Decussation (Definition + Functions)

Decussation is very complex. According to the decussation, the side of the body opposite the lesion will be affected by a lesion disrupting the fibers above the crossing. On the other side of the body is the extremities’ limited voluntary movement if the cerebrum’s corticospinal tract is disrupted. The same side, or ipsilateral, is affected by voluntary movement when there is a lesion below the decussation.

Decussation is the intersection of the right and left corticospinal tracts. The uncrossed anterior corticospinal tract refers to the few, variable-number fibers that do not cross. A corticospinal tract’s lateral portion is its major portion. The lateral region of a corticospinal tract is the most important part.

Decussation is an important part of the human body and plays several roles. If you want to find out what decussation is and how it works, you’ve come to the right place. This article will discuss everything about this incredible system.

Decussation – Definition & Functions

The junction of the right and left corticospinal tracts are referred to as decussation. The term ” uncrossed anterior corticospinal tract ” refers to the small number of fibers that do not cross.

Due to the uppercase ‘X’ shape of the Roman numeral for ten, which in Latin is known as decussis and is derived from the words decem, which means “ten,” and as, which means “as,” decussation is used to depict a crossing in biological contexts. Decussation, as in decussatio pyramidum, is a Latin anatomical term.

Likewise, the anatomical term chiasma takes its name from the Greek letter X in capital letters (chi). A decussation, on the other hand, is a crossing of the central nervous system.

We will discuss the following in this article; below;

  1. Posterior Commissure Development & Sub-Commissural Organ
  2. Decussation Of The Auditory System
  3. Decussation Of Pyramids
  4. Sensory Decussation
  5. Sensory Levels Of Decussation In The Caudal Medulla
  6. Decussation Of Retroambiguus and Ambiguus Nuclei
  7. Why Does The Nervous System Decussate?

1.     Posterior Commissure Development & Sub-Commissural Organ

Numerous axonal guidance cues regulate the intricate process of commissure and decussation development in the CNS. Their creation is governed by a collection of glial and, to a smaller degree, neuronal cell types situated in the midline. One of them, the SCO, which is situated near the midline of the diencephalic roof, has been linked to the creation of the PC.

The SCO’s secretion of the SCO-spondin, a large glycoprotein member of the thrombospondin superfamily , is its most notable characteristic. In contrast to other midline glial cell types, SCO cells haven’t gotten much attention for their function in developing the PC.

The development of this commissure has received little attention, and only a few researchers have focused on the axonal guiding process during its formation, even though it is one of the first axonal scaffolds in the vertebrate’s embryonic brain.

2.     Decussation Of The Auditory System

Decussation of the human auditory system causes unexpected hearing loss following an ischemic stroke in the midbrain and upper brain stem.

Ischemic stroke above the low brain stem level is more likely to result in auditory-processing imbalances rather than peripheral-type hearing loss because of the numerous decussations of the auditory system and its distinctive escalating duplication from the periphery to the cortex, but there are exceptions.

The right lateral lemniscus and the inferior colliculus were both affected by a lacunar infarction of the superior cerebellar vascular territory in a case reported by Cerrato et al. (2005). Bilateral hearing loss was one of the predominant symptoms at presentation.

On audiometry, the left had a more severe hearing loss than the right, and the left had a longer ABR interwave III-V interval. Because acoustic fibers are primarily crossed at the level of the inferior colliculus, the authors hypothesized that this would explain hearing abnormalities on the side opposite the stroke.

In addition, they described two patients who had an upper brainstem infarction on magnetic resonance imaging (MR) and had ipsilateral hearing loss but no other signs, such as palinacusis or tinnitus. Both of these cases had a hearing loss similar to that of a cochlear implant, while the other likely had both.

3.     Decussation Of Pyramids

Pyramidal decussation, also known as decussation of the pyramids, is the crossing of corticospinal tract fibers from the one side of the central nervous system to the other between the intersection of the medulla and the spinal cord. Corticobulbar and corticospinal fibers, two types of motor fibers that leave the brain and travel to the medulla oblongata and medulla spinalis, are found in the two pyramids.

When these pyramidal fibers are followed downward, it is discovered that at least three-fifths of them depart the pyramids in successive bundles and decussate in the medulla oblongata’s anterior median fissure, creating what is known as the pyramidal decussation or motor decussation. After crossing the midline, they descend to the lateral cerebrospinal fasciculus in the back of the lateral funiculus.

4.     Sensory Decussation

The gracile nucleus  and cuneate nuclei , which control fine touch, vibration, proprioception, and two-point discrimination of the body, are responsible for the sensory decussation or decussation of the lemnisci. The internal arcuate fibers of this decussation are located in the superior aspect of the closed medulla, above the motor decussation.

It is a component of the second neuron inside the medial lemniscus-posterior column pathway. The gracile nucleus and cuneate nucleus are two sizable nuclei located in the posterior white column at the level of the closed medulla. The two ascending tracts, fasciculus gracilis and fasciculus cuneatus, send the impulse to the two nuclei.

The extensively myelinated fibers emerge and climb anteromedially around the periaqueductal gray after the two tracts meet at these nuclei as internal arcuate fibers. The sensory decussation is the process by which these fibers decussate (cross) to the contralateral (opposite) side.

The medial lemniscus is the ascending bundle that comes after the decussation. Medial lemniscus fibers do not produce collateral branches as they move down the brainstem, in contrast to other ascending tracts of the brain. Proprioception, two-point discrimination, and fine touch are all functions of the body’s sensory decussation, except for the head.

5.     Sensory Levels Of Decussation In The Caudal Medulla

The posterior column-medial lemniscus, a significant ascending sensory channel, crosses the midline at the location known as the sensory decussation. The gracile and cuneate nuclei have largely taken the place of the posterior columns (gracile and cuneate fasciculi) at this level of the medulla.

The gracile and cuneate nuclei are the sites where fibers carrying vibratory and tactile sensations from lower and upper levels of the body correspondingly end. The internal arcuate fibers are formed by the axons of these cells swinging anteromedially. These fibers then cross the centerline as the sensory decussation and gather on the contralateral side to create the medial lemniscus.

The anterior portion of the medial lemniscus transmits information from the lower extremities (gracile cell axons). In contrast, the posterior portion of the medial lemniscus transmits information from the upper limbs (cuneate cell axons). Next to the cuneate nucleus is the supplementary cuneate nucleus.

Its cells receive fundamental sensory information through cervical spinal nerves, which they then transmit as Cuneo cerebellar fibers to the cerebellum. In doing so, they serve as the posterior spinocerebellar tract’s upper extremity analog. The lateral medulla continues to house the spinal trigeminal tract and nucleus (pars caudalis).

The area of the spinal trigeminal nucleus caudal to the obex level is known as the pars caudalis. However, at this level, the origins of the restiform body can already be seen since posterior spinocerebellar fibers have moved posteriorly to envelop the spinal cord. A short column of motor neurons called the nucleus ambiguus can be seen immediately medial to the spinal trigeminal nucleus.

These SE cells’ axons run down the vagus (X) and glossopharyngeal (IX) nerves. The anterolateral medulla houses the rubrospinal tract and anterolateral system fibers. The anterolateral system’s neighbor, the lateral reticular nucleus, is a separate cell group that takes input from the spinal cord and sends it to the cerebellum.

6.     Decussation Of Retroambiguus and Ambiguus Nuclei

The RAmb begins as a scattering of AChE-positive cells embedded in the region of the IRt that is divided from the rest by the decussating corticospinal fibers below the pyramidal decussation. It goes up to the level of the LRt, after which the Amb takes over.

Cells with a diffuse spindle form are characteristic of RAmb. In contrast, Amb possesses massive multipolar neurons with big Nissl granules and intense AChE staining. Only a small number of cells at area postrema levels correspond to the Amb.

It widens ventrolaterally to match the arcuate shape of the IRt at the level of the caudal pole of the dorsal accessory olive. It turns into a rounded cluster just below the rostral pole of the hypoglossal nucleus. It reaches its largest size close to the glossopharyngeal nerve’s roots.

The surrounding cell-poor region is engulfed at this threshold by the Amb’s associated AChE reactivity. RAmb and Amb are not infiltrated by catecholamine- or NPY-containing processes, in contrast to other regions of the IRt, and neither do they contain catecholamine or NPY cells.

On the other hand, the human embryo Amb has large levels of somatostatin receptors, and the human adult Amb has serotonin immunoreactivity.

7.     Why Does The Nervous System Decussate?

Recently, two studies independently put forth similar theories to account for the wide decussations that all vertebrates’ neurological systems have between their forebrains and the remainder of their nervous systems as an axial twist.

Both theories contend that this twist is a “spandrel,” a natural result of a different evolutionary event. Neuroscientists and evolutionary biologists vigorously disagree on this. The subject of numerous theories is why tracts intersect in the human neurological system. However, one view focuses on the lineage of vertebrates and how they have evolved.

The “somatic twist” concept claims that the transition from having a ventral (belly-side) to a dorsal (back-side) nerve cord, which is referred to as a “decussation” in scientific terms, is what led to neural crosses. 96% of animal species currently recognized are invertebrates, meaning bone vertebrae do not cover their spinal cords.

This group includes ants, crabs, squid, jellyfish, butterflies, worms, sponges, snails, and scallops. Intriguingly, none of these species display the pattern of crossing you indicate. Only vertebrates are capable of decussations. Some invertebrates, like sponges, lack any discernible neural system.

Invertebrates with radial body designs, such as jellyfish, are a little more developed and have a widespread “nerve net” but no decussating central nervous system. However, bilateral invertebrates typically have a long nerve cord (or cords) that extends the entire length of the animal and a significant concentration of neurons close to the front of the body (the brain).

Similar to how our brain is located, the primitive brain of bilateral invertebrates is located above (dorsal to) their mouths. But after that, something quite new takes place. The spinal cord travels along the back of the body in humans and other vertebrates (including sharks, frogs, crocodiles, owls, and kangaroos).

Bilateral invertebrates have a central nervous system (CNS) that originates in the brain and travels via the digestive system to the underside (belly) of the animal. The animal’s belly is lined with the invertebrate spinal cord. The kidneys (nephridia) are located directly above the nerve cord in this body layout.

The digestive system and the primary circulatory pumping organs—the invertebrate equivalent of the heart—are located above the kidneys.

If you are interested in anatomy, you might have observed that this structure is completely different from the one found in vertebrates. We have a dorsal nerve cord, the kidneys are ventral to that, the digestive tract is below that, and the heart is below the digestive tract.

The ”  somatic twist ” refers to this significant reorganization of the overall body design. According to the theory, the complete body plan turned 180 degrees concerning the brain at some point during evolution, perhaps close to when the first vertebrates appeared. Researchers studying evolution have sought hints about this shift in very old vertebrates.

Many of the signaling molecules that define the dorsal-to-ventral development of the nervous system in both vertebrates and invertebrates are conserved. For instance, the nerve cord consistently resides on the side of the body where chordin protein is abundant during  embryonic development. Even our nearest invertebrate ancestors (the acorn worm) have their mouths on this side of the body (ventral). However, this configuration is upside-down in even the most basic vertebrates, with all the organ systems turning 180 degrees.

Theodore T.

Theodore is a professional psychology educator with over 10 years of experience creating educational content on the internet. PracticalPsychology started as a helpful collection of psychological articles to help other students, which has expanded to a Youtube channel with over 2,000,000 subscribers and an online website with 500+ posts.