Motor Systems

Question 1

What is a motor unit?

Answer 1

All muscle fibers innervated by a lower motor neuron. Groups of motor neurons often work together to coordinate the contractions of a single muscle.

Question 2

What does the size principle state?

Answer 2

LMNs are recruited in order of size and force.

Question 3

How much does the motor cortex contribute to the CST? What contributes to the rest of it?

Answer 3

The primary motor cortex is only 50% of the CST. The remainder of CST fibers come from frontal motor and parietal areas.

Question 4

Where is the corticobulbar tract located in the internal capsule?

Answer 4

It is located anterior to the CST

Question 5

Which are the cranial nerve nuclei that do not receive direct corticobulbar innervation?

Answer 5

oculomotor, trochlear, and abducens

Question 6

What is the difference between an upper and lower motor neuron?

Answer 6

An upper motor neuron originates in the cerebral cortex and carries information down to the spinal cord to activate lower motor neurons, which directly signal skeletal muscles to contract.