Pyramidal system

Pyramidal system: Corticonuclear and Corticospinal tractsAnatomy and function of the pyramidal motor system
~ 5 min

In this note, we will continue discussing 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

Let’s examine in more detail the pyramidal tracts — the lateral corticospinal tract

Lateral corticospinal tract
Lateral corticospinal tract

and the anterior corticospinal tract.

Anterior corticospinal tract
Anterior corticospinal tract

Moreover, within this group, the corticonuclear (or corticobulbar) tract is distinguished separately.

All of these pathways send impulses to skeletal muscles, providing our voluntary movements.

These pathways have first- and second-order neurons.

The fibers originate from the upper motor neurons (or Betz cells) in the primary motor cortex of the precentral gyrus.

Precentral gyrus - lateral aspect
Precentral gyrus - lateral aspect
Precentral gyrus - superior aspect
Precentral gyrus - superior aspect

Like the postcentral gyrus, it is organized in a particular way of somatotopic arrangement. Its upper and medial parts are responsible for the movement of the lower limbs, and the lower and lateral parts, respectively, of the head and neck.

This particular pattern, this concept, is called Penfield’s motor homunculus.

Moreover, the proportions of the homunculus differ from those of a real person. These differences are due to the number of motor units controlling a specific area of the body. For example, the muscles of the face and hands are capable of performing more complex movements, so the area of the cortex responsible for them will be significantly wider.

Then the tracts pass within the corona radiata,

Corona radiata
Corona radiata

the genu of the internal capsule,

Genu of the internal capsule
Genu of the internal capsule

and the posterior limb of the internal capsule.

Posterior limb of the internal capsule
Posterior limb of the internal capsule

The fibers of the corticonuclear pathway decussate while passing within the brainstem. They innervate the motor nuclei of cranial nerves on the ipsilateral and contralateral side. Specifically, the motor nuclei of the trigeminal (V) nerve (muscles of mastication), facial (VII) nerve (muscles of facial expression), glossopharyngeal (IX), vagus (X) (muscles of the soft palate, pharynx, and larynx), accessory (XI) , and hypoglossal (XII) nerve (muscles of the tongue).

The nuclei of the oculomotor (III), trochlear (IV), and abducens (VI) nerves also receive innervation from the corticonuclear pathway, but indirectly via the reticular formation and the medial longitudinal fasciculus.

Medial longitudinal fasciculus
level of the midbrain
Medial longitudinal fasciculus<br />
level of the midbrain
Medial longitudinal fasciculus
level of the pons
Medial longitudinal fasciculus<br />
level of the pons
Medial longitudinal fasciculus
level of the medulla oblongata
Medial longitudinal fasciculus<br />
level of the medulla oblongata

It should also be noted that the facial nerve nucleus is innervated in a special manner. The upper part receives bilateral innervation, while the lower part is innervated only from the contralateral side.

The same goes for the hypoglossal nucleus. It is innervated only from the opposite side.

The corticonuclear tract terminates here.

Then only the corticospinal tract continues.

In the medulla oblongata, on the anterior surface, it forms enlargements known as pyramids.

Pyramid of medulla oblongata
Pyramid of medulla oblongata
Pyramid of medulla oblongata
Pyramid of medulla oblongata

Then 80% of the fibers of the corticospinal tract decussate, forming the pyramidal decussation or motor decussation.

Pyramidal decussation
Pyramidal decussation
Pyramidal decussation
Pyramidal decussation

These decussated fibers then continue as the lateral corticospinal tract, which travels in the lateral funiculus of the spinal cord.

Lateral funiculus
Lateral funiculus

It synapses to the lower motor neuron within the anterior horn of the spinal cord, at all of its levels.

Anterior horn
Anterior horn

The remaining 20% of fibers do not decussate in the medulla but continue on the ipsilateral side in the anterior funiculus of the spinal cord, forming the anterior corticospinal tract.

Anterior funiculus
Anterior funiculus

These fibers decussate via the anterior white commissure and synapse to a lower motor neuron in the anterior horn of the spinal cord, but only in the cervical and upper thoracic regions.

Anterior white commissure
Anterior white commissure
Golosary

Pyramidal system: Corticonuclear and Corticospinal tracts

lateral corticospinal tract
tractus corticospinalis lateralis
anterior corticospinal tract
tractus corticospinalis anterior
corticonuclear tract
tractus corticonuclearis
precentral gyrus
gyrus precentralis
corona radiata
corona radiata
genu of the internal capsule
genu capsulae internae
posterior limb of the internal capsule
crus posterius capsulae internae
pyramidal decussation / motor decussation
decussatio pyramidum
anterior horn of the spinal cord
cornu anterius
anterior funiculus of the spinal cord
funiculus anterior
anterior white commissure
commissura alba anterior
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