Substantia Nigra

SUBSTANTIA NIGRA Soemmeringi, s. locus niger, is part of the extrapyramidal system. The substantia nigra is located in the midbrain and diencephalon: in the midbrain, it is in the cerebral peduncle, between the tegmentum and the base, between the nucleus ruber and the pyramidal system (see Cerebral Peduncles, Fig. 1), and in the diencephalon, it is in the regio subthalamica, under the thalamus opticus and corpus Luysi (see Regio subthalamica, Fig. 2). The substantia nigra is formed by gray matter and has the shape of a band, wider in the middle part and narrowing at the ends; it is curved with the concavity directed backward. In humans, the substantia nigra can be divided into two zones that are somewhat different in structure: a dorso-medial part, which is more compact in structure (formatio compacta), formed by cells rich in pigment (Schwarzzone according to Spatz), and a ventro-medial part, which is less compact (formatio reticularis), having, like the nucleus ruber, an orange-reddish color (Rotezone according to Spatz), and is rich in myelin fibers, in which, as in the pallidum, large cells devoid of pigment are embedded. This part is designated by Meynert as the stratum intermedium. The inner part of the substantia nigra is traversed by the roots of the oculomotor nerve (n. III), and somewhat outwardly by fibers to the corpora mamillaria. The cells of the substantia nigra are very significant in size, oblong, multipolar, and very rich in dendrites, which form very rich ramifications that connect with neighboring cells; the cells contain a nucleus, chromatophilic substance, and the pigment melanin, which occupies the greater part of the cell protoplasm in the form of delicate granulation; it is sometimes observed in the dendrites. The cells are surrounded by a large number of fibers. The substantia nigra is connected: 1) with the cerebral cortex by centripetal and centrifugal fibers, mainly with the central gyri and the posterior sections of the frontal lobes; these fibers pass through the capsula interna; 2) with the corpus striatum by means of the fibrae striae nigrae; these fibers run from the pallidum to the upper-outer pole of the substantia nigra; the ansa lenticularis sends fibers to the substantia nigra; 3) with the corpus Luysi; 4) with the thalamus opticus; 5) with the nucleus ruber. The efferent pathways of the substantia nigra are partially directed through the commissura posterior to the opposite side, while another part of the fibers has a descending direction; pathways to the anterior colliculus and to the region of the lemniscus medialis in the tegmentum have been established; the substantia nigra has a relationship to the motor formations located below. The substantia nigra receives blood from the arteria cerebri posterior and from the arteria chorioidea anterior. At birth, the cells of the substantia nigra do not contain pigment, which appears only in the 3rd or 4th year of life. The fibers of the substantia nigra are covered with myelin later than other formations of the extrapyramidal system—at the end of intrauterine life, so that at birth they are covered with myelin. The substantia nigra of mammals resembles the human substantia nigra. In the extrapyramidal system, the substantia nigra occupies a special place due to its direct connections with the cortex. A lesion of the substantia nigra is accompanied by a severe hypokinetic hypertonic state with tremor, which appears as an early symptom of the disease; alongside the disturbance of tone, there is a disturbance in the sequence of movements. Its innervation influences are bilateral, but with a predominance on the opposite side. In view of the connections of the substantia nigra with the nuclei of the tegmentum and with the nucleus ruber, it is assumed that it exerts a regulating influence on them. Jakob, considering the presence of connections of the substantia nigra with the cortex, assumes that alongside the cortico-spinal tract runs a cortico-nigral tract, the task of which is to provide every motor act with a tonicizing impulse. In the substantia nigra, changes are found in paralysis agitans and in parkinsonism after epidemic encephalitis; these changes consist of inclusions in the cells in the form of round bodies of various sizes; some authors express the assumption of their connection with pigment (for figures, see Cerebral Peduncles, Fig. 1).