The descending tracts of the spinal cord are motor pathways that carry signals from the brain down to the body. They fall into two main groups: pyramidal tracts, which control voluntary movement, and extrapyramidal tracts, which handle automatic functions like posture, balance, and muscle tone. If you encountered this question on an exam or quiz, the key descending tracts to know are the corticospinal tract (pyramidal) and the rubrospinal, vestibulospinal, reticulospinal, and tectospinal tracts (extrapyramidal).
Pyramidal Tracts: Voluntary Movement
The pyramidal system includes two tracts: the corticospinal tract and the corticobulbar tract. The corticospinal tract is the one that descends through the spinal cord, making it the most tested example of a descending spinal cord tract. It originates in the brain’s motor and sensory cortex, then travels downward through the brainstem into the medulla, where something important happens: roughly 85 to 90% of its fibers cross over to the opposite side of the body at a point called the pyramidal decussation.
This crossing creates two separate pathways. The fibers that cross over form the lateral corticospinal tract, which runs in the side (lateral) portion of the spinal cord. This tract is the primary pathway for voluntary movement of your arms and legs. Because the fibers have already crossed, the left side of your brain controls the right side of your body, and vice versa. These fibers connect to motor neurons in the front part of the spinal cord’s gray matter at whichever level they need to reach.
The remaining 10 to 15% of fibers that don’t cross in the medulla continue straight down as the anterior corticospinal tract, running near the front of the spinal cord. This smaller tract controls muscles of the trunk and shoulder/hip girdle. It only extends down to about the lower thoracic level, and its fibers eventually do cross over through the spinal cord’s midline before connecting to motor neurons. So nearly all corticospinal signals ultimately reach the opposite side of the body.
The corticobulbar tract is also pyramidal, but it terminates in the brainstem rather than the spinal cord, controlling muscles of the face, jaw, and throat. It would not be the correct answer to a question about descending tracts in the spinal cord specifically.
Extrapyramidal Tracts: Automatic Motor Control
The extrapyramidal tracts originate in various brainstem nuclei rather than the cortex. They don’t pass through the medullary pyramids, which is why they’re called “extrapyramidal.” These pathways manage the things you don’t consciously think about: staying upright, adjusting muscle tone, and reflexively turning your head toward a sound.
Rubrospinal Tract
This tract originates from the red nucleus in the midbrain and descends in the lateral part of the spinal cord, right alongside the lateral corticospinal tract. It promotes flexor muscle activity while inhibiting extensor (antigravity) muscles. In many animals, the rubrospinal tract is a major motor pathway, but in humans it has long been considered somewhat reduced in importance. More recent research suggests it still plays a meaningful role, particularly for automated movements. The corticospinal tract appears to dominate when you’re learning a new movement, while the rubrospinal tract may be more active during well-practiced, automatic ones. In primates, rubrospinal fibers project mainly to the lower cervical and upper thoracic segments of the cord, corresponding to the upper limbs.
Vestibulospinal Tract
Originating from the vestibular nuclei in the brainstem (which receive input from the inner ear), this tract controls balance. It has lateral and medial divisions. The lateral vestibulospinal tract activates extensor muscles throughout the body to keep you upright against gravity. The medial vestibulospinal tract coordinates head and neck position in response to changes in balance.
Reticulospinal Tract
This tract arises from the reticular formation, a diffuse network in the brainstem. It regulates spinal reflex circuits and helps maintain the baseline muscle tone you need for standing and walking. It’s part of the medial motor system and plays a role in modulating how strongly your spinal reflexes fire.
Tectospinal Tract
The tectospinal tract originates from the superior colliculus in the midbrain, a structure that processes visual and auditory information. Its job is to orient your head and eyes toward something you see or hear. It’s responsible for reflexive movements like turning your head toward a sudden noise or tracking an object with your eyes. This tract descends through the medial part of the spinal cord and primarily influences neck muscles.
Lateral vs. Medial Organization
Beyond the pyramidal/extrapyramidal division, descending tracts are also organized by their position in the spinal cord, which reflects what body parts they control. The lateral motor system (lateral corticospinal and rubrospinal tracts) runs along the outer side of the spinal cord and controls limb muscles, especially fine, skilled movements of the hands and feet. The medial motor system (anterior corticospinal, vestibulospinal, reticulospinal, and tectospinal tracts) sits closer to the midline and controls trunk muscles, posture, and head position.
One interesting finding from recent primate research is that fibers within the lateral corticospinal tract don’t appear to be neatly organized by body region. Fibers controlling the arm and fibers controlling the leg overlap extensively as they travel through the tract, rather than being sorted into distinct layers. Leg fibers simply pass through the cervical cord without branching off, then make their connections once they reach the lower spinal cord.
What Happens When Descending Tracts Are Damaged
Because descending tracts carry commands from the brain, damage to them produces a pattern called upper motor neuron syndrome. This is distinct from damage to the nerves that directly connect to muscles (lower motor neurons). The hallmarks of upper motor neuron damage include:
- Spasticity: muscles resist being stretched, especially during quick movements. You may notice a “clasp-knife” quality where resistance suddenly gives way, like folding a pocketknife blade.
- Hyperreflexia: reflexes become exaggerated because the brain’s normal dampening signals are no longer reaching the spinal cord.
- Clonus: rhythmic, involuntary muscle contractions (around 5 to 7 beats per second), most easily seen at the ankle.
- Babinski sign: the big toe extends upward when the sole of the foot is stroked, instead of curling downward as it normally would in adults.
- Characteristic weakness pattern: weakness tends to affect arm extensors and leg flexors specifically, rather than all muscles equally.
Lower motor neuron damage, by contrast, causes floppy (flaccid) weakness, muscle wasting, and diminished reflexes. Recognizing this distinction is one of the most clinically useful applications of understanding descending tract anatomy.
The Hypothalamospinal Tract
There is one additional descending pathway worth knowing. The hypothalamospinal tract carries autonomic (involuntary nervous system) signals from the hypothalamus down through the spinal cord. It controls functions like sweating and pupil size. Damage to this tract in the cervical spinal cord can cause Horner syndrome, a combination of a constricted pupil, drooping eyelid, and decreased sweating on one side of the face. These fibers travel in the lateral part of the cervical cord, relatively close to the surface.