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Flashcards in Physiology - Skeletal Muscle Deck (21)
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1

Duration of a muscle contraction is based on...

...the function of the muscle Ex: Ocular muscle - extremely rapid contraction to maintain fixation of eye on a specific object Gastrocnemius - contracts moderately rapidly to provide movement for running and jumping Soleus Muscle - slow contraction for long-term support of body against gravity

2

Muscle fibers are insulated = ?

Allows for coordination of movement with the least amount of exertion b/c one muscle does not affect another

3

Tropomyosin

Protein within actin filament Covers active sites of F-actin - prevents interactions with myosin Troponin T binds Tropomyosin = uncover active sites of F-actin = myosin is free to bind = muscle contraction

4

Mechanism of Muscle Contraction

1) At rest = active sites of actin are covered by Tropomyosin ADP + phosphate ion are attached to the myosin head 2) Calcium released from the Sarcoplasmic Reticulum binds to Troponin C creating a conformational change in Tropomyosin which detaches from the active site of F-actin 3) Myosin head binds to the active sites of actin 4) Myosin heads instantly ratchet pulling actin filaments along myosin (also pulling Z-lines closer) 5) ADP and phosphate ion are released from myosin and myosin binds ATP causing a detachment from actin 6) ATP is cleaved to ADP + phosphate ion and myosin head (reloads) Further contraction is dependent on calcium levels within the cell (high = myosin heads rebind causing more contraction and low = tropomyosin blocks active site and muscle relaxes)

5

Physical anatomy of skeletal muscle

Composed of muscle fibers (hundreds of thousands)

6

The molecular mechanism of muscle contraction is know as...

...a Sliding Filament Mechanism - contraction results from the sliding action of interdigitating actin and myosin filaments Filaments contract pulling Z-lines in as well

7

Muscle Contraction of Different Force

1) Multiple Fiber Summation - results from an increase in the number of motor units contracting simultaneously (fiber recruitment) 2) Frequency Summation and Tetanization - as frequency increases there is a point where each new contraction occurs before the preceding one is over (total strength of contraction rises with increasing frequency

8

Different types of muscle remodeling

Hypertrophy - increase in the number of sarcomeres Hyperplasia - increase in the number of muscle fibers Lengthening - elongation of muscle fibers Atrophy - loss of muscle fiber and sarcomeres

9

Contraction of muscle depends on levels of...

Calcium High = myosin heads rebind causing more contraction Low = tropomyosin blocks active site and muscle relaxes

10

Actin Filament

Within Sarcomere Globular protein (G-actin - actin monomers) 3 different proteins within actin filament: F-actin, Tropomyosin, and Troponin (G-actin attaches ATP and polymerizes = F-actin)

11

Rigor Mortis

Body is in a state of contraction b/c no ATP = no more myosin release

12

Muscle Fibers

Elongated Multinucleated Contain Myofibrils (parallel to cell): Contains light and dark badns (striated) Within is the sarcomere (between 2 Z-lines) which is composed of actin and myosin filaments Organelles exist between myofibrils

13

Types of skeletal muscle

Based on speed of twitch contraction = dependent on Vmax of myosin ATPase (how quickly it will release) 1) Fast Twitch - High Vmax (white muscle) Glycolytic: large diameter low myoglobin content low capillary density few mitochondria high glycolytic enzyme content 2) Slow Twitch - Low Vmax (red muscle) Oxidative: small diameter high myoglobin density high capillary density many mitochondria low glycolytic enzyme content most muscles contain both types (proportions differ)

14

Myosin Filament

Multiple myosin molecules - 6 polypeptide chains (tail and 2 heads) ATPase activity in myosin head (ATPase hydrolyzes ATP to ADP + Phosphate ion = release of energy used for muscle contraction

15

Sarcomere

Located between Z-lines within the myofibrils Muscle functional unit Contains: Actin Filaments that is attached to proteins on Z-lines Myosin Filament = between 2 actin filaments Titin - protein that keeps actin and myosin filaments aligned

16

Tetanization

At a critical frequency successive contractions eventually become so rapid that they fuse together and muscle contraction appears smooth and continuous

17

Source of energy for muscle contraction

ATP Sources: Phosphocreatine = high energy phosphate bonds (no oxygen) Glycolysis of Glycogen to Glucose to pyruvate and lactate (no oxygen; fast but not efficient = used for fast-twitch muscles) Oxidative Metabolism (use oxygen) - slower but produce more ATP = used for slow-twitch muscles Carbohydrate Metabolism Fat Metabolism

18

Troponin

Protein within actin filament Regulates myosin activity 3 Types: Troponin I = binds to actin Troponin T = binds to Tropomyosin when stimulated by calcium binding in order to detach tropomyosin from active site of F-actin = myosin binding to active site = muscle contraction Troponin C = binds to calcium from Sarcoplasmic Reticulum; allows for muscle contraction

19

F-actin

Protein that is created through the polymerization of G-actin Double stranded helix Myosin heads bind to active sites

20

Sarcoplasmic Reticulum

Special type of smooth endoplasmic reticulum found in striated muscles Surrounds myofibrils Contain a lot of calcium Stimulated by Acetylcholine to release calcium = acts on sarcomere to cause muscle contraction Calcium returns to Sarcoplasmic Reticulum in order for muscle to relax

21

Motor Unit

All muscles are innervated by a single nerve fiber (! nerve per fiber BUT potential for multiple fibers innervated by same nerve = must be fiber of the same type (i.e. fast vs slow twitch muscles)