Increasing Muscle Speed Drives Changes in the Neuromuscular Transform of Motor Commands during Postnatal Development in Songbirds

Abstract

We would like to thank Sam Sober for comments on an earlier version of our manuscript.

Syringeal vocal folds do not have a voice in zebra finch vocal development

“…Until 25 DPH the syrinx of male and female zebra finches is not clearly distinguishable. However, after 25 DPH, the syringeal muscles start to exhibit sexually dimorphic features, such as increased muscle mass 10 and speed in males 11 , which leads to a changing transformation from motor neuron spikes to force, often referred to as the neuromuscular transform 13 . Furthermore, muscle force exerted on bones is known to redirect bone deposition and thus can lead to significant bone remodelling 92 .…”

“…However, the vocal organ, the syrinx, is also undergoing significant anatomical and functional changes during song development 10 – 13 . Coinciding with the transition from the sensory to the sensorimotor phase of learning around 20 DPH, syringeal muscles in male zebra finches increase in mass and cross-sectional area 10 .…”

“…Coinciding with the transition from the sensory to the sensorimotor phase of learning around 20 DPH, syringeal muscles in male zebra finches increase in mass and cross-sectional area 10 . Additionally, the contractile properties of male syringeal muscles increase in speed 11 , which affects the transformation from neural commands to muscle force 13 . Thus, over development functional changes occur in the syrinx that change the transformation from neural signals into force which thereby change motor control of the vocal organ.…”

Resurgent Na+ currents promote ultrafast spiking in projection neurons that drive fine motor control

“…Finally, we note that a subclass of large Betz-type L5PNs in primate motor cortex 9,10 also exhibit narrow spikes that may fine tune them for rapid and precise motor control 3 . We thus propose that the specialized “hardware” of RAPNs facilitate their precise in vivo spike burst firing that is required for activation of lower motor neurons and contraction of the superfast syrinx muscles involved in singing 33 .…”

“…Song maturation thus correlates with reduced variability in the timing of increasingly high frequency bursts with a refinement of single spike firing precision. Importantly, nerve firing rates of >75 Hz are required for force summation in the superfast syrinx muscles 33 . This high spike frequency in RAPNs during song production is energetically demanding as indicated by increased staining for cytochrome C in maturing males, but not female zebra finches 34 .…”