Week 1: skeletal muscle growth & function

Question 1

Review the cell structure of skeletal muscle.

Answer 1

• endomysium surrounds myofiber, perimysium surrounds muscle fascicles, epimysium surrounds whole muscle
• myofibers have multiple nuclei, located peripherally to receive signals from cell surface receptors
• satellite cells-reserve cells
• myofibers contain myofibrils, each surrounded by SER and T tubules

Question 2

Describe the structure of a sacromere

Answer 2

• between to Z discs
• dark A band: actin and myosin overlapping
• light I band: actin filaments from 2 adjacent sacromeres
• actin anchored to Z discs, myosin connected to M line
• they slide over each other when contracted. I bands shorten, A bands don’t

Question 3

Describe muscle fiber types.

Answer 3

red: thin, rich in myoglobin and mitochondria. Slow contraction, but fatigue resistant, aerobic metabolism
White: large, fast twitch but faster fatigue, less myoglobin, use anaerobic
intermediate
-postural muscles have more slow twitch
-eye muscles are fast twitch

Question 4

Describe muscle innervation via a-motor neurons

Answer 4

• a-motor neurons in ventral horn of spinal cord
• a-motor neurons fan out a cluster of terminal boutons that form motor end plate when reaches muscle fiber
• release Ach–>depolarization of sarcolemma and T tubules
• T tubules convey action potentials–>release of Ca2+ from adjacent terminal cistern of SER–>shortening of sarcomere via myosin bind to actin

Question 5

What are intrafusal fibers?

Answer 5

• extrafusal fibers: ordinary muscle cells
• intrafusal fibers: make up muscle spindle
• intrafusal fibers with neural elements wrapped around it
  1. Nuclear bag fibers (static and dynamic):
  2. Nuclear chain fibers

Question 6

Describe nerve fibers in muscle spindles.

Answer 6

Sensory nerve fibers
1. static (II): interact with chain fibers and static nuclear bag fibers
2. dynamic (Ia): connect with all intrafusal fibers
Motor innervation by y-motor (gamma) neurons
1. static y-motor neuron: chain fibers and static nuclear bag fibers (stimulation–>increases overall responsiveness in nuclear chain fibers and static nuclear bag fibers, but decreases dynamic nuclear bag fiber response)
2. dynamic y-motor neurons: innervate only dynamic nuclear bag fibers (increases tension on these fibers, increases sensitivity to rate of muscle stretch).
-modulate tension on intrafusal fibers to adjust sensitive of muscle spindle

Question 7

What happens in the muscle spindle when a muscle is stretched?

Answer 7

• sensory nerve fibers activated
• particularly Ia fibers (innervate dynamic nuclear bag fibers), respond to rate of change of muscle stretch
• activates contraction of extrafusal fibers via a-motor neurons

Question 8

Describe Golgi tendon organ.

Answer 8

• monitors force of muscle contraction
• free Ib sensory nerve endings in collagen fibers of tendon connecting muscle to bone
• increased tension causes firing of Ib nerve fibers–>neg feedback to a-motor neurons (causes muscle relaxation so heavy load is dropped)