
What is the recruitment order of motor units?
Weakest to strongest (meaning type-I, then IIA, then IIB since the velocity relates to force)
Relationship between electrical resistance and cell surface area
inversely related(so small cells have less electrical resistance and depolarize first)
Maximum number of degrees of freedom in a joint (movement)
6 (3 rotational and 3 translational)
How does the nervous system allow muscles to make rapid changes in torque since the decay of cross bridging is slow?
The agonist is activated vigorously and the antagonist is activated briefly after to counteract the excess torque
The fastest reflex arc still takes about
100ms
Has does muscle co-contraction help maintain joint stability?
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.
How is force over distance and force over time different?
1. force over distance is work
2. force over time is momentum