Questions: Spinal Cord and Peripheral Nerve Anatomy

Question 1

A patient suffers a right-sided hemisection of the spinal cord at T6 (Brown-Séquard syndrome). Which pattern of deficits below the lesion is expected?

• A. Right-side motor loss and right-side pain/temperature loss; left-side proprioception loss
• B. Right-side motor loss and right-side proprioception loss; left-side pain/temperature loss
• C. Left-side motor loss and left-side pain/temperature loss; right-side proprioception loss
• D. Bilateral motor loss with preserved sensation on both sides

Answer: (B)

Dorsal column fibers (carrying proprioception and fine touch) ascend ipsilaterally and decussate only in the medulla — so a right-sided cord lesion cuts the right dorsal columns, producing right-side proprioception loss. Corticospinal tract fibers have already crossed in the medullary pyramids, so right-sided cord damage causes right-side motor loss. Spinothalamic tract fibers, however, decussate within 1–2 segments of entering the cord, so they are already traveling on the left side at T6 — cutting the right side disrupts the left-crossing fibers, producing left-side pain and temperature loss. This crossed sensory dissociation is the hallmark of Brown-Séquard.

Question 2

Dorsal root ganglia are located just outside the spinal cord and contain which type of neuron cell body?

• A. Alpha motor neurons that drive skeletal muscle contraction
• B. Preganglionic sympathetic neurons projecting to the sympathetic chain
• C. Sensory (afferent) neurons carrying signals from the body to the CNS
• D. Interneurons that coordinate reflex arcs within the gray matter

Answer: (C)

Bell's law: dorsal = sensory, ventral = motor. The dorsal root ganglion (DRG) houses the cell bodies of primary sensory neurons — their peripheral axons receive signals from skin, muscle, and viscera, and their central axons enter the spinal cord through the dorsal root. Motor neuron cell bodies sit in the ventral horn of the spinal gray matter and their axons exit through the ventral root. The DRG's external location (outside the CNS proper) is clinically significant: conditions like shingles (varicella-zoster reactivation) target these ganglia.

Question 3

The dorsal horns of the spinal cord gray matter contain alpha motor neuron cell bodies that directly drive skeletal muscle.

• T. True
• F. False

Answer: False

This reverses the dorsal-ventral functional organization. The dorsal horns process incoming sensory (afferent) information — they receive and relay signals arriving via the dorsal root. Alpha motor neurons, whose axons exit via the ventral root to innervate skeletal muscle, reside in the ventral horns. The mnemonic: SAME DAVE — Sensory Afferent, Motor Efferent; Dorsal Afferent, Ventral Efferent.

Question 4

A right-sided hemisection of the spinal cord at T4 would produce loss of pain and temperature sensation on the right side of the body below the lesion.

• T. True
• F. False

Answer: False

Pain and temperature are carried by the spinothalamic tract, whose fibers decussate within 1–2 segments of entering the cord — so they are already traveling contralaterally by the time they ascend. Cutting the right side of the cord at T4 damages the left-ascending spinothalamic fibers (which crossed earlier), producing pain/temperature loss on the LEFT side below the lesion. Contrast this with proprioception and fine touch (dorsal columns), which travel ipsilaterally all the way to the medulla — those would be lost on the RIGHT side.

Question 5

Why does a hemisection of the spinal cord (Brown-Séquard syndrome) produce ipsilateral motor and proprioception loss but contralateral pain and temperature loss?

Model answer: Different tracts decussate at different levels. Corticospinal (motor) fibers cross in the medullary pyramids before descending, and dorsal column fibers (proprioception, fine touch) ascend ipsilaterally to the medulla before crossing — so both produce ipsilateral deficits when the cord is cut on one side. Spinothalamic fibers carrying pain and temperature cross within 1–2 spinal segments of entering the cord, so by any higher level they are already traveling on the opposite side, producing contralateral deficits.

This pattern is the direct clinical read-out of tract anatomy. The two major ascending sensory systems decussate at entirely different levels: dorsal columns stay ipsilateral until the brainstem; spinothalamic fibers cross almost immediately. A hemisection therefore 'catches' the two systems at different stages of their crossing, producing the distinctive dissociation. Knowing this, a clinician can infer the cord level and side of a lesion from the pattern of sensory and motor deficits without imaging.