Stage 3

In stage 3 the disease process crosses the upper limit of the pontine tegmentum and makes inroads into the mesencephalic tegmentum and basal forebrain (Fig. 5a). The Lewy pathology in previously involved sites worsens, and inclusion bodies appear in the catecholaminergic neurons of both the dorsal vagal area (A2 group) and intermediate reticular zone (A1 group), as well as in nerve cells close to the roof of the fourth ventricle that form the cerebellar portion of the coeruleus nucleus (A4 group). LNs and LBs also develop in the central subnucleus of the amygdala (Fig. 15 a,c), posterolateral subnucleus of the substantia nigra (pars compacta; Fig. 16c-g), and in the pedunculopontine tegmental nucleus (Fig. 17 a-d), upper raphe nuclei (Fig. 17e,f), hypothalamic tuberomamillary nucleus (Fig. 19d,e), and magnocellular nuclei of the basal forebrain (Figs. 15a,c and 16a,b).

At this point, the entire tectum and thalamus are uninvolved (see Sect. 7.1). Slight changes are visible in the somatomotor nuclei of the eye muscles (cranial nerves III, IV, VI). Phylogenetically older components of the limbic system, such as the habenular nuclei, interpeduncular nucleus, and dorsal tegmental nucleus of Gudden (asterisk in Fig. 14a) as well as extremely phylogenetically recent dien-cephalic structures, e.g., the hypothalamic lateral tuberal nucleus (see Sect. 6.2 and Fig. 19d, e) remain intact for the duration of the disease. Equally striking is that both the diencephalic and mesencephalic components of the striatal and cerebellar circuits that generate moderately to thickly myelinated axons (pallidum, subthalamic nucleus, red nucleus) are also resistant.