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

and the 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.

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,

the genu of the internal capsule,

and the 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.

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.

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

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

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

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.

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.

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