Figure 4
Neurolucida 360 tracing and digitizing of the TH-IR bundles and axons innervating the RA and LA (connected). (a) Tracing of TH-IR bundles revealed ~ 4 major bundles entered the atria mainly at the SVC, LPCV or LA-PV junction then bifurcated into small bundles that eventually ramified into individual varicose axons. Two bundles extended the TH-IR axons towards the RA while the other two bundles mainly innervated the LA. These four bundles innervated distinct regions with a certain degree of overlap. (b) TH-IR bundle that extended projections mainly towards the base of SVC, SAN, AVN, the lower part of the RAu. (c) A TH-IR bundle that extended projections towards the upper part of the RAu, SAN, LA and RA junction, and the medial part of the LA. (d) A TH-IR bundle that accessed the LA through the LPCV and mainly innervated the LPCV, the LAu, and the middle of the LA. (e) TH-IR bundles entered the LA between LPCV and the left PV to mainly innervate the LPCV, the left and middle PVs, the middle part of the LA and the junction of LA and RA. Arrows indicate the beginning of the parent axon bundle. RA right atrium, LA left atrium, LPCV left precaval vein, SVC superior vena cava. Scale bar: 500 μm.
