Lower Motor Neurons and Combined LMN/UMN Lesions

Question 1

What are the two subtypes of alpha motoneurons?

Answer 1

1. Phasic - larger, high firing rate, and fast, but difficult to excite
2. Tonic - smaller, low firing rate, and slower, but easy to excite

Question 2

What is the order of recruitment of spinal lower motor neurons?

Answer 2

1. Gamma (to intrafusal fibers)
2. Tonic alpha - to tonic alphamotoneurons
3. Phasic alpha - to phasic alphamotoneurons

Question 3

What are the hallmarks of LMN syndrome?

Answer 3

1. Flaccid paralysis
2. Hypotonia
3. Hyporeflexia or areflexia (regarding deep tendon reflexes)
4. Muscle atrophy
5. Fasciculations

Question 4

What are fasciculations?

Answer 4

Random contractions of single motor units before muscle degenerates

Question 5

What are two causes of LMN syndrome?

Answer 5

1. Injury - i.e. trauma

2. Disease - i.e. Polio or ALS

Question 6

How does polio generally differ from ALS?

Answer 6

Polio - affects exclusively lower motor neurons, sparing upper motor neurons

ALS - combined UMN / LMN disease

Neither typically affects visceral motor neurons

Question 7

In what areas are the UMN / LMN most commonly affected in ALS?

Answer 7

UMN - to the lower limbs

LMN - to the upper limbs as well as laryngeal + pharyngeal constrictor muscles

Question 8

What is one clinical complication of LMN degradation to throat in ALS?

Answer 8

Often leads to respiratory pneumonia or choking.

Question 9

Why does lesion of the oculomotor nucleus result in a dilated pupil, ptosis, and loss of accommodation?

Answer 9

CN3 carries the preganglionic PANS before synapsing in ciliary ganglion and going to the pupillary sphincter. Also causes loss of direct light reflex + ipsilateral loss of direct light reflex.

Accommodation - ciliary muscle is also parasympathetic, travelling with CN3.

Question 10

What is Weber’s syndrome (superior alternating hemiplegia)?

Answer 10

lesion of PT tract + CN3 nucleus. Will manifest in PT syndrome (spastic hemiparesis of contralateral side of body) + CN3 symptoms + lower facial paralysis since corticobulbar tract is included (upper face has bilateral contribution).

Question 11

What does CN4 lesion cause?

Answer 11

Patient to tilt head away from affected eye. The trochlear nerve fibers will cross midline though. The superior oblique muscle intorts and brings the eye down.

Question 12

How does ipsilateral abducens motor nucleus lesion differ from abducens nerve lesion?

Answer 12

Motor nucleus lesion leads to destruction of interneuron for nucleus of 6 which controls medial rectus contraction when trying to gaze towards the effected side. Thus, both eyes will look straight when trying to look to affected side during the “lateral gaze syndrome”.

Question 13

How does middle alternating hemiplegia differ from superior alternating hemiplegia?

Answer 13

Middle affects PT tract at level of caudal pons, where there will be no involvement with the corticobulbar tract and thus no facial paralysis. Will still have symptoms of unilateral 6 lesion + hemiparesis of contralateral side of body, however.

Question 14

What are two manifestations of unilateral nerve V lesion?

Answer 14

Hyperacusis, deviation of jaw towards affected side due to protrusion of jaw working better on unaffected side (muscles of mastication)

Question 15

Where does trigeminal alternating hemiplegia occur? Will there be facial paralysis?

Answer 15

Midpontine level, no facial paralysis because corticobulbar tract not affected. PT syndrome symptoms + CNV lesion will occur.

Question 16

What is Bell’s palsy?

Answer 16

Unilateral nerve 7 lesion - loss of ipsilateral face movements, hyperacusis (stapedius), ipsilateral corneal reflex, diminished taste on anterior 2/3, dry eye (loss of PANS before giving to greater petrosal nerve)

Question 17

What is Millard-Gubler syndrome?

Answer 17

Lesion affecting 6, 7, and causing PT syndrome on contralateral side.

Question 18

What is the ambiguus nucleus?

Answer 18

Narrow column of cells in reticular formation at rostral and mid medulla levels, between spinal nucleus of V and inferior olivary complex. Contains neurons for CNs 9 / 10

Question 19

What are the clinical manifestations of unilateral ambiguus lesions?

Answer 19

Paralysis of pharyngeal and laryngeal muscles + soft palate, leading to deviation of uvula towards unaffected side, difficulties in swallowing (dysphagia) and vocalizing (dysarthria)

Question 20

What are the clinical manifestations of bilateral ambiguus lesion?

Answer 20

bulbar palsy - same symptoms of pseudobulbar palsy, which is knocking out corticobulbar tract to ambiguus nucleus.

Aspiration pneumonia, asphyxia, and eventual death

Question 21

What are the clinical signs of a unilateral CN12 lesion?

Answer 21

Protrusion of tongue to paralyzed side, and atrophy + fasciculations of 1/2 of tongue

Question 22

What is inferior alternating hemiplegia?

Answer 22

CN12 lesion + PT syndrome with no facial paralysis

Question 23

Where is the spinal accessory nucleus?

Answer 23

Found in gray horn transition between medulla and spinal cord from C1-C5 levels, axons leave spinal cord between dorsal and ventral roots.

Question 24

What does unilateral CN11 lesion cause?

Answer 24

trapezius and SCM paralysis - downward and outward rotation of ipsilateral scapula + difficulties turning head to opposite side of lesion against resistance (SCM) and shrugging shoulder

Question 25

What is the difference between UMN and LMN?

Answer 25

UMN - does not directly innervate muscles, but directly or indirectly affects LMN (i.e. corticospinal + corticobulbar tracts)

LMN - directly innervates muscles, includes gamma and alpha