Descending Tracts

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Descending tracts carry motor signals down the brainstem and spinal cord. A descending motor pathway typically involves two neurons called the upper and lower motor neuron. The upper motor neuron begins with a soma in the cerebral cortex or brainstem and has an axon that terminates on a lower motor neuron in the brainstem or spinal cord. The axon of the lower motor neuron then leads the rest of the way to the muscle or other target organ. The names of most descending tracts consist of a word root denoting the point of origin in the brain, followed by the suffix -spinal. The major descending tracts are described here.

  • The corticospinal (COR-tih-co-SPY-nul) tracts carry motor signals from the cerebral cortex for precise, finely coordinated limb movements. The fibers of this system form ridges called pyramids on the ventral surface of the medulla oblongata, so these tracts were once called pyramidal tracts. Most corticospinal fibers decussate in the lower medulla and form the lateral corticospinal tract on the contralateral side of the spinal cord. A few fibers remain uncrossed and form the ventral corticospinal tract on the ipsilateral side (fig. 13.6). Fibers of the ventral tract decussate lower in the spinal cord, however, so even they control contralateral muscles.
  • The tectospinal (TEC-toe-SPY-nul) tract begins in a midbrain region called the tectum and crosses to the contralateral side of the brainstem. In the lower medulla, it branches into lateral and medial tectospinal tracts of the upper spinal cord. These are involved in reflex movements of the head, especially in response to visual and auditory stimuli.
  • The lateral and medial reticulospinal (reh-TIC-you-lo-SPY-nul) tracts originate in the reticular formation of the brainstem. They control muscles of the upper and lower limbs, especially to maintain posture and balance. They also contain descending analgesic pathways that reduce the transmission of pain signals to the brain (see chapter 16).
  • The vestibulospinal (vess-TIB-you-lo-SPY-nul) tract begins in a brainstem vestibular nucleus that receives impulses for balance from the inner ear. The tract passes down the ventral column of the spinal cord and controls limb muscles that maintain balance and posture.

Rubrospinal tracts are prominent in other mammals, where they aid in muscle coordination. Although often pictured in illustrations of human anatomy, they are almost nonexistent in humans and have little functional importance.

Chapter 13 The Spinal Cord, Spinal Nerves, and Somatic Reflexes 489

Corticospinal Tracts Anatomy
Figure 13.6 Two Descending Pathways of the CNS. The lateral and ventral corticospinal tracts, which carry signals for voluntary muscle contraction. Nerve signals originate in the cerebral cortex at the top of the figure and carry motor commands down the spinal cord.

Saladin: Anatomy & I 13. The Spinal Cord, Spinal I Text I © The McGraw-Hill

Physiology: The Unity of Nerves, and Somatic Companies, 2003

Form and Function, Third Reflexes Edition

490 Part Three Integration and Control

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