Sherlock Holmes and the curious case of the human locomotor central pattern generator

Abstract

Evidence first described in reduced animal models over 100 years ago led to deductions about the control of locomotion through spinal locomotor central pattern-generating (CPG) networks. These discoveries in nature were contemporaneous with another form of deductive reasoning found in popular culture, that of Arthur Conan Doyle's detective, Sherlock Holmes. Because the invasive methods used in reduced nonhuman animal preparations are not amenable to study in humans, we are left instead with deducing from other measures and observations. Using the deductive reasoning approach of Sherlock Holmes as a metaphor for framing research into human CPGs, we speculate and weigh the evidence that should be observable in humans based on knowledge from other species. This review summarizes indirect inference to assess "observable evidence" of pattern-generating activity that leads to the logical deduction of CPG contributions to arm and leg activity during locomotion in humans. The question of where a CPG may be housed in the human nervous system remains incompletely resolved at this time. Ongoing understanding, elaboration, and application of functioning locomotor CPGs in humans is important for gait rehabilitation strategies in those with neurological injuries.

Contralateral seventh cervical nerve transfer can affect the pennation angle of the lower limb in spastic hemiplegia patients: An observational case series study

“…The neural control mechanisms of interlimb coordination within the spinal cord mainly include intrinsic pathways in the spinal cord, various supraspinal pathways, and somato‐sensory feedback from the limbs (Frigon, ; Ramirez‐Jarquin & Tapia, ). The basic locomotor output may be controlled by the central pattern generators (CPGs) located in cervical and lumbar, and there are strong interactions between cervical and lumbar CPGs (Frigon, ; Kaupp et al, ; Klarner & Zehr, ; Minassian, Hofstoetter, Dzeladini, Guertin, & Ijspeert, ; Ramirez‐Jarquin & Tapia, ; Figure ). However, the studies on neural control mechanisms of interlimb coordination were all conducted in animal models, which may have limitations as humans are very different in coordinating the limbs.…”

“…Schematic indicating the interlimb pathways and central pattern generator (CPG) with reference to literature (Kaupp et al, ; Klarner & Zehr, ; Minassian et al, ). Neuronal connections were represented with arrows and can be either excitatory or inhibitory.…”

Muscular Activity Modulation During Post-operative Walking With Hybrid Assistive Limb (HAL) in a Patient With Thoracic Myelopathy Due to Ossification of Posterior Longitudinal Ligament: A Case Report

“…The spinal cord has been considered to be housing the spinal locomotor network which coordinates the rhythmic limb motion during locomotion, based on evidences from non-human species and partly from humans (32,33). For the recovery of spinal locomotor function after injury, the importance of sensory input of movement based on activity is suggested (34,35).…”

Subthreshold Electrical Noise Applied to the Plantar Foot Enhances Lower-Limb Cutaneous Reflex Generation

“…Similar patterns of activation occur following electrical stimuli applied to the heel (Zehr et al, 2014 ). This phase-dependency of cutaneous reflexes during gait is thought to be driven by central pattern generator activity (Klarner and Zehr, 2018 ). While all four types of cutaneous mechanoreceptors found in the glabrous skin of the foot can generate cutaneous reflex responses (Fallon et al, 2005 ; Bent and Lowrey, 2013 ), fast adapting type-I (FAI) receptors are believed to be particularly important for the maintenance of balance.…”