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The Classic Experiments That Changed Everything
The Classic Experiments That Changed Everything
Sherrington’s Classic Experiment
Sherrington’s Classic Experiment
In the early 1900s, British neurophysiologist Sir Charles Sherrington changed the way we understand movement. He experimented on cats and used a method called decerebration, which involves removing the higher brain centers while keeping the brainstem and spinal cord intact. In some cases, he even performed a complete spinal cord injury (spinalization or transection), cutting off all brain communication with the body.
What happened? Cats presented alternating limb movements when stimulated. Sherrington called this a “reflex chain”, where sensory inputs from the skin, joints, and muscles triggered motor responses, like stepping. He also introduced the concept of reciprocal innervation: when one muscle contracts (like an extensor), its opposite (the flexor) relaxes, and vice versa. This back-and-forth pattern is essential for walking.
Brown’s Groundbreaking Experiment
Brown’s Groundbreaking Experiment
In 1911, Thomas Graham Brown, a former student of Charles Sherrington, conducted a series of experiments that reshaped our understanding of movement. Working with spinalized and decerebrated cats, Brown isolated the spinal cord from all descending signals. He then eliminated sensory input from the muscles, tendons, skin, and joints by cutting the dorsal roots below the transection (rhizotomy, deafferentation). To further eliminate sensory input, he removed all nerves innervating the hindlimb muscles, leaving only the tibialis anterior (ankle flexor) and gastrocnemius muscles (ankle extensors) intact.
When Brown pinched the isolated spinal cord, he recorded spontaneous alternating contractions between the intact flexor and extensor ankle muscles. These observations provided the first direct evidence that the spinal cord alone can generate the flexor–extensor alternation characteristic of locomotion, without input from the brain or sensory feedback.
From these results, Brown proposed the Half-Center Hypothesis: the spinal cord contains two groups of neurons, each controlling either flexor or extensor motoneurons, which mutually inhibit each other to produce rhythmic, alternating movement. These neural networks later became known as central pattern generators (CPGs).
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