Questions: Spinal Reflex Circuits

Question 1

A patient with a complete spinal cord injury at the thoracic level — severing all descending connections from the brain — is supported on a treadmill. Under what circumstances might rhythmic stepping movements appear in the legs?

• A. Never — voluntary locomotion requires intact corticospinal tracts from the motor cortex to the spinal cord
• B. Only if sensory feedback from the legs is artificially eliminated, removing inhibitory input
• C. Rhythmic stepping can occur because central pattern generators in the lumbar spinal cord can generate locomotor rhythms intrinsically, without descending commands
• D. Only with electrical stimulation of the motor cortex, which can bypass the injury and trigger spinal circuits

Answer: (C)

Central pattern generators (CPGs) are spinal interneuron networks that generate rhythmic, alternating motor patterns without requiring continuous commands from the brain. This has been demonstrated directly in isolated spinal cord preparations — the rhythm is intrinsic to the spinal circuitry. The brain normally modulates CPG output (initiating walking, adjusting speed and gait) but does not generate the fundamental pattern. After spinal cord injury above the lumbar CPG, rhythmic stepping can sometimes be elicited by treadmill loading and sensory input to the spinal cord alone.

Question 2

When you step on a sharp object, your foot jerks away and your opposite leg simultaneously stiffens. What spinal circuit mechanism produces the stiffening of the opposite leg?

• A. The pain signal travels to the brain, which sends a rapid descending command to stiffen the opposite leg
• B. Crossed-extension: spinal interneurons simultaneously activate extensors and inhibit flexors in the contralateral limb, coordinated at the spinal level
• C. The stretch reflex in the opposite leg is triggered automatically by the sudden shift in body weight
• D. GABAergic interneurons suppress the crossed-extension response on the ipsilateral side, disinhibiting the opposite leg

Answer: (B)

Crossed-extension is a polysynaptic spinal reflex requiring no brain involvement. Pain afferents activate interneurons that drive ipsilateral flexors (pulling the foot away) and inhibit ipsilateral extensors, while simultaneously crossing the midline via commissural interneurons to activate contralateral extensors and inhibit contralateral flexors. The result is the opposite leg stiffening to bear weight, preventing a fall. This coordinated bilateral response demonstrates that spinal circuits can integrate information across both sides of the body to produce functionally coherent behavior.

Question 3

Central pattern generators can produce coordinated rhythmic motor patterns even in isolated spinal cord preparations with all descending input from the brain removed.

• T. True
• F. False

Answer: True

This has been demonstrated experimentally in cats, lampreys, and other animals: an isolated spinal cord (or even a section of it) bath-applied with neuromodulators like dopamine or NMDA can produce rhythmic, alternating patterns of motor activity resembling locomotion, with no input from supraspinal structures. The CPG rhythm is intrinsic to the spinal interneuron network. This finding fundamentally changed the understanding of locomotion: the brain initiates and modulates walking, but the basic stepping pattern is generated locally.

Question 4

The knee-jerk reflex (patellar tendon reflex) is a polysynaptic reflex because it involves rapid coordination of multiple muscles.

• T. True
• F. False

Answer: False

The knee-jerk reflex is the canonical example of a *monosynaptic* reflex. Ia afferent fibers from muscle spindles in the quadriceps enter the spinal cord and synapse directly onto alpha motor neurons — just one synapse. This is why it is so fast (~30 ms). Polysynaptic reflexes, like the flexor withdrawal reflex, interpose interneurons between sensory input and motor output, which allows for more complex coordination (including reciprocal inhibition and crossed-extension) but introduces additional synaptic delays.

Question 5

What does the existence of central pattern generators reveal about the relationship between the brain and spinal cord in generating locomotion?

Model answer: CPGs demonstrate that the spinal cord is not a passive relay of brain commands but an active computational structure capable of generating complex, rhythmically coordinated motor patterns on its own. The brain's role in locomotion is modulatory and supervisory — it initiates walking, selects gait, and makes real-time adjustments — but the fundamental rhythm of alternating flexion and extension is generated by spinal interneuron circuits. This division of labor means locomotion can persist (in altered form) after spinal injury, and that the spinal cord encodes substantial motor 'knowledge' independently.

This insight has significant clinical implications. Rehabilitation strategies after spinal cord injury now target CPG reactivation through treadmill training and epidural stimulation, exploiting the spinal cord's intrinsic locomotor circuitry. It also explains why decerebrate cats can still walk on a treadmill and why premature infants show stepping movements — the spinal circuitry for locomotion develops and functions before cortical control is fully established.