AN7.1-8 | Introduction to the nervous system — Summary & Reflection

REFLECT

Return to the two cases in the hook:

1. The stroke patient has a UMN lesion. List 4 features that distinguish it from an LMN lesion.
2. The radial nerve injury patient cannot extend his wrist (wrist drop). Which anterior horn cell segments supplied the radial nerve? Why is there no Babinski sign?
3. In Guillain-Barré syndrome — which component of the spinal nerve is primarily affected? Which glial cell is damaged?
4. A patient in a rural Tamil Nadu clinic has small muscle wasting of the hand with fasciculations. Is this UMN or LMN? Which condition should you consider?

Discussion: UMN = spasticity, hyperreflexia, Babinski+, late wasting. LMN = flaccidity, areflexia, Babinski−, early wasting + fasciculations. Radial nerve = C5–T1; wrist drop = LMN, Babinski absent. GBS = peripheral nerve demyelination (Schwann cells). Hand wasting + fasciculations = LMN → consider motor neuron disease (ALS/amyotrophic lateral sclerosis).

KEY TAKEAWAYS

Key Takeaways — Introduction to the Nervous System

• CNS = brain + spinal cord; PNS = 12 CN + 31 spinal nerves; ANS = sympathetic (T1–L2) + parasympathetic (craniosacral)
• Neuroglia: Oligodendrocytes (CNS myelin), Schwann cells (PNS myelin), Astrocytes (BBB), Microglia (CNS immune), Ependyma (CSF)
• Neurons: Multipolar (motor/interneurons), Bipolar (special sense), Pseudounipolar (DRG — sensory)
• Spinal nerve: Dorsal root (sensory, DRG) + Ventral root (motor) → mixed spinal nerve → dorsal ramus + ventral ramus
• UMN lesion: Spastic paralysis, hyperreflexia, Babinski+; LMN lesion: Flaccid paralysis, areflexia, fasciculations, early wasting
• Synapses: Chemical (most common, unidirectional) vs Electrical (gap junctions, bidirectional)
• Ganglia: Sympathetic = multipolar motor relay; DRG = pseudounipolar sensory cell bodies, no synapse