Since the 1970s, a detailed description of the anatomical pathways of the vibrissae system has evolved in the literature. The process has been facilitated by the ability to discern topographical arrangements at each stage of the somatosensory pathway by virtue of the barrel pattern itself and by the careful application of dye-tracing techniques by a number of laboratories. The connectivity of the barrel cortex still remains a challenge because of its intricate complexity, and it has only recently become ammenable to description as a result of dual intracellular recording and glutamate uncaging methodologies. In this chapter, we describe the pathways leading from the whisker follicles (Section 2.1) through the brainstem (Section 2.2) and thalamus (Section 2.3) to the barrel cortex (Section 2.4). Where appropriate, the physiology of subcortical pathways are discussed but a more detailed consideration of barrel cortex physiology is reserved for subsequent chapters. We begin here at the periphery by considering the anatomy of the vibrissa follicle.

Whisker follicle innervation

The follicle itself is composed of a series of concentrically arranged membranes. Closest to the vibrissa itself lies a thick basement membrane, often referred to as the glassy membrane, which ensheaths the vibrissa hair (Figure 2.1). Outside the glassy membrane is a mesenchymal sheath that forms the inner surface of the follicle sinus. Outside this lies the vascular sinus itself, which is bounded on the outside by another thick highly collagenous membrane that forms the outside of the follicle sinus.