Evidence for Specialized Rhythm-Generating Mechanisms in the Adult Mammalian Spinal Cord

Abstract

Locomotion and scratch are characterized by alternation of flexion and extension phases within one hindlimb, which are mediated by rhythm-generating circuitry within the spinal cord. By definition, the rhythm generator controls cycle period, phase durations, and phase transitions. The aim was to determine whether rhythm-generating mechanisms for locomotion and scratch are similar in adult decerebrate cats. The regulation of cycle period during fictive scratching was evaluated, as were the effects of specific sensory inputs on phase durations and transitions during spontaneous fictive locomotion and pinna-evoked fictive scratching. Results show that cycle period during fictive scratching varied predominantly with flexion phase duration, contrary to spontaneous fictive locomotion, where cycle period varied with extension phase duration. Ankle dorsiflexion greatly increased extension phase duration and cycle period during fictive locomotion but did not alter cycle period during scratching. Moreover, stimulating the plantaris (ankle extensor muscle) nerve during flexion reset the locomotor rhythm to extension but not the scratch rhythm. Stimulating the plantaris nerve during extension prolonged the extension phase and cycle period during fictive locomotion but not during fictive scratching. Stimulating the sartorius nerve (hip flexor muscle) during early flexion reduced the flexion phase and cycle period during fictive locomotion, but considerably prolonged the flexion phase and cycle period during fictive scratching. These data indicate that cycle period, phase durations, and phase transitions are not regulated similarly during fictive locomotion and scratching, with or without sensory inputs, providing evidence for specialized rhythm-generating mechanisms within the adult mammalian spinal cord.

Cutaneous inputs from the back abolish locomotor-like activity and reduce spastic-like activity in the adult cat following complete spinal cord injury

“…The locomotor-like pattern resumed immediately after terminating the back pinch. The reset to flexion suggests that sensory inputs generated by pinching the skin of the back have direct access to the rhythm-generating circuitry [recently reviewed in (Frigon et al, 2010a; Frigon et al, 2010b; Gossard et al, 2011; Frigon, 2012)]. Spontaneous locomotor-like activity was observed in all cats following clonidine administration and pinching the back effectively stopped locomotor-like activity in 100% of trials.…”

Sensory-evoked perturbations of locomotor activity by sparse sensory input: a computational study

“…Such selection may be dependent on the motor task or even on the phase of a given motor behavior. For example, group I ankle extensor afferents can initiate a new extensor phase in locomotion (Gossard et al 1994), yet these afferents do not perturb scratch (Frigon and Gossard 2010). On the other hand, sartorius group II afferents affect both forms of rhythmic motor activity (Frigon and Gossard 2010).…”

“…For example, group I ankle extensor afferents can initiate a new extensor phase in locomotion (Gossard et al 1994), yet these afferents do not perturb scratch (Frigon and Gossard 2010). On the other hand, sartorius group II afferents affect both forms of rhythmic motor activity (Frigon and Gossard 2010). Sparse distribution of sensory inputs to a central motor network would readily enable central sensory selection mechanisms.…”

“…A number of neuronal populations, defined by anatomical, physiological, or genetic characteristics, have been identified as key components of spinal locomotor networks (reviewed by Arber 2012; Grillner and Jessell 2009;Grossmann et al 2010;Kiehn 2011). Part of the difficulty of describing the CPG is that no population of neurons has been found to be critical to rhythm generation (Crone et al 2008;Gosgnach et al 2006;Kwan et al 2009;Lanuza et al 2004;Zhang et al 2008). Thus the search for sources of rhythmogenesis in spinal locomotor CPGs has concentrated on cells exhibiting rhythmogenic or pacemaker characteristics (Dyck et al 2012;Wilson et al 2005).…”

Reflex modulation during rhythmic limb movements in humans

“…These findings imply that we can utilize an optimal motor task from three basic patterns to rhythm, indicating that the corticospinal tract has direct access to the rhythm-generating circuitry 54) . In addition, group I ankle extensor afferents from the foot appear to have direct access to the rhythm-generating circuitry of the spinal locomotor CPG, which was inferred by their ability to reset or entrain the fictive locomotor rhythm in adult cats 44,[55][56][57] . Cutaneous inputs from the plantar surface of the paw can also reinforce extensor activity in decerebrate cats walking on a treadmill 58) or during fictive locomotion 59) .…”