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Figure

Sensory impulses originating in skin touch receptors ascend in the fasciculus cuneatus tract and cross over in the medulla of the brain. Pain and temperature information ascends in the lateral spinothalamic tract, which crosses over in the spinal cord.

fibers cross over (decussate) from one side to the other—that is, those ascending on the left side of the spinal cord pass across to the right side, and vice versa. As a result, the impulses originating from sensory receptors on the left side of the body reach the right side of the brain, and those originating on the right side of the body reach the left side of the brain (fig. 11.12).

2. Spinothalamic (spi"no-thah-lam'ik) tracts. The lateral and anterior spinothalamic tracts are located in the lateral and anterior funiculi, respectively. Impulses in these tracts cross over in the spinal cord. The lateral tracts conduct impulses from various body regions to the brain and give rise to sensations of pain and temperature (fig. 11.12). Impulses carried on fibers of the anterior tracts are interpreted as touch and pressure.

3. Spinocerebellar (spi"no-ser"e-bel'ar) tracts. The posterior and anterior spinocerebellar tracts lie near the surface in the lateral funiculi of the spinal cord. Fibers in the posterior tracts remain uncrossed, whereas those in the anterior tracts cross over in the medulla. Impulses conducted on their fibers originate in the muscles of the lower limbs and trunk and then travel to the cerebellum of the brain. These impulses coordinate muscular movements.

Descending Tracts The major descending tracts of the spinal cord are shown in figure 11.11. They include the following:

1. Corticospinal (kor"ti-ko-spi'nal) tracts. The lateral and anterior corticospinal tracts occupy the lateral and anterior funiculi, respectively. Most of the fibers of the lateral tracts cross over in the lower portion of the medulla oblongata. Some fibers of the anterior tracts cross over at various levels of the spinal cord. The corticospinal tracts conduct motor impulses from the brain to spinal nerves and outward to various skeletal muscles. Thus, they help control voluntary movements (fig. 11.13).

The corticospinal tracts are sometimes called pyramidal tracts after the pyramid-shaped regions in the medulla oblongata through which they pass. Other descending tracts are called extrapyramidal tracts, and they include the reticulospinal and rubrospinal tracts.

2. Reticulospinal (re-tik"u-lo-spi'nal) tracts. The lateral reticulospinal tracts are located in the lateral funiculi, whereas the anterior and medial reticulospinal tracts are in the anterior funiculi. Some fibers in the lateral tracts cross over, whereas others remain uncrossed. Those of the anterior and medial tracts remain uncrossed. Motor impulses transmitted on the reticulospinal tracts originate in the brain and control muscular tone and activity of sweat glands.

3. Rubrospinal (roo"bro-spi'nal) tracts. The fibers of the rubrospinal tracts cross over in the brain and pass through the lateral funiculi. They carry motor impulses from the brain to skeletal muscles, and coordinate muscles and control posture.

Motor cortex of cerebrum

Cerebrum

Essentials of Human Physiology

Essentials of Human Physiology

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