The nucleus basalis of Meynert, the major source of cholinergic innervation of the cerebral cortex, was morphometrically investigated in 58 cases of neuropsychiatric disorders and compared to 14 controls. The results demonstrate a loss of neurons in the nucleus basalis of Meynert in Alzheimer's disease (70%), paralysis agitans (77%), and Korsakoff's disease (47%) but no marked reduction of neurons in postencephalitic parkinsonism, Huntington's disease, chronic alcoholism without dementia, schizophrenia and infantile brain damage. Neurons of the three subdivisions of the nucleus basalis of Meynert (the nucleus septi medialis, the nucleus of the diagonal band of Broca and the nucleus basalis Meynert neurons in the substantia innominata) may be affected in a different manner in different patients within a single group homogeneous with respect to the usual clinical and neuropathological diagnostic criteria. Cell loss in the basal forebrain is restricted to the large neurons of the nucleus basalis, the immediately adjacent neurons of the globus pallidus externus not being affected. The selective degeneration of these neurons provides the morphological correlate of the cortical cholinergic deficiency in these neuropathological conditions. The degeneration of this discrete cholinergic neuronal population in several disorders of higher cortical function is probably directly related to the progressive deterioration of memory and cognitive processes in affected patients.

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A morphometric analysis of neuronal loss during normal aging was performed in the nucleus basalis Meynert complex of the basal forebrain (Nbm) (nucleus septi medialis, nucleus of Broca’s diagonal band, nucleus basalis) and the ciliary ganglion, a peripheral cholinergic structure, in patients free of neurological and psychiatric illness. As a basis for morphometric evaluation of the Nbm complex, a three-dimensional reconstruction of this complex structure was made. Neuronal counts in the Nbm complex and the ciliary ganglion remained stable up to the age of 60 or 50 years, respectively. After this age the number of neurons declined moderately in ciliary ganglion in all cases studied as well as in the Nbm complex in some cases (–20 and –25%, respectively, at about 90 years of age). In 3 out of 8 cases older than 60 years, neuronal counts in the Nbm complex were not reduced, so that no significant decline in neuronal number is apparent from the mean values of the 17 cases studied. No age-related changes were found in the neuronal distribution amongst the different subgroups of Nbm neurons using the alternative nomenclature of Mesulam et al. [J. comp. Neurol. 214: 170–197, 1983]. Our results provide no evidence that the cortical cholinergic projection system and peripheral cholinergic neurons might be especially vulnerable during normal aging. The severe degeneration of the cholinergic cortical projection system in SDAT is probably caused by mechanisms different from those acting during normal aging.

The deficiency of the cholinergic cortical projection system arising in the different basal forebrain structures collectively referred to as nucleus basalis of Meynert complex is a constant finding in Alzheimer's disease, a disorder which is neuro-pathologically characterised by the appearance of three intracerebral formes of twisted beta-pleated sheet (amyloid) fibrils, neurofibrillary tangles, amyloid-containing neuritic plaques and congophilic amyloid angiopathy. In the present study the quantitative relationship between these hallmarks of the disease, amyloid deposition and neuronal loss in the cholinergic basal forebrain system, was investigated in ten cases of Alzheimer's disease. Besides a constant involvement of the cerebral cortex and hippocampus, all cases of Alzheimer's disease show a large amount of amyloid in the medial septal nucleus, in the diagonal band nucleus and in the substantia innominata which is correlated with neuronal loss in these areas. These amyloid deposits in the basal forebrain are due to congophilic angiopathy associated with plaques and neurofibrillary tangles. The distribution of amyloid deposition in the basal forebrain is restricted entirely to those neuronal clusters which represent the origin of cholinergic innervation of the cerebral cortex and hippocampus. Immediately adjacent structures are not affected. These findings suggest a pathogenetic role of amyloid deposition in the mechanism of degeneration of the cholinergic basal forebrain system.