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Previously, we reported an immediate emergence of new lower jaw input to the anterior forepaw barrel subfield (FBS) in primary somatosensory cortex (SI) following forelimb deafferentation. However, a delay of 7 weeks or more post‐amputation results in the presence of this new input to both anterior and posterior FBS. The immediate change suggests pre‐existing latent lower jaw input in the FBS, whereas the delayed alteration implies the involvement of alternative sources. One possible source for immediate lower jaw responses is the neighboring lower jaw barrel subfield (LJBSF). We used anatomical tracers to investigate the possible projection of LJBSF to the FBS in normal and forelimb‐amputated rats. Our findings are as follows: (1) anterograde tracer injection into LJBSF in normal and amputated rats labeled fibers and terminals exclusively in the anterior FBS; (2) retrograde tracer injection in the anterior FBS in normal and forelimb‐amputated rats, heavily labeled cell bodies predominantly in the posterior LJBSF, with fewer in the anterior LJBSF; (3) retrograde tracer injection in the posterior FBS in normal and forelimb‐amputated rats, sparsely labeled cell bodies in the posterior LJBSF; (4) retrograde tracer injection in anterior and posterior FBS in normal and forelimb‐amputated rats, labeled cells exclusively in ventral posterior lateral (VPL) nucleus and posterior thalamus (PO); (5) retrograde tracer injection in LJBSF‐labeled cell bodies exclusively in ventral posterior medial thalamic nucleus and PO. These findings suggest that LJBSF facilitates rapid lower jaw reorganization in the anterior FBS, whereas VPL and/or other subcortical sites provide a likely substrate for delayed reorganization observed in the posterior FBS. [ABSTRACT FROM AUTHOR]

Barrel subfields in rodent primary somatosensory cortex (SI) are important model systems for studying cortical organization and reorganization. During cortical reorganization that follows limb deafferentation, neurons in deafferented forelimb SI become responsive to previously unexpressed inputs from the lower jaw. Although the lower jaw barrel subfield (LJBSF) is a likely source of the input, this subfield has received little attention. Our aim was to describe the structural and functional organization of the normal LJBSF. To investigate LJBSF organization, a nomenclature for lower jaw skin surface was developed, cytochrome oxidase (CO) was used to label flattened‐cut LJBSF sections, microelectrodes were used to map the lower jaw skin surface representation in SI, and electrolytic lesions, recovered from electrode penetrations, were used to align the physiological map to the underlying barrel map. LJBSF is a tear‐shaped subfield containing approximately 24 barrels, arranged in eight mediolateral rows and a barrel‐free zone capping the anterior border. The representation of the lower jaw skin consisting of chin vibrissae and microvibrissae embedded in common fur is somatotopically organized in a single map in the contralateral SI. This physiological map shows that the activity from the vibrissae aligns with the CO‐staining of the underlying LJBSF. LJBSF barrels receive topographically ordered barrel‐specific input from individual vibrissa and microvibrissae in the lower jaw but not from trident whiskers. The barrel‐free zone receives topographically ordered input from the lower lip. These data demonstrating that the LJBSF is a highly organized subfield are essential for understanding its possible role in cortical reorganization. [ABSTRACT FROM AUTHOR]