The complexity of the brain can be compelling, leaving one completely perplexed. However, it can also be obscure, making one's understanding of the brain a difficult task to accomplish. Nevertheless, there is no need to fret as we are here to help you begin your journey to learn more about the organ that controls every outcome in your daily life by starting with the medullary pyramid.
The medullary pyramid consists of grouped white matter formations at the intersection between the brain and the spinal cord. It contains two motor fibers (responsible for motor functions of the body): the corticospinal and corticobulbar tracts. These two components together are the pyramidal tracts.
We can analyze it further by starting at the outer circle of focus and slowly reel ourselves into the center. Following this method will properly analyze this section of your brain. Let us start with the structure known as the medullary pyramid.
Analyzing The Medullary Pyramid
You can find the medullary pyramid (pyramids medullae oblongata) at the lower side of the medulla oblongata. These strong structures traverse along the medulla oblongata while also being centrally beside the anterior median fissure.
The location of an anterolateral sulcus is along the medullary pyramid's side borders, where the hypoglossal nerve emerges. Additionally, there is a noticeable bulge known as an olive situated on the side of each pyramid. Lastly, situated at the back of the formation are fibers within the posterior column that convey sensory and proprioceptive signals.
Now, I know that was a lot of scientific terms and might have been confusing. So, to clear the field, let us look at some of the biological terms we used and deliver a clear definition for each of them.
The word anterolateral differentiates it from another sulcus. However, a sulcus is a minor tear in the surface of the limb or an organ. Regarding the brain specifically, sulci assist in separating various parts of the brain. However, a larger sulcus rather gets referred to as a fissure.
The hypoglossal nerve transfers brain signals to achieve motor control over the external muscles of the tongue. This nerve also processes movements like clearing saliva within the mouth and involuntary motions.
Like the medullary pyramid, olivary Bodies are structures containing the olivary nuclei. Within the olive are two types of nuclei: the inferior olivary nucleus and the superior olivary nucleus. Below is an explanation of both nuclei and their functions.
- Inferior olivary nucleus – in tandem with the olivo-cerebellar (neural fibers) system, it participates in cerebellar motor learning and function.
- The superior olivary nucleus – associated with the pons and the auditory system, assists in precepting sound.
Additionally, the inferior olive individually has three main nuclei. Below is an explanation for each of these nuclei.
- The primary olivary nucleus (PO) – involves the major laminar structure.
- The medial accessory olivary nucleus (MAO) – is located between the PO and the pyramid, forming a curved lamina (flat plates of bone of a vertebra).
- The dorsal accessory olivary nucleus (DAO) – is a small structure that appears as a curved lamina behind the PO.
Clinical Injuries Related To The Medullary Pyramid
It is crucial to avoid any potential injuries that may occur to the medullary pyramid. Should the pyramids be damaged, you could encounter symptoms such as weakness in all four primary limbs, difficulty speaking, and difficulty swallowing.
One way to injure the medullary pyramid is from whiplash resulting from a car accident. This event could bring about dislocation at the occiput (spinal level C1). Additionally, quick hyperextension of the neck region could cause damage to the pyramids via pull and tears in the tissue.
A bilateral infection can also develop in the medullary pyramids, which ends in motor quadriplegia (paralysis from the neck down). However, this illness is rare, with only four cases ever reported.
The Pyramidal Tracts: Corticospinal And Corticobulbar Tracts
The corticospinal and corticobulbar tract combination is known as the pyramidal tracts. It obtained this name because it passes through the medullary pyramid. In simplistic terms, the corticospinal tract controls voluntary muscle movements of the body. The corticobulbar tract involves the movement of head muscles, including facial maneuvering and the tongue.
The Corticospinal Tract
The corticospinal tract transfers signals from the brain to the spinal cords relating to voluntary movements. Most of the fibers associated with the corticospinal tract travel across the medulla oblongata. Hence the functions of your muscles get controlled by the opposite side of the brain. This formation means signals are sent to muscles by the side of the brain opposite the muscles.
The nerve fibers that form the corticospinal tract mostly come from pyramidal cells in layer V of the cerebral cortex (outer layer of neural tissue). The remaining nerve fibers are from the primary motor cortex (around 30%), parietal lobe, somatosensory cortex, supplementary motor area, and premotor cortex (around 30%), and the cingulate gyrus provides the rest.
Nerve axons (efferent nerve fibers of the upper motor neurons) traverse down the tract and then through the white substance of the spinal cord until they arrive at the vertebral level of the muscle. Then, the axons proceed to synapse with lower motor neurons.
However, only a few of the nerves synapse directly with the lower motor neurons. Instead, most nerves synapse with an interneuron and proceed to the lower motor neurons. This process generally happens in the anterior grey column (front column of grey matter in the spinal cord).
The Corticobulbar Tract
Fibers from the ventral motor cortex travel along the corticospinal tract. However, they cease in numeral places, one of these destinations being the corticobulbar tract (medulla oblongata). The upper motor neurons associated with the corticobulbar tract then synapse with interneurons or lower motor neurons.
Within the motor cranial nerve nuclei, the motor neurons get distributed to the oculomotor, trochlear, abducens, facial nerve, accessory nerves, and motors nucleus of the trigeminal nerve. Within the nucleus ambiguus, the motor neurons get sent to the hypoglossal, vagus, and accessory nerves.
Both tracts contain over 1 million axons, most of which go through a process known as myelination. In this process, the brain produces layers of myelin that surround the neuronal axons and form a layer of insulation to enable the transmission of electric impulses (action potentials) down the neuronal axon.
Clinical Injuries Related To The Pyramid Tracts
Upper motor neuron syndrome comes from damage to the fibers of the corticospinal tract, anywhere from the cerebral cortex to the lower end of the spinal cord. Injury to the upper motor neurons can produce symptoms including hyperactive reflexes, sensory problems, Babinski sign (an extensor plantar response), spasticity, and a loss of the ability to achieve fine movements.
Certain disorders like strokes, cerebral palsy, abscesses, tumors, and subdural hemorrhage could occur. Additionally, one could develop various neurodegenerative diseases, such as multiple system atrophy, trauma to the spinal cord (slipped discs), and inflammation like meningitis and multiple sclerosis.
If the corticobulbar tract gets damaged on one side, the only affected area is the lower face region. However, should damage strike both sides, including the left and right tracts, the result is generally pseudobulbar palsy which develops issues with swallowing, speaking, and emotional lability.
Damaging that causes severe disabling involuntary movements such as hemiballismus, or severe chorea could exhaust the patient and potentially lead to a life-threatening situation. However, this condition was treated by a partial section of the pyramidal tract either at the cerebral crus (pedunculotomy) or the primary motor cortex.
The Process Of Decussation
The corticospinal and corticobulbar fibers pass through three sections: the brain, the medulla oblongata, and the spinal cord. 90% of these fibers depart the pyramids in consecutive groups and decussate (cross over) in the anterior median fissure of the medulla oblongata, which results in pyramidal decussation or motor decussation.
These fibers then proceed traversing down till they reach the lateral funiculus. However, 10% of the remaining fibers stay within the anterior corticospinal tract. The pyramidal decussation indicates the border between the spinal cord and the medulla oblongata.
The Medulla Oblongata
Now that we have explored the medullary pyramid's deeper workings let us look at the section where it resides. This section of the brain is known as the medulla oblongata (medulla for short), a stem-like formation located at the lower part of the brainstem.
This cone-shaped neuronal mass performs involuntary functions like vomiting and sneezing. In addition, the medulla holds the cardiac, respiratory, vomiting, and vasomotor centers, which control autonomic functions like breathing, heart rate, blood pressure, and even your sleep-wake cycle.
The word bulbar gets used in modern clinical terminology when addressing terms relating to the medulla oblongata, particularly when referring to medical conditions. The word also refers to nerves and tracts linked to the medulla.
The medulla consists of two main parts: the upper open part (superior part) and the lower closed part (inferior part). The superior part is where the dorsal surface of the medulla gets created by the fourth ventricle. The inferior part is where the fourth ventricle has thinned at the obex in the caudal medulla, surrounding a part of the central canal.
Medulla Blood Supply
Multiple arteries provide blood to the medulla. Below is a list of all the arteries involved and where they supply blood.
- Anterior spinal artery – supplies the entire medial section of the medulla oblongata.
- Vertebral artery (direct branches) – supplies a certain area between the anterior spinal and posterior inferior cerebellar arteries.
- Posterior spinal artery – supplies the dorsal column of the closed part of the medulla, which holds fasciculus gracilis, gracile nucleus, fasciculus cuneatus, and cuneate nucleus.
- The posterior inferior cerebellar artery (direct branch of the vertebral artery) – supplies the posterolateral part of the medulla (main sensory tracts) and part of the cerebellum.
When the medulla finishes developing, at week 20 gestation, neuroblasts from the alar plate will produce sensory nuclei, and basal plate neuroblasts will produce motor nuclei. Below is a list providing information on the formations both plates produce.
- The alar plate neuroblasts form.
- Solitary nucleus - consists of general visceral afferent fibers for taste and the special visceral afferent column.
- Cochlear and vestibular nuclei – involves the special somatic afferent column.
- Spinal trigeminal nerve nuclei – incorporates the general somatic afferent column.
- Inferior olivary nucleus – transfers to the cerebellum.
- Dorsal column nuclei – consists of the gracile and cuneate nuclei.
- The basal plate neuroblasts form.
- Hypoglossal nucleus – involves general somatic efferent fibers.
- Nucleus ambiguus – creates the special visceral efferent.
- The dorsal nucleus (vagus nerve) and the inferior salivatory nucleus - both generate the general visceral efferent fibers.
Functions Of The Medulla
The medulla plays a big part as it forms the connection between the higher levels of the brain and the spinal cord. It is also responsible for various functions relating to the autonomous nervous system, which include:
- Control of ventilation – achieved through signals from the carotid and aortic bodies. Respiration gets controlled by bundles of chemoreceptors (sensors that detect changes in acidity in the blood).
- The cardiovascular center – contains the sympathetic (fight-or-flight) and parasympathetic (rest-and-digest) nervous systems.
- Vasomotor center – involves baroreceptors (sensors that monitor blood pressure).
- Reflexes – these reflexes can include vomiting, coughing, sneezing, and swallowing.
Clinal Injuries Related To The Medulla Oblongata
A blood vessel blockage, also known as a stroke, will injure the pyramidal tract, medial lemniscus, and hypoglossal nucleus. These injuries then develop, causing a syndrome known as a medial medullary syndrome. Below are the circumstances that would occur should one of these areas be severely damaged.
- Hypoglossal nerve fibers – ipsilateral muscle weakness
- Pyramidal tracts – limb weakness
- Medial lemniscus – loss of discriminative touch, vibration sense, and conscious proprioception
The obstruction of the posterior inferior cerebellar artery or the vertebral arteries can cause lateral medullary syndrome.
Progressive bulbar palsy (PBP) is a neurodegenerative disease that attacks the nerves connected to the bulbar muscles. This disease can also occur in children, and this illness is known as infantile progressive bulbar palsy.
You cannot deny the importance of the medullary pyramid, as it enables voluntary motor movements of both the body and the head. However, one should not neglect this section of the brain as the injury's consequences can be quite unpleasant, resulting in body disorders and certain neurodegenerative diseases.