Activity of descending propriospinal axons in the turtle hindlimb enlargement during two forms of fictive scratching: phase analyses

In the present study, we examine the activity patterns of and synaptic inputs to Renshaw cells (RCs) during fictive locomotion in the newborn mouse using visually guided recordings from GABAergic cells expressing glutamic acid decarboxylase 67-green fluorescent protein (GFP). Among the GFP-positive neurons in the lumbar ventral horn, RCs were uniquely identified as receiving ventral rootevoked short-latency EPSPs that were markedly reduced in amplitude by nicotinic receptor blockers mecamylamine or tubocurarine. During locomotor-like rhythmic activity evoked by bath application of 5-HT and NMDA, 50% of the recorded RCs fired in-phase with the ipsilateral L2 flexor-related rhythm, whereas the rest fired in the extensor phase. Each population of RCs fired throughout the corresponding locomotor phase. All RCs received both excitatory and inhibitory synaptic inputs during the locomotor-like rhythmic activity. Blocking nicotinic receptors with mecamylamine markedly reduced the rhythmic excitatory drive, indicating that these rhythmic inputs originate mainly from motor neurons (MNs). Inhibitory synaptic inputs persisted in the presence of the nicotinic blocker. Part of this inhibitory drive and remaining excitatory drive could be from commissural interneurons because the present study also shows that RCs receive direct crossed inhibitory and excitatory synaptic inputs. However, rhythmic synaptic inputs in RCs were also observed in hemicord preparations in the presence of mecamylamine. These results show that, during locomotor activity, RC firing properties are modulated not only by MNs but also by the ipsilateral and contralateral locomotor networks.

Section: Discussionmentioning

confidence: 99%

Activity of Renshaw Cells during Locomotor-Like Rhythmic Activity in the Isolated Spinal Cord of Neonatal Mice

Nishimaru¹,

Restrepo²,

Kiehn³

2006

“…To calculate dualreferent phase-averaged membrane potential oscillations, any action potentials were deleted from the intracellular recording (and the missing voltage values were interpolated from the values just before and after the action potential), the recording was then linearly smoothed with a time constant of 200 ms, and the signal was dual-referent phase normalized (with respect to rectified and smoothed bursts and interburst intervals of the ipsilateral hip flexor nerve) and averaged for all cycles of that form of fictive scratching or swimming, using Datapac 2000 (Berkowitz and Stein, 1994;Berkowitz, 2005). The form of ipsilateral scratching with the largest phase-averaged oscillation for each cell was used for averages across cells.…”

Section: Methodsmentioning

confidence: 99%

Spinal Interneurons That Are Selectively Activated during Fictive Flexion Reflex

Berkowitz¹

2007

Behavioral choices in invertebrates are mediated by a combination of shared and specialized circuitry, including neurons that are inhibited during competing behaviors. Less is known, however, about the neural mechanisms of behavioral choice in vertebrates. The spinal cord can appropriately select among several types of limb movements, including limb withdrawal (flexion reflex), scratching, and locomotion, and thus is conducive to examination of vertebrate mechanisms of behavioral choice. Flexion reflex can interrupt and reset the rhythm of scratching and locomotion, suggesting that a combination of shared and specialized circuitry contributes to these behaviors, but little is known about the interneurons involved. Here, I used in vivo intracellular recording and dye injection to identify a group of spinal interneurons that are strongly activated during fictive flexion reflex but inhibited during fictive scratching and fictive swimming. These flexion-selective interneurons are typically rhythmically hyperpolarized during fictive scratching and fictive swimming. This hyperpolarization can be maximal during the ipsilateral hip flexor bursts of rhythmic limb motor patterns, although these cells are strongly activated during the ipsilateral hip flexor bursts of fictive flexion reflex. Thus, these interneurons are relatively specialized for fictive limb withdrawal, rather than contributing to the hip flexor phase of multiple types of limb movements. These flexion-selective cells are physiologically and morphologically distinguishable from a recently described group of spinal interneurons (transverse interneurons) that are strongly activated during both fictive flexion reflex and fictive scratching. Thus, spinal interneurons with distinct behavioral roles may to some extent be morphologically distinguishable.

“…7). Dual-referent phase measurements are appropriate for periodic events with a variable duty cycle (Berkowitz and Stein, 1994b). Circular statistics (Batschelet, 1981) were used to analyze phase measurements.…”

Section: Rectified-smoothed and Rectified-integrated Eng Recordingsmentioning

confidence: 99%

“…The onsets of right HF bursts were defined by phase values of 0.0 and 1.0; offsets were defined by a phase value of 0.5. The mean vector and angular deviation (ϭ circular analog of SD) were obtained for each set of phase measurements using vector addition (Batschelet, 1981;Berkowitz and Stein, 1994b;Stein et al, 1995). We used the Watson U 2 test (Batschelet, 1981) to determine whether there was a statistically significant difference between phase values before and after the spinal cord was superfused with strychnine.…”

Section: Rectified-smoothed and Rectified-integrated Eng Recordingsmentioning

confidence: 99%

Glycinergic Inhibition Contributes to the Generation of Rostral Scratch Motor Patterns in the Turtle Spinal Cord

Currie

Lee

1997

Cutaneous stimulation within the rostral scratch receptive field in a low spinal-immobilized turtle elicits a fictive rostral scratch reflex characterized by robust rhythmic motor output from ipsilateral hindlimb muscle nerves and weaker, alternating motor discharge in contralateral nerves. Simultaneous bilateral stimulation elicits bilateral rostral scratch motor patterns in which activity on the right and left sides alternates.We investigated the role of glycinergic inhibition in the generation and coordination of fictive rostral scratch motor patterns. Glycine (2 or 5 mM) and strychnine (5-50 M), a glycine antagonist, were superfused over the anterior spinal hindlimb enlargement while fictive rostral scratch motor output was recorded bilaterally from hindlimb muscle nerves in the form of electroneurograms (ENGs). Although glycine reduced rostral scratch burst frequencies, strychnine tended to increase burst frequency. Strychnine also changed the shape of hip flexor ENG bursts, resulting in more abrupt burst onsets, indicating an earlier recruitment of motor neurons with large ENG spikes. During bilateral stimulation, strychnine increased the variability of interlimb phase values (left vs right hip flexor bursts) but did not abolish right-left alternation.These results indicate that glycinergic neurons in or near the anterior hindlimb enlargement contribute to the overall timing of the rostral scratch rhythm and to the recruitment timing of individual hip flexor motor neurons within each scratch burst. Our data also indicate that glycinergic mechanisms contribute to, but are not critically important for, maintaining an alternating interlimb coordination during bilateral scratch motor patterns.