Exam #6: Introduction to Motor Systems

Question 1

What is an alpha motoneuron?

Answer 1

This is a lower motor neuron that innervates skeletal muscle; the cell body is located in the anterior (or ventral) horn of the spinal cord

Question 2

What is the motoneuron pool?

Answer 2

All of the motor neurons that innervate a single skeletal muscle

*****Note that these motor units are recruited from smallest to largest

Question 3

What is the motor unit?

Answer 3

One alpha motor neuron and all of the skeletal muscle fibers it innervates

Question 4

What is a fibrillation?

Answer 4

Spontaneous activity within a single muscle fiber

Question 5

What is a fasciculation?

Answer 5

Visible twitches of muscles that can be seen as ripples under the skin

Question 6

What is the difference between a fibrillation & a fasciculation?

Answer 6

• Size: fibrillations involve a single muscle fiber vs. a fasciculation that involves a muscle
• Presentation: cannot see a fibrillation, can see a fasciculation

Question 7

What size (large, small) motor units are expected in the extra ocular muscles?

Answer 7

Small

Question 8

What size (large, small) motor units are expected in the muscles controlling individual fingers?

Answer 8

Small

Question 9

What size (large, small) motor units are expected in the large postural muscles?

Answer 9

Large

Question 10

What size (large, small) motor units are expected in the limb muscles that have been affected by polio?

Answer 10

Large

Question 11

What is a lower motor neuron?

Answer 11

A motor neuron that directly innervates a skeletal muscle

- Originates from the brainstem or spinal cord

Question 12

What is an upper motor neuron?

Answer 12

A neuron in a higher motor area that synapses with a lower motor neuron or its interneuron

Question 13

What are the effects of lower motor neuron damage i.e. describe the presentation of a lower motor neuron lesion?

Answer 13

1) Paresis or paralysis
2) Muscle atrophy/ decreased muscle tone
3) Hyporeflexia or absent reflexes
4) Fibrillations or fasciculations

*****Note that there is an increase in post-synaptic ACh receptors when there has been LMN damage

Question 14

What are the effects of upper motor neuron damage i.e. describe the presentation of an upper motor neuron lesion?

Answer 14

1) Hypertonia
2) Hyperreflexia & clonus
3) Resurgence of primitive reflexes i.e. Babinski sign
4) Spasticity i.e. velocity-dependent increase in muscle tone

Question 15

Generally, what happens if the spinal cord is transected?

Answer 15

• Permanent total paralysis for all muscles innervated by spinal segments below the level cut
• Anesthesia below the level cut

Question 16

When the spinal cord is transected, what effects are seen immediately? What is spinal shock?

Answer 16

Initially there is “spinal shock” which is characterized by:

1) Flaccid paralysis
2) Areflexia
3) Loss of ANS function

**Note that loss of ANS function below the transected level can lead to HYPOTENSION that can be life-threatening

Question 17

When the spinal cord is transected, what effects are permanent?

Answer 17

1) Paralysis below the level

2) Anesthesia below the level of the injury

Question 18

How would a patient’s condition change over time in response to transection of the spinal cord?

Answer 18

1) The type of paralysis changes over time
- Transition from FLACCID paralysis to SPASTIC paralysis
2) Transition from hypotonia to hypertonia with time
3) Areflexia–>hyperreflexia

Question 19

What is the difference between voluntary muscle movement and reflex activity following spinal cord transection?

Answer 19

• Patient will have total loss of voluntary muscle movement
• Reflexes e.g. patellar tendon reflex will not only be intact, it will be HYPERREFLEXIVE if enough time has passed since the injury–this is NOT a voluntary movement

Question 20

What is Babinski’s sign? What is the difference between the extensor plantar response and the normal plantar (flexor) response?

Answer 20

Babinski= upward movement of the great toe in response to stroking of the foot

• Extensor Plantar Response= Babinksi Sign
• Flexor Plantar Response= downward movement of the great toe in response to stroking of the foot

Question 21

What is the difference between paralysis and paresis?

Answer 21

Paralysis= complete loss of voluntary contraction 
Paresis= partial loss of voluntary contraction