Lower Motor Neurons (Glendinning)

Question 1

Identify the location of LMNs in the spinal cord.

Answer 1

LMN = alpha motor neuron (innervates muscles)

Cell bodies located in brainstem (cranial nerve nuclei) and spinal cord (ventral horns)

As opposed to UMN which begin in the cortex or brainstem and project in long-pathways to cranial nerve nuclei or spinal cord ventral horns

Question 2

List the cranial nerves that have LMN components.

Answer 2

called “bulbar” motor neurons

III, IV, VI - muscles of the eye

V - trigeminal jaw opening

VII - facial nerve expression

IX, X, XI - laryngeal and pharyngeal muscles (speaking and swallowing) / trapezius and sternocleidomastoid

XII - hypoglossal - tongue muscles (sticking tongue out)

Question 3

Spinal cord LMN with important functions (3)

Answer 3

C3-C5: motor neurons to PHRENIC NERVE controlling diaphragm (damage to the spinal cord here will affect breathing)

S3-S4: ONUF’S NUCLEUS - motor neurons innervating urethral and external anal sphincter -> enable voluntary control of urination and defecation

S2-S4: Motor neurons to pelvic floor muscles (w sacral and caudal lesions, can see bowel and bladder incontinence issues)

Question 4

Distinguish between the 3 types of alpha motor units.

Answer 4

FF (fast, fatigable) - ATPase strong reaction

• Stronger and faster
• Greatest # of muscle fibers
• Fibers contract rapidly

FR (fast, fatigue resistant) - ATPase mild reaction
- Less strong and fast

S (slow-fatigue resistant) - ATPase weak reaction

• Weakest and slow
• Least # of muscle fibers
• Fibers contract slowly

Question 5

Explain the mechanisms by which motor neurons increase the force of muscle contractions (motor-unit recruitment).

Answer 5

1. “orderly” motor unit recruitment by size: when you are making low force contractions, you just use size S - as you gradually increase contraction, you get into the FR and then the FF - only bring in FF when you need a forceful contraction
2. after a motor unit is recruited, its firing rate also increases

THUS, recruiting bigger units AND increasing firing rate

Question 6

Define fibrillation and fasciculation and explain their clinical significance.

Answer 6

Fibrillation: only detected with EMG

• Short-duration, spontaneous biphasic or triphasic potentials produced by single muscle FIBERS
• Indicative of denervated muscle
• Thought to represent unstable muscle fiber cell membrane (both occur in peripheral nerve injuries, axonal neuropathies, motor neuron disorders, some myopathies)

Fasciculations: larger potentials caused by spontaneous activity in a motor UNIT or several motor units

• Caused by LMN lesions (particularly in ventral horn cell disease - ALS)
• Large potentials suggestive of denervation and rein nervation

Question 7

Describe and recognize the post-polio syndrome and its clinical features.

Answer 7

Virus attacks alpha-motor neurons (polio virus is actually taken up peripherally, brought back to alpha motor neurons -> causes LMN syndrome)

Diagnosis of PPS based on:

1. LMN syndrome: muscle weakness, dec. reflexes, dec. muscle tone
2. Many LMNs affected
3. Period of years during which she was neurologically stable (usually ~15 years) followed by progressive weakness in the same muscles originally affected / recovery and stable period due to sprouting by neighboring motor neurons

post-polio period occurs years later b/c those new sprouts cannot be sustained

Question 8

Describe the pathological findings associated with denervation.

Answer 8

Type grouping and group atrophy

Question 9

Describe the LMN syndrome (sites and symptoms) and explain the physiological basis for each of the symptoms.

Answer 9

Lesions in:

1. Cell bodies in CNS (ventral horn - lamina IX or brainstem nuclei)
2. Axons in the PNS in ventral root, spinal nerve, peripheral nerve, or cranial nerve

Symptoms:
1. Weakness or paralysis
2. Atrophy
w/o motor neuron innervation, muscles cannot contract so lose mass - also lose trophic support

3. Hyporeflexia or areflexia
4. Decreased tone (resistance to passive movement - floppy - “flaccid paresis/paralysis”)
   use 0+ to 5+ scale
5. Fibrillation, positive sharp waves or fasciculations measured by EMG
   measures of denervation: needle electrodes inserted into muscle -> study muscle at rest (normal: no activity) -> study muscle during a contraction

DECREASED TONE + WEAKNESS

Symptoms will be in muscles that are innervated by the motor neurons (not a pathway)

Question 10

Describe the common CAUSES of LMN disorders (7).

Answer 10

1. Peripheral nerve, spinal nerve, cranial nerve lesions
2. Cauda equina lesions
3. Strokes or tumors affecting alpha motor neurons in ventral horn or brainstem
4. Polio (viral infection of alpha motor neurons)
5. Amyotrophic lateral sclerosis (loss of LMNs in ventral horn)
6. Guillain-barre syndrome (demyelinating disease - mostly motor)
7. Werdnig-Hoffman disease (degeneration of anterior horn)

Question 11

What is Spinal Muscle Atrophy (SMA I-IV)?

Describe the clinical features and pathophysiology of Werdnig-Hoffman disease.

Answer 11

• A group of diseases caused by degeneration of anterior horns
• Progressive and begin in infancy
• Due to abnormalities in chromosome 5
• Motor neurons affected in spaniel cord and cranial nerve motor nuclei

Most common (and severe) type of spinal muscle atrophy = SMA I Werdnig-Hoffman disease

• “floppy-baby”
• Median age of death: 6-7 mo.
• weakness and muscle wasting in limbs, respiratory, bulbar muscles (sucking, swallowing breathing)