Cellular mechanisms of motor control in the vibrissal system

Abstract

In this article we discuss the experimental advantages that the vibrissal motor system offers for analysis of motor control and the specializations of this system related to the unique characteristics of whisker movements. Whisker movements are often rhythmic, fast, and bilateral. Movements of individual whiskers have simple characteristics, whereas, movements of the entire vibrissae array are complex and sophisticated. In the last few years, powerful methods for high precision tracking of whisker movements have become available. The whisker musculature is arranged to permit forward movements of individual whiskers and consists-depending on the species -mainly or exclusively of fast contracting, fast fatigable muscle fibers. Whisker motor neurons are located in the lateral facial nucleus and their cellular properties might contribute to the rhythmicity of whisking. Numerous structures provide input to the lateral facial nucleus, the most mysterious and important one being the putative central pattern generator (CPG). Although recent studies identified candidate structures for the CPG, the precise identity and the functional organization of this structure remains uncertain. The vibrissa motor cortex (VMC) is the largest motor representation in the rodent brain, and recent work has clarified its localization, subdivisions, cytoarchitectonics, and connectivity. Single-cell stimulation experiments in VMC allow determining the cellular basis of cortical motor control with unprecedented precision. The functional significance of whisker movements remains to be determined.

What single-cell stimulation has told us about neural coding

“…The rodent's vibrissal motor system offers several advantages for analysis of motor control, primarily owing to the ability to accurately measure speed and displacement of single whisker movements, availability of powerful methods for high-speed tracking such movements, and the ease of manipulation of single whiskers [60]. The intrinsic and extrinsic muscles surround the whiskers and produce rhythmic movements via consecutive contractions that cause whisker protraction and retraction, respectively [61 -63].…”

Topographical Organization of the Facial Motor Nucleus in Florida Manatees <i>(Trichechus manatus latirostris)</i>

“…It has been accepted that direct descending corticomotor neurons projections to the facial motonucleus are present only in cararrhine primates, which would presumably be important in the evolution of 'enhanced mobility and flexibility in facial expression' [Sherwood, 2005]. However, recent data in rats has demonstrated direct connections from rat vibrissa motor cortex (VMC) to the facial motor nucleus (FN) [Grinevich et al, 2005;Brecht et al, 2006]. Presumably, the "evolution of the VMC to FN projections is related to the rich repertoire of whisker movements in rodents, whereas simple synchronous movements of whiskers are observed during free whisking rats [exhibit] complex movements of subsets of whiskers when they are in object contact" [Grinevich et al, 2005].…”

“…Similarly, the motor neuron pools innervating the musculature of the pinnae in these taxa are concomitantly large relative to the motor neuron pools of other facial muscles, and relative to musculature of other taxa in which pinnae are not mobile. Such brain structure-behavior relationships have also been demonstrated in taxa that possess well-developed and mobile vibrissae, such as rodents, that exhibit an expanded representation in their lateral facial motor nuclei [Huber, 1930a, b;Ashwell, 1982;Watson et al, 1982;Komiyama et al, 1984;Klein and Rhoades, 1985;Semba and Egger, 1986;Brecht et al, 2006].…”

Sensory experience modifies spontaneous state dynamics in a large-scale barrel cortical model

“…During active exploration of their environment, rodents rhythmically sweep their whiskers over objects at a frequency of 5-25 Hz (Carvell & Simons 1990, Lee & Woolsey 1975, Jin et al 2004, Brecht et al 2006. In order to mimic the protraction/retraction motion of the whiskers during an exploration event, we consider an external stimulus that consists of blocks with 10 repetitions per second of the same pattern of white noise appended to its mirror image ( Figure 2B lower panel, light and dark grey).…”