The corticobulbar tract conducts impulses from the brain to the cranial nerves.[1] These nerves control the muscles of the face and neck and are involved in facial expression, mastication, swallowing, and other motor functions.
The corticospinal tract conducts impulses from the brain to the spinal cord. It is made up of a lateral and anterior tract. The corticospinal tract is involved in voluntary movement. The majority of fibres of the corticospinal tract cross over in the medulla oblongata, resulting in muscles being controlled by the opposite side of the brain. The corticospinal tract contains the axons of the pyramidal cells, the largest of which are the Betz cells, located in the primary motor cortex.
The pyramidal tracts are named because they pass through the pyramids of the medulla oblongata. The corticospinal fibers converge to a point when descending from the internal capsule to the brain stem from multiple directions, giving the impression of an inverted pyramid. Involvement of the pyramidal tract at any level leads to pyramidal signs.
The myelination of the pyramidal fibres is incomplete at birth and gradually progresses in cranio-caudal direction and thereby progressively gaining functionality. Most of the myelination is complete by two years of age and thereafter it progresses very slowly in cranio-caudal direction up to twelve years of age.
The nerve axons traveling down the tract are the efferent nerve fibers of the upper motor neurons. These axons travel down the tracts in the white matter of the spinal cord until they reach the vertebral level of the muscle that they will innervate.[5] At this point, the axons synapse with lower motor neurons. The majority of axons do not directly synapse with lower motor neurons, but instead synapse with an interneuron that then synapses with a lower motor neuron. This generally occurs in the anterior grey column.[2] Nerve axons of the lateral corticospinal tract that did not cross over in the medulla oblongata do so at the level of the spinal cord they terminate in.[6]
These tracts contain more than 1 million axons and the majority of the axons are myelinated. The corticospinal tracts myelinate largely during the first and second years after birth. The majority of nerve axons are small (<4μm) in diameter. About 3% of nerve axons have a much larger diameter (16μm) and arise from Betz cells, mostly in the leg area of the primary motor cortex. These cells are notable because of their rapid conduction rate, over 70m/sec, the fastest conduction of any signals from the brain to the spinal cord.[2]
Horizontal section through the lower part of the pons, showing the fibers of the corticospinal tract (#19) passing through the pontine nuclei
The nerves within the corticospinal tract are involved in movement of muscles of the body. Because of the crossing-over of fibres, muscles are supplied by the side of the brain opposite to that of the muscle.[2] The nerves within the corticobulbar tract are involved in movement in muscles of the head. They are involved in swallowing, phonation, and movements of the tongue.[6] By virtue of involvement with the facial nerve, the corticobulbar tract is also responsible for transmitting facial expression.[5] With the exception of lower muscles of facial expression, all functions of the corticobulbar tract involve inputs from both sides of the brain.[5]
The extrapyramidal system refers to tracts within the spinal cord involved in involuntary movement but not part of the pyramidal tracts.[2] Their functions include the control of posture and muscle tone.[citation needed]
Damage to the fibres of the corticospinal tracts, anywhere along their course from the cerebral cortex to the lower end of the spinal cord, can cause an upper motor neuron syndrome.
A few days after the injury to the upper motor neurons, a pattern of motor signs and symptoms appears, including spasticity, hyperactive reflexes, a loss of the ability to perform fine movements, and an extensorplantar response known as the Babinski sign.[7]
Symptoms generally occur alongside other sensory problems.
If the corticobulbar tract is damaged on only one side, then only the lower face will be affected, however if there is involvement of both the left and right tracts, then the result is pseudobulbar palsy. This causes problems with swallowing, speaking, and emotional lability.[4]
Severe disabling involuntary movements such as hemiballismus or severe chorea might exhaust the patient and become a life-threatening situation.
In the past, this condition was treated by partial section of the pyramidal tract either at the primary motor cortex or at the cerebral crus (pedunculotomy).[11]
Additional images
Dissection of brain-stem. Lateral view.
Superficial dissection of brain-stem. Ventral view.
The motor tract.
Diagram of the principal fasciculi of the spinal cord, from Gray's anatomy
In popular culture
In National Lampoon's European Vacation, the Griswold family wins a vacation on a game show called Pig in a Poke when their opponents fail to correctly answer a question about the pyramidal tracts, despite Clark Griswold (played by Chevy Chase) mistakenly answering that they are a housing development outside Cairo.[12]
References
↑Chapter 9 of "Principles of Physiology" (3rd edition) by Robert M. Berne and Mathew N. Levy. Published by Mosby, Inc. (2000) ISBN0-323-00813-5.
1234567Hall, Arthur C. Guyton, John E. (2005). Textbook of medical physiology (11thed.). Philadelphia: W.B. Saunders. pp.687–690. ISBN978-0-7216-0240-0.{{cite book}}: CS1 maint: multiple names: authors list (link)
123Fauci, Anthony S.; Harrison, T. R., eds. (2008). Harrison's principles of internal medicine (17thed.). New York: McGraw-Hill Medical. pp.147–149. ISBN978-0-07-147692-8.
↑Neuroscience (2.ed.). Sunderland, Mass: Sinauer Assoc. 2001. pp.Damage to Descending Motor Pathways: The Upper Motor Neuron Syndrome. ISBN0-87893-742-0. Archived from the original on 2018-05-03.
