Gray Substance and Intrinsic System
The posterior horn is formed by the nucleusproprius (A1), the major portion of the pos-terior horn from which the dorsal nucleus (Clarke’s nucleus) (A2) is set apart. The gelat-inous substance (Rolando’s substance) (A3)borders dorsally on the nucleus proprius. On it sits like a cap the end of the posterior horn, the marginal zone (nucleus postero-marginalis) (A4). The posterior horn is sepa-rated from the surface of the spinal cord through the posterolateral tract (Lissauer’stract) (A5). Between posterior horn andanterior horn lies the intermediate gray mat-ter (A6) and lateral to it thelateral horn(A7).The border to the white matter between posterior horn and lateral horn is diffuse (reticular formation) (A8).
In the anterior horn, the motor neurons are arranged in groups of nuclei.
Anteromedial nucleus (A9)
Posteromedial nucleus (A10)
Anterolateral nucleus (A11)
Posterolateral nucleus (A12)
Retroposterolateral nucleus (A13)
For example, in the cervical spinal cord (B), the anterior horn is subdivided somatotopi-cally so that the neurons of the medial group of nuclei supply neck and back muscles, intercostal, and abdominal muscles (B14). The neurons of the antero-lateral nucleus supply the muscles of shoulder girdle and upper arm (B15), and the neurons of the posterolateral nucleus supply the muscles of lower arm and hand (B16). Finally, the retroposterolateral nu-cleus contains particularly large motor neu-rons that supply the small finger muscles (B17).
The neurons for the extensor muscles (B18) lie in the anterior field of the anterior horn, and those for the flexor muscles (B20) lie posterior to them. The somatotopic subdivi-sions do not occupy a single plane in the anterior horn but are spread over a certain height in such a way that the neurons for the shoulder girdle lie at a higher level, below them those for the upper arm, and still deeper those for the lower arm and hand. Diagram (C) illustrates the nerve supply to all body muscles.
To bring about an orderly movement during contraction of a muscle group, there must be simultaneous relaxation of the corre-sponding antagonists. This is achieved through inhibition of the corresponding anterior horn cells (D). For example, if an impulse is passed on by a neuron of the ex-tensor muscles (D18), it is simultaneouslytransmitted by an axon collateral to inhibi-tory interneurons, the Renshaw cells (D19), which then inhibit the neurons of the flexormuscles (D20).
Other interneurons mediate the spread of impulses over several levels, either on the same side or on the opposite side. Their as-cending and descending fibers run in basicbundles,fasciculi proprii(E21), which borderdirectly on the gray matter. In general, the ascending and descending fibers reach only one or two root levels. However, the fasciculi proprii also contain long fibers connecting the cervical spinal cord and the lumbar spinal cord (as shown in cats and monkeys). These fibers transmit excitatory and inhibitory impulses to anterior horn motor cells, a fact that is thought to be im-portant for coordinated movement of the anterior and posterior extremities during locomotion. About half of the posterolateral tract (Lissauer’s tract) (E5) consists of fibers of the intrinsic system.
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