A patient with a known spinal cord ependymoma presents to his neurologist for a check up. He complains that he has had difficulty walking, which he attributes to left leg weakness. On exam, he is noted to have 1/5 strength in his left lower extremity, as well as decreased vibration and position sensation in the left lower extremity and decreased pain and temperature sensation in the right lower extremity. Which of the following spinal cord lesions is most consistent with his presentation?

Left-sided Brown-Sequard (hemisection)

Right-sided Brown-Sequard (hemisection)

A 23-year-old man presents to the emergency room following a stab wound to the back. He was in a bar when he got into an argument with another man who proceeded to stab him slightly right of the midline of his back. He is otherwise healthy and does not take any medications. He has one previous admission to the hospital for a stab wound to the leg from another bar fight 2 years ago. His temperature is 99°F (37.2°C), blood pressure is 115/80 mmHg, pulse is 100/min, and pulse oximetry is 99% on room air. Cardiopulmonary and abdominal exams are unremarkable; however, he has an abnormal neurologic exam. If this wound entered his spinal cord but did not cross the midline, which of the following would most likely be seen in this patient?

Ipsilateral flaccid paralysis at the level of the lesion

Contralateral spasticity below the level of the lesion

Ipsilateral loss of pain and temperature sensation below the lesion

Contralateral loss of tactile, vibration, and proprioception below the lesion

Contralateral loss of sensation at the level of the lesion

Motor pathways for USMLE Step 3: High-Yield Anatomy Lessons

Corticospinal Tract - The Main Highway

Principal pathway for voluntary, skilled, and fine motor control, originating from the primary motor cortex (precentral gyrus).

Decussation: ~90% of fibers cross in the medulla to form the lateral tract; the remaining 10% form the anterior tract.

⭐ Clinical Pearl: Lesions above the decussation (e.g., stroke) cause contralateral motor deficits. Lesions below the decussation (e.g., spinal cord injury) cause ipsilateral deficits.

Corticospinal Tract Pathway with UMN and LMN

Extrapyramidal Tracts - The Side Roads

Function: Modulate motor activity, especially posture, balance, and reflexes, without direct cortical input.
Key Tracts & Roles:
- Vestibulospinal: From vestibular nuclei; crucial for balance and head position.
- Reticulospinal: From reticular formation; governs postural control and muscle tone.
- Rubrospinal: From red nucleus; facilitates upper limb flexion.
- Tectospinal: From superior colliculus; coordinates head/neck movements in response to visual stimuli.

Extrapyramidal tracts and motor pathways

⭐ Clinical Pearl: Lesions above the Red Nucleus can cause decorticate (flexor) posturing. Lesions at or below it cause decerebrate (extensor) posturing, which typically indicates a more severe brainstem injury.

Lesion Clues - UMN vs LMN

Feature	UMN Lesion	LMN Lesion
Weakness	Spastic paralysis	Flaccid paralysis
Reflexes	↑ Hyperreflexia	↓ Hyporeflexia
Tone	↑ Hypertonia	↓ Hypotonia
Babinski	Positive (Upgoing)	Negative (Downgoing)
Atrophy	Late (disuse)	Severe & early
Fasciculations	Absent	Present

⭐ The Babinski sign (dorsiflexion of the big toe with fanning of other toes) is a key indicator of UMN damage, specifically to the corticospinal tract.

UMN vs LMN Lesions: Clinical Presentation by Location

Clinical Syndromes - Pathway Problems

Brown-Séquard Syndrome (Hemicord Lesion)
- Ipsilateral loss of UMN motor control, proprioception, and vibration below the lesion.
- Contralateral loss of pain and temperature, starting 1-2 levels below the lesion.
Amyotrophic Lateral Sclerosis (ALS)
- Combined UMN (spasticity, ↑reflexes) and LMN (atrophy, fasciculations) signs.
- Sensation and cognition are typically spared.
⭐ Superoxide dismutase 1 (SOD1) gene mutation is a well-known cause of familial ALS.
Anterior Cord Syndrome
- Bilateral loss of motor function, pain, and temperature.
- Spares dorsal columns (proprioception, vibration).

Spinal Cord Syndromes: Brown-Séquard, Anterior, Central

High‑Yield Points - ⚡ Biggest Takeaways

The corticospinal tract is the principal pathway for voluntary movement, decussating at the medullary pyramids.

Upper Motor Neuron (UMN) lesions cause spastic paralysis, hyperreflexia, and a positive Babinski sign.

Lower Motor Neuron (LMN) lesions result in flaccid paralysis, fasciculations, and severe muscle atrophy.

The basal ganglia modulate movement; lesions lead to disorders like Parkinson's or Huntington's disease.

The cerebellum coordinates movement and balance; damage causes ataxia and intention tremors.

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Motor pathways