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Neuroanatomy - The Corticospinal Tract in 3D

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Author: Alexandru Andrușca MD, PhD * Reviewer: Nicola McLaren MSc
Last reviewed: July 27, 2023
Reading time: 13 minutes

Corticospinal and corticonuclear tract (Tractus corticospinalis et corticonuclearis)

The pyramidal tract provides voluntary control of muscular movements. It consists of two distinct pathways, the corticonuclear tract and the corticospinal tract.

The corticospinal tract carries motor signals from the primary motor cortex in the brain, down the spinal cord, to the muscles of the trunk and limbs. Thus, this tract is involved in the voluntary movement of muscles of the body.

The corticonuclear tract carries efferent, motor, information from the primary motor cortex to the muscles of the face, head and neck. It does this by synapsing with motor cranial nerves in the brainstem. Therefore the corticonuclear tract is responsible for innervating the muscles of the face, head and neck, as well as the muscles involved in swallowing, phonation and facial expression.

It is important to note that according to the official terminologia anatomica (TA), the term 'pyramidal tract' refers to only the corticospinal tract, while the corticonuclear is deemed as a separate tract. However, more often than not, the literature and pratice uses the pyramidal tract as an umbrella term for both the corticospinal and corticonuclear tracts, and we go by that premise in this article.

This article will describe the anatomy and function of the corticonuclear and corticospinal tracts.

Corticonuclear tract

Definition: motor pathway from the motor cortex of the brain to the motor nuclei of cranial nerves within the brainstem.
Function: responsible for voluntary movement of the muscles of the face (CN. VII), head and neck (CN. XI). Also involved in phonation, swallowing and facial expression. (CN. VII and IX)

Corticospinal tract

Definition: motor pathway from the brain's motor cortex to lower motor neurons located in the anterior horn of the spinal cord's gray matter. Divided into the anterior corticospinal tract (supplies axial muscles) and the lateral corticospinal tract (supplies muscles of the limbs).
Function: responsible for voluntary movement of the muscles of the limbs and trunk.

Corticonuclear (corticonuclear) tract

Structure

The corticonuclear tract is part of the pyramidal system. It is sometimes also called the corticobulbar tract.

This tract originates from upper motor neurons (UMN) located in the lateral aspect of the primary motor cortex. Specifically, this tract originates from the V-th layer of the cerebral cortex, where the giant pyramidal cells of Betz reside. Most of these cell bodies are found in Brodmann area 4 (primary motor), area 6 (premotor cortex), areas 3, 1, 2 (postcentral gyrus) and area 5 (parietal cortex). The arising fibers converge to pass within the corona radiata, the knee "genu" of the internal capsule, then continue inferiorly through the cerebral peduncle of the midbrain, to reach the brainstem.

While descending through the brainstem, the UMNs terminate on motor nuclei of certain cranial nerves within the midbrain, pons and medulla. These motor nuclei are considered the lower motor neurons (LMN) of the corticonuclear pathway, they carry efferent signals directly to the muscles of the face, head and neck. In the pons, fibers dedicated to the facial nerve leave the corticonuclear tract, cross the midline right above the facial nuclei and synapse with its motor nuclei. A similar pathway is followed by fibers connecting with the motor nuclei of the trigeminal nerve, at the level of the mid-pons. In the same manner, corticonuclear tract fibers project to the glossopharyngeal and accessory nerves by synapsing with their respective motor nuclei in the medulla oblongata. The corticonuclear tract also indirectly affects the nuclei of oculomotor, trochlear and abducens nerves within the midbrain, through intermediate structures such as the medial longitudinal fasciculus and reticular formation.

Function

The corticonuclear tract provides voluntary control over the muscles of the face, head and neck. This is in contrast to the corticospinal tract which controls the movement of the torso and limbs.

Specifically, the corticonuclear tract carries upper motor neuron input to the motor nuclei of the trigeminal, facial, glossopharyngeal and accessory cranial nerves. This then supplies voluntary control over a number of head and neck functions;

Trigeminal nerve (CN V): the motor component of trigeminal nerves supplies the muscles of mastication.

Facial nerve (CN VII): the motor nucleus has both dorsal and ventral regions. Neurons in the dorsal region innervate the muscles of the upper face, while neurons in the ventral region innervate the muscles of the lower face.

Glossopharyngeal nerve (CN IX): innervates the muscles of the pharynx and larynx.

The accessory nerve (CN XI): supplies the sternocleidomastoid and trapezius muscles.

The corticonuclear tract also influences the activity of muscles supplied by the third, fourth and sixth cranial nerves, indirectly modulating their activity. It does this by synapsing with intermediate structures from the brainstem such as the medial longitudinal fasciculus and reticular formation.

If your goal is to remember the pathways of the pyramidal system, find out about the common mistakes that hinder your anatomy learning.

Corticospinal tract

The corticospinal tract is a motor pathway that carries efferent information from the cerebral cortex to the spinal cord. It is responsible for the voluntary movements of the limbs and trunk.

The path starts in the motor cortex, where the bodies of the first-order neurons lie. These specialized upper motor neurons are called the pyramidal cells of Betz. Axons of Betz cells descend and converge to join a white matter sheet within each cerebral hemisphere called the " corona radiata''. The fibers then pass inferiorly through the anterior two-thirds of the posterior arm of the internal capsule (a white matter structure, located between the thalamus and the basal ganglia), through the cerebral peduncles of the midbrain, the pons and into the medulla.

In the anterior aspect of the lower medulla, the majority of corticospinal fibers decussate ( pyramidal decussation). The crossed fibers form the lateral corticospinal tract while the uncrossed fibers enter the anterior corticospinal tract. The former is responsible for providing voluntary motor information to the muscles of the limbs while the latter supplies the axial muscles of the trunk. Both tracts run along the spinal cord, synapsing with lower motor neurons in the anterior gray horn on the same side. The lower motor neurons leave the spinal cord through the ventral root and form peripheral nerves which innervate the musculature of the body.

More details about the pyramidal tracts are provided below:

Anterior corticospinal tract (ACST)

The anterior corticospinal tract (also called the " Bundle of Turck") emerges from the un-decussated fibers of the corticospinal tract, at the level of the bulbomedullary junction. The UMN axons descend in the anterior funiculus of the spinal cord. At their spinal cord level of innervation, the axons cross over to the contralateral side. Thus this is their point of decussation. The UMNs then synapse in the anterior horn with lower motor neurons. Their axons pass through the anterior root of the spinal nerve and terminate in the endplate of skeletal muscles. The neural impulses are conveyed to the muscle cells through the motor endplates of the neuromuscular junction (myotomes), resulting in muscle movement.

The anterior corticospinal tract is usually small, varying inversely in size with the lateral corticospinal tract. Lying close to the anterior median fissure, it is present only in the upper part of the spinal cord, gradually diminishing in size as it descends. The tract ends around the level of the mid-thoracic region.

Function

The anterior corticospinal tract is primarily responsible for gross and postural movement of the trunk and proximal musculature ( axial muscles).

Lateral corticospinal tract

The lateral corticospinal tract (LCST) is the largest descending motor pathway in the human body, it spans the entire length of the spinal cord, eventually supplying motor signals to all the skeletal muscles of our upper and lower limbs.

As the upper motor neurons (UMN) fibers pass through the caudal medulla, the majority of fibers (80%) decussate (pyramidal decussation) to the contralateral side and enter the lateral corticospinal tract. The tract then descends in the lateral funiculus along the entire spinal cord, synapsing with second-order, lower motor neurons (LMN) in the ventral horn at each level of the spinal cord. Impulses generated in the second-order motor neurons pass through the anterior roots of the spinal nerve, through the peripheral nerve plexuses (in the cervical, brachial and lumbosacral regions), and along individual peripheral nerves on their way to the skeletal muscles.

Function

The lateral corticospinal tract is responsible for the voluntary movement of the contralateral upper and lower limbs.

The upper motor neurons of the LCST, the giant pyramidal cells of Betz, preserve a somatotopic organization, called the motor homunculus. This homunculus represents the somatotopic motor distribution of the human body, along the motor cortex. From medial to lateral we see the motor processing regions corresponding to the structures of the face, then hand and arm, next the shoulder, head, neck and trunk, the lower limb and most laterally, the genitalia. This somatotopic organization is preserved all along the corticospinal tracts, whereby the more medial part of the LCST is responsible for innervation of the cervical region and the lateral part of the tract sends efferent output to the lower thoracic, lumbar and sacral regions, respectively.

Take our quiz on the pyramidal tracts to solidify your knowledge.

Spastic weakness (paresis) when voluntary movement is attempted

Increased muscle spasticity

Exaggerated tendon reflexes

Positive pathological pyramidal signs (e.g. Plantar reflex "Babinski")

Flaccid weakness (paresis) of the limb

Loss of muscle tone

Loss of tendon reflexes

Atrophy of the muscle

Fasciculation (random muscles twitches)

References:

Haines, D: (2004). Neuroanatomy (6th ed.). Philadelphia: Lippincott Williams & Wilkins.

Kiernan, J., Barr, M., & Rajakumar, N: (2013). Barr's The Human Nervous System (10th ed.). Philadelphia, PA: Lippincott Williams & Wilkins.

Snell, R: (2010). Clinical Neuroanatomy (7th ed.). Philadelphia: Wolters Kluwer Health/Lippincott Williams & Wilkins.

Standring, S. & Gray, H: (2008). Gray's Anatomy. [Edinburgh]: Churchill Livingstone/Elsevier.

M. Baehr, Duus Topical Diagnosis in Neurology

Walter J. Hendelman, Atlas of Functional Neuroanatomy, third edition

Richard S. Snell. Clinical Neuroanatomy, 7th edition

Wilson-Pauwels, Akesson, Stewart, Spacey. Cranial Nerves in health and disease. Second Edition.

Illustrators:

Corticospinal and corticonucelar tract - Paul Kim

Corticonuclear and corticospinal tracts: want to learn more about it?

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Corticospinal and corticonuclear are responsible for the voluntary movement of the body. Learn more about these tracts at Kenhub.

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Sindromul de neuron motor central - SNMC

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Sindromul de neuron motor central - SNMC

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I. TULBURARI DE MOTILITATE

1. Tulburari de motilitate activa

- deficitul motor da pareze si plegii care , in general, sunt extinse realizand:

•       hemipareze – hemiplegii

•       parapareze – paraplegii

•       dipareze – diplegii

•       terapareze - tetraplegii

2. Tulburari de motilitate pasiva

- cand leziunea se instaleaza masiv si brutal, se poate remarca o hipotonie musculara, dar aceasta se transforma in decurs de cateva saptamani in contractura

- modificarea caracteristica este hipertonia:

a) este extinsa la fel ca si deficitul motor

b) predomina distal

c) este electiva la membr. sup pe flexori, iar la membr. inf pe extensori

d) este elastica, dupa o tentativa de mobilizare a membrului acesta revine imediat la pozitia initiala

e) fenomenul ”lamei de briceag” – la mobilizarea pasiva, rezistenta intampinata este mult mai mare la inceputul tentativei de mobilizare pentru ca la un moment dat sa cedeze brusc

f) contractura piramidala – la mobilizare cedeaza in mod continuu su nu in sacade ca in cea extrapiramidala.

3. Tulburari de motilitate automata – membrul superior paralizat nu se balanseaza pe langa corp

4. Tulburari de motilitate asociata patologiei – sincinezii globale, de imitatie, de coordonatie etc.

·      Sincineziile: ex: - Globale – contractia unui grup muscular al mb paralizate declansate de acte reflexe bruste: tuse, stranut

-De imitatie – mb. paralizate pot realiza involuntar o miscare ce este realizata voluntar de mb sanatoase

II. TULBURARI ALE REFLEXELOR

1) ROT sunt exagerate, difuze, polichinetice si adesea cu difuziune contralaterala. Exagerarea acestora se datoreaza eliberarii arcului refelx spinal simplu, care este lipsit de controlul scoartei cerebrale, prin intreruperea cailor piramidale.

•       Exista eventualitatea diminuarii episodice a ROT in perioada initial, flasca a sdr. piramidale instalate brusc

2) RCA sunt abolite

3) R. de postura sunt diminuate

4) Semnul Babinski = indiciu pretios.

III. TULBURARI TROFICE

•       pot exista si tulburari trofice de tip:

-       edem, cianoza, ↓ temperaturii cutanate la nivelul membr. paralizate

-       artropatii mai ales la membre. sup.

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Calea piramidala controleaza motilitatea voluntara

Calea piramidala are un traiect descendent extrem de lung trecand prin centrul oval → capsula interna (genunchi + brat post) → piciorul pedunculului cerebral → al protuberantei si → al bulbului→ cordon ant/lat al maduvei spinarii

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Rappel anatomic:

NMC isi are sediul in scoarta cerebrala, fiind reprezentat de:

-Celule piramidale mari din str. V (cel Betz)

-Celule piramidale mici din str. III din aria 4 Brodmann din frontala ascendenta

Dar fibrele care constituie fasc piramidal isi au originea si in neuronii din:

- Aria 6 frontala

- Ariile parietale 3, 1, 2 si 7

La nivelul frontalei ascendente exsita o SOMATOTOPIE bine definita - inervatia motorie:

A capului – corespunde portiunii inferioare a circumvolutiei frontale descendente

A mb inferioare – portiunii mijlocii

A mb inferioare – portiunea sup si la niv lobului paracentral

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O1- însușirea terminologiei legate de bolile neuronului motor central
O2- sa puteți explica apariția semnelor și simptomelor clinice în afecțiunile piramidale
O3- sa puteți formula ipoteze diagnostice pe baza investigațiilor clinice
O4- sa creionate un plan de tratament al unui pacient cu afecțiune de neuron motor central

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Metode didactice: Expunerea, Conversația, Exemplificarea

Mijloace didactice: . Powerpoint, Laptop, Videoproiector, prezentarea pacientilor cu deficite motorii