Muscle Physiology

Question 1

Transverse (T-) Tubules

Answer 1

Invaginations of the sarcolemma
Transmit action potential into interior muscle cell
Closely apposed to sarcoplasmic reticulum (SR)

Question 2

Sarcoplasmic Reticulum (SR)

Answer 2

Membranous sac underlying the sarcolemma
Responsible for calcium storage, release and reuptake
Integral to muscle contraction

Question 3

Terminal Cisternae

Answer 3

Bulbous enlargements of the SR

Store and release calcium

Question 4

“Triade”

Answer 4

SR, T Tubules, Terminal Cisternae

Question 5

Sarcolemma

Answer 5

Muscle cell membrane

Contains sarcoplasm, cellular proteins, organelles and myofibrils

Question 6

Myofibrils

Answer 6

Divided into individual contractile units – sarcomeres
Thick filaments (myosin)
Thin filaments (actin)
Troponin and tropomyosin are located on actin protein

Question 7

Titin / Connectin

Answer 7

Serves as a spring to link Z-disk and M-line
Maintains actin/myosin position
Molecular blueprint

Question 8

Nebulin

Answer 8

Molecular ruler
Incorporated into and co-extensive with actin
Extends from Z-disk to end of actin myofilament
Precisely regulates actin length (caps actin, determines length)

Question 9

Obscurin

Answer 9

Intimately surrounds sarcomere, primarily at Z-disk and M-band regions
Coordinates assembly and organization of SR with myofilaments

Question 10

Myosin

Answer 10

2 heavy chain polypeptides (MHC)
Light meromyosin (LMM):  intertwine in double helix formation to form molecular backbone
Heavy meromyosin (HMM): project outward to form neck (S2) and globular head (S1)
MHC isoforms (I, IIa, IIx, and IIb) are determined by ATPase activity and contribute to contraction velocity

Question 11

Neck region - Myosin

Answer 11

4 light chain polypeptides (MLC)
Each S1/S2 complex contains 1 essential (ELC) and 1 regulatory (RLC) light chain
MLC isoforms fine tune contraction velocity

Question 12

Actin

Answer 12

Comprises majority of thin myofilament
Arranged in double helix formation
Contains myosin binding sites

Question 13

Tropomyosin

Answer 13

Resides in groove along length of actin protein

Blocks myosin binding site under resting conditions

Question 14

Troponin

Answer 14

Spaced at regular intervals along length of actin protein
3 distinct subunits (TI, TC, and TT)
Regulates position of tropomyosin relative to myosin binding site (Steric Block Model)

Question 15

Sequence of Events:

Answer 15

1 Action potential is propagated along the sarcolemma and into the T-tubules
2 This stimulates the release of calcium (Ca+) from the SR
3 Ca+ binds to Troponin C, resulting in a conformation change that pulls tropomyosin away from myosin binding site on actin filament
4 Hydrolysis of ATP “cocks” myosin head
5 “Cocked” myosin head binds to actin and contraction occurs
6 Hydrolysis of ATP detaches myosin head from actin
7 This sequence continues as long as Ca+ is available
Ca+ is released as long as action potentials are present
Ca+ is resorbed and recycled by the SR
8 In the absence of action potentials, the SR resorbs Ca+ from the sarcoplasm
9 In the absence of Ca+ troponin and tropomyosin return to their resting states, blocking the myosin/actin binding site

Question 16

Histochemical analysis of metabolic characteristics:

Answer 16

Type I: Slow oxidative
Type IIa: Fast oxidative-glycolytic
Type IIb/x: Fast glycolytic

Question 17

Muscle Fiber Type - Changes?

Answer 17

Myosin isoforms are genetically determined

In general, exercise training does not override a myocyte’s intrinsic, genetically determined qualities (e.g. type I ↔ II)

However, exercise may alter histochemical characteristics, resulting in intermediate changes (e.g. type IIa ↔ IIb)

Question 18

Myoplasticity

Answer 18

Changes in use and environment can generate alterations in structural and enzymatic protein content
Predominantly due to changes in gene expression
Some post-translational alterations contribute

Gene expression can be influenced by:
Contractile activity (e.g. endurance exercise, e-stim, denervation)
Loading conditions (e.g. resistance training, microgravity)
Substrate availability (e.g. nutrition)
Hormones (e.g. insulin, IGF, thyroid, etc)
Environment (e.g. hypoxia, trauma)

Question 19

Myoplastic adaptation can occur at the level of:

Answer 19

Structure (e.g. cross-sectional area)
Type (e.g. MHC isomer)
Metabolism (e.g. mitochondrial density, oxidative capacity)
Energy storage (e.g. intracellular lipid content)
Capillary density, capillary : fiber ratio
Function (force production, contraction velocity, fatigue resistance

Question 20

Adaptations to Endurance Training

Answer 20

Increased oxidative capacity
Increased mitochondrial density
Increased expression of type I fibers
Reduced expression of type IIb fibers (in days)
Reduced expression of type IIa fibers (in years)
Little change in CSA or glycolytic capacity

These adaptations result in:
Delayed onset of metabolic acidosis
Increased fatigue resistance
Increased oxygen consumption

Question 21

Adaptations to Resistance Training:

Answer 21

Increased CSA (in all fiber types)
Increased number of nuclei / cell
Reduced mitochondrial density (due to hypertrophy)
Reduced expression of type I fibers
Reduced expression of type IIb fibers
Increased expression of type IIa fibers
Little change in capillarity or enzymatic capacity (oxidative or glycolytic)

These adaptations result in:
Increased contractility
Improved elasticity
Improved neuromotor recruitment

Question 22

Duchenne’s and Physical Therapy

Answer 22

Have pt perform resistance exercises that strengthen and tone Mm
Stronger muscles can help to delay the impending weakness
Range of motion exercises and stretching
Flexibility can help ease the severity of joint contractures
Evaluate for braces
Keep tendons and muscles stretched and avoiding painful contractures
Aquatic therapy
Water exercises and swimming help to tone and strengthen muscles and joints
Decreases stress where the body is already weak
Place emphasis on mobility
Goal of physical therapy
PTs treat people with muscular dystrophy to provide them with independence for as long as possible by focusing on movement.
Develop large muscle groups improve strength and increase endurance