Muscle Biology & Physiology

Author:

http://upload.orthobullets.com/topic/9021/images/Anatomy Epimysium_moved.jpg
http://upload.orthobullets.com/topic/9021/images/image013[1]_moved.jpg
http://upload.orthobullets.com/topic/9021/images/excitation-contraction_coupling.jpg
Overview
  • Key topics of this chapter include
    1. Gross anatomy
    2. Muscle contraction
    3. Muscles type
    4. Muscle metabolism
    5. Types of contraction
    6. Muscle training
    7. Nutritional training
    8. Muscle injury
Gross Anatomy
  • Myotendinous junction
    • weak link in muscle and often site of tears (especially with eccentric contraction)
    • involution of muscles cells maximized surface area for attachment
  • Noncontractile elements
    • Epimysium surrounds muscle bundles
    • Perimysium surrounds muscle fascicles
    • Endomysium surrounds individual fibers
Muscle Contraction
  • Contractile elements
    • derived from myoblasts
    • the muscles fiber (an elongated cell) is the basic unit of contraction
    • a myofibril is a collection of sarcomeres
  • Sarcomere composition
    • filaments
      • thick myosin filaments
      • thin actin filaments
    • bands
      • H band is myosin only
      • I band is actin only
      • A band is both actin and myosin
      • Z line flanks each sarcomere and acts as site of attachment for actin filament
      • during muscle contraction
        • A band stays the same length
        • I band reduces in length
        • H zone reduces in length
  • Action stimulation
    • nerve cell body delivers electrical signal to motor endplate (junction between muscle and nerve)
      • nerve action potentials are started with passage of sodium ions through voltage gated channels 
    • Ach is released and diffuses across synaptic cleft to bind to Ach receptor
      • myasthenia gravis patient has shortage of Ach receptors
      • botox blocks release of Ach from end plate
    • Ach binding triggers depolarization of sarcoplasmic reticulum and release of calcium into muscles cytoplasm
    • excitation-contraction coupling  
      • in low calcium environment
        • tropomyosin blocks myosin-binding sites on actin 
      • in high calcium environment
        • calcium binds to troponin (on thin filaments) leading to a configuration change of tropomyosin (on thin filaments) 
        • exposing myosin-binding sites on actin filament
        • actin forms cross-bridges to myosin, and the ATP breakdown, the two fibers contract past one another
  • Types of muscle contraction
    • isometric
      • muscle contracts with constant length (e.g. pushing against an immovable object)
    • isokinetic
      • muscle contracts with constant speed (requires specific equipment like cybex machines)
    • plyometric
      • rapid lengthening followed by contraction of muscle groups (e.g. jumping up and down on boxes)
    • isotonic - muscle contract with constant tension
      • concentric
        • muscle shortens during contraction (e.g. biceps curl) 
      • eccentric
        • muscle lengthens during contraction (e.g. "negative" of a biceps curl)
  • Force generation
    • force generated is most dependent on muscle cross-sectional area  
    • muscle fiber size increases with strength conditioning 
  • Contraction speed
    • duration and speed of contraction is most dependent on fiber type
Muscle Types  

 
Type I muscle 
(slow twich - ST)
"slow red ox muscles"
Type II muscle
(fast twitch - FT)
Metabolism • aerobic / oxidative • anaerobic / glycolytic
Energy source
• Aerobic system (oxidative phosphorolation via Krebs cycle)
• ATP-CP system
Exercise duration
• endurance (distance running) 
• low strength of contraction 
• low speed of contraction 
• first to atrophy with deconditoning
• high strength of contraction 
• high speed of contraction (large force generation per cross sectional area) 
• fatigue rapidly 
• sprinting is example
Note

• high yield ATP
• requires O2 and thus more vascular 
• increase mitochondria in cells 

• high yield ATP (increased ATPase)
• low intramuscular triglycerine stor
es

Metabolic Systems
  • Three systems are used to generate energy for muscles
    • ATP-CP anaerobic system
      • (adenosine triphosphate-creatinine phosphate system, "phosphagen system") 
        • basis for creatine phosphate supplementation (main side effect: muscle cramping)
      • used for intense metabolic exercise lasting less than 20 seconds (e.g., 100 meter sprint)
      • converts carbohydrates stored within muscle into energy
      • anaerobic (does not use oxygen and does not produce lactate)
      • formulas
        • ATP –» ADP + P + energy
        • ADP –» AMP + P + energy
    • lactic anaerobic system (lactic acid metabolims)
      • intense muscle activity lasting 20 to 120 seconds (e.g., 400 meter sprint)
      • involves hydrolysis of one glucose molecule
      • formula
        • glucose –» lactic acid + energy
    • aerobic system
      • used in longer duration and lower intensity exercises
      • Krebs cycle generates ATP from glucose and fatty acids through oxidative phosphorylation
Muscle Injury
  • Muscles soreness
    • caused by edema and inflammation in the connective tissue
      • neutrophils are the most abundant cells early on after acute injury 
        • generates free radicals that possibly increase muscle damage
    • worse with unaccustomed eccentric exercise
    • peaks at 24-48 hours
    • elevated CK levels seen in serum
  • Muscles strain
    • occur at myotendinous junction (off during eccentric contraction which produces highest forces in skeletal muscle)
    • pathoanatomy in inflammation followed by fibrosis
  • Muscle atrophy
    • caused by disuse or nerve injury
    • leads to fatty infiltration and increased fatigability 
    • muscles crossing a single joint atrophy faster
    • loss of cross-sectional area leads to decreased force generation 
 

Please rate topic.

Average 4.3 of 18 Ratings

Questions (5)

(OBQ12.109) Progressive overloading of muscles in adults during exercise leads to which of the following? Review Topic

QID:4469
1

Increased muscle fiber length

4%

(85/2154)

2

Decreased musculotendinous junction length

1%

(29/2154)

3

Slowed peak contraction velocity

5%

(114/2154)

4

Muscle fiber hypertrophy

87%

(1884/2154)

5

Decreased sarcomere length

1%

(27/2154)

Select Answer to see Preferred Response

PREFERRED RESPONSE 4

Strength training is achieved by incremental progressive loading of muscles, in effort to increase muscle fiber contraction coordination and eventually hypertrophy of the muscle fibers themselves.

Kraemer et al. provide an American College of Sports Medicine position statement on appropriate training regimens. They recommend that loads corresponding to 8-12 repetition maximum (RM) be used in novice training. For intermediate to advanced training, it is recommended that individuals use a wider loading range, from 1-12 RM in a periodized fashion. For local muscular endurance training, it is recommended that light to moderate loads (40-60% of 1 RM) be performed for high repetitions (> 15) using short rest periods (< 90 s).

Booth et al. review the adaptation of muscle after exercise. They note that increased in muscle fiber coordination occur initially with training, and increases in power thereafter are from muscle fiber hypertrophy.

Illustration A is a diagram showing the connection between muscle size and number of myonuclei. Previously untrained muscles acquire newly formed nuclei by fusion of satellite cells preceding the hypertrophy (this is permanent). The elevated number of nuclei in muscle fibers that had experienced a hypertrophic episode would provide a mechanism for muscle memory, explaining the long-lasting effects of training and the ease with which previously trained individuals are more easily retrained.

Incorrect Answers:
Answer 1,2,4,5: Overloading of muscle with strength conditioning does not lead to any of these options.

ILLUSTRATIONS:

Please rate question.

Average 3.0 of 12 Ratings

Question COMMENTS (3)

(OBQ12.181) The cross-sectional area of a muscle is the factor most responsible for which of the following? Review Topic

QID:4541
1

Amount of maximal tension

87%

(2536/2906)

2

Speed of contraction

3%

(92/2906)

3

Duration of contraction

3%

(79/2906)

4

Type of contraction

2%

(62/2906)

5

Fatigability

4%

(117/2906)

Select Answer to see Preferred Response

PREFERRED RESPONSE 1

Force generation, or the amount of maximal tension that can be generated by a given skeletal muscle is most dependent on the cross-sectional area of the muscle.

The cross-sectional area is the main determining factor in force generated by the muscle and is controlled by the number of myofibrils that contract. Weight lifting can lead to muscle hypertrophy, increased cross-sectional area, and increased force (ability to lift heavier weights). Fiber types have less to do with the force of contraction and more to do with the duration and speed of contraction.

Baroni et al. investigated the chronology of neural and morphological adaptations to knee extensor eccentric training. After 12 training weeks, significant increases in strength and anatomical cross-sectional area (19%) were seen.

Illustration A shows how muscle hypertrophy from strength training increases cross-sectional area.

Incorrect Responses:
The other functional attributes of a muscle, such as speed and duration of contraction and fatigability are more predicated on muscle fiber type than on the area.

ILLUSTRATIONS:

Please rate question.

Average 3.0 of 12 Ratings

Question COMMENTS (2)

(OBQ09.127) A collegiate football player is hit in the thigh by an opposing player's helmet. Radiographs are unremarkable and a clinical image of the injury is shown in Figure A. Which of the following cells appear first at the site of injury? Review Topic

QID:2940
FIGURES:
1

Osteoblasts

0%

(6/1359)

2

Neutrophils

89%

(1215/1359)

3

Myoblasts

4%

(59/1359)

4

Lymphoblasts

3%

(46/1359)

5

Eosinophils

2%

(28/1359)

Select Answer to see Preferred Response

PREFERRED RESPONSE 2

Neutrophils are the first cells to appear following acute muscle injury.

Tidball showed that mononucleated cells that normally reside in muscle are activated resulting in chemotactic signaling to inflammatory cells. These signaling molecules cause a massive influx of neutrophils to move to the injury site releasing inflammatory cytokines, resulting in swelling. Following this initial phase is an increase in macrophages that phagocytose debris. Finally, there is an increase in a second subpopulation of macrophages associated with muscle regeneration and scarring. Neutrophils and their generation of free radicals is thought to be associated with increased scarring and muscle damage.

Figure A demonstrates a quadriceps contusion and these are treated with icing immobilization in 120 degrees of knee flexion for 24 hours followed by therapy.


Please rate question.

Average 2.0 of 20 Ratings

Question COMMENTS (1)

(SBQ07.4) Regarding skeletal muscles, which of the following is true? Review Topic

QID:1389
1

Force generated is most dependent on muscle length

5%

(75/1520)

2

Force generated is most dependent on muscle fiber type

2%

(23/1520)

3

Type I muscle is comprised of fast twitch fibrils

6%

(88/1520)

4

Duration and speed of contraction are most dependent on cross-sectional area

13%

(205/1520)

5

Duration and speed of contraction are most dependent on muscle fiber type

74%

(1123/1520)

Select Answer to see Preferred Response

PREFERRED RESPONSE 5

The duration and speed of contraction is most dependent on the muscle fiber type. The force generated by the muscle is most dependent on the cross-sectional area of the muscle.

Fiber types have less to do with the force of contraction and more to do with the duration and speed of contraction. The cross-sectional area of a muscle determines to a great extent the force generated by the muscle and is controlled by the number of myofibrils that contract. Muscle length affects contraction force through the Blix curve. The morphology of a muscle can affect the cross-sectional area by varying the angle of the fibers in relation to the force vector.

Incorrect Answers:
1. Force generated is most dependent on muscle cross-sectional area.
2. Force generated is most dependent on muscle cross-sectional area.
3. Type I is "slow twitch" and Type II is "fast twitch"
4. Duration and speed of contraction is most dependent on fiber type.


Please rate question.

Average 4.0 of 15 Ratings

Question COMMENTS (1)

(OBQ05.163) While flexing the elbow to perform a biceps curl, what type of muscle contraction is occuring? Review Topic

QID:1049
1

Isometric

3%

(11/315)

2

Isokinetic

4%

(14/315)

3

Plyometric

1%

(3/315)

4

Eccentric

3%

(10/315)

5

Concentric

88%

(276/315)

Select Answer to see Preferred Response

PREFERRED RESPONSE 5

Concentric muscle contractions occur when a muscle shortens during contraction, as in the upward motion when performing a biceps curl. An eccentric contraction occurs when a muscle lengthens with contraction, as in the "negative" or lowering motion of a biceps curl. An example of an isometric (muscle contracts while maintaining constant length) contraction would be pushing against an immovable object. An example of an isokinetic (muscle has constant speed of contraction) occurs with specialized equipment like Cybex machines. Plyometric contractions occur when a muscle rapidly lengthens just prior to contraction - like during repetitive box jumping.

Woo and Buckwalter describe the mechanisms, barriers, and molecular processes involved in ligament and tendon injury and repair.


Please rate question.

Average 3.0 of 13 Ratings

Question COMMENTS (0)
Sorry, this question is available to Virtual Curriculum members only.

Click HERE to learn more and purchase the Virtual Curriculum today!


This is a Never-Been-Seen Question that can only be seen in Milestone Exams
for Virtual Curriculum members.

Click HERE to learn more and purchase the Virtual Curriculum today!


EVIDENCE & REFERENCES (10)
Topic COMMENTS (0)
Private Note