Final answer:
Different types of motor units have different basic twitch and fatigability properties. Slow-twitch motor units contract slowly and are resistant to fatigue, while fast-twitch motor units contract rapidly and fatigue relatively quickly.
Step-by-step explanation:
There are different types of motor units in skeletal muscles, which have different properties. Motor units are composed of a motor neuron and the muscle fibers it innervates. The basic twitch properties of different types of motor units are determined by the size and characteristics of the motor neurons and muscle fibers.
The basic twitch properties can be classified into two main categories:
- Slow-twitch (Type I) motor units: These motor units have smaller motor neurons and muscle fibers. They contract slowly and are resistant to fatigue. They primarily use aerobic metabolism to produce ATP.
- Fast-twitch (Type II) motor units: These motor units have larger motor neurons and muscle fibers. They contract rapidly and fatigue relatively quickly. They can be further divided into:
- Fast-oxidative-glycolytic (FOG) motor units: These motor units have intermediate characteristics and use both aerobic and anaerobic metabolism to produce ATP.
- Fast-glycolytic (FG) motor units: These motor units have the largest motor neurons and muscle fibers. They contract rapidly and use anaerobic metabolism to produce ATP, but fatigue quickly.