69 - Motor Systems

Question 1

Alpha motor neuron

Answer 1

Alpha motoneuron - also called lower motoneuron; can be called anterior or ventral horn cell when in spinal cord

Question 2

Motor neuron pool

Answer 2

The group of motor neurons that innervate a muscle

Question 3

Motor unit

Answer 3

Motor unit definition - one alpha motor neuron plus all the skeletal muscle fibers it innervates

Question 4

Fibrillation

Answer 4

Fibrillations - spontaneous activity within single muscle fibers. They are not visible clinically since they are too small to be seen as movement through the skin.

Fibrillations can be recognized only by electromyography.

Question 5

Fasciculation

Answer 5

Fasciculations - visible twitches of muscle that can be seen as ripples under the skin. Can involve one or more motor units.

Benign fasciculations are not a sign of lower motor neuron disorder. An example of a benign fasciculation is the eyelid twitching often seen during periods of fatigue or eye strain.

Question 6

What size motor units would you find in extra ocular eye muscles

Answer 6

Small

Question 7

What size motor units would you find in muscles controlling individual fingers?

Answer 7

Small

Question 8

What size motor units would you find in large postural muscles?

Answer 8

Large

Question 9

What size motor units would you find in limb muscles that have been affected by polio?

Answer 9

Large

Question 10

How can you determine if a motor unit for a particular muscle is large or small?

Answer 10

Many small motor units when need fine control of a muscle

Large motor units (~1,000 muscle fibers) can be found in large, strong muscles

Question 11

Lower motor neuron

Answer 11

Lower motor neurons -

They are the motor neurons of the spinal cord and brain stem that directly innervate skeletal muscle

Question 12

Upper motor neuron

Answer 12

Upper motor neurons (UMN)

An UMN is not actually a motor neuron since it does not synapse on skeletal muscle fibers. An upper motor neuron is a neuron in a higher motor area (e.g. motor cortex, premotor cortex, brainstem center) that synapses on a lower motor neuron or on an interneuron which synapses on a lower motor neuron.

Question 13

LMN lesion

Answer 13

• Atrophy
• Decreased muscle tone
• Weak or absent reflexes (hyporeflexia)
• Fasiculations
• Fibrillations
• Paralysis of muscle if too many LMNs to that muscle are damaged

Question 14

Paralysis vs paresis

Answer 14

Paralysis means complete loss of voluntary contraction of a muscle while paresis refers to partial loss of voluntary contraction or voluntary muscle weakness.

Question 15

UMN lesion

Answer 15

• Spasticity
• Overactive deep tendon reflexes (DTR)
• Positive Babinski sign (extensor plantar reflex)

Question 16

When would a patient with an UMN lesion experience flaccid vs spastic paralysis?

Answer 16

The patient would have flaccid paralysis due to spinal shock immediately after the lesion and then later develop spastic paralysis

Question 17

Spasticity

Answer 17

Spasticity is defined as “ a motor disorder characterized by a velocity- dependent increase in tonic stretch reflexes (muscle tone) with exaggerated tendon jerks, resulting from hyperexcitability of the stretch reflex, and is one component of the upper motor neuron syndrome.”

Question 18

Partial vs total UMN loss

Answer 18

• If there is a total loss of descending UMN axons due to a total transection of the spinal cord there will be paralysis of all muscles innervated by lower motor neurons below the level of the spinal cord transection.
• If only some upper motor neurons controlling a muscle are damaged then the patient might have voluntary muscle weakness and develop spastic paresis.

Question 19

What happens immediately after a spinal cord transection?

Answer 19

Initial effect of transection is spinal shock

Question 20

Spinal shock

Answer 20

Spinal shock is the temporary loss of spinal cord reflex activity that occurs below a total (or near total) spinal cord injury.

Question 21

Effects of spinal shock

Answer 21

a. flaccid paralysis - joints offer no resistance to being bent; no stretch reflexes
b. areflexia includes loss of all spinal reflexes below level of transection (so micturition reflex lost)
c. loss of autonomic function below level of transection; (can cause hypotension, loss of temperature control)

Question 22

Permanent effects of complete transection of the spinal cord

Answer 22

• Permanent total paralysis for all muscles innervated by spinal segments below level of cut. Paralysis means loss of voluntary movement . The paralysis is permanent in that voluntary movement is lost forever. The paralysis is initially a flaccid paralysis and later changes to a spastic paralysis.
• Permanent anesthesia (lack of feeling below level of transection)

Question 23

Abnormal reflexes following a recovery from spinal shock

Answer 23

Patient with upper motor neuron lesion (such as a patient who has recovered from spinal shock) can show certain abnormal reflexes. (Clonus and the clasp-knife reflex)

This is NOT voluntary movement