What is the definition of "motor unit"

Motor Unit is made up of a motor neuron and the skeletal muscle fibers innervated by that motor neuron's axonal terminals. Groups of motor units often work together to coordinate the contractions of a single muscle; all of the motor units within a muscle are considered a motor pool.

1. Over-hunting
2. A disappearance of a primary food source
3. Drought
4. A more competitive species taking food
5. Different environment conditions that is difficult for the species to survive
6. Something killing off a certain gender of a species
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