Muscle Physiology

Question 1

Principales of whole muscle contraction/ Terminology

Answer 1

▪ Muscle tension – force exerted by a contracting muscle on an object.

▪ Load – force exerted on a muscle by the weight of the object to be moved
(force opposing muscle tension)

Question 2

The motor unit

Answer 2

a single neuron and all of the muscle fibers it innervates.
very few (4) to several hundread

Question 3

Motor unit size : 1

Answer 3

▪ Small motor units have few muscle fibers.

◦ produce precise movements
◦ highly excitable
◦ usually activated first.
- ex. muscles controlling eyes & fingers

Question 4

Motor unit size - 2

Answer 4

▪ Large motor units have many muscle fibers.

◦ produce gross (large, strong) movements.
◦ are activated only as demand increases.
- ex. muscles controlling leg movements

Question 5

Graded muscle contractions :

Answer 5

Our muscle responses can be adjusted or graded to:

◦ provide varying strengths of muscle contractions needed to meet different demands

◦ promote smoothness of muscle contractions

Question 6

Graded muscle contractions - two factors

Answer 6

Two factors affect the force generated by muscular contraction:

1.Number of motor units activated
(Recruitment)

Question 7

Graded Muscle contractions - two factors pt 2

Answer 7

2. Frequency of the nerve impulse firing rate by the motor neuron (Wave summation)

Question 8

Graded muscle contractions process pt 1

Answer 8

1 - The number of motor units activated

•The more motor units that are activated, the greater the force or muscle tension generated.

This is called recruitment or multiple motor unit summation.

Question 9

Graded muscle contractions process pt 1

Answer 9

▪ Recruitment in the body is not random but is based on the size of motor units.

…Remember, smaller motor units are activated first.
▪ Most commonly, motor units are activated asynchronously*.
◦ allows for prolonged contraction & delays fatigue
◦ contributes to smoothness of contractions

Question 10

Graded muscle contraction process pt 2

Answer 10

The frequency of the nerve impulse firing rate by the motor neuron

•An increase in the firing rate causes a greater force (muscle tension) to be generated.

This is called wave summation.

Question 11

Graded muscle contractions process pt 2

Answer 11

Wave summation:

▪ When two identical impulses are delivered rapidly to a muscle, the force of the second contraction will be stronger than the first.

Helps produce smooth, continuous contractions.
(Second stimulus must arrive after absolute refractory period.)

Question 12

Muscle tone

Answer 12

▪ Even “relaxed” muscles are almost always slightly contracted.

▪ Nerve stimuli are almost continually sent so that at any one time some fibers are contracting.

▪ Occurs via spinal reflexes (not conscious).

Question 13

Importance of muscle tone:

Answer 13

▪ stabilizes joints and maintains posture

▪ keeps muscles healthy and ready to respond to stimulation

▪ generates body heat

▪ does not produce active movements

Question 14

Types of contractions:

Answer 14

two types

Question 15

Isotonic contractions:

Answer 15

▪ Contraction that causes movement
▪ Tension of the muscle stays the same but the length of the muscle changes

Question 16

Isometric contractions :

Answer 16

▪ Contraction against a force that is too great for a muscle to move

▪ Length of muscle stays the same even though the tension changes

Question 17

Energy sources

Answer 17

Main ATP

Question 18

ATP fuels muscular contraction:

Answer 18

ATP provides muscle tissue with energy to:
- move and detach myosin cross bridges
- operate the calcium pump in the SR
- operate the Na+-K+ pump in the plasma membrane
▪ ATPase of myosin heads breaks ATP down:
ATP → ADP + P + energy
▪ Muscle fibers store a very small amount of ATP in sarcoplasm or bound to myosin heads.
▪ Energy for first 4-6 sec. of contraction.

Question 19

Other sources of ATP for muscle :

Answer 19

3

Question 20

1- creatine phosphate - can regenerate ATP creatine + ADP - creatine + ATP phosphate

Answer 20

▪ Creatine phosphate is a high energy molecule stored in muscle.
▪ Muscle cells store 4-5 times as much CP as ATP.
▪ Stored ATP and creatine phosphate provide energy for around 15 seconds of contraction.

Question 21

2- Anaerobic glycolysis of glucose to form ATP
1 glucose- 2 lactic+ 2 ATP

Answer 21

▪ Does not require oxygen (anaerobic).
▪ Occurs when the blood cannot supply oxygen to the muscle fast enough.
▪ Used for short term exertion (sprints).
▪ Provides ATP very rapidly but is wasteful.
▪ Liver converts lactic acid in blood back to glucose or glycogen when oxygen is available again.

Question 22

3 Aerobic breakdown of glucose to form ATP.
1 glucose+ O2 - pyruvic- CO2+H2O+ 32 ATP

Answer 22

▪ Requires oxygen (aerobic).
▪ Used for long term exertion (marathons).
▪ Is relatively slow but provides lots of ATP.

Fat metabolism:
▪ Fatty acids are the main fuel source for resting muscle.

Question 23

Types of skeletal muscle fibers:

Answer 23

There are three different types of muscle fibers.
Each serve somewhat different functions.
Fibers are classified on the basis of:
1. Shortening velocity (fast or slow)
2. Major pathway used to form ATP (anaerobic or aerobic)

Question 24

Fast glycolytic fibers ( white fibers)

Answer 24

▪ contract rapidly because myosin heads break down ATP fast
▪ rely on anaerobic glycolysis
▪ have high glycogen content which supplies plenty of glucose
▪ respond quickly but fatigue quickly
▪ have few capillaries (little incoming glucose & O2)
▪ are pale in color because they have little myoglobin and few capillaries
▪ best for rapid, short-term, intense movements
▪ ex. lifting a weight

Question 25

slow oxidative fibers ( red fibers)

Answer 25

▪ contract slowly because myosin heads break down ATP slowly
▪ rely primarily on aerobic breakdown of glucose
▪ have high myoglobin content (stores O2)
▪ resist fatigue & have high endurance
▪ have many capillaries
▪ are red in color because of the myoglobin and capillaries
▪ best for endurance type activities
▪ ex. maintaining posture

Question 26

Fat oxidative fibers:

Answer 26

▪ intermediate between the other two types
▪ myosin heads break down ATP rapidly
▪ rely on both aerobic and anaerobic pathways
▪ have intermediate amounts for glycogen and myoglobin
▪ most versatile
▪ ex. walking

Question 27

summary of the 3 fiber types

Answer 27

▪ Most muscles have all three types of fibers.

▪ Proportions differ in each muscle in each person.
- determines who is a good sprinter and who is a good marathoner.
- genetically initiated, but modified by exercise
▪ All muscle fibers in one motor unit are of the same type.

Question 28

Cardiac muscle : characteristics

Answer 28

• only found in heart
• Branchoing cells with single nucleus
• Striated (because action & myosin are arranged in sarcomeres)

Question 29

Cardiac muscle characteristics pt 2

Answer 29

-Involuntray ( not under conscious control)

-Pacemaker cells in myocardium generate rhythmic electrical impulses without nervous stimulation.

• Most of the cardiac muscle cells are NOT innervated

Question 30

Cardiac muscle characteristics pt 3

Answer 30

Has intercalated discs cointaining desmosomes and gap juntions located at the junction of two cardiac muscle cells.

Question 31

Intercalated discs contents -2

Answer 31

-Desmosomes - anchoring proteins that keep neighboring cells from SEPARATING

-Gap junctions- CHANNELS that connect the sarcoplasm of adjacent cells so ions, and thus electrical charge, can flow form cell to cell

Question 32

Characteristics of smooth muscle

Answer 32

▪ Found in walls of hollow organs (except heart)
ex. blood vessels, bronchi, and organs of GI tract

▪ Short, spindle-shaped
cells with single nucleus

▪ No striations
Why? – lacks sarcomeres
▪ Involuntary

Question 33

Smooth muscle contractions

Answer 33

▪ Relies on calcium from outside of cell
▪ Lacks troponin, but has a calcium-binding protein called calmodulin instead
▪ When calcium binds to calmodulin, it causes myosin to be phosphorylated which activates the myosin.
▪ This triggers contraction.

Question 34

Effects of exercises on muscles

Answer 34

3 different types - Aerobic( endurance) , resistance, disuse atrophy.

Question 35

Aerobic ( Endurence ) exercise :

Answer 35

Regular aerobic exercise (such as running, swimming, biking) results in changes in skeletal muscles including increased:
▪ number of capillaries surrounding muscles
▪ number of mitochondria within muscle fibers
▪ myoglobin content within fibers
Overall these changes lead to:
greater endurance, strength, and resistance to fatigue.

Question 36

Resistance Exercise - anaerobic

Answer 36

Regular high-intensity resistance exercise (such as weight-lifting) can result in skeletal muscle fibers containing more:
▪ mitochondria, myofilaments, and myofibrils
(This leads to larger muscle fibers.)
▪ glycogen
▪ connective tissue between fibers
Overall these changes lead to:
increases in muscle strength & size
“muscle hypertrophy”

Question 37

Disuse Atrophy

Answer 37

▪ Degeneration of muscle tissue and loss of muscle mass

▪ Caused by complete immobilization or by loss of neural stimulation

▪ Lost muscle tissue is replaced by fibrous c.t.