In lower motor neuron injuries, spasticity is typically not present. Which statement is true?

In lower motor neuron injuries, spasticity is typically not present. Which statement is true?

• A. True
• B. False
• C. Spasticity is variable in LMN injuries
• D. Spasticity is always present in LMN injuries

Answer: (A)

Spasticity is driven by upper motor neuron damage, which removes brain signals that normally modulate and dampen spinal reflexes. When the upper motor neurons are injured, the result is a hyperexcitable stretch reflex that increases muscle tone in a velocity-dependent way. In lower motor neuron injuries, the final common pathway—the motor neurons or peripheral nerves to the muscle—is damaged, so muscles tend to be flaccid with reduced or absent reflexes, and there’s often atrophy and fasciculations. Because the circuit that generates spastic tone is interrupted at the level of the LMN, spasticity is not typically present. Rare cases with mixed or evolving injuries can show different signs, but the classic LMN presentation is flaccid rather than spastic.

Spasticity is driven by upper motor neuron damage, which removes brain signals that normally modulate and dampen spinal reflexes. When the upper motor neurons are injured, the result is a hyperexcitable stretch reflex that increases muscle tone in a velocity-dependent way. In lower motor neuron injuries, the final common pathway—the motor neurons or peripheral nerves to the muscle—is damaged, so muscles tend to be flaccid with reduced or absent reflexes, and there’s often atrophy and fasciculations. Because the circuit that generates spastic tone is interrupted at the level of the LMN, spasticity is not typically present. Rare cases with mixed or evolving injuries can show different signs, but the classic LMN presentation is flaccid rather than spastic.