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
Muscle fibers are insulated = ?
Allows for coordination of movement with the least amount of exertion b/c one muscle does not affect another
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
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)
Physical anatomy of skeletal muscle
Composed of muscle fibers (hundreds of thousands)
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
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
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
Contraction of muscle depends on levels of...
Calcium High = myosin heads rebind causing more contraction Low = tropomyosin blocks active site and muscle relaxes
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)
Body is in a state of contraction b/c no ATP = no more myosin release
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
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)
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
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
At a critical frequency successive contractions eventually become so rapid that they fuse together and muscle contraction appears smooth and continuous
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
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
Protein that is created through the polymerization of G-actin Double stranded helix Myosin heads bind to active sites
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
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)