Upper and lower neuron lesions & Lesion of pyramidal tracts at different sites (unilateral lesion of posterior limb of internal capsule)

🧠 Lecture Notes: Upper vs. Lower Motor Neuron Lesions (UMNL vs. LMNL)

🖇 I. Basic Definitions & Structure

📌 Upper Motor Neurons (UMN):

• Originate from motor cortex (Area 4 & 6) or brainstem nuclei (e.g., basal ganglia, vestibular nuclei).
• Axons form descending tracts → end on LMNs in brainstem/spinal cord.

📌 Lower Motor Neurons (LMN):

• Include cranial motor nuclei & anterior horn cells.
• Their fibers supply skeletal muscles directly.

🟣 II. Motor System Divisions

1️⃣ Lateral Motor System:

• Tracts: Lateral corticospinal & rubrospinal.
• Controls: Distal muscles (hands/fingers).
• Function: Fine, skilled movements.

2️⃣ Medial Motor System:

• Tracts: Anterior corticospinal, vestibulospinal, reticulospinal.
• Controls: Axial muscles (trunk, shoulders).
• Function: Posture, gross movements, background tone for fine motion.

🟠 III. Rigidity & Abnormal Postures

📌 Decerebrate Rigidity:

• Lesion between superior & inferior colliculi.
• ↑ Tone in anti-gravity muscles (extensors of limbs).

🚨 Types of Rigidity:

• Gamma Rigidity (Clasp-knife):
• • Due to ↑ gamma motor activity.
  • Seen in UMNL.
  • Resistance ↑ with movement velocity.
• Alpha Rigidity (Lead-pipe / Cogwheel):
• • Due to ↑ alpha motor activity.
  • Affects all muscles equally.
  • Seen in Parkinsonism (basal ganglia lesion).

🖇 IV. Key Comparison: UMNL vs. LMNL

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📌 V. Important Focus Points (Exam Tips)

🟣 1. Core Comparison:

• Table above = high-yield for MCQs & essays.

🟣 2. Cause of Spasticity (UMNL):

• Due to loss of inhibitory control from cortex → facilitatory centers dominate → hypertonia & hyperreflexia.

🟣 3. Decerebrate Rigidity:

• Lesion between colliculi → affects anti-gravity muscles (extensors).

🟣 4. Clonus Significance:

• Clonus = hallmark of UMNL (ankle or patellar).

🟣 5. Electrical Testing:

• LMNL: “Reaction of degeneration.”
• • Faradic: ❌ No response
  • Galvanic: ↓ Weak response
  • Chronaxie ↑ (prolonged), Rheobase defined as minimal current for response.

🟣 6. Atrophy Difference:

• UMNL: Mild (due to disuse).
• LMNL: Severe (loss of trophic factors & denervation hypersensitivity).

🚨 Quick Clinical Reminders

• UMNL: Stroke, spinal cord injury, multiple sclerosis.
• LMNL: Polio, peripheral nerve injury, motor neuron disease.
• Babinski Sign: Pathognomonic for UMNL (toes extend & fan).
• Lead-pipe rigidity: Think Parkinsonism.