Muscle Anatomy & Physiology

Question 1

Structure of Muscle

Answer 1

Epimysium (surrounds entire muscle & continuous w/ tendon) –> perimysium (surrounds fascicle) –> endomysium (surrounds individual myocyte) –> sarcolemma (muscle cell membrane)

Question 2

Each myocyte contains…

Answer 2

Sarcolemma: muscle cell membrane, including:

• Transverse (T) tubules: bring AP into interior of muscle fiber
• Sarcoplasmic Reticulum (SR): stores Ca
• Terminal Cisternae: bulbous enlargements of SR responsible for storage and release of Ca

Myofibrils: divided into contractile units (sarcomere)
- actin (thin filaments) & myosin (thick)

Question 3

Myosin

-MHC & MLC

Answer 3

MHC = myosin heavy chain (2)
- light: forms molecular backbone (double helix)
- heavy: forms neck (S2) & head (S1)
These MHC isoforms determined by ATPase activity and contribute to contraction velocity (type I, IIa or b)

MLC - myosin light chain (4)

• each S1/S2 complex contain 1 essential and 1 regulatory chain
• MLC isoforms fine tune contraction velocity (less contribution)

Question 4

Actin

Answer 4

contains myosin binding sites & troponin & tropomyosin

• Troponin: regulates position of tropomyosin (spaced at regular intervals)
• Tropomyosin: blocks myosin binding site under resting condition (resides in actin protein)

Question 5

Sliding Filament Theory

Answer 5

1. AP travels along sarcolemma & down T-tubules
2. Triggers release of Ca from SR
3. Ca binds to troponin C which removes tropomyosin from myosin binding sites on actin
4. Hydrolysis of ATP “cocks” myosin head
5. Myosin head attaches to actin and contraction occurs
6. Hydrolysis of ATP detaches myosin head
7. Returns to resting state if no AP & Ca is resorbed by SR

Question 6

Slow twitch vs. Fast twitch

Answer 6

histochemical analysis of myosin-ATPase (S1 head - MHC isoforms)

Slow = low levels of activity w/ lower maximum velocity & longer time to peak tension

Fast = high levels of activity w/ higher maximum velocity & shorter time to peak tension

Question 7

Type I vs Type II

Answer 7

histochemical analysis of metabolic characteristics

Type I = slow, oxidative, fatigue resistant, long delayed activity being primary purpose (posture muscles)
Type IIa = fast, oxidative-glycolytic
Type IIb = fast glycolytic

Question 8

Muscle Adaptations due to ENDURANCE training & what they result in

Answer 8

Adaptations:
Increased - oxidative capacity, mitochondrial density & expression of type I fibers
Reduced - expression of type IIb & IIa

Resulting in….

• delayed onset of metabolic acidosis
• increased fatigue resistance
• increased O2 consumption

Question 9

Muscle Adaptations due to STRENGTH training & what they result in

Answer 9

Adaptations:
Increased - CSA, number of cell/nuclei, expression of type IIa fibers
Reduced - mitocondrial density (due to hypertrophy) & expression of type I and IIb fibers

Resulting in…

• increased contractility
• improved elasiticity
• improved neuromotor recruitment

Question 10

Hypertrophy

Answer 10

occurs when protein is degraded to to mechanical overload which stimulates protein synthesis
= increase in size & amount of contractile units & # of sarcomeres in parallel
= increase in CSA & strength

Question 11

Protein recommendations

Answer 11

Sedentary adult = 0.8 g/kg
Active adult - 1.0
Endurance athletes = 1.2 - 1.4
Strength athletes = 1.4 - 1.8
MAX for adults = 2.0

Adolescent athletes = 1.0 - 2.0

Question 12

Hyperplasia

Answer 12

proliferation of myocytes

Question 13

Effects of Immobilization on muscles

Answer 13

muscle composition changes occur w/n HOURS!

• decrease in protein synthesis = loss of muscle mass & contractile/SR protein
• Type I fibers atrophy FIRST
• 3.4% loss of strength/day for 1st week

Question 14

Effects of Aging on muscles

Answer 14

Muscle mass peaks btwn 25-30 years

• loss of 8% per decade
• 25-30% loss by age 65 due to fiber atrophy, fiber loss & motor unit remodeling (type I predominates)