Chapter 17: Rhythmical Movements

Rhythmical Movements

Rhythmical, repetitive movements are used by virtually all creatures for propulsion through their environments.
- Examples include:
  - Undulations of the body wall for swimming.
  - Flapping of wings for flying.
  - Wiggling for crawling.
  - Cyclic leg motions for walking and running.
Rhythmical motions serve various purposes, like scratching, mastication, and breathing.
The chapter focuses on the neural control of mammalian terrestrial locomotion but notes that principles are similar across different rhythmical movements.

Sherrington's Reflex Theory:
- Proposed that repetitive stepping in locomotion arises from linked sequences of different reflexes.
- Suggested that extensor (E) and flexor (F) reflexes alternate during stepping.
- Believed proprioceptive stimulations from the limb itself trigger these reflexes.
Graham Brown's Challenge:
- Experimentally challenged Sherrington's reflex-based concept.
- Demonstrated stepping activity in spinalized animals even after dorsal roots (carrying somatosensory axons) were lesioned.
- This indicated that the rhythm's source wasn't the limb itself or reflexes.
- Summarized that locomotion is primarily a central (CNS) rather than a peripheral phenomenon.
- Circuits controlling rhythmic motor acts reside within the CNS and don't require sensory input.
- Graham Brown's findings were not embraced initially, possibly due to Sherrington's prominence.

Discovery:
- In the 1960s, USSR investigators identified a small region (~1 mm diameter) in the dorsal midbrain that, when stimulated electrically, evoked sustained walking movements in decerebrate cats.
- This region was named the midbrain locomotor region (MLR).
- Stimulation of the MLR in immobile cats caused them to stand and begin moving, even accelerating a treadmill belt.
MLR Output:
- The output of the MLR passes through the reticular formation (RF) in the pons and medulla.
- The RF then projects to the spinal cord.

Setup:
- Decerebrate cats were held by a frame over a treadmill.
- Movement sensors on limbs and EMG electrodes in muscles recorded the evoked locomotion.
Findings:
- MLR-evoked locomotion in decerebrate cats was similar to that in intact cats walking on a treadmill.
- Increased MLR stimulation intensity led to increased walking speed and gait changes (walking to trotting to galloping).
- Increased treadmill speed with constant MLR stimulation also altered gaits appropriately.
- This implied sensory feedback from limb movement plays a key role in adapting gait patterns to treadmill speed. However, it was not necessary for the initiation of locomotion.

Grillner's Experiments:
- Sten Grillner replicated Graham Brown's experiments by evaluating MLR stimulation's ability to produce locomotion in decerebrate cats with severed dorsal roots.
Results:
- EMG recordings showed similar muscle coordination patterns during MLR-evoked locomotion in cats with and without intact dorsal roots.
- This supported the conclusion that complex motor patterns in locomotion don't require sensory input.
- However, Grillner and Zangger (1984) found that after several coordinated steps in cats with severed dorsal roots, the activity pattern could become variable or break down.
- This suggested that sensory feedback is important for maintaining and adjusting the locomotor pattern to variations in limb placement and the environment.

Question:
- Where in the CNS is the locomotor pattern generated?
Experiment:
- Grillner and colleagues lesioned the spinal cords of kittens at a thoracic level, causing hindlimb paralysis.
- Kittens were then trained to walk on a treadmill, with experimenters manually moving their hindlimbs.
Results:
- After training, kittens began producing rhythmic hindlimb movements on their own.
- Movement and muscle activity patterns resembled those of intact cats walking on a treadmill.
Conclusion:
- The circuitry for controlling complex locomotor activity resides in the spinal cord.

Concept:
- CPGs are low-level central circuits that produce complex spatiotemporal patterns of muscle activity for rhythmic behaviors without needing sensory input.
- Locomotion can involve up to 100 muscles, and CPGs orchestrate this complex activity with little conscious involvement.
Cortical Involvement:
- The cerebral cortex initiates locomotion as a volitional act.
- Commands likely originate from the motor cortex and activate the basal ganglia (bg).
- The basal ganglia then operate on the subthalamic locomotor region (SLR) in the diencephalon, potentially housed within the zona incerta.
- Output from the SLR activates the MLR, which drives projections from the reticular formation (RF) to excite spinal CPGs.
Activation Pathway:
- Cerebral cortex → basal ganglia → SLR → MLR → RF → spinal CPGs.
- This pathway acts as a