Correct Answer: C. Rigidity occurs in all the muscles of the body
Decerebrate rigidity, as described by Herrington's classification, is a state of sustained muscle contraction resulting from removal of inhibitory influences from higher brain centers. The key discriminating fact is that decerebrate rigidity is selective and not universal—it predominantly affects extensor muscles (antigravity muscles) while sparing or even reducing tone in flexor muscles. This selective pattern occurs because the vestibular nuclei and reticular formation, which drive extensor motor neurons, are released from cerebral inhibition. The rigidity is NOT present in all muscles equally; flexors of the limbs and trunk remain relatively spared or hypotonic. Herrington's classification specifically emphasizes that decerebrate rigidity follows a pattern: extensors of the neck, trunk, and limbs show marked rigidity, while flexors show less involvement. This is clinically relevant in Indian neurology practice when assessing brainstem lesions (e.g., pontine hemorrhage, severe head trauma)—the characteristic "decerebrate posture" with extended limbs and arched back reflects this selective extensor predominance, not global muscle rigidity. The other three options (A, B, D) are all accurate features of Herrington's classification and thus correctly describe decerebrate rigidity.
Why the other options are wrong
A. Increase in the rate of discharge of the γ efferent neuron — This is a TRUE feature of decerebrate rigidity per Herrington. Gamma efferent (fusimotor) neurons increase their discharge rate, enhancing spindle sensitivity and maintaining muscle contraction. This is a core mechanism of the rigidity and is correctly included in Herrington's classification, making it a valid descriptor of decerebrate rigidity. B. Increased excitability of the motor neuron pool — This is a TRUE feature of decerebrate rigidity. The motor neuron pool becomes hyperexcitable due to loss of descending inhibitory pathways from the cerebrum. Vestibular and reticular inputs drive alpha motor neurons into a state of sustained depolarization. This mechanism is explicitly recognized in Herrington's classification as a hallmark of decerebrate rigidity. D. Decerebration produces no phenomenon akin to spinal shock — This is a TRUE feature per Herrington. Unlike spinal transection (which causes initial spinal shock with flaccidity), decerebration produces immediate rigidity without a shock phase because the vestibular and reticular nuclei remain intact and active. This distinguishes decerebrate rigidity from spinal rigidity and is a key point in Herrington's classification.
High-Yield Facts
- Decerebrate rigidity is selective—predominantly affects extensor muscles (antigravity muscles), NOT all muscles equally
- Gamma efferent hyperactivity increases spindle sensitivity and maintains the rigidity in Herrington's mechanism
- No spinal shock phase occurs with decerebration (unlike spinal transection) because vestibular and reticular nuclei remain functional
- Extensor predominance creates the characteristic decerebrate posture: extended neck, trunk, and limbs with arched back
- Flexor muscles are relatively spared or show reduced tone compared to extensors in decerebrate rigidity
Mnemonics
DECEREBRATE = Extensors Extended Decerebrate rigidity → Extensors are Extended (antigravity muscles contract). Flexors are Flaccid (relatively spared). Remember: loss of cerebral inhibition releases vestibular drive to extensor motor neurons. Herrington's 3 Pillars Gamma ↑ (fusimotor drive) + Alpha ↑ (motor neuron excitability) + Vestibular release (no spinal shock) = Selective extensor rigidity (NOT universal).
NBE Trap
NBE pairs "decerebrate rigidity" with "all muscles" to exploit the common misconception that decerebration causes global muscle rigidity. In reality, Herrington's classification emphasizes the selective extensor predominance, making universal muscle involvement the classic wrong answer in this question type.
Clinical Pearl
In Indian ICU practice, a patient with pontine hemorrhage or severe head trauma presenting with extended limbs, arched back, and rigid extensors (but relatively mobile flexors) is displaying decerebrate rigidity—a sign of brainstem dysfunction. Recognizing the selective extensor pattern (not global rigidity) helps localize the lesion and guides urgent neurosurgical intervention.
_Reference: Guyton & Hall Textbook of Medical Physiology, Ch. 55 (Motor Control); Harrison's Principles of Internal Medicine, Ch. 23 (Coma and Altered Consciousness)_