Key Terms
ISOTONIC
Muscle tension stays constant; muscle length changes; load is moved
ISOMETRIC
Muscle produces tension without changing length; load is not moved
Motor unit
One motor neuron + all the muscle fibers it innervates. Each muscle fiber is innervated by only one motor neuron.
RECRUITMENT
Adding more motor units to increase muscle force.
Twitch
Single contraction in response to one action potential. Three phases:
GRADED MUSCLE RESPONSE
Variable tension controlled by:
WAVE SUMMATION
Second stimulus arrives before muscle fully relaxes from first; Ca++ levels in sarcoplasm remain elevated; stronger cont
INCOMPLETE TETANUS
Rapid stimulation; short relaxation phases between contractions; tension ~3-4x a single twitch.
COMPLETE TETANUS
Stimulation so rapid that relaxation phase disappears entirely; contraction is continuous until fatigue.
TREPPE (staircase effect)
Muscle dormant, then stimulated with repeated signals; first contractions weaker than later ones; tension builds stepwis
MUSCLE TONE
Low-level continuous contractions even when muscle is not producing movement. A few motor units active at a time in rota
HYPERTROPHY
Increase in muscle mass; structural proteins added to existing fibers; diameter increases. Cell count does not increase.
ATROPHY
Loss of structural proteins; muscle mass decreases. Sarcomeres and myofibrils are lost; fiber count remains the same.
SARCOPENIA
Age-related atrophy. Irreversible.
Location
Walls of hollow organs (bladder, stomach, intestines, uterus), blood vessel walls, respiratory and reproductive tracts,