Corticospinal Tract -Course, Function And Lesions
Corticospinal tract (CST) also referred to as pyramidal tracts were the first tracts to be found in man. Corticospinal tract of spinal cord are the descending tracts involved with voluntary motor activities of the body. There are two corticospinal tract, the anterior corticospinal tract and lateral corticospinal tract. Even though running from cerebral cortex towards spinal cord, the fibers of these two tracts provide the appearance of a Pyramid on the top portion of anterior surface of medulla oblongata thus the name pyramidal tracts.
Nerve fibre –
All the fibers of the CST are present since birth. However, myelination of these fibers is finished in about two years after birth.
The pyramidal tracts on each side have more than a thousand fibers.
About 70 percent of these fibers are large myelinated fibers having a diameter of 4 to 22 micron. Substantial fibers of pyramidal tracts have the tendency To disappear at old age.
Considering these tracts are Concerned with management of voluntary movements, the Disappearance of these fibers of pyramidal tracts triggers automatic shivering moves in older age.
Fibers of CST Originate from Subsequent cells or regions of cerebral cortex
• Giant cells or Betz cells or pyramidal cells from precentral gyrus of the motor cortex. These Cells are situated in area 4 (primary motor place ) of frontal lobe.
• Other areas of motor cortex namely, premotor Region (area 6) and supplementary motor regions
• Other parts of frontal lobe
• Somatosensory areas of parietal lobe. It is believed that 30 percent of pyramidal fibers arise from primary motor area (area 4) and supplementary motor areas, another 30 percent from premotor region (area 6) and the remaining 40% of fibers arise from somatosensory areas. All the aforementioned fibers form fibers of upper motor neurons of motor pathway.
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Course of corticospinal tract –
Corona Radiata –
• After taking origin, the nerve pathways run downwards in a diffused manner through white matter of cerebral hemisphere and converge in the Kind of a fan-like arrangement along with ascending fibers, which endeavor from thalamus to cerebral cortex. This fan-like structure is called corona radiata. Therefore, corona radiata contains Both ascending fibers from thalamus and descending fibers in cerebral cortex.
Internal Capsule –
• while passing down towards the brainstem that the corona radiata converges in the kind of inner capsule. It is Located in between thalamus and caudate nucleus on the medial side and lenticular nucleus in the lateral side.
In Pons –
• The fibers descend down through inner capsule, midbrain and pons. While shifting through pons, the fibers are divided into different bundles by the at lower edge of pons, the fibers are grouped once again into a compact bundle then descend down into medulla oblongata.
In Medulla –
• This compact bundle of corticospinal fibers provides the look of a pyramid in the anterior surface of upper part of medulla. In the lower border of medulla, pyramidal tract on each side is divided into two packages of unequal sizes. About 80 percent of fibers from each side cross to the other side. While crossing the midline, the fibers of either side form the pyramidal decussation.
In Spinal Cord –
• Fibers which cross the midline and shape pyramidal decussation descend through posterior part of lateral White column of spinal cord. This package of crossed fibers is known as the crossed pyramidal tract or lateral corticospinal tract or indirect corticospinal tract.
• Remaining 20% of fibers don’t cross to the opposite side but descend through the anterior white pillar of the spinal cord. This bundle of uncrossed fibers is called the uncrossed pyramidal tract or anterior CST or direct CST. This tract is nicely marked in cervical region.
• Considering that, the fibers of this tract terminate in various segments of spinal cord, this tube generally gets thinner while descending through the successive segments of spinal cord. Fibers of this tract are absent mostly below the mid thoracic amount.
• Before conclusion, majority of the fibers of the anterior corticospinal tract cross into the opposite side at different levels of spinal cord.
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CST are concerned with voluntary movements of the body. Fibers of the pyramidal tracts (CST) transmit motor impulses from motor area of cerebral cortex to the anterior motor neurons of the spinal cord. These two tracts are responsible for nice, skilled movements.
• Recent developments have improved the Comprehension of the source and termination of the CST neurons –
• Rest of the fibers originate in the supplementary motor area (SMA), premotor cortex (PMA), regions of the somatosensory regions (S1 and S2) and parts of the posterior parietal cortex.
• On account of the various origins that contribute to the CST, it’s believed that this tract not only forms part of motor system, but also has a big sensory role too.
• Here they synapse using interneurons that receive input from somatosensory receptors and therefore are thought to regulate data from peripheral receptors within the spinal cord.
Thus, the CST may act as a ‘gate‘, modulating or inhibiting information that is deemed useful or insignificant.
• The corticobulbar tract carries higher motor neuron enter to the motor nuclei of the trigeminal, facial, glossopharyngeal and accent cranial nerves. This then provides voluntary management over a variety of head and neck features;
• Trigeminal nerve (CN V): the motor element of trigeminal nerves provides the muscles of mastication.
• Facial nerve (CN VII): the motor nucleus has each dorsal and ventral areas. Neurons within the dorsal area innervate the muscle tissues of the higher face, whereas neurons within the ventral area innervate the muscle tissues of the decrease face.
• Glossopharyngeal nerve (CN IX): innervates the muscles of the larynx & pharynx.
• The accent nerve (CN XI): supplies the sternocleidomastoid and trapezius muscles.
• The corticobulbar tract additionally influences the exercise of muscle tissues provided by the third, fourth and sixth cranial nerves, not directly modulating their exercise. It does this by synapsing with intermediate constructions from the brainstem such because the medial longitudinal fasciculus and reticular formation.
• Patients with central wire syndrome have compression of the dorsal column tracts, lateral corticospinal tracts, and spinothalamic tracts. The compression of the dorsal column tracts causes bilateral sensory impairments under the extent of harm.
• Sensory impairment is usually skilled in a “cape-like’ distribution throughout the higher again and down the posterior aspect of the higher limbs. Damage to the lateral CST causes bilateral weak spot of the higher physique, however sufferers retain energy within the decrease limbs.
• Compression of the spinothalamic tracts leads to the bilateral lack of ache/temperature for the higher physique greater than the decrease physique. Deficits affecting higher physique features greater than decrease physique end result from compression of the central side of the wire.
• Both corticospinal and spinothalamic tracts have homunculi wherein the higher physique is positioned centrally whereas the decrease physique is positioned extra peripherally inside the spinal wire.
• Patients with anterior spinal wire syndrome have bilateral deficits to the corticospinal tracts and spinothalamic tracts. They expertise bilateral paralysis and paresis under the location of the lesion. They additionally expertise bilateral ache/temperature and lightweight contact deficits as a result of harm of the spinothalamic tract. Sacral sparing of the spinothalamic tract happens as a result of a twin blood provide by the posterior spinal arteries.
• The posterior spinal arteries wrap across the peripheral side of the wire. This secondary provide allows the total performance of the peripheral spinothalamic tract, which transmits ache, temperature, and lightweight contact for the ft.
• Patients with Brown-Sequard syndrome have a unilateral deficit of the lateral wire. Deficits related to this syndrome embody ipsilateral paralysis, paresis, and hypertonia; ipsilateral proprioception loss; and contralateral lack of ache and temperature sensation.
• In any of those syndromes, the locus of the harm may cause further signs. If the harm is at or above T1-L2, a compression will disrupt sympathetic neurons inside the intermediolateral nucleus within the lateral horn of the spinal wire and trigger an ipsilateral Horner syndrome.
• Similarly, accidents inside the decrease lumbar area may cause autonomic dysfunction that induces bladder, bowel, or sexual dysfunction.
• There are numerous causes for spinal cord injuries: trauma, ischemic events, and infection are the most common procedures of damage. It results from hyperextension of this cord typically within the cervical region. This type of injury is not uncommon in shaken baby syndrome.
•Another cause may be motor vehicle accidents where the head hyper-extends due to contact with the vehicle or air-bag, or higher impact contact sports such as football.Anterior spinal cord syndrome is the consequence of obstruction or damage of the anterior spinal artery. The spinal cord has one anterior spinal artery and two lateral spinal arteries. The anterior spinal artery supplies the anterior 2/3 of the spinal cord. Thrombosis or an embolism may lead to damage.
• One of the more common sites of anterior spinal injury is at the artery of Adamkiewcz. This radicular artery branches from the aorta at the level of their 9 to 12 intercostal spaces in most people. In a small percentage of individuals, it originates between L1-L2 or T5-T8.
•The artery of Adamkiewcz terminates in an acute angle that makes it more prone to damage. Because the artery of Adamkiewcz is more prone to damage from surgical procedures that involve the retroperitoneal area, identification and preservation of this artery is essential for many surgical procedures (e.g., thoracoabdominal aortic aneurysm repair, thoracic or lumbar spine operation, removal of intramedullary tumors, and retroperitoneal processes ). Other kinds of harm which cause cervical spinal cord syndrome include celiac disease, spinal muscular atrophy, multiple sclerosis, or illness.
• Brown-Sequard syndrome is a condition where the left or right half of the spinal cord is damaged. Its normal cause is traumatic injuries such as gunshot and traumatic wounds, automobile accidents, or fractured vertebra because of falls. Other causes of this disease include vertebral disc herniation, cervical spondylosis, tumors, multiple sclerosis, decompression illness, cystic disease, in addition to ailments (e.g., meningitis, tuberculosis, transverse myelitis, and herpes zoster).
• During embryologic improvement, there may be an overgrowth of axons distributed all through the cortex, which incorporate into the CST , and as improvement progresses, many of those axons are eradicated.
• Gray matter improvement begins just a few weeks after the CST axons attain the spinal wire. As development continues, the corticospinal tract axons will attain the decrease a part of the cervical spinal wire by 24 weeks gestation. After beginning, the corticospinal tract continues to develop. The tract is then refined, and motor management develops.
•The analysis proposes that refinement of the corticospinal tract occurs by way of the elimination of transient termination and development throughout the grey matter of the spinal cord, that is adopted by growing management of the CST’s position in voluntary motor operate.
• The tract continues improvement by way of puberty, which is when the gender variations in white matter emerge. Studies have proven that androgens play a task in axonal improvement by way of the proliferation of neural cell our bodies and the prevention of cell demise following axonal damage.
• As a end result, the event of white matter in women and men diverges throughout adolescence.
Lesions of corticospinal tract –
• Lesion in the neurons of motor cortex as well as the fibers of pyramidal tracts is called the upper motor neuron lesion.
• In human beings, pure pyramidal tract lesions do not happen. Lesion of pyramidal fibers occurs most commonly in stroke (cardiovascular accident) because of hemorrhage and thrombosis. During these lesions, many extrapyramidal fibers are also damaged along with pyramidal fibers.
• Following are the Consequences of lesion –
1. Voluntary movements –
• Voluntary motions of the human body are very much influenced. Originally, there’s loss of voluntary motions in the extremities. Afterwards, it entails the different areas of the body like shoulder and hip.
2. Muscle Tone –
• Muscle tone is raised resulting in spasticity. Muscles are also paralyzed. This sort of paralysis of muscles is known as the spastic paralysis.
• The spasticity is because of this collapse of inhibitory impulses from upper motor neurons, particularly the nerves of extrapyramidal system to achieve exactly the γ-motor neurons in spinal cord.
3. Reflexes –
• Each of the superficial reflexes are missing and the deep springs are exaggerated. Abnormal plantar reflex called Babinski sign is current (Babinski sign positive).
BABINSKI SIGN –
• Abnormal plantar reflex is called Babinski sign. It is also called Babinski reflex or phenomenon. It is named after the discoverer Joseph Babinski. In normal plantar reflex, a gentle scratch over the outer edge of the sole of foot causes plantar flexion and adduction of all toes.
• But in Babinski sign, there is dorsiflexion of great toe and fanning of other toes.
When Babinski reflex is present, the condition is commonly called Babinski positive sign and when it is negative, the condition is called Babinski negative sign. Babinski sign is present in upper motor neuron lesion. Physiological conditions when Babinski sign is present are infancy and deep sleep. It is present in infants because of non-myelination of pyramidal tracts
Effects of Lesion at Several Levels –
1. Cerebral Cortex –
• Lesion of pyramidal tract fibers in cerebral cortex Triggers hypertonia, spasticity and contralateral monoplegia (paralysis of one limb) or contralateral hemiplegia
(paralysis of one side of the body).
2. Internal capsule –
• Lesion of pyramidal tract fibers in posterior limb of internal capsule results in contralateral hemiplegia.
3. Brainstem –
• Lesion at brainstem involves not only pyramidal tract fibers but also other constructions like VI and VII cranial nerve nuclei. Hence the lesion leads to contralateral hemiparesis (weakness of muscles in one side of the body) along with VI and VII cranial nerve palsies.
4. Spinal Cord –
• Unilateral lesion of lateral corticospinal fibers in upper cervical segment causes ipsilateral hemiplegia and bilateral lesion causes quadriplegia (paralysis of four limbs) and paralysis of respiratory muscles.
•Bilateral lesion of those fibers in thoracic and lumbar segments results in paraplegia (paralysis of both lower limbs) without paralysis of respiratory muscles.
What is the function of corticospinal tract?
Corticospinal tract (CST) are concerned with voluntary movements of the body. Fibers of the CST transmit motor impulses from motor area of cerebral cortex to the anterior motor neurons of the spinal cord.
What is the corticospinal tract?
Corticospinal tract (CST) also referred to as pyramidal tracts were the first tracts to be found in man. CST of spinal cord are the descending tracts involved with voluntary motor activities of the body. There are two CST, the anterior CST and lateral CST.
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