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Difficulty: HardNervous Coordination and Sense Organs

A patient involved in a workplace injury retains normal cutaneous pain and tactile sensations in the right foot, but experiences total loss of motor function and muscle contraction in that same limb. Which of the following nerve structures has most likely suffered localized damage?

  1. Ventral root of the spinal nerveAnswer
  2. B
    Dorsal root of the spinal nerve
  3. C
    Dorsal root ganglion
  4. D
    Cutaneous sensory receptors in the foot

Answer

Ventral root of the spinal nerve
The ventral root of a spinal nerve exclusively conducts motor (efferent) nerve impulses from the spinal cord to muscle effectors. Consequently, an isolated lesion of the ventral root produces complete paralysis of the innervated muscles while leaving sensory impulses travelling along the dorsal root completely unaffected.

Step-by-Step Solution

1
Analyze the patient's neurological symptoms.
Sensory perception (pain and touch) is functional, while motor execution (muscle contraction) is lost.
This establishes that the sensory (afferent) pathway is uninjured while the motor (efferent) pathway is compromised.
2
Identify the anatomical roles of the spinal nerve roots.
The dorsal root carries sensory impulses into the spinal cord, whereas the ventral root carries motor impulses outward to effectors.
According to the Bell-Magendie law of spinal nerve function, sensory and motor fibers enter and exit through distinct roots.
3
Deduce the localized site of injury.
The structural lesion must reside strictly along the ventral (motor) root.
Only a ventral root injury selectively abolishes motor response without destroying incoming sensory signals.

Key Concept

Functional differentiation of spinal nerve roots (dorsal vs. ventral roots)
Estimated Time:1m 30s
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Topics

Nervous Coordination and Sense Organs

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