A complete transection of the brainstem between the superior and inferior colliculi permits the brainstem pathways to function independent of their input from higher brain structures.

• This is called a midcollicular decerebration 
• This lesion interrupts all input from the cortex (corticospinal and corticobulbar tracts) and red nucleus (rubrospinal tract), primarily to distal muscles of the extremities.
• The excitatory and inhibitory reticulospinal pathways (primarily to postural extensor muscles) remain intact.
• Pontine reticulospinal nucleus is a tonically active nucleus. This leads to hyperactivity in extensor muscles in all four extremities, which is called decerebrate rigidity
• The excitatory input from the reticulospinal pathway activates Gamma-motor neurons, which indirectly activate ŷ-motor neurons (via Ia spindle afferent activity). This is called the gamma loop.

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Physiology Pearls

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