Muscular System Pt.2

Question 1

Motor unit

Answer 1

One motor neuron and all of the muscle fibers it innervates

Question 2

Muscle contraction results from

Answer 2

Individual and combined actions of motor units

Question 3

Varies from “ “ per motor unit depending on need for fine control

Answer 3

3-1000 fibers

Question 4

Motor neuron pool

Answer 4

The collection of all alpha motor neurons innervating a single muscle

Question 5

Larger motor units=

Answer 5

Worse fine motor control

Question 6

Smaller motor units=

Answer 6

Better fine motor control

Question 7

Force is controlled by

Answer 7

1. Varying firing rate of motor neurons
   -One action potential elicits one twitch in the muscle fiber
   -Sustained contraction requires many action potentials)
2. Recruiting additional synergistic motor units
   -Most muscles have range of motor unit sizes
   -First to be recruited are small, easily excited neurons innervating fewer fibers (for fine control)
   -Last to be recruited are large, less easily excited
   neurons innervating many fibers (for higher force)

Question 8

Threshold Stimulus

Answer 8

Is the minimum stimulation required to cause a contraction.

Question 9

One motor neuron firing generally

Answer 9

Releases enough Ach to produce threshold stimulus in a muscle fiber

Question 10

The length of a muscle fiber before stimulation

Answer 10

Determines the amount of force it can develop

Question 11

Resting length of a fiber is the

Answer 11

Optimum starting length

Question 12

Stretched fibers produce less force because

Answer 12

Myosin heads can’t reach binding sites on actin

Question 13

Compressed fibers produce less force because

Answer 13

Compressed sarcomeres can’t further shorten

Question 14

In summation

Answer 14

The force of individual muscle fiber twitches combine

Question 15

Tetanus

Answer 15

Is sustained muscle contraction due to frequent stimulation
-Unfused tetanus, stimuli are far enough apart to allow muscle to relax slightly between tetanus
-Complete tetanus, muscle reaches steady tension

Question 16

Motor units differ based on function

Answer 16

-Muscle fibers vary in metabolic activity and force/speed of contraction
-Alpha motor neurons vary in size, conduction speed, and firing

Question 17

What are the three types of muscle fibers

Answer 17

1. Slow-twitch (Type I)
2. Fast-oxidative (Type IIa)
3. Fast-glycolytic (Type IIb)

Question 18

Most muscles have “ “ but each motor unit
“ “

Answer 18

“All three types” “only has one type”

Question 19

Cellular respiration

Answer 19

Is the production of ATP from nutrients (primarily glucose).

Question 20

Slow- twitch (type-1) (definition)

Answer 20

Fibers are slow to contact but can sustain contraction without fatigue

Question 21

Slow-twitch (type-1)

Answer 21

-Small and have many mitochondria and myoglobin
(O2-carrying molecule) for aerobic metabolism (giving red color)
-Can produce large quantities of ATP over time
-Used for sustained movement like postural control
-Motor neurons innervating these are small, conduct slowly, and have steady, low-frequency activity

Question 22

Fast-glycolytic (type llb) (definition)

Answer 22

Fibers contract rapidly and powerfully, but fatigue quickly

Question 23

Fast-glycolytic (type llb)

Answer 23

-Large and have fewer mitochondria/less myoglobin (appear white)
-Primarily use anaerobic glycolysis and have large stores of
glycogen (polymer of glucose)
-Produce rapid, powerful contractions for movements like sprinting
-Motor neurons innervating these are large, conduct quickly, and have occasional bursts of activity

Question 24

Fast-oxidative (intermediate, type lla)

Answer 24

-Have high levels of mitochondria and intermediate levels of myoglobin (appear light red)
-Produce ATP via aerobic respiration, but have faster/stronger contraction than slow-twitch fibers
-Useful for intermediate movement like walking
-Motor neurons innervating these are medium in size, conduction speed, and firing rate

Question 25

Changes in motor neuron activity can change

Answer 25

Muscle fiber characteristics

Question 26

Hypertrophy

Answer 26

Is increase in muscle fiber size- occurs due to increased motor neuron activity

Question 27

Atrophy

Answer 27

Is decrease in muscle fiber size -occurs due to decreased muscle or motor neuron activity

Question 28

What are the three types of muscle?

Answer 28

1. Smooth muscle
2. Cardiac muscle
3. Skeletal muscle

Question 29

Smooth muscle is? and is controlled by?

Answer 29

Lines digestive tract, arteries, etc. and is controlled by autonomic nervous system

Question 30

Cardiac muscle is? and is controlled by?

Answer 30

Found in the heart and rate of contraction controlled by autonomic nervous system

Question 31

Skeletal muscle is? and is controlled by?

Answer 31

Found throughout body plan and voluntary contraction controlled by somatic nervous system

Question 32

Smooth muscle cells are

Answer 32

Located in walls of organs and blood vessels, iris of eye, and arrector pili muscles

Question 33

Smooth muscle cell characteristics?

Answer 33

-Short with tapered ends
-Have single, centrally-located nucleus
-Not striated (thick and thin filaments exist, but are randomly organized)
-Slow to contract and relax

Question 34

Smooth muscle contraction is involuntary or voluntary?

Answer 34

Involuntary

Question 35

Smooth muscle contraction is stimulated by?

Answer 35

1. Neurotransmitters: Acetylcholine OR norepinephrine
2. Hormones (example: oxytocin stimulating uterine contraction)
3. Cell stretch (example: partially digested food triggers gut contraction)

Question 36

Smooth muscle can be classified by

Answer 36

Location, contraction pattern, or communication with neighboring cells

Question 37

Smooth muscle can be classified by location

Answer 37

1. Vasculature
2. GI tract
3. Urinary system
   4.Respiratory system
   5.Reproductive system
4. Eye

Question 38

Smooth muscle can be classified by contraction pattern

Answer 38

1. Phasic smooth muscles alternate between contraction and relaxation
2. Tonic smooth muscles always maintain some level of muscle tone

Question 39

Smooth muscle can be classified by communication with neighboring cells

Answer 39

1. In multi-unit smooth muscle, fibers function independently
   -Cells less organized
   -Examples: in iris of eye and walls of blood vessels
2. In single-unit smooth muscle, fibers are connected by gap junctions and contract together
   -Cells organized into sheets
   -Examples: walls of hollow organs

Question 40

Smooth muscle is different from skeletal muscle because

Answer 40

Controlled by autonomic nervous system
-Initiated by a variety of signals (electrical, chemical, stretch)
-Contractile fibers not arranged by sarcomeres
-Ca2+ signal initiates a cascade that ends
with phosphorylation of myosin light chain
and activation of myosin ATPase

Question 41

For smooth muscle contraction (5)

Answer 41

1. Variety of signals increase intracellular Ca2+ concentration
2. Ca2+ binds calmodulin, a regulatory protein
3. Calmodulin activates myosin light chain kinase (MLCK), which phosphorylates myosin light chain
4. Phosphorylation enhances myosin ATPase activity, which activates crossbridging
5. Phosphorylation is removed via myosin light chain phosphatase (MLCP), and muscle relaxes

Question 42

Cardiac muscle cells (fibers)

Answer 42

Are only located in the heart

Question 43

Cardiac muscle fibers…

Answer 43

-Fibers are branched
-Fibers are joined together by intercalated discs
-Each fiber contains a single nucleus
-Fiber networks contract involuntarily and rhythmically as a unit