Medullary Pyramids

Overview:

The human brain is a very complex, yet amazingly structured organ that is in charge of a multitude of human body functions. Different parts of the brain are responsible for various tasks that need to be performed correctly to ensure the working and functioning of a healthy body.

The medullary pyramids are two white matter formations in the medulla oblongata of the brainstem that carry motor fibres from the corticospinal and corticobulbar tracts, which are commonly understood as the pyramidal tracts. When the fibres cross, the pyramids’ lowest limit is marked (decussate).

In this article, the role of the medullary pyramids and their structural significance concerning motor functions of the brain will be discussed and explained. A few complications that arise from injury to the spinal cord or brainstem will also be mentioned.

Summary:

  • The medullary pyramids are located in the ventral region of the medulla oblongata. The anterior median fissure separates these two crest-shaped structures that run along the length of the medulla oblongata.
  • The corticobulbar and corticospinal pathways are motor fibres found in the medullary pyramids.
  • The corticospinal tracts are located in the pyramids’ area.
  • Motor impulses start in the precentral gyrus and go via the internal capsule to the medulla oblongata and pyramids via these tracts.
  • Extrapyramidal tracts are motor tracts that do not traverse the thalamus.
  • In between the medullary pyramid and the inferior cerebellar peduncle, the vagus nerve leaves the medulla oblongata.
  • The vascular supply to the vagus and glossopharyngeal nerves is mainly repetitive due to their physical proximity. The artery for the medulla oblongata’s lateral fossula gives birth to arterioles that supply the vagus nerve roots.
  • The motor fibres that travel from the brain to the medulla oblongata and spinal cord are housed in the two pyramids. The pyramidal tracts are made up of corticobulbar and corticospinal fibres.
  • Approximately ninety per cent of these fibres exit the pyramids in consecutive packages and cross over or ‘decussate’ in the medulla oblongata’s anterior median fissure as pyramidal decussation or motor decussation.
  • The corticobulbar tracts emerge from the internal capsule as many bundles and join the basilar region of the pons
  • The Corticospinal tract (CST) controls afferent signals, spinal responses, and nerve cell activities, the most significant of which is the regulation of intentional distal motions.
  • When the top nerve cells of the corticospinal tract are injured, it can result in a group of impairments known as upper motor nerve cell syndrome.
  • Neck hyperextension can pull and rupture the pyramids, causing symptoms such as paralysis in all four limbs, problem swallowing food or water, and speech problems. Motor quadriplegia can develop from a bilateral infarction in the medulla pyramids.

Location:

The medullary pyramids are two white matter formations in the medulla oblongata of the brainstem that include motor fibres from the corticobulbar tract corticospinal tract, which are known collectively as the pyramidal tracts. When the fibres intersect, the pyramids’ lowest limit is defined (decussate).

Structure:

The medullary pyramids are located in the ventral region of the medulla oblongata. The anterior median fissure separates these two crest-shaped structures that run along the length of the medulla oblongata (Figure 1). Each has an anterolateral sulcus along its lateral wall, from which the hypoglossal nerve exits. 

Also, there is a noticeable protrusion termed as olive on the side of every pyramid. The behind of pyramids on the medulla oblongata is present the fibres of the posterior column that carry sensory and proprioceptive input.

Function:

The corticobulbar and corticospinal pathways are motor fibres found in the medullary pyramids. The corticospinal tracts are located in the pyramids’ area. Motor impulses start in the precentral gyrus and go via the internal capsule to the medulla oblongata and pyramids via these tracts. Extrapyramidal tracts are motor tracts that do not traverse the thalamus.

Extrapyramidal tracts are motor tracts that do not pass via the medullary pyramids. Corticospinal axons decussate (or intersect over) the central line and proceed down the spinal cord on the contralateral area at the pyramids’ most caudal end. (Pyramid – Pyramis medullae oblongatae; Pyramis bulbi)

Fibres that decussate migrate down the lateral corticospinal tract, whereas fibres (that do not decussate) move down the anterior corticospinal tract. Almost ninety per cent of the fibres decussate and move down the lateral corticospinal tract, with the remaining ten per cent travelling down the anterior corticospinal tract.

The motor fibres that travel from the brain to the medulla oblongata and spinal cord are housed in the two pyramids. The pyramidal tracts are made up of corticobulbar and corticospinal fibres. Approximately ninety per cent of these fibres exit the pyramids in consecutive packages and cross over or ‘decussate’ in the medulla oblongata’s anterior median fissure as pyramidal decussation or motor decussation.

After crossing the middle line, the lateral corticospinal tract descends into the lateral funiculus as the lateral corticospinal tract. The remaining ten per cent of fibres in the anterior corticospinal tract remain uncrossed. The pyramidal decussation separates the spinal cord from the medulla oblongata.

The motor fibres of the pyramidal tracts – the corticospinal and corticobulbar tracts – are found in the medullary pyramids.

The corticospinal tract:

The Corticospinal Tract (CST), also called the Pyramidal Tract, is a network of axons that connects the cerebral cortex to the spinal cord. It is a component of the descending spinal tract system, which stems from the cortex or brainstem.

Upper nerve cells are the nerve cells that move along the corticospinal tract; they synapse on lower motor nerve cells in the spinal cord, which communicate directly with skeletal muscle to generate muscular contraction.

The corticospinal tract (CST) is one of the principal channels for transmitting movement-related data from the brain to the spinal cord, and it contains around one million nerve fibres (mean transmission velocity of roughly 60m/s using glutamate as their transmission material).

Signalling all along the corticospinal tract is associated with a variety of actions, including strolling and grasping, but it is particularly significant for fine finger gestures, such as typing, writing, or knitting garments.

In human beings, it shows the actual highest order of motor function and is most effective in the control of precise, digital motions.

The corticospinal tract (CST) originates in a variety of cortical areas; around half of these axons stretch from nerve cells in the main motor cortex, but many others begin in nonprimary motor sections of the brain and also parietal lobe areas such as the somatosensory cortex.

The corticospinal tract (CST) axons drop into the brainstem as a component of enormous fibre bundles known as cerebral peduncles.

The tract goes further down into the medulla, where it produces two huge clusters of axons called the pyramids, which create noticeable bumps on the brainstem’s external surface.

About ninety per cent of the fibres in the corticospinal tract decussate, or crossover over to the opposite side of the brainstem, near the foot of the pyramids in a tangle of axons known as the pyramidal decussation.

The decussated fibres form the lateral corticospinal tract; these would join the spinal cord and induce motion on the part of the body that is contralateral to the hemisphere from which they arose.  

The remaining ten per cent of corticospinal tract fibres do not decussate; instead, they keep going down into the ipsilateral spinal cord; this portion of the corticospinal tract is termed as the anterior (or ventral) corticospinal tract. The majority of the axons of the anterior corticospinal tract decussate in the spinal cord right before synapsing with lower motor nerve cells.

The Corticospinal tract (CST) controls afferent signals, spinal responses, and nerve cell activities, the most significant of which is the regulation of intentional distal motions.

Primary motor cortex (M1) projections serve to the Corticospinal tract (CST) via links to excitatory monosynaptic alpha nerve cells; polysynaptic connections onto gamma nerve cells (involved in muscle spindle length regulation); and polysynaptic linkages through interneurons inside the spinal cord.

When the top nerve cells of the corticospinal tract are injured, it can result in a group of impairments known as upper motor nerve cell syndrome.

A lesion of the corticospinal tract (CST) cranial to the pyramid decussation results in impairments on the contralateral side. (Corticospinal Tract)

A lesion of the corticospinal tract (CST) caudal to the pyramids’ decussation results in impairments on the ipsilateral side.

After spinal cord damage, both voluntary (sensory-motor) and unintentional control can be compromised, with the degree of restoration varying according to the seriousness of the lesion. Motor deficits will be ipsilateral to the injury site since the corticospinal tract (CST) has already decussated.

The corticobulbar tract:

The corticobulbar tract is a downward channel that runs parallel to the corticospinal tract and innervates multiple cranial nerves (Figure 2).

The corticobulbar tracts emerge from the internal capsule as many bundles and join the basilar region of the pons. The fibres exit the cerebral crus near the corticospinal tract. The fibres can go in a variety of directions and terminate in a variety of ways:

  • End on alpha nerve cells or interneurons that innervate alpha nerve cells in the brainstem. These regulate somatic motor activity in the brain, such as the muscles that are involved in mastication, expressiveness, and movement of the eyes. 
  • Axons that innervate motor nerve cranial nuclei can decussate (cross) before terminating, causing them to innervate contralateral muscles. Because some decussate and several descend ipsilaterally, bilateral descending control is achieved.
  • Innervate cranial nerves directly or through interneurons, — in other words through the corticospinal tract

The cranial nerves innervate the facial muscles, tongue, muscles of the jaw, and muscles of the pharynx.

– The corticobulbar tract innervates the nuclei of the cranial nerves directly:

Cranial nerves motor areas of X in the nucleus ambiguus

VII-  Facial muscles 

XII- Hypoglossal- tongue muscles – 

V – Trigeminal masticatory muscles

Because the lower motor nerve cells of the brain stem receive bilateral corticobulbar innervation, unilateral corticobulbar lesion normally has no therapeutic significance on the neck and head muscles. These rules, however, have two exceptions:

  • The lower face muscles get contralateral information from the opposite motor cortex via the facial nucleus (VII). As a result, contralateral injuries to the motor cortex/internal capsule cause weakness in the face muscles on the opposite side of the face. They will, nevertheless, be capable of wrinkling their brow because the corticobulbar tract innervates it bilaterally.
  • The genioglossus muscle (a muscle involved in pushing out the tongue) obtains innervation from the contralateral motor cortex via the hypoglossal nucleus (XII). As a consequence, a lesion affecting the right motor cortex/internal capsule would cause weakness in the left hypoglossal muscle. As a result of the weakness on the left part of the tongue, the tongue is pushed forward by the powerful muscle on the right side.

Since corticospinal fibres produce the medullary pyramids in mammals, it has been referred to as the ‘pyramidal tract.’ The use of such a phrase may generate some misunderstanding because the cells of derivation are, by chance, pyramidal nerve cells in the cerebral cortex. Furthermore, it appears that corticospinal fibres in monotreme mammals do not form a paramedian pyramid as they do in the rest of them.

In between the medullary pyramid and the inferior cerebellar peduncle, the vagus nerve leaves the medulla oblongata. The vascular supply to the vagus and glossopharyngeal nerves is mainly repetitive due to their physical proximity. The artery for the medulla oblongata’s lateral fossula gives birth to arterioles that supply the vagus nerve roots.

The jugular branch of the ascending pharyngeal artery provides arterial supply at the region of the jugular ganglion, and this is where the nerve leaves the skull base via the pars venosa of the jugular foramen. 

Complications involving Medullary Pyramids:

Trauma caused by an automobile collision can lead to brainstem damage affecting the pyramids of the medulla oblongata. These pyramid wounds are typically caused by an occiput or spinal level C1 displacement. Sudden hyperextension of the neck can potentially produce damage to the medulla oblongata pyramids (cervical region of the spine). 

Neck hyperextension can pull and rupture the pyramids, causing symptoms such as paralysis in all four limbs, problem swallowing food or water, and speech problems. Motor quadriplegia can develop from a bilateral infarction in the medulla pyramids.

Motor quadriplegia:

Quadriparesis is a sickness in which all four extremities are feeble (both legs and arms). Tetraparesis is another name for it. The weakening could be either transient or lifelong.

Quadriparesis is not the same as quadriplegia. A person with quadriparesis can still move and sense their extremities. A person with quadriplegia has fully lost the capacity to operate their extremities.

Quadriplegia impacts the human body from the neck down, reducing a person’s freedom severely. (Eske, 2020)

A person with tetraplegia may have the following symptoms, based on the intensity of their paralysis:

  • Arm and hand function issues are restricted or absent
  • Arm and hand function issues are restricted or absent talking, eating, or trying to breathe on their own is difficult
  • difficulties with daily duties such as washing, bathing, and dining
  • Having difficulties getting out of bed or a wheelchair without help or the usage of a gadget
  • Pressure ulcers are also referred to as pressure sores or bedsores.
  • loss of bone and muscle

Although paralysis is not treatable, some patients can recover some or whole control of damaged parts with time.

Presently available therapies include:

  • decrease the long-term consequences of central nervous system impairment
  • slow the course of any illness
  • avoidance and management of extra difficulties
  • enhance a patient’s quality of life

Bilateral infarction in the medulla pyramids:

Medial medullary infarcts result in less than one per cent of vertebrobasilar strokes and are seldom bilateral.   Hemiparesis sparing the face, dysarthria, and hypoglossal nerve palsy is frequently present.

Conclusion:

The medullary pyramids are located in the ventral region of the medulla oblongata. The anterior median fissure separates these two crest-shaped structures that run along the length of the medulla oblongata. The corticobulbar and corticospinal pathways are motor fibres found in the medullary pyramids. The corticospinal tracts are located in the pyramids’ area. Motor impulses start in the precentral gyrus and go via the internal capsule to the medulla oblongata and pyramids via these tracts. In between the medullary pyramid and the inferior cerebellar peduncle, the vagus nerve leaves the medulla oblongata. The vascular supply to the vagus and glossopharyngeal nerves is mainly repetitive due to their physical proximity. The artery for the medulla oblongata’s lateral fossula gives birth to arterioles that supply the vagus nerve roots.

Approximately ninety per cent of these fibres exit the pyramids in consecutive packages and cross over or ‘decussate’ in the medulla oblongata’s anterior median fissure as pyramidal decussation. Neck hyperextension can pull and rupture the pyramids, causing symptoms such as paralysis in all four limbs, problem swallowing food or water, and speech problems. Motor quadriplegia can develop from a bilateral infarction in the medulla pyramids.

References:

  • Corticospinal Tract. (n.d.). Retrieved from physiopedia: https://www.physio-pedia.com/Corticospinal_Tract
  • Eske, J. (2020, March 25). What are the differences between paraplegia and quadriplegia? Retrieved from MedicalNewsToday: https://www.medicalnewstoday.com/articles/paraplegia-vs-quadriplegia
  • Pyramid – Pyramis medullae oblongatae; Pyramis bulbi. (n.d.). Retrieved from IMAIOS: https://www.imaios.com/en/e-Anatomy/Anatomical-Parts/Pyramid