descending tracts

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Descending Tracts Dr Rania Gabr

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Descending Tracts. Dr Rania Gabr. Objectives. Define the terms upper and lower motor neurons with examples Describe the corticospinal (pyramidal) tract and the direct motor pathways from the cortex to the trunk and limbs. - PowerPoint PPT Presentation

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Page 1: Descending Tracts

Descending Tracts

Dr Rania Gabr

Page 2: Descending Tracts

Objectives

• Define the terms upper and lower motor neurons with examples

• Describe the corticospinal (pyramidal) tract and the direct motor pathways from the cortex to the trunk and limbs.

• Briefly describe the indirect motor pathways from the cortex to the trunk and limbs through extrapyramidal tracts such as rubrospinal and reticulospinal tracts..

• Compare the signs and symptoms of the upper and lower motor neuron lesions.

Page 3: Descending Tracts

Descending tracts

• Are segregated bundles of nerve fibres in the white matter of the spinal cord descending from the supraspinal centresreferred to as upper motor

neurons ( UMN )• They are concerned with

somatic and visceral motor activities.

• Their cells of origin lie in the cerebral cortex and brain stem

Page 4: Descending Tracts

They regulate LMN activity:hey regulate the LMN activityThey are concerned with the control of movement, muscle tone, spinal reflexes, spinal autonomic functions and the modulation of sensory transmission to higher centers.

Page 5: Descending Tracts

Upper motor neurons ( UMN )

• Are the descending supraspinal pathways that influence the activity of the LMN

• They control :1-voluntary motor activity2-maintenance of posture & equilibrium 3- muscle tone and 4- reflex activity• generally exert their effect on groups of

muscles ( not on one specific muscle ) reciprocally on agonist and antagonist muscle group

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cerebral cortex – midbrain - pons - medulla oblongata

descending tracts

LMN

sensory inputs

UMN

Page 7: Descending Tracts

Lower motor neurons ( LMN )

Are motor neurons that innervate the voluntary muscles located in1- anterior grey column of the spinal cord , and 2- motor nuclei of brainstemform the final common pathway

LMN

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Descending Tracts

1- Corticospinal tracts(Lateral and anterior)2- Corticobulbar tracts3- Rubrospinal tract 4-Tectospinal tract5- Reticulospinal tracts(Lateral and medial) 6-Vestibulospinal tract

Page 9: Descending Tracts

Corticospinal TractsThe corticospinal tracts are particularly concerned with the control of voluntary, discrete, skilled movements, especially those of the distal parts of the limbs. Corticospinal tract neurons arise from cell bodies in the cerebral cortex. The cells of origin are widely distributed in the motor and sensory cortices, including the precentral gyrus or primary motor cortex.large Betz cells give rise to the largest diameter corticospinal axons.

Page 10: Descending Tracts

Course of the tract• 1- Corticospinal axons

leave the cerebral hemispheres :Pyramidal cells

• 2- They pass through the the corona radiata

• 3-Then through post. Limb of internal capsule

• 4- They enter the crus cerebri of the midbrain. (Cerebral peduncles)

Page 11: Descending Tracts

• 5- They pass through the ventral portion of the pons

• 6- Fibers reach the medulla oblongata, where they form two prominent columns on the ventral surface.

• They are called the pyramids and for this reason the term pyramidal tract is used as an alternative name for the corticospinal tract.

• In the caudal medulla, the fibers of the pyramids undergo subtotal decussation.

• ( Motor decussation)

Page 12: Descending Tracts

• About 75-90% of fibers decussate and enter the contralateral lateral corticospinal tract, which is located in the lateral part of the spinal white matter.

• 10-25% of pyramidal fibres remain ipsilateral and enter the ventral corticospinal tract located lateral to the ventral median fissure.

Page 13: Descending Tracts
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Rubrospinal Tract• Originate from nerve cells in

red nucleus

nerve fibres / axons – cross the mid line – descend as rubrospinal

tract • through pons and medulla

oblongata • Terminate in the anterior grey

column of spinal cord

( facilitate the activity of flexor muscles )

Page 15: Descending Tracts

• The rubrospinal tract originates from the red nucleus of the midbrain tegmentum.

• It exerts control over the tone of limb flexor muscles, being excitatory to the motor neurons of these muscles.

• Axons leave the cells of the red nucleus course ventromedially and cross in the ventral tegmental decussation,

• after which they descend to the spinal cord where they lie ventrolateral to, the lateral corticospinal tract.

Page 16: Descending Tracts

• Red nucleus receives afferent fibers from motor cortex and from cerebellum.

• The rubrospinal tract represents a non-pyramidal route by which the motor cortex and cerebellum can influence spinal motor activity.

• The tract facilitates the activity of the flexors and inhibits the activity of the extensors and the antigravity muscles.

Page 17: Descending Tracts

• Tectospinal fibers arise from the superior colliculus of the midbrain.

• Axons pass ventromedially around the periaqueductal grey matter and cross in the dorsal tegmental decussation.

• In the spinal cord, descending tectospinal fibers lie near the ventral median fissure and terminate predominantly in cervical segments.

• The superior colliculus receives visual input and the tectospinal tract is thought to mediate reflex movements in response to visual stimuli.

Tectospinal tract

Page 18: Descending Tracts

• nerve cells in the vestibular nuclei (in the pons and medulla oblongata– receive afferents from the

inner ear and cerebellum

• axons descend uncrossed – through medulla and

through the length of spinal cord

• synapse with neurons in the anterior gray column of the spinal cord

( balance by facilitate the activity of the extensor

muscles )

Vestibulospinal Tracts

Page 19: Descending Tracts

• Vestibulospinal fibres arise from the vestibular nuclei

• They are Medial and Lateral Vestibular Nuclei

• They are situated in the pons

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The vestibular nuclei receive input from the labyrinthine system by way of the vestibular nerve and from the cerebellum.

Page 21: Descending Tracts

Axons arise from cells of lateral vestibular nucleus Descend ipsilaterally as the lateral vestibulospinal tract.Located in the ventral funiculus. Lateral vestibulospinal tract fibres mediate excitatory influences upon extensor motor neurons. They serve to control extensor muscle tone in the antigravity maintenance of posture.

Page 22: Descending Tracts

The medial vestibular nucleus contributes descending fibers to the ipsilateral medial longitudinal fasciculus, also known as the medial vestibulospinal tract, which is located adjacent to the ventral median fissure.

Page 23: Descending Tracts

The reticular formation of the pons and medulla gives rise to reticulospinal fibres. Axons arising from the pontine reticular formation descend ipsilaterally as the medial (or pontine) reticulospinal tract. Axons from the medulla descend bilaterally in the lateral (or medullary) reticulospinal tracts. Both tracts are located in the ventral and lateral white columns respectively.They influence the voluntary movement

Reticulospinal Tracts

Page 24: Descending Tracts

• Pyramidal tractrefers to corticospinal tracts

• Extrapyramidal tractother than corticospinal tract( VeSt, ReSt, TeSt, RuSt )

Page 25: Descending Tracts

Lesions of the Spinal Cord• Lesions of the dorsal columns

• The loss of proprioception leads to a high steppage and unsteady gait (sensory ataxia), which is exacerbated when the eyes are closed (Romberg's sign).

• Subacute combined degeneration

• Deficiency of vitamin B12• The degeneration of the

dorsal columns produces sensory ataxia.

• The degeneration of lateral columns produces weakness and spasticity of the limbs

• It is combined.

Page 26: Descending Tracts

• Multiple Sclerosis

• Immune disease, leads to damage of the fasciculus cuneatus of the cervical spinal cord

• loss of proprioception in the hands and fingers, causing profound loss of dexterity

• (Astereognosis).:is the inability to identify an object by touch without visual input

Spinothalamic Tract Lesions

• Syringomyelia :Enlarged central canal compressing adjacent nerve fibers.

• Neurons carrying pain and temperature are damaged as they decussate in the ventral white commissure, close to the central canal

• Selective loss of pain and temperature below the lesion

• Dissociated sensory loss, since light touch and proprioceptive sensation are retained

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Friedreich's Ataxia

Inherited degenerative disease (autosomal recessive)Affect the spinocerebellar tractsLeads to incoordination of the arms (intention tremor) and a wide-based, reeling or drunken gait (ataxia).

Page 28: Descending Tracts

Poliomyelitis

• Acute viral infection of ventral horn neurons

• Rapid paralysis and wasting of the limb and respiratory muscles.

Hereditary spastic paraparesis

• Inherited degenerative disorder (autosomal dominant)

• Degeneration of the lateral columns, including the lateral corticospinal tract

• It causes weakness of legs, marked stiffness of gait.

• It results in spastic paraparesis with hyperreflexia and extensor plantar responses

Page 29: Descending Tracts

Brown-Sequard syndrome• Ipsilateral LMNL at same level• Ipsilateral UMNL below the level• Ipsilateral loss of posterior column functions:propioception,vibration• Contralateral loss of spinothalamic tract functions: pain and temperature

Amyotrophic Lateral Sclerosis

• Degeneration of descending pathways

• weakness and spasticity of the limb muscles

(Upper Motor Neuron Lesion)• Degeneration of ventral horn Leads to weakness, wasting, hypotonia (Lower Motor Neuron Lesion)

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