Cardiology I
Thursday, October 31, 2019 8:17 PM
All of the muscle fibers in a single motor unit are composed of the same fiber type and
muscles are composed of multiple motor units of different types
- Force and duration of contraction can be varied as a result of:
○ Changing the types of motor units that are active
○ Changing the number of motor units responding at any one time
The force of contraction in skeletal muscle can be increased by recruiting additional
motor units
- Recruitment is controlled by the nervous system and proceeds in a standardized
sequence
1. A weak stimulus directed onto a pool of somatic motor neurons in the
CNS activates only neurons with the lowest thresholds, which control
slow-oxidative fibers (type 1) that generate minimal force
2. With increasing stimulus strength, additional motor neurons with higher
thresholds begin to fire and stimulate the fast-oxidative glycolytic fibers
(type II a)
i. Greater force has been generated in the muscle as more motor
units have been recruited
3. As the stimulus increases to higher levels, the fast-glycolytic fibers (type II
b), that generate more force but fatigue rapidly, are recruited
i. You cannot maintain a contraction at maximal force for an extended
period of time
Asynchronous recruitment of motor units is used by the nervous system to avoid
fatigue during sustained contraction
- The nervous system modulates the firing rates of the motor neurons so that
different motor units take turns maintaining muscle tension
- We do not notice the cycling of different motor units because different motor
units in the muscle are contracting and relaxing at slightly different times --
these contractions & relaxations average out to appear as one smooth
contraction
Thursday, October 31, 2019 8:17 PM
All of the muscle fibers in a single motor unit are composed of the same fiber type and
muscles are composed of multiple motor units of different types
- Force and duration of contraction can be varied as a result of:
○ Changing the types of motor units that are active
○ Changing the number of motor units responding at any one time
The force of contraction in skeletal muscle can be increased by recruiting additional
motor units
- Recruitment is controlled by the nervous system and proceeds in a standardized
sequence
1. A weak stimulus directed onto a pool of somatic motor neurons in the
CNS activates only neurons with the lowest thresholds, which control
slow-oxidative fibers (type 1) that generate minimal force
2. With increasing stimulus strength, additional motor neurons with higher
thresholds begin to fire and stimulate the fast-oxidative glycolytic fibers
(type II a)
i. Greater force has been generated in the muscle as more motor
units have been recruited
3. As the stimulus increases to higher levels, the fast-glycolytic fibers (type II
b), that generate more force but fatigue rapidly, are recruited
i. You cannot maintain a contraction at maximal force for an extended
period of time
Asynchronous recruitment of motor units is used by the nervous system to avoid
fatigue during sustained contraction
- The nervous system modulates the firing rates of the motor neurons so that
different motor units take turns maintaining muscle tension
- We do not notice the cycling of different motor units because different motor
units in the muscle are contracting and relaxing at slightly different times --
these contractions & relaxations average out to appear as one smooth
contraction
