In a classic experiment, an animal's spinal cord is surgically disconnected from the brain (spinalized), and the animal is placed on a treadmill. What does the observed outcome reveal about motor control?
AThe animal cannot produce any coordinated limb movement, confirming that the brain commands each muscle individually
BThe animal shows only simple withdrawal reflexes, not rhythmic movement
CThe animal can produce coordinated stepping movements, demonstrating that central pattern generators reside in the spinal cord
DThe animal walks normally, showing that the brain plays no role in locomotion
Spinalized animals placed on treadmills produce coordinated stepping patterns, demonstrating that central pattern generators (CPGs) for locomotion are built into spinal circuitry and can operate without descending brain input. The brain normally initiates, modulates, and stops CPG activity — but the moment-to-moment pattern generation (alternating flexors and extensors, left-right coordination) is handled autonomously by spinal interneuron networks. This is direct evidence that the spinal cord is a computational layer, not merely a relay.
Question 2 Multiple Choice
A spinal cord injury disrupts the reciprocal inhibition interneurons in the lumbar region. What movement problem would most likely result?
ALoss of all voluntary movement below the injury due to severed motor pathways
BLoss of proprioceptive feedback from the legs, impairing balance
CInability to coordinate antagonist muscle relaxation during joint movement, causing co-contraction and rigidity
DSelective loss of descending corticospinal commands, with reflexes preserved
Reciprocal inhibition is the spinal interneuron mechanism that automatically relaxes the antagonist muscle when the agonist contracts. Without this wiring, activating the biceps would not simultaneously suppress the triceps — both could contract together (co-contraction), producing joint stiffness and impaired smooth movement. Voluntary motor commands and sensory pathways are anatomically separate from this local inhibitory circuit. This illustrates how much of 'voluntary' movement coordination is actually handled by automatic spinal circuitry — not by the brain micromanaging every muscle.
Question 3 True / False
The brain is expected to continuously send signals down the spinal cord for a person to sustain rhythmic movements like walking or swimming.
TTrue
FFalse
Answer: False
False. Central pattern generators (CPGs) in the spinal cord can sustain rhythmic locomotor patterns without continuous descending brain input once they are initiated. The brain's role is to turn CPGs on and off, adjust their speed, and modify them for terrain or task demands — not to command each individual muscle activation. Evidence from spinalized animals demonstrating treadmill stepping is the clearest demonstration of this spinal autonomy. Continuous cortical input is needed for fine voluntary motor tasks (like piano playing), but not for stereotyped rhythmic patterns like locomotion.
Question 4 True / False
Proprioceptive feedback from muscle spindles and Golgi tendon organs allows the spinal cord to make real-time corrections to ongoing movement without requiring cortical involvement in each correction.
TTrue
FFalse
Answer: True
True. Muscle spindles (detecting muscle length and stretch velocity) and Golgi tendon organs (detecting muscle tension) send sensory signals to the spinal cord, where local interneurons can generate corrective motor responses within milliseconds — far faster than a cortical loop would allow. If your foot unexpectedly catches on an obstacle mid-stride, spinal circuits can trigger a flexion response and adjust CPG timing before any cortical signal could arrive. This is a feature of the spinal cord's semi-autonomous function: it monitors sensory state and corrects discrepancies locally.
Question 5 Short Answer
How does the concept of reciprocal inhibition illustrate that the spinal cord performs genuine computation, rather than simply relaying brain commands to muscles?
Think about your answer, then reveal below.
Model answer: Reciprocal inhibition is a spinal interneuron circuit that automatically relaxes the antagonist muscle whenever the agonist is activated — a coordination computation performed locally. When the brain sends a 'flex' signal to the biceps, the spinal cord distributes this into activation of the biceps AND simultaneous inhibition of the triceps. The brain does not need to send a separate 'relax triceps' command; the spinal circuit transforms a simple motor command into the coordinated push-pull activation of an antagonist pair. This local transformation is computation, not relay.
A pure relay station would pass whatever signal arrived from above directly to motor neurons without modification. Instead, the spinal cord interprets descending commands and adds coordination logic: reciprocal inhibition resolves the agonist-antagonist problem automatically, CPGs generate rhythmic patterns autonomously, and sensory feedback triggers corrections without cortical involvement. The result is that the brain can operate at the level of goals and strategies ('move leg forward') while the spinal cord handles the detailed muscle-level implementation — a hierarchical division of labor that makes complex movement tractable.