L56. Neuroscience - UMNs and LMNs

• Upper motor neurons (UMN) and lower motor neurons (LMN) make up distinct systems
• Upper motor neurons (UMN) originate in the primary motor cortex.
• Lower motor neurons (LMN) innervate muscles and are the final common pathway

Functional Organization of UMNs and LMNs

• Neuronal cell bodies of UMNs are in the primary motor cortex's inferolateral part of the precentral gyrus.
• UMNs innervate cranial nerve nuclei via the corticobulbar tract.
• Other parts of the precentral gyrus contain neuronal cell bodies that innervate motor neurons in the ventral horn of the spinal cord via the corticospinal pathway.
• LMN cell bodies are located in the nuclei of cranial nerves (V, VII, IX, X, XI, XII) and ventral horn motor neurons of the spinal cord.
• Knowing the location and function of cranial nerves is crucial for localizing lesions
• The two types of motor neurons in the brainstem cranial nerve nuclei and spinal cord are alpha(α) and gamma(γ).
• Alpha (α) motor neurons are large, multipolar neurons with prominent Nissl bodies and innervate extrafusal motor fibers
• Gamma (γ) motor neurons have small, multipolar cell bodies with fine Nissl bodies, innervating the polar regions of the muscle spindle which causes stretch of the intrafusal fibers and discharge of annulospiral fibers when activated.

LMN Lesions

• Destruction of LMNs from injury or disease leads to "LMN syndrome."
• LMN syndrome is characterized by:
• Flaccid paralysis (flabby muscles)
• Hypotonia or atonia, depending on the severity of the lesion
• Hyporeflexia or areflexia (decreased or absent reflexes)
• Muscle atrophy and eventual reduction of muscle mass
• Fasciculations (random contractions of single motor units before muscle degeneration)

UMN Lesions

• Symptoms of UMN include:
• Increased Deep Tendon Reflexes (DTRs)
• Loss of superficial reflexes:
  • Cremasteric reflex: Stroking inner thigh causes cremaster muscle to contract and pull up the ipsilateral testicle.
  • Abdominal Reflexes: Stroking the abdomen causes the abdominal wall muscle to contract, pulling the umbilicus towards the stimulus.
• Clonus, a self-sustained, oscillating stretch reflex induced by briskly stretching a hyperreflexic muscle.
• Spastic Paralysis (begins a few days after the injury).
• Hypertonia, increased muscle tone.
• Clasp Knife Response, a rapid decrease in resistance when attempting to flex a joint.
• Babinski Sign, where stroking the lateral sole of the foot causes the toes to flair and the big toe to move up (normal in infants).
• Hoffman sign (finger flexor reflex), tapping or flicking the end of the 3rd or 4th finger, positive suggests the flexion and adduction of the end of the thumb.

Signs of UMN vs LMN Damage

• UMN damage includes:
  • Increased Deep Tendon Reflexes (DTR).
  • Loss of Cremasteric & Abdominal Reflexes.
  • Clonus.
  • Spastic Paralysis.
  • Hypertonia.
  • Clasp Knife Response.
  • Babinski Sign.
  • Hoffman sign.
• LMN damage includes:
  • Decreased Reflexes.
  • Muscle Fasciculation.
  • Muscle Atrophy.
  • Flaccid Paralysis.
  • Hypotonia.

Oculomotor Nuclear Complex Lesions

• V-shaped structure next to the midline of the rostral midbrain.
• The Edinger-Westphal nucleus caps the rostral pole.
• Axons exit at the interpeduncular fossa, coursing through the midbrain tegmentum
• Unilateral CN III lesion results in:
  • Ipsilateral ptosis (drooping eyelid).
  • Lateral strabismus (diplopia/double vision).
  • Dilated pupil (mydriasis).
  • Loss of direct light reflex.
  • Ipsilateral accommodation.
• Unilateral CN III + PT lesion results in:
  • All of the above symptoms.
  • PT syndrome.
  • Contralateral lower facial paralysis.
  • Called superior alternating hemiplegia (Weber's syndrome) because the corticobulbar tract is included in the lesion at this level.

Trochlear Nucleus Lesions

• Small, round nucleus ventral to the periaqueductal gray at caudal midbrain.
• Axons cross the midline and exit at midbrain/pons transition to innervate the superior oblique muscle (intorts the eye).
• Unilateral CN IV lesion:
  • Ipsilateral extortion (eyeball slightly up and out).
  • Diplopia.
  • Patient compensates by tilting head to the non-paralyzed side.

Trigeminal Motor Nucleus Lesions

• Large, round nucleus located just medial to the chief sensory nucleus of V at mid-pontine levels.

• Axons from motor V leave the pons with the Vth nerve and mandibular (V3) division to innervate:

  • Ipsilateral muscles of mastication.
  • Tensor tympani.
  • Tensor veli palatini.

• Unilateral CN V lesion results in:

  • Paralysis of ipsilateral muscles of mastication with jaw deviation to the paralyzed side upon protrusion.
  • Hyperacusis (paralysis of the tensor tympani).

• Sensory loss to the ipsilateral face.

• Unilateral CN V + PT (trigeminal alternating hemiplegia) combines the above signs + PT syndrome signs and may occur from a single lesion located in the basilar portion of the pons at mid-pontine levels, and no facial paralysis.

Abducens Nucleus Lesions

• Fairly large, round nucleus located close to the midline of the caudal pons.
• Fibers of the internal genu of VII wrap around it.
• Axons from motor VI course ventrally in the tegmentum and exit at the junction of the pons and medulla.
• Unilateral CN VI lesion:
  • Ipsilateral medial strabismus with diplopia.
• Unilateral abducens motor nucleus lesion:
  • "Lateral gaze syndrome" where neither eye can turn towards the lesion side and both eyes deviate conjugately to the opposite side. Unilateral CN VI + PT lesion (middle alternating hemiplegia):
  • Occurs at the basilar portion of the caudal pons.
  • Symptoms of unilateral CN VI lesion + PT syndrome.
  • Facial paralysis does not occur (corticobulbar fibers to motor VII do not travel with PT at this level).

Facial Nucleus Lesions

• Large nucleus in the caudal pontine tegmentum at the same level as abducens motor nucleus.
• Axons from motor VII course dorsomedial, wrap around the abducens nucleus (internal genu), and course ventrolaterally between spinal nucleus of V and facial motor nucleus, finally exiting the pons at the cerebellopontine angle.
• Motor VII is divided into dorsal ( upper face muscles innervation) and ventral ( lower face muscles innervation).
• The stapedius muscle is also innervated
• Unilateral CN VII lesion results in:
  • Bell's palsy (inability to smile, show teeth, close the eyelid, or wrinkle forehead on the paralyzed side).
  • Hyperacusis (paralysis of stapedius).
  • Ipsilateral corneal reflex abolished.
• Lower face is only affected in UMN lesions

Nucleaus Ambiguous Lesions

• A long, narrow column of cells in the medullar reticular formation.
• Located halfway between spinal nucleus of V and the inferior olivary complex.
• Axons join cranial nerves IX and X.
• The ipsilateral pharyngeal constrictors (for swallowing) and laryngeal muscles are innervated (for vocalization)
• Muscle innervation also include soft palate
• Unilateral ambiguus lesion results in:
  • Ipsilateral paralysis of related muscles.
  • Dysphagia.
  • Dysarthria.
• Deviation of uvula to the nonparalyzed side during vocalization.
• Bilateral ambiguus results in:
  • Bulbar palsy.
  • Aspiration.
  • Asphyxia.
• Pseudobulbar palsy differs from bilaterial lesion, which is bilateral corticobulbar tract lesions.

Hypoglossal Nucleus Lesions

• Column of cells next to the midline at mid and caudal medulla levels.
• Runs ventrally, lateral to med lemniscus then lateral to pyramid, exiting at the preolivary sulcus.
• Unilateral CN XII (or nucleus) results in:
• Ipsilateral paralysis of intrinsic and extrinsic tongue muscles.
• Protruded tongue deviates towards paralyzed side.
• Muscle fasciculations and atrophy
• Unilateral CN XII and PT lesion results in:
  • Inferior alternating hemiplegia.
  • Signs of CN XII lesion + PT syndrome.
  • No facial paralysis.

Spinal Accessory Nucleus Lesions

• Long, narrow column of cells in the gray horn at the medulla and spinal levels.
• Axons leave the spinal cord, ascend into the skull via the foramen magnum, and exit via the jugular foramen.
• Innervates the ipsilateral sternocleidomastoid and trapezius muscles.
• The upper motor neurons (UMN) innervating this are part of the corticobulbar tract.
• Unilateral CN XI lesion:
  • Ipsilateral paralysis of the sternocleidomastoid and trapezius muscles.
  • Downward and outward rotation of ipsilateral scapula.
  • Difficulty turning the head to the opposite side of the lesion.
  • Shrugging at the shoulder. Alpha and gamma motor neurons at all cross-sectional levels of the spinal cord are considered LMNs.

Upper vs Lower face paralysis

Corticalbulbar synapses on cranial motor Neurons:

• CN V nuceli - innervated bilaterally but > contralaterally Transient jaw deviation.
  • lower and/or upper motor neuron will deviate jaw to leisioned side with difficulty chewing and or hyperacusis.
• CN VII:
  • dorsal 1/2 is bilaterally innervated
• CN IX and X nucleus :
  • bilateral innervation for gag reflex
• CN XII:
  • bilateral > contra with tongue deviating to opposite side of lesion on protrusion

Motor involvment in Brown-Sequard Syndrome:

• Brown-Sequard syndrome causes lower motor neuron symptom at the site of injury. This leads to damage of the lateral corticospinal tract, resuting in symptons below the leision. No clinical signs of damage will be presented with the destuction of the anterior corticospinal tract.