Pyramidal tracts

Last updated
Pyramidal tracts
Gray684.png
Deep dissection of brain-stem. Lateral view. ("pyramidal tract" visible in red, and "pyramidal decussation" labeled at lower right.)
Spinal cord tracts - English.svg
Spinal cord tracts, with pyramidal tracts labeled at upper left
Details
Decussation Many fibres in pyramids of medulla oblongata
From Cerebral cortex
To Spinal cord (corticospinal) or brainstem (corticobulbar)
Identifiers
Latin tractus pyramidalis
tractus corticospinalis
MeSH D011712
NeuroNames 1320
NeuroLex ID birnlex_1464
TA98 A14.1.04.102
A14.1.06.102
TA2 6040
FMA 72634
Anatomical terms of neuroanatomy

The pyramidal tracts include both the corticobulbar tract and the corticospinal tract. These are aggregations of efferent nerve fibers from the upper motor neurons that travel from the cerebral cortex and terminate either in the brainstem (corticobulbar) or spinal cord (corticospinal) and are involved in the control of motor functions of the body.

Contents

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 cerebral 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.

Structure

Pyramidal tracts Bentralki (1).tif
Pyramidal tracts

The term pyramidal tracts refers to upper motor neurons that originate in the cerebral cortex and terminate in the spinal cord (corticospinal) or brainstem (corticobulbar). Nerves emerge in the cerebral cortex, pass down and may cross sides in the medulla oblongata, and travel as part of the spinal cord until they synapse with interneurons in the grey column of the spinal cord. [2]

There is some variation in terminology. The pyramidal tracts definitively encompass the corticospinal tracts, and many authors also include the corticobulbar tracts. [3]

Corticospinal tract

Nerve fibres in the corticospinal tract originate from pyramidal cells in layer V of the cerebral cortex. Fibres arise from the primary motor cortex (about 30%), supplementary motor area and the premotor cortex (together also about 30%), and the somatosensory cortex, parietal lobe, and cingulate gyrus supplies the rest. [2] The cells have their bodies in the cerebral cortex, and the axons form the bulk of the pyramidal tracts. [4] The nerve axons travel from the cortex through the posterior limb of internal capsule, through the cerebral peduncle and into the brainstem and anterior medulla oblongata. Here they form two prominences called the medulla oblongatary pyramids. Below the prominences, the majority of axons cross over to the opposite side from which they originated, known as decussation. The axons that cross over move to the outer part of the medulla oblongata and form the lateral corticospinal tract, whereas the fibres that remain form the anterior corticospinal tract. [2] About 80% of axons cross over and form the lateral corticospinal tract; 10% do not cross over and join the tract, and 10% of fibres travel in the anterior corticospinal tract. [ citation needed ]

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 Lower pons horizontal KB.svg
Horizontal section through the lower part of the pons, showing the fibers of the corticospinal tract (#19) passing through the pontine nuclei

Corticobulbar tract

Fibres from the ventral motor cortex travel with the corticospinal tract through the internal capsule, but terminate in a number of locations in the midbrain (cortico-mesencephalic tract), pons (Corticopontine tract), and medulla oblongata (cortico-bulbar tract). [6] The upper motor neurons of the corticobulbar tract synapse with interneurons or directly with the lower motor neurons located in the motor cranial nerve nuclei, namely oculomotor, trochlear, motor nucleus of the trigeminal nerve, abducens, facial nerve and accessory and in the nucleus ambiguus to the hypoglossal, vagus and accessory nerves. [6] These nuclei are supplied by nerves from both sides of the brain, with the exception of the parts of the facial nerve that control muscles of the lower face. These muscles are only innervated by nerves from the contralateral (opposite) side of the cortex. [6]

Function

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 ]

Clinical significance

  1. 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.
  2. 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 extensor plantar response known as the Babinski sign. [7]
  3. Symptoms generally occur alongside other sensory problems.
  4. Causes may include disorders such as strokes, [8] cerebral palsy, [9] [10] subdural hemorrhage, abscesses and tumours, neurodegenerative diseases such as multiple system atrophy, inflammation such as meningitis and multiple sclerosis, and trauma to the spinal cord, including from slipped discs. [4]
  5. 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]
  6. Severe disabling involuntary movements such as hemiballismus or severe chorea might exhaust the patient and become a life-threatening situation.
  7. 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

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]

Related Research Articles

<span class="mw-page-title-main">Motor neuron</span> Nerve cell sending impulse to muscle

A motor neuron is a neuron whose cell body is located in the motor cortex, brainstem or the spinal cord, and whose axon (fiber) projects to the spinal cord or outside of the spinal cord to directly or indirectly control effector organs, mainly muscles and glands. There are two types of motor neuron – upper motor neurons and lower motor neurons. Axons from upper motor neurons synapse onto interneurons in the spinal cord and occasionally directly onto lower motor neurons. The axons from the lower motor neurons are efferent nerve fibers that carry signals from the spinal cord to the effectors. Types of lower motor neurons are alpha motor neurons, beta motor neurons, and gamma motor neurons.

<span class="mw-page-title-main">Medulla oblongata</span> Structure of the brain stem

The medulla oblongata or simply medulla is a long stem-like structure which makes up the lower part of the brainstem. It is anterior and partially inferior to the cerebellum. It is a cone-shaped neuronal mass responsible for autonomic (involuntary) functions, ranging from vomiting to sneezing. The medulla contains the cardiac, respiratory, vomiting and vasomotor centers, and therefore deals with the autonomic functions of breathing, heart rate and blood pressure as well as the sleep–wake cycle.

The motor system is the set of central and peripheral structures in the nervous system that support motor functions, i.e. movement. Peripheral structures may include skeletal muscles and neural connections with muscle tissues. Central structures include cerebral cortex, brainstem, spinal cord, pyramidal system including the upper motor neurons, extrapyramidal system, cerebellum, and the lower motor neurons in the brainstem and the spinal cord.

Pronator quadratus is a square-shaped muscle on the distal forearm that acts to pronate the hand.

<span class="mw-page-title-main">Brainstem</span> Posterior part of the brain, adjoining and structurally continuous

The brainstem is the stalk-like part of the brain that interconnects the cerebrum and diencephalon with the spinal cord. In the human brain, the brainstem is composed of the midbrain, the pons, and the medulla oblongata. The midbrain is continuous with the thalamus of the diencephalon through the tentorial notch.

<span class="mw-page-title-main">Internal capsule</span> White matter structure situated in the inferomedial part of each cerebral hemisphere of the brain

The internal capsule is a white matter structure situated in the inferomedial part of each cerebral hemisphere of the brain. It carries information past the basal ganglia, separating the caudate nucleus and the thalamus from the putamen and the globus pallidus. The internal capsule contains both ascending and descending axons, going to and coming from the cerebral cortex. It also separates the caudate nucleus and the putamen in the dorsal striatum, a brain region involved in motor and reward pathways.

<span class="mw-page-title-main">Neural pathway</span> Connection formed between neurons that allows neurotransmission

In neuroanatomy, a neural pathway is the connection formed by axons that project from neurons to make synapses onto neurons in another location, to enable neurotransmission. Neurons are connected by a single axon, or by a bundle of axons known as a nerve tract, or fasciculus. Shorter neural pathways are found within grey matter in the brain, whereas longer projections, made up of myelinated axons, constitute white matter.

<span class="mw-page-title-main">Spinothalamic tract</span> Sensory pathway from the skin to the thalamus

The spinothalamic tract is a part of the anterolateral system or the ventrolateral system, a sensory pathway to the thalamus. From the ventral posterolateral nucleus in the thalamus, sensory information is relayed upward to the somatosensory cortex of the postcentral gyrus.

<span class="mw-page-title-main">Dorsal column–medial lemniscus pathway</span> Sensory spinal pathway

The dorsal column–medial lemniscus pathway (DCML) is a sensory pathway of the central nervous system that conveys sensations of fine touch, vibration, two-point discrimination, and proprioception from the skin and joints. It transmits information from the body to the primary somatosensory cortex in the postcentral gyrus of the parietal lobe of the brain. The pathway receives information from sensory receptors throughout the body, and carries this in nerve tracts in the white matter of the dorsal column of the spinal cord to the medulla, where it is continued in the medial lemniscus, on to the thalamus and relayed from there through the internal capsule and transmitted to the somatosensory cortex. The name dorsal-column medial lemniscus comes from the two structures that carry the sensory information: the dorsal columns of the spinal cord, and the medial lemniscus in the brainstem.

<span class="mw-page-title-main">Corticobulbar tract</span> Motor pathway in the brain connecting the motor cortex to the medullary pyramids

In neuroanatomy, the corticobulbartract is a two-neuron white matter motor pathway connecting the motor cortex in the cerebral cortex to the medullary pyramids, which are part of the brainstem's medulla oblongata region, and are primarily involved in carrying the motor function of the non-oculomotor cranial nerves. The corticobulbar tract is one of the pyramidal tracts, the other being the corticospinal tract.

<span class="mw-page-title-main">Upper motor neuron</span> Neurons in the brain that carry signals to lower motor neurons

Upper motor neurons (UMNs) is a term introduced by William Gowers in 1886. They are found in the cerebral cortex and brainstem and carry information down to activate interneurons and lower motor neurons, which in turn directly signal muscles to contract or relax. UMNs represent the major origin point for voluntary somatic movement.

<span class="mw-page-title-main">Precentral gyrus</span> Motor gyrus of the posterior frontal lobe of the brain

The precentral gyrus is a prominent gyrus on the surface of the posterior frontal lobe of the brain. It is the site of the primary motor cortex that in humans is cytoarchitecturally defined as Brodmann area 4.

<span class="mw-page-title-main">Facial motor nucleus</span>

The facial motor nucleus is a collection of neurons in the brainstem that belong to the facial nerve. These lower motor neurons innervate the muscles of facial expression and the stapedius.

<span class="mw-page-title-main">Lateral corticospinal tract</span>

The lateral corticospinal tract is the largest part of the corticospinal tract. It extends throughout the entire length of the spinal cord, and on transverse section appears as an oval area in front of the posterior column and medial to the posterior spinocerebellar tract.

<span class="mw-page-title-main">Anterior corticospinal tract</span>

The anterior corticospinal tract is a small bundle of descending fibers that connect the cerebral cortex to the spinal cord. Descending tracts are pathways by which motor signals are sent from upper motor neurons in the brain to lower motor neurons which then directly innervate muscle to produce movement. The anterior corticospinal tract is usually small, varying inversely in size with the lateral corticospinal tract, which is the main part of the corticospinal tract.

<span class="mw-page-title-main">Alpha motor neuron</span>

Alpha (α) motor neurons (also called alpha motoneurons), are large, multipolar lower motor neurons of the brainstem and spinal cord. They innervate extrafusal muscle fibers of skeletal muscle and are directly responsible for initiating their contraction. Alpha motor neurons are distinct from gamma motor neurons, which innervate intrafusal muscle fibers of muscle spindles.

<span class="mw-page-title-main">Medullary pyramids (brainstem)</span> White matter structures within the brainstems medulla oblongata

In neuroanatomy, the medullary pyramids are paired white matter structures of the brainstem's medulla oblongata that contain motor fibers of the corticospinal and corticobulbar tracts – known together as the pyramidal tracts. The lower limit of the pyramids is marked when the fibers cross (decussate).

<span class="mw-page-title-main">Spinal cord</span> Long, tubular central nervous system structure in the vertebral column

The spinal cord is a long, thin, tubular structure made up of nervous tissue that extends from the medulla oblongata in the brainstem to the lumbar region of the vertebral column (backbone) of vertebrate animals. The center of the spinal cord is hollow and contains a structure called the central canal, which contains cerebrospinal fluid. The spinal cord is also covered by meninges and enclosed by the neural arches. Together, the brain and spinal cord make up the central nervous system.

<span class="mw-page-title-main">Primary motor cortex</span> Brain region

The primary motor cortex is a brain region that in humans is located in the dorsal portion of the frontal lobe. It is the primary region of the motor system and works in association with other motor areas including premotor cortex, the supplementary motor area, posterior parietal cortex, and several subcortical brain regions, to plan and execute voluntary movements. Primary motor cortex is defined anatomically as the region of cortex that contains large neurons known as Betz cells, which, along with other cortical neurons, send long axons down the spinal cord to synapse onto the interneuron circuitry of the spinal cord and also directly onto the alpha motor neurons in the spinal cord which connect to the muscles.

<span class="mw-page-title-main">Corticospinal tract</span> Pyramidal white matter motor pathway

The corticospinal tract is a white matter motor pathway starting at the cerebral cortex that terminates on lower motor neurons and interneurons in the spinal cord, controlling movements of the limbs and trunk. There are more than one million neurons in the corticospinal tract, and they become myelinated usually in the first two years of life.

References

  1. ↑ Chapter 9 of "Principles of Physiology" (3rd edition) by Robert M. Berne and Mathew N. Levy. Published by Mosby, Inc. (2000) ISBN   0-323-00813-5.
  2. 1 2 3 4 5 6 7 Hall, Arthur C. Guyton, John E. (2005). Textbook of medical physiology (11th ed.). Philadelphia: W.B. Saunders. pp. 687–690. ISBN   978-0-7216-0240-0.{{cite book}}: CS1 maint: multiple names: authors list (link)
  3. ↑ Anthoney, Terence R. (1994). Neuroanatomy and the neurologic exam : a thesaurus of synonyms, similar-sounding non-synonyms, and terms of variable meaning. Boca Raton: CRC Press. pp. 458–460. ISBN   9780849386312. Archived from the original on 2018-05-03.
  4. 1 2 3 Fauci, Anthony S.; Harrison, T. R., eds. (2008). Harrison's principles of internal medicine (17th ed.). New York: McGraw-Hill Medical. pp. 147–149. ISBN   978-0-07-147692-8.
  5. 1 2 3 Arslan, Orhan (2001). Neuroanatomical Basis of Clinical Neurology. CRC Press. p. 368. ISBN   1439806136. Archived from the original on 2018-05-03.
  6. 1 2 3 4 5 Young, Paul A. (2007). Basic clinical neuroscience (2nd ed.). Philadelphia, Pa.: Lippincott Williams & Wilkins. pp. 69–70. ISBN   9780781753197. Archived from the original on 2017-03-13.
  7. ↑ Neuroscience (2. ed.). Sunderland, Mass: Sinauer Assoc. 2001. pp. Damage to Descending Motor Pathways: The Upper Motor Neuron Syndrome. ISBN   0-87893-742-0. Archived from the original on 2018-05-03.
  8. ↑ Topcuoglu, MA; Saka, E; Silverman, SB; Schwamm, LH; Singhal, AB (1 September 2017). "Recrudescence of Deficits After Stroke: Clinical and Imaging Phenotype, Triggers, and Risk Factors". JAMA Neurology. 74 (9): 1048–1055. doi:10.1001/jamaneurol.2017.1668. PMC   5710180 . PMID   28783808.
  9. ↑ Vitrikas, K; Dalton, H; Breish, D (15 February 2020). "Cerebral Palsy: An Overview". American Family Physician. 101 (4): 213–220. PMID   32053326.
  10. ↑ Farag, Sara M.; Mohammed, Manal O.; EL-Sobky, Tamer A.; ElKadery, Nadia A.; ElZohiery, Abeer K. (March 2020). "Botulinum Toxin A Injection in Treatment of Upper Limb Spasticity in Children with Cerebral Palsy". JBJS Reviews. 8 (3): e0119. doi: 10.2106/JBJS.RVW.19.00119 . PMC   7161716 . PMID   32224633.
  11. ↑ DeMyer, William (1998). Neuroanatomy. Williams & Wilkins. ISBN   9780683300758. Archived from the original on 2018-03-03.
  12. ↑ "Quotes from "National Lampoon's European Vacation"". Archived from the original on 25 March 2017. Retrieved 3 May 2018– via www.imdb.com.
This page is based on this Wikipedia article
Text is available under the CC BY-SA 4.0 license; additional terms may apply.
Images, videos and audio are available under their respective licenses.