Skeletal muscle growth and function

Question 1

Myofiber

Answer 1

• 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

Question 2

Types of muscle connective tissue

Answer 2

• 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

Question 3

Myofibril

Answer 3

• 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

Question 4

Sarcomere

Answer 4

• 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)

Question 5

Fiber types

Answer 5

• 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

Question 6

Motor innervation of muscle

Answer 6

• 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

Question 7

Sensory innervation of muscle

Answer 7

• 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)

Question 8

Intrafusal fiber sensory innervation

Answer 8

• 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

Question 9

Intrafusal fiber motor innervation

Answer 9

• 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

Question 10

Golgi tendon organ

Answer 10

• 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