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.

Descending tracts Are segregated bundles of nerve fibres in the white matter of the spinal cord descending from the supraspinal centres referred 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

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

Upper motor neurons ( UMN ) Are the descending supraspinal pathways that influence the activity of the LMN They control : 1-voluntary motor activity 2-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

descending tracts sensory inputs UMN cerebral cortex – midbrain - pons - medulla oblongata descending tracts sensory inputs LMN

Lower motor neurons ( LMN ) Are motor neurons that innervate the voluntary muscles located in 1- anterior grey column of the spinal cord , and 2- motor nuclei of brainstem form the final common pathway LMN

Descending Tracts 1- Corticospinal tracts (Lateral and anterior) 2- Corticobulbar tracts 3- Rubrospinal tract 4-Tectospinal tract 5- Reticulospinal tracts (Lateral and medial) 6-Vestibulospinal tract

Corticospinal Tracts The 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.

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)

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)

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.

( facilitate the activity of flexor muscles ) 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 )

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.

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.

Tectospinal tract 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.

Vestibulospinal 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 fibres arise from the vestibular nuclei They are Medial and Lateral Vestibular Nuclei They are situated in the pons

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

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.

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.

Reticulospinal 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

Pyramidal tract refers to corticospinal tracts Extrapyramidal tract other than corticospinal tract ( VeSt, ReSt, TeSt, RuSt )

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.

Spinothalamic Tract Lesions 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 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

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

Hereditary spastic paraparesis Poliomyelitis Acute viral infection of ventral horn neurons Rapid paralysis and wasting of the limb and respiratory muscles. 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

Amyotrophic Lateral Sclerosis 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 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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