Kandel Motor Unit And Muscle Action

Front 1

Dimensions of a typical mammalian muscle fiber

Back 1

Diameter 50-100 microns
Length 2-6 cm

Front 2

Where are motor neurons located in general?

Back 2

Cranial nerves or ventral roots

Front 3

A typical muscle is controlled by about how many motor neurons?

Back 3

100

Front 4

Define muscle unit

Back 4

Muscle fibers innervated by a single neuron

Front 5

Define motor unit

Back 5

A motor neuron and the muscle fibers it innervates

Front 6

General organization for innervation of single muscle fiber

Back 6

Usually innervated by only one motor neuron and towards the middle of the fiber (some exceptions in development)

Front 7

Main neurotransmitter at the motor end plate

Back 7

Acetylcholine

Front 8

How does activity at the motor end plate help muscle fibers "reset" quickly

Back 8

Synaptic cleft is full of acetylcholinesterase which works rapidly

Front 9

The membrane of the muscle fiber is called the

Back 9

sarcolemma

Front 10

Speed of propagation in a muscle (general) out from the end plate region

Back 10

3-5 meters per second

Front 11

Does the propogation of action potential in muscle fiber require high or low transmembrane currents?

Back 11

High

Front 12

Why is the activity of one motor neuron recordable on the surface of the body?

Back 12

One motor neuron often activates hundreds of motor fibers

Front 13

Muscle fibers are made of _______ which are made of _______

Back 13

myofibrils, sarcomeres

Front 14

What is the functional unit of the muscle?

Back 14

The sarcomere

Front 15

What are the "myofilaments" in muscle?

Back 15

The contractile proteins

Front 16

The physiologic range of length of a sarcomere

Back 16

1.5-3.5 microns

Front 17

Give the general setup of the sarcomere in terms of z-disks, thick filaments and thin filaments

Back 17

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

Front 18

General components of the thin filament

Back 18

* 1. F actin (pair of polymerized actin monomers)
2. tropomyosin
3. troponin

Front 19

General components of the thick filament

Back 19

Myosin (about 250 molecules per filament)

Front 20

Which muscle element has the "heads" and how are they arranged?

Back 20

The thick filament has the myosin with it's heads; these are arranged towards the edges and the middle is more smooth

Front 21

What is the "sliding filament hypothesis" and who developed it?

Back 21

This is actin and myosin sliding past each other; Huxley in the 1950s.

Front 22

How does myosin become "cocked"

Back 22

Each head has an ATPase that uses ATP for conformation change

Front 23

About how much (distance) does the myosin "head" move

Back 23

About 0.06 microns

Front 24

How can the myosin "head" detach?

Back 24

The movement takes the "cocked" position away and the head of the myosin can release from the actin

Front 25

Is myosin "head" detachment an active or passive process

Back 25

active

Front 26

What two molecules/fuel are required for the release of the myosin "head"?

Back 26

ATP and calcium (the ATPase only works in the presense of Ca)

Front 27

What is the physiologic cause of rigor mortis?

Back 27

Myosin heads are unable to detach because the cell no longer can regenerate the ATP

Front 28

In muscle, what are conectins and what do they do?

Back 28

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

Front 29

Endomysial connective tissue is mostly what?

Back 29

collagen (parallel)

Front 30

When the myosin "head" is cocked is it attached or detached in relation to the thin filament?

Back 30

detached

Front 31

What contributes to the total force output of a muscle at the muscle spindle level?

Back 31

1. passive tension from connective tissue
2. active tension from cross bridges

Front 32

What are the three physiologic elements of active muscle tension?

Back 32

1. the number of cross bridges
2. the force from each cross bridge
3. the velocity of the cross bridge motion

Front 33

Actin-binding sites at rest are covered by

Back 33

troponin-tropomyosin complexes

Front 34

What part of the thin filament binds to calcium?

Back 34

the troponin

Front 35

Describe the sarcoplasmic reticulum

Back 35

longitudinal tubules and chambers that sequester and release calcium

Front 36

What is the state of calcium in the resting muscle cell?

Back 36

Intracellular calcium is low since there is active pumping into the sarcoplasmic reticulum

Front 37

Where is calcium stored in the resting muscle cell?

Back 37

In the terminal cisternae of the sarcoplasmic reticulum, which are bound tightly to the transverse tubules

Front 38

Describe the general release of calcium into the muscle cell

Back 38

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

Front 39

General time frame from calcium release to full formation of cross-bridges

Back 39

25-50ms

Front 40

Calcium reuptake is fast. How long does it take to decrease cross-bridging and stop the contractile force?

Back 40

80-200 ms

Front 41

General time difference betwen onset and decay of muscle contraction

Back 41

Onset is much faster and relaxation from decreased cross bridging takes more time

Front 42

How are summation contractions and tetnus possible?

Back 42

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

Front 43

Why is maximal contraction of muscle at the middle length (in terms of actin and myosin)?

Back 43

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

Front 44

Which produces more force at the same level of activation: lengthening muscle or isometric contraction?

Back 44

Lengthening

Front 45

What determines the rate of energy consumption for cross bridging?

Back 45

The velocity

Front 46

Does a fastly contracting, fastly cycling sarcomere produce more or less force than its slow neighbor?

Back 46

Less

Front 47

Why do "fatigued" muscles take longer to relax?

Back 47

Because relaxation is an active process requiring ATP

Front 48

Type I muscle fibers are

Back 48

slow twitch

Front 49

Time and tension characteristics of type I muscle fibers

Back 49

Produce small amounts of tension for long periods of time

Front 50

Why are slow twitch muscles red?

Back 50

They rely on the glucose and oxygen of oxidative/aerobic metabolism and thus have lots of capillaries

Front 51

Why do type I muscle fibers have less contractile force than fast twitch?

Back 51

the fibers are smaller and have fewer contractile elements

Front 52

List the 3 types of muscle fibers

Back 52

Slow twitch (type I)
Fast fatigue resistant (Type IIA)
Fast fatigable (Type IIB)

Front 53

General characteristics of type IIA muscle fibers

Back 53

1. Fast fatigue resistant
2. Enough aerobic capacity to sustain action for several minutes

Front 54

General characteristics of type IIB muscle fibers

Back 54

1. Fast fatigable
2. Glycogen stores for anaerobic metabolism into lactic acid
3. short lived
4. take hours to recover

Front 55

General formula for the contractile force of a motor unit

Back 55

contractile force = (force of fiber type) * (number of muscle fibers innervated). Also dependent on firing rate

Front 56

How fast does a large fat neuron conduct compared to its skinny neighbor?

Back 56

Larger cell body and larger diameter equals faster conduction

Front 57

Relative force produced by slow-twitch fibers compared to fast-twitch fibers

Back 57

slow twitch often produce about 1% as much force

Front 58

What is the recruitment order of motor units?

Back 58

Weakest to strongest (meaning type-I, then IIA, then IIB since the velocity relates to force)

Front 59

Relationship between electrical resistance and cell surface area

Back 59

inversely related(so small cells have less electrical resistance and depolarize first)

Front 60

Maximum number of degrees of freedom in a joint (movement)

Back 60

6 (3 rotational and 3 translational)

Front 61

How does the nervous system allow muscles to make rapid changes in torque since the decay of cross bridging is slow?

Back 61

The agonist is activated vigorously and the antagonist is activated briefly after to counteract the excess torque

Front 62

The fastest reflex arc still takes about

Back 62

100ms

Front 63

Has does muscle co-contraction help maintain joint stability?

Back 63

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.

Front 64

How is force over distance and force over time different?

Back 64

1. force over distance is work
2. force over time is momentum