por What is the pyramidal pathway?
The pyramidal way, or pyramidal tract, are a group of nerve fibers that originate in the cerebral cortex and end in the spinal cord. They direct the voluntary control of the musculature of the whole body.
This pathway includes two tracts: the corticospinal and the corticobulbar. The first ends in the brain stem, and the second in the spinal cord.
The pyramidal pathway is a descending pathway, that is, it sends impulses from the brain to the motor neurons of the body. The latter directly innervate the muscles so that we can move them.
It differs from the extrapyramidal pathway in that it directs involuntary and automatic muscle control, such as coordination, balance, muscle tone, posture, etc.
There are no synapses (neuronal connections) within the pyramidal pathway. The cell bodies are in the cerebral cortex, or else in the brain stem.
The neurons in this pathway are called upper motor neurons, since once completed, they connect to lower motor neurons that directly control the muscles.
The pyramidal pathway is so named because its fibers pass through the pyramids of the medulla oblongata. In this area, the fibers converge in many directions, taking the appearance of an inverted pyramid.
tracts of the pyramidal pathway
The pyramidal pathway can be functionally subdivided into two parts: the corticobulbar tract and the corticospinal tract. Next, we explain what each of them consists of.
corticobulbar tract
This tract directs the muscles of the head and neck. Thanks to this structure, we can control facial expression, chew, produce sounds and swallow.
It arises on the lateral side of the primary motor cortex. The fibers then converge on the internal capsule of the brainstem.
From there, they travel to the motor nuclei of the cranial nerves. In these nerves they connect with the lower motor neurons to innervate the muscles of the face and neck.
Typically, fibers from the left primary motor cortex control neurons bilaterally. That is, they direct the right and left trochlear nerves. However, there are exceptions.
An example is the motor neurons of the hypoglossal cranial nerve, which are innervated contralaterally (on the opposite side).
corticospinal tract
The corticospinal tract controls voluntary movement of the body. It begins in the cerebral cortex, specifically, from the pyramidal cells of layer V.
The fibers arise from several structures: the primary motor cortex, the premotor cortex, and the supplementary motor area. It also receives nerve impulses from the somatosensory area, the parietal lobe, and the cingulate gyrus, although to a lesser extent.
Nerve fibers converge on the internal capsule, which is located between the thalamus and the basal ganglia.
From there, they pass through the cerebral peduncle, pons, and medulla oblongata. In the lower part of the medulla, the corticospinal tract divides into two: the lateral and anterior corticospinal tracts.
Fibers from the former cross to the other side of the central nervous system and descend to the ventral horn of the spinal cord. Once there, they connect to the lower motor neurons that direct the muscles directly.
On the other hand, the anterior corticospinal tract is ipsilateral. That is, the right side activates the right part of the body (as with the left). It travels down the spinal cord, ending in the ventral horn of the cervical and thoracic segments. In that place, it connects with the lower motor neurons present there.
The corticospinal tract contains a special type of cells that do not exist in any other part of the body. They are called Betz cells, and they are the largest pyramidal cells in the entire cortex.
From them arise large-diameter axons, which mainly control the legs. Its characteristics allow nerve impulses to travel very quickly.
This tract contains more than a million axons, most of which are covered with myelin.
Development of the pyramidal pathway
When we are born, the pyramidal pathway is not fully myelinated. Little by little it is myelinated from below (trunk or medulla) upwards (cortex). As it is covered with myelin, each time we make more refined and precise movements.
This pathway completes myelination at 2 years of age, although it continues to progress gradually in the opposite direction until 12 years of age.
Structure of the pyramidal pathway
The pyramidal pathway is made up of upper motor neurons that originate in the cerebral cortex and terminate in the brainstem (corticobulbar tract) or spinal cord (corticospinal tract). The pathway itself is made up primarily of axons.
The axons that run through the tracts are called efferent nerve fibers, because they send information from the cerebral cortex to the muscles (if they received the information instead of sending it, they would be called afferent).
They can cross in the medulla oblongata and travel through the spinal cord. There, they usually connect with interneurons in the middle area of the spinal cord, called the gray matter.
Interneurons are normally small and have a short axon. They serve to connect two different neurons. They usually link sensory and motor neurons.
These interneurons connect to lower motor neurons, which control muscles. Although, in some cases, the axons travel through the white matter of the spinal cord until they reach the vertebral level of the muscle they are going to direct.
Once there, the axons connect to the lower motor neurons.
pyramidal pathway lesions
The pyramidal pathway can be damaged as it extends through almost the entire central nervous system. An especially vulnerable area is the internal capsule. It is common for strokes to occur in this area.
Damage to the pyramidal pathway can be due to both strokes and hemorrhages, abscesses, tumors, inflammation, multiple sclerosis… As well as spinal cord trauma or herniated discs.
Lesions can give different symptoms if they affect the corticospinal or corticobulbar tract.
Damage to the corticospinal tract produces upper motor neuron syndrome. If only one side of the corticospinal tract is damaged, symptoms will be seen on the side of the body away from the injury. Some of them are:
– Increased muscle tone (hypertonia).
– Muscular weakness.
– Increased muscle reflexes (hyperreflexia).
– Babinski sign.
– Clonus, which refers to rhythmic and involuntary muscle contractions.
– Problems making fine movements.
In contrast, a lesion in the corticobulbar tract, if unilateral, would produce mild muscle weakness in the face or neck. Although this changes depending on the nerves affected:
– Hypoglossal nerve: is responsible for directing the movements of the tongue. If damaged, a spastic paralysis would occur to one side of it, causing it to drift to one side.
– Facial nerve: its lesion would give rise to spastic paralysis of the muscles of the lower quadrant of the face, on the opposite side of the lesion.
If the lesion of the corticobulbar tract is complete, pseudobulbar palsy may occur. It consists of difficulties in pronouncing, chewing and swallowing, in addition to sudden mood swings.
References
Pyramidal pathway (nd). Retrieved from quizlet.com.
Pyramidal tracts (nd). Retrieved from sciencedirect.com.