Extrapyramidal system

Extrapyramidal systemAnatomy and function of the rubro-, tecto-, vestibulo-, and reticulospinal tracts
~ 7 min

In this note, we will continue examining the anatomy and function of the tracts of the brain and spinal cord.

Let’s move one to the descending tracts.

They carry motor (efferent) information from the structures of the central nervous system to effector organs. In the somatic division of the nervous system, which we are studying right now, effectors are represented by the skeletal muscles.

The descending tracts are functionally divided into two major groups:

  • Pyramidal (voluntary) tracts
  • Extrapyramidal (involuntary) tracts

The second group includes the basal ganglia, which we have discussed in detail in the corresponding topic from the “Central Nervous System” section,

as well as several pathways:

  • Rubrospinal tract
  • Tectospinal tract
  • Vestibulospinal tracts (lateral and medial)
  • Reticulospinal tracts (from the pons and medulla oblongata)

The activity of these pathways provides the involuntary control of skeletal muscles. Mostly this involves the muscles of the neck, trunk, and proximal muscles of the limbs.

Rubrospinal tract

The rubrospinal tract is the only pathway of the extrapyramidal system involved in the regulation of movements of the distal musculature. Specifically, it activates the flexor muscles and inhibits the extensors of the upper limbs.

Rubrospinal tract
Rubrospinal tract
Rubrospinal tract - cross-section
Rubrospinal tract - cross-section

The first-order neurons are located in the red nuclei within the tegmentum of the midbrain.

Red nucleus
Red nucleus

These nuclei receive proprioceptive information from the cerebellum.

Almost immediately after emerging from the nuclei, the fibers decussate, forming the anterior tegmental decussation.

The tract then continues within the lateral funiculus,

Lateral funiculus
Lateral funiculus

eventually synapsing to the second-order neuron in the anterior horn of the cervical segments of the spinal cord.

Anterior horn
Anterior horn

Tectospinal tract

Tectospinal tract
Tectospinal tract
Tectospinal tract
Tectospinal tract

The tectospinal tract originates from neurons of the superior colliculi.

Superior colliculi
Superior colliculi
Superior colliculi - cross-section
Superior colliculi - cross-section

The fibers of this pathway decussate, forming the posterior tegmental decussation.

Then the tract travels within the anterior funiculus of the spinal cord, eventually synapsing to the second-order neuron in the anterior horn of the cervical segments.

Due to its connections with the superior colliculus, the tectospinal tract provides a motor response to sudden visual stimuli.

Vestibulospinal tracts

The vestibulospinal tracts include the lateral and medial pathways. The vestibular nuclei are located in the lateral part of the rhomboid fossa. These nuclei receive sensory information from the cerebellum and vestibular receptors of the inner ear.

Vestibular nuclei
Vestibular nuclei

As a result, this complex activates the antigravity muscles, in particular the muscles of the vertebral column and the extensors of the limbs.

The lateral vestibulospinal tract originates from the lateral vestibular nucleus.

The medial vestibulospinal tract primarily originates from the medial vestibular nucleus.

The fibers of both tracts travel within the anterior funiculus of the spinal cord, and in the anterior horn, they synapse to the second-order neuron.

The medial tract affects the muscles of the head and neck, while the lateral tract activates the axial musculature and the muscles of the pelvic and shoulder girdle.

Reticulospinal tract

The reticulospinal tracts include the pontine and medullary pathways.

Reticulospinal tracts
Reticulospinal tracts

These pathways originate from the reticular formation of the brainstem and descend into the spinal cord.

Reticulospinal tracts - level of the pons
Reticulospinal tracts - level of the pons
Reticulospinal tracts - level of the medulla oblongata
Reticulospinal tracts - level of the medulla oblongata

The pontine (medial) pathway emerges from the neurons of the reticular formation of the pons, while the lateral (medullary pathway) originates from the reticular formation of the medulla oblongata (in particular the gigantocellular nucleus).

The fibers of the medial pathways travel within the anterior funiculus, and the fibers of the lateral pathways within the lateral funiculus of the spinal cord, and synapse to the second-order neuron in the anterior horn.

The pontine pathway activates the antigravity muscles, activating the muscles of the vertebral column and the limb extensors, while the medullary pathway activates the flexor muscles. This helps us to maintain posture.

Golosary

Extrapyramidal system

basal nuclei
nuclei basales
rubrospinal tract
tractus rubrospinalis
tectospinal tract
tractus tectospinalis
red nucleus
nucleus ruber
anterior tegmental decussation
decussatio tegmentalis anterior
lateral funiculus
funiculus lateralis
superior colliculi
colliculi superiores
posterior tegmental decussation
decussatio tegmentalis posterior
anterior funiculus
funiculus anterior
vestibular nuclei
nuclei vestibulares
lateral vestibulospinal tract
tractus vestibulospinalis lateralis
medial vestibulospinal tract
tractus vestibulospinalis medialis
lateral vestibular nucleus
nucleus vestibularis lateralis
medial vestibular nucleus
nucleus vestibularis medialis
pontine reticular formation
formatio reticularis pontis
medullary reticular formation
formatio reticularis medullae oblongatae
pontine reticulospinal tract
tractus pontoreticulospinalis
medullary reticulospinal tract
tractus bulboreticulospinalis
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