Central control of interlimb coordination and speed-dependent gait expression in quadrupeds

Area of Science:

• Neuroscience
• Computational Biology
• Locomotion

Background:

• Quadrupeds exhibit distinct gaits (walk, trot, gallop, bound) that change with locomotion speed.
• The underlying neural mechanisms for these speed-dependent gait transitions are not fully understood.
• Spinal circuits, including central pattern generators and commissural interneurons (CINs), are crucial for coordinating limb movements and interlimb coordination.

Purpose of the Study:

• To develop a computational model of spinal locomotor circuits to explain speed-dependent gait transitions.
• To investigate the roles of different types of commissural interneurons (CINs) in gait control.
• To reproduce and explain experimental data on gait expression in intact and mutant mice.

Main Methods:

• A computational model of four spinal rhythm generators (RGs) with specified interconnections was developed.
• The model incorporated left-right interactions mediated by V0 and V3 CINs, and fore-hind inhibition.
• Simulations involved increasing excitatory drives to RGs and V3 CINs to observe changes in simulated locomotor speed and gait.

Main Results:

• Increasing excitatory drive to the model's RGs and V3 CINs resulted in progressive increases in simulated locomotor speed.
• The model accurately reproduced sequential gait transitions from walk to trot, gallop, and bound.
• Simulations of V0 CIN-deficient mutants matched experimental observations, with V0_V removal preventing trot and complete V0 removal allowing only bound.

Conclusions:

• The model provides a plausible explanation for how spinal circuit interactions generate speed-dependent gaits.
• Commissural interneurons play critical roles in coordinating interlimb movements and enabling specific gaits.
• The model offers insights into spinal cord organization and proposes testable predictions for future research.