M02.03.002 The Descending Tracts

The descending tracts, also known as motor pathways or efferent pathways, are neural pathways that transmit motor commands from the brain to the spinal cord and ultimately to the muscles and other effectors. These tracts allow us to initiate and control voluntary movements. Here are the major descending tracts:

1. Corticospinal Tract (Pyramidal Tract):
   • Location: This tract originates in the primary motor cortex (precentral gyrus) of the cerebral cortex.
   • Pathway: The corticospinal tract consists of two parts: the lateral corticospinal tract and the anterior corticospinal tract. The majority of fibers form the lateral corticospinal tract. The upper motor neurons originate in the primary motor cortex and descend through the internal capsule, cerebral peduncles of the midbrain, and the medullary pyramids. At the level of the medulla, about 85% of the fibers decussate (cross over) and descend contralaterally as the lateral corticospinal tract. The remaining 15% of fibers form the anterior corticospinal tract, which descends ipsilaterally until they synapse with lower motor neurons in the spinal cord.
2. Rubrospinal Tract:
   • Location: This tract originates in the red nucleus of the midbrain.
   • Pathway: The rubrospinal tract descends contralaterally in the brainstem and spinal cord. The upper motor neurons originate in the red nucleus, cross over at the level of the midbrain, and descend to synapse with lower motor neurons in the spinal cord. It plays a role in controlling fine motor movements, particularly in the upper extremities.
3. Vestibulospinal Tracts:
   • Location: These tracts originate in the vestibular nuclei of the brainstem.
   • Pathway: The vestibulospinal tracts consist of the lateral vestibulospinal tract and the medial vestibulospinal tract. The lateral vestibulospinal tract originates from the lateral vestibular nucleus and descends ipsilaterally to innervate motor neurons controlling antigravity muscles, mainly in the lower limbs. The medial vestibulospinal tract originates from the medial vestibular nucleus and has both ipsilateral and contralateral projections to the spinal cord, influencing neck and upper body posture.
4. Reticulospinal Tracts:
   • Location: These tracts originate in the reticular formation of the brainstem.
   • Pathway: The reticulospinal tracts consist of the pontine reticulospinal tract and the medullary reticulospinal tract. The pontine reticulospinal tract descends ipsilaterally and influences voluntary and reflexive movements, as well as muscle tone. The medullary reticulospinal tract has both ipsilateral and contralateral projections and plays a role in controlling posture and muscle tone.

These descending tracts are involved in transmitting motor commands from the brain to the spinal cord, where they synapse with lower motor neurons that innervate muscles, resulting in voluntary movements. Understanding the anatomy of these tracts is crucial for diagnosing and treating motor disorders and for comprehending the complex mechanisms of motor control.