lower motor neurons 2/4

PI: a unilateral lesion of which of the following results in right upper limb flaccid paralysis?

1

two other terms for lower motor neuron

somatic efferent and alpha

flaccid paralysis

muscle paralysis (weakness) and hypotonicity (decreased tone)

PI: during a neurologic examination on a 7 year old boy, the physician taps the patellar tendon and elicits a simple knee-jerk reflex that is also called the quadriceps stretch reflex. this is one example of a myotatic reflex. which of the following is characteristic of this type of reflex?

1. Is a three-neuron reflex; the afferent receptor is the Golgi tendon organ, which is innervated by Ib nerve fibers, and the afferent endings are the neuromuscular junctions, which are innervated by lower motor neurons

2. Requires glutaminergic (excitatory) input to the extensor and flexor motor neurons

3. Relies on nociceptive input to the primary sensory ending

4. Causes withdrawal on the ipsilateral limb and extension on the contralateral limb

5. Is a two-neuron reflex; the afferent receptor is the muscle spindle, which is innervated by Ia and II nerve fibers, and the efferent endings are neuromuscular junctions, which are innervated by lower motor neurons

5

muscle spindle

detects muscle stretch and responds (myotatic reflex receptor)

encapsulated receptor within muscle

consists of small, encapsulated nuclear bag and nuclear chain intrafusal muscle fibers connected in parallel with large extrafusal fibers

what types of fibers innervate the nuclear bag and nuclear chain (intrafusal fibers)?

Is and II afferent and gamma motor neurons

PI: a 78 year old man presents with weakness of the muscles of mastication and of facial expression, both on the same side of the face. the examining physician concludes that the lesion involves the nuclei of cranial nerves V and VII. which of the following parts of the nervous system is the most likely site of this lesion?

1. medulla

2. pons

3. midbrain

4. thalamus

5. telencephalon

2

which cranial nerves don’t have LMNs?

olfactory, optic, vestibulocochlear

where would you find lower motor neurons?

brainstem and spinal cord

tract/fasiculus

bundle of axons in the CNS

PI: a 5 year old girl presents with medial deviation of her left eye. when asked to abduct her left eye, it doesn’t move. these signs are characteristic of which of the following?

1. oculomotor nerve palsy

2. trochlear nerve palsy

3. abducens nerve palsy

4. trigeminal nerve palsy

5. facial nerve palsy

3

exotropia

medial deviation of the eye

palsy

weakness or paralysis of a nerve

diplopia

double vision

PI: a unilateral lesion of which of the following results in sagging of ipsilateral palatal arch and vocal muscle paralysis?

B

PI: a lesion of which of the following cranial nerves results in diplopia with an ipsilateral ptosis, down and out eye, and mydriasis?

5

PI: a unilateral lesion of which of the following results in the ability to close ipsilateral eye and retract corner of mouth?

D

facial nerve lesions cause…

flaccid paralysis of muscles of facial expression around the eyes AND the mouth

which axons arch over the abducens nucleus?

facial

PI: a unilateral lesion of which of the following results in the inability to depress eye in the adducted position and compensatory tilting of the head?

F

PI: a lesion of which of the following cranial nerves results in paralysis and atrophy of ipsilateral muscles of the tongue and, when protruded, the tongue deviates to the side of lesion?

9

lower motor neurons

brainstem or spinal cord motor neurons whose axon carries impulses to skeletal muscle

aka somatic body efferent or alpha motor neuron

axons go directly into peripheral nerves (spinal or cranial) and synapse on skeletal muscles (extrafusal fibers)

what does a lesion of lower motor neurons or their axons result in?

lower motor neuron syndrome

lower motor neuron syndrome

flaccid paralysis

decreased or absent reflexes

severe muscle atrophy

where are spinal LMN cell bodies?

ventral horn, specifically 2 main cell columns (medial and lateral) forming lamina IX

cell bodies of spinal LMNs are arranged

somatotopically

LMN organization

medial (proximal) to lateral (distal) and dorsal (flexors) to ventral (extensors)

efferent limbs of spinal reflexes

LMNs

3 types of spinal reflexes

stretch (myotatic) reflex—knee jerk

golgi tendon (inverse myotatic) reflex

flexor withdrawal reflex

stretch (myotatic) reflex

monosynaptic

muscle stretches→stretching stimulates Ia afferent fibers→synapse on alpha motor neurons→contraction in stretched muscle

nuclear bag and Ia afferents

detect the rate of change—beginning of stretch

nuclear chain and II afferents

detect static changes—maintained stretch

gamma motoneurons

keeps the muscle spindle taut and increases the sensitivity of the muscle spindle to contraction or stretching, innervate the ends of intrafusal fibers

golgi tendon (inverse myotatic) reflex

disynaptic

active muscle contraction (tension) stimulates the golgi tendon organs and Ib afferent fibers→stimulate inhibitory interneurons in spinal cord→inhibit alpha motoneurons→relax contracting muscle

Clasp-knife reflex (in spastic muscles)

flexor withdrawal reflex

polysynatic

pain afferents stimulate spinal interneurons (inhibitory and excitatory)→flexion on ipsilateral side and extension of contralateral side

examples of myotatic reflex

biceps reflex (musculocutaneous nerve)

triceps reflex (radial nerve)

patellar reflex (femoral nerve)

achilles tendon reflex (tibial & sciatic nerves)

where are spinal alpha motor neurons found?

more than one spinal cord segment for neurons innervating any muscle

extrafusal fibers

contract to allow movement to occur

brainstem LMNs

found within medulla, pons, and midbrain

LMNs of the open medulla

hypoglossal and ambiguus

CNS nuclei

clusters of neuronal cell bodies

hypoglossal nuclei

originate just deep to the dorsal surface of the open medulla

intramedullary rootlets

axons from the hypoglossal nuclei coursing through the medulla to emerge on the ventral surface between the olive and the medullary pyramid as the hypoglossal nerve

nucleus ambiguus

ventral and lateral to hypoglossal nuclei

give rise to LMNs that enter 3 cranial nerves—majority to vagus n., caudal to cranial accessory n. (merges w/ vagus), rostral to glossopharangeal n.

nuclei of the caudal pons

abducens and facial

abducens nuclei

just deep to the facial colliculus

facial nuclei

ventral and lateral to abducens nuclei

axons course towards midline of pons, arch over abducens nuclei to form facial colliculus, then continue laterally and ventrally to emerge as nerves

nuclei of the mid pons

motor trigeminal

motor trigeminal nuclei

nuclei of the caudal midbrain

trochlear

trochlear nuclei

nerve emerges dorsally

nuclei of the rostral midbrain

oculomotor

oculomotor nuclei

axons emerge as nerves from interpeduncular fossa

atrophy of one side of tongue, deviation to ipsilateral side

hypoglossal nucleus/rootlets/nerve

sagging of pharyngeal muscles on one side, uvula deviated to contralateral side, partial vocal muscle paralysis

vagus nerve or nucleus ambiguus, glossopharyngeal, cranial accessory rootlets & nerves

facial muscle paralysis on one side—no closing eye, no retraction of corner of mouth

facial nerve/nucleus/rootlets

medial deviation of eye, limited eye abduction, diplopia (double vision)

abducens nerve/nucleus/rootlets→flaccid paralysis of lateral rectus (isotropia)

trouble chewing, atrophy of ipsilateral cheek, mandible deviated to atrophied side

motor trigeminal nucleus/rootlets/nerve

tilting head to contralateral side, diplopia in affected eye, cannot depress eye fully when adducted

trochlear nerve/nucleus/rootlet→flaccid paralysis of superior oblique

cannot open affected eye, pupil depressed and abducted and dilated

oculomotor nerve/nucleus/rootlets→flaccid paralysis of superior rectus, inferior rectus, inferior oblique, medial rectus, superior levator (causes ptosis) of upper eyelid—superior oblique and lateral rectus unaffected

dilation caused by preganglionic parasympathetics that go to ciliary ganglion and postganglionic parasympathetic dilates—lesion will cause constriction

Bell’s palsy

facial nerve lesion