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Flashcards in Skeletal muscle growth and function Deck (10):
1

Myofiber

-Individual muscle cell w/ associated connective tissue
-From in -> out: sarcolemma (muscle cell membrane)-> basal lamina-> endomysium
-Each cell is surrounded by basal lamina
-Moving toward periphery of muscle each layer of connective tissue (endomysium->perimysium->epimysium) becomes more dense

2

Types of muscle connective tissue

-Endomysium surrounds each myofiber (rich in reticular fibers)
-Perimysium separates muscle fascicles (bundles of myofibers)
-Epimysium is dense irregular CT at the muscle periphery
-Sarcomeres within myofibers are connected to the sarcolemma, which is connected to a tendon (DICT) via reticular fibers and collagen of the 3 muscle CT layers
-Muscles usually connect to bone via tendons, but bone is not necessary

3

Myofibril

-Dozens of myofibrils w/in each myofiber
-Each myofibril is surrounded by sarcoplasmic reticulum and T-tubules
-About 100 sarcomeres w/in each myofibril, all in parallel
-Mitochondria interspersed btwn myofibrils and organelles are concentrated near nuclei
-The T-tubule + sarcoplasmic reticulum formt he triad, which separates each myofibril
-These look like a fly, with the body of the fly being the T-tubule and the wings of the fly being the SR

4

Sarcomere

-Overlapping actin and myosin filaments
-Actin filaments (thin) attach at Z line and form I bands which are actin filaments from 2 adjacent sarcomeres
-The myosin filaments (thick) attach at M line
-The length of the thick filaments make up the A band, which at rest contains a small amount of overlap w/ the thin filaments
-Upon contraction the M line and Z lines move closer together, thus shortening the I bands (amount of non-overlapping thin filaments) while the A bands remain the same (no change in myosin length)

5

Fiber types

-Red fibers: thin, rich in myoglobin + mito, slow contraction but resistant to fatigue, aerobic metabolism, for endurance
-White fibers: large, fast twitch, faster fatigue, less myoglobin, fewer mito, anaerobic, for resistance
-Postural muscles: red fibers
-Eye muscles: white fibers
-Relative expression of different genes (ACTN3) can effect potential for different muscle types

6

Motor innervation of muscle

-Motor units controlled by aMNs in the ventral horn of the SC
-aMNs fan out in muscle to control many myofibers, they release Ach to cause contraction
-This is due to Ach-induced depolarization of the myofiber's membranes down the T-tubules causing a release of Ca from the terminal cisternae of the SR
-Ca initiates contraction of myofibers by binding to TnC, causing a change in conformation of tropomyosin
-This reveals the actin binding site and causes actin and myosin to pull sarcomeres closer together
-There are 2 triads per sarcomere

7

Sensory innervation of muscle

-Sensory bodies: muscle spindle and golgi tendon organ both convey proprioception
-Extrafusal fibers are the ordinary muscle fibers that cause contraction
-Intrafusal fibers make up the muscle spindle together w/ the spindle fibers
-Intrafusal fibers are innervated gamma-MNs (g-MN)

8

Intrafusal fiber sensory innervation

-Types of intrafusal fibers: nuclear bag fibers (static and dynamic) and nuclear chain fibers
-Sensory nerve fibers: static (II) and dynamic (Ia)
-Dynamic sensory fibers (Ia) innervate all intrafusal fibers, while static nerve fibers (II) innervate nuclear chain fibers and static nuclear bag fibers (just not dynamic nuclear bag fibers)
-Static nerves (II) innervate the same intrafusal fibers as the static g-MNs: both innervate all intrafusal fibers except dynamic nuclear bag fibers

9

Intrafusal fiber motor innervation

-gMNs can be either static or dynamic
-Static gMNs innervate nuclear chain fibers and static nuclear bag fibers (just not dynamic nuclear bag fibers)
-Dynamic gMNs innervate only dynamic nuclear bag fibers
-gMNs modulate tension on intrafusal fibers to adjust the sensitivity of the muscle spindle to stretch
-They do this by maintaining intrafusal fiber tautness during extrafusal contraction so they can respond to stretch even when in shortened position

10

Golgi tendon organ

-Monitor the force of muscle contraction
-Consist of sensory nerve ending in collagen fibers of tendons
-As muscles contract the tension on the tendons increases, thereby causing firing of the nerve fibers
-This provides a negative feedback to aMNs in SC to protect the muscle from excessively heavy loads (causes muscle to relax and drop load)
-Besides this spinal reflex the golgi tendon info is also relayed to higher brain centers for proprioception