Lower Motor Neurons

What are LMNs also called

α-motoneurons

What is the only direct connection between CNS and skeletal muscle?

Lower Motor Neurons (LMNs)

What junctions do LMNs form with muscle fibers?

Neuromuscular junctions (NMJs)

What neurotransmitter do neuromuscular junctions always use?

Acetylcholine (ACh), always excitatory

Why are LMNs called the “final common pathway”?

1) LMNs are the only link between the CNS and skeletal muscles.

2) All voluntary movement, reflexes, and automatic motor activity must pass through LMNs to reach muscle.

3) Without LMNs, no muscle contraction can happen.

Define a motor unit.

One LMN + all the muscle fibers it innervates

What size motor units give fine control? Example?

Small motor units (<12 fibers); e.g., extraocular, tongue, laryngeal muscles

What size motor units give gross control? Example?

Large motor units (thousands of fibers); e.g., gluteus maximus, soleus

Where do LMNs originate?

Brainstem (cranial nerves) or spinal cord (spinal nerves)

Do LMNs innervate ipsilateral or contralateral muscles?

Ipsilateral

Which cranial nerves get bilateral UMN input?

CN V, VII (upper face), IX, X, XI

Which cranial nerves get primarily contralateral UMN input?

CN VII (lower face), CN XII

Where is the motor trigeminal nucleus (cranial nerve V, mandibular branch) located?

Dorsolateral tegmentum, mid-pons

What are symptoms of a lesion to the motor trigeminal nucleus or nerve?

Ipsilateral paralysis of jaw-closing muscles

What swallowing-related deficit occurs with a motor trigeminal lesion?

Impaired Eustachian tube opening during swallow/yawn (tensor veli palatini)

What ear-related symptom occurs with a motor trigeminal lesion?

Sound distortion or hyperacusis-like effect from tensor tympani dysfunction

Where is the facial nucleus (cranial nerve VII) located?

Lateral tegmentum, caudal pons

What happens with a lesion to the facial nucleus or nerve?

Ipsilateral paralysis of facial expression muscles (cannot close eye, retract mouth)

What condition results from inflammation of the facial nerve in the facial canal?

Bell’s Palsy (usually recovers in 1–2 months)

What causes hyperacusis in a facial nerve lesion?

Paralysis of stapedius muscle

Where is the nucleus ambiguus (cranial nerves IX, X, XI) located?

Ventrolateral medullary reticular formation

What symptom occurs from a rostral lesion of the nucleus ambiguus?

Dysphagia from pharyngeal muscle paralysis (ipsilateral)

What symptom occurs from a caudal lesion of the nucleus ambiguus?

Soft palate and laryngeal paralysis (ipsilateral)

What happens with bilateral lesions of the nucleus ambiguus?

Severe deficits; may require tracheostomy

Where is the hypoglossal nucleus (cranial nerve XII) located?

Floor of medullary 4th ventricle, near midline

What happens with a lesion of the hypoglossal nucleus or nerve?

Ipsilateral paralysis and atrophy of tongue muscles

On protrusion, how does the tongue deviate in a hypoglossal lesion?

Toward the side of the lesion

How are proximal and distal muscles organized in spinal lower motor neuron innervation?

More medial = proximal muscles; more lateral = distal muscles

What are the symptoms of a lower motor neuron lesion?

Flaccid paralysis, hypotonia, decreased/absent reflexes, fasciculations, atrophy

What type of speech disorder results from lower motor neuron lesions?

Flaccid dysarthria (slurred, weak, breathy, slow speech)

In a central nervous system lesion, what is damaged?

Lower motor neuron cell bodies or intramedullary rootlets

In a peripheral nervous system lesion, what is damaged?

Axons in peripheral nerves

Why are paralysis and atrophy called “segmental” in lower motor neuron lesions?

They are limited to muscles directly innervated by the damaged lower motor neuron

What is flaccid paralysis?

Loss of voluntary muscle movement due to interruption of the final common pathway

What is hypotonia in lower motor neuron syndrome?

Decreased muscle tone

What reflex changes occur in lower motor neuron syndrome?

Decreased or absent superficial and deep reflexes

What are fasciculations in lower motor neuron syndrome?

Visible muscle twitches caused by spontaneous lower motor neuron discharges

What is atrophy in lower motor neuron syndrome?

Wasting of denervated muscles over weeks to months

Where can lesions occur in lower motor neuron syndrome within the central nervous system?

Cell bodies in brainstem nuclei or spinal anterior horn, or intramedullary rootlets

Where can lesions occur in lower motor neuron syndrome within the peripheral nervous system?

Lower motor neuron axons in peripheral nerves after exiting the central nervous system