PYRAMIDAL LESIONS by prof charles.pptx

Health

1. Disorders of motor functions:pyramidal lessions by Prof.Charles 18-May-24 1

2. DESCENDING TRACTS OF SPINAL CORD Descending tracts of the spinal cord are formed by motor nerve from brain into the spinal cord. Descending tracts of spinal cord are of two types: A. Pyramidal tracts B. Extrapyramidal tracts 18-May-24 2

3. PYRAMIDAL TRACTS / corticospinal tracts Pyramidal tracts were the first tracts to be found in man. Pyramidal tracts of spinal cord are the descending tracts concerned with voluntary motor activities of the body. Also known as corticospinal tracts. There are two corticospinal tracts, the anterior corticospinal tract and lateral corticospinal tract. running from cerebral cortex towards spinal cord, the fibers give the appearance of a pyramid on the upper part of anterior surface of medulla oblongata hence the name pyramidal tracts 18-May-24 3

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5. Nerve Fibers All the fibers of the pyramidal tracts are present since birth. However, myelination of these fibers is completed in about 2 years after birth. The pyramidal tracts on each side have more than a million fibers. About 70% of the fibers are large myelinated fibers having a diameter of 4 to 22 micron. 18-May-24 5

6. Large fibers of pyramidal tracts have the tendency to disappear at old age. Since these tracts are concerned with control of voluntary movements, the disappearance of the fibers of pyramidal tracts causes automatic shivering movements in old age. Fibers of pyramidal tracts are the axons of upper motor neurons. 18-May-24 6

7. Origin Fibers of pyramidal tracts arise from following cells or a reas of cerebral cortex: 1. Giant cells or Betz cells or pyramidal cells in precentral gyrus of the motor cortex.These cells are situated in area 4 (primary motor area) of frontal lobe. 2. Other areas of motor cortex namely, premotor area (area 6) and supplementary motor areas 3. Other parts of frontal lobe 4. Somatosensory areas of parietal lobe. 18-May-24 7

8. It is believed that 30% of pyramidal fibers arise from primary motor area (area 4) and supplementary motor areas, another 30% from premotor area (area 6) and the remaining 40% of fibers arise from somatosensory areas. All the above fibers form fibers of upper motor neurons of motor pathway 18-May-24 8

9. C ourse Corona radiata After taking origin, the nerve fibers run downwards in a diffused manner through white matter of cerebral hemisphere and converge in the form of a fan-like structure along with ascending fibers, which project from thalamus to cerebral cortex. This fan-like structure is called corona radiata 18-May-24 9

10. Thus, corona radiata contains both ascending fibers from thalamus and descending fibers from cerebral cortex Internal capsule While passing down towards the brainstem the corona radiata converges in the form of internal capsule. It is situated in between thalamus and caudate nucleus on the medial side and lenticular nucleus on the lateral side . 18-May-24 10

11. In pons The fibers descend down through internal capsule,midbrain and pons. While descending through pons, the fibers are divided into different bundles by the nuclei of pons. At lower border of pons, the fibers are grouped once again into a compact bundle and then descend down into medulla oblongata. 18-May-24 11

12. In medulla This compact bundle of corticospinal fibers gives the appearance of a pyramid in the anterior surface of upper part of medulla . So, the corticospinal tracts are called the pyramidal tracts. 18-May-24 12

13. At the lower border of medulla, pyramidal tract on each side is divided into two bundles of unequal sizes. About 80% of fibers from each side cross to the opposite side. While crossing the midline, the fibers of both sides form the pyramidal decussation. 18-May-24 13

14. In spinal cord Fibers which cross the midline and form pyramidal decussation descend through posterior part of lateral white column of spinal cord. This bundle of crossed fibers is called the crossed pyramidal tract or lateral corticospinal tract or indirect corticospinal tract. Remaining 20% of fibers do not cross to the opposite side but descend down through the anterior white column of the spinal cord. This bundle of uncrossed fibers is called the uncrossed pyramidal tract or anterior corticospinal tract or direct corticospinal tract . 18-May-24 14

15. . T hey a well marked in cervical region. Since, the fibers of this tract terminate in different segments of spinal cord, this tract usually gets thinner while descending through the successive segments of spinal cord. Fibers of this tract are absent mostly below the mid thoracic level. Before termination, majority of the fibers of this anterior corticospinal tract cross to the opposite side at different levels of spinal cord. 18-May-24 15

16. Termination All the fibers of pyramidal tracts, both crossed and uncrossed fibers, terminate in the motor neurons of anterior gray horn either directly or through internuncial neurons. Pyramidal tract fibers terminate on both α-motor neurons and β-motor neurons. 18-May-24 16

17. Axons of the motor neurons leave the spinal cord as spinal nerves through anterior nerve roots and supply the skeletal muscles. Neurons giving origin to the fibers of pyramidal tract are called the upper motor neurons. Anterior motor neurons in the spinal cord are called the lower motor neurons. 18-May-24 17

18. Function Pyramidal tracts are concerned with voluntary movements of the body. Fibers of the pyramidal tracts transmit motor impulses from motor area of cerebral cortex to the anterior motor neurons of the spinal cord. These two tracts are responsible for fine, skilled movements 18-May-24 18

19. Effects of lesion Lesion in the neurons of motor cortex and the fibers of pyramidal tracts is called the upper motor neuron lesion. In human beings, pure pyramidal tract lesions do not occur. Lesion of pyramidal fibers occurs most commonly in stroke (cardiovascular accident) due to hemorrhage and thrombosis. During such lesions, many extrapyramidal fibers are also damaged along with pyramidal fibers. Because of this reason, neurologists often consider the lesion as upper motor neuron lesion and not as pyramidal tract lesion. 18-May-24 19

20. Following are the effects of lesion: 1. Voluntary movements Voluntary movements of the body are very much affected. Initially, there is loss of voluntary movements in the extremities. Later, it involves the other parts of the body like hip and shoulder. 2. Muscle tone Muscle tone is increased leading to spasticity. Muscles are also paralyzed. This type of paralysis of muscles is called the spastic paralysis. 18-May-24 20

21. The spasticity is due to the failure of inhibitory impulses from upper motor neurons, particularly the neurons of extrapyramidal system to reach the γ-motor neurons in spinal cord. However, hypotonia occurs in pure pyramidal tract lesion, which is very rare. 3. Reflexes All the superficial reflexes are lost and the deep reflexes are exaggerated. Abnormal plantar reflex called Babinski sign is present (Babinski sign positive). 18-May-24 21

22. Effects of Lesion at Different Levels Cerebral cortex Lesion of pyramidal tract fibers in cerebral cortex causes hypertonia, spasticity and contralateral monoplegia (paralysis of one limb) or contralateral hemiplegia(paralysis of one side of the body). Internal capsule Lesion of pyramidal tract fibers at posterior limb of internal capsule results in contralateral hemiplegia. 18-May-24 22

23. Brainstem Lesion at brainstem involves not only pyramidal tract fibers but also other structures such as VI and VII cranial nerve nuclei. So the lesion results in contralateral hemiparesis (weakness of muscles in one side of the body) along with VI and VII cranial nerve palsies. 18-May-24 23

24. Spinal cord Unilateral lesion of lateral corticospinal fibers at upper cervical segment causes ipsilateral hemiplegia and bilateral lesion causes quadriplegia (paralysis of all four limbs) and paralysis of respiratory muscles. Bilateral lesion of these fibers in thoracic and lumbar segments results in paraplegia (paralysis of both lower limbs) without paralysis of respiratory muscles. 18-May-24 24

25. Refferences K sembulingam essential of medical physiology. Gayton and Hall textbook of medical physiology. Ganong review of medical physiology. 18-May-24 25