Descending tracts D.Nimer D.Rania Gabr D.Safaa D.Elsherbiny

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.

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

Motor system Pyramidal system Corticospinal tract. Corticobulbar tract. Extrapyramidal system Vestibulospinal,reticulospinal,olivospinal, rubrospinal and tectospinal Motor pathway to cerebellum: corticopontocerebellar tract

Pyramidal system

Cerebral cortex Brain stem Spinal cord Coricobulbar tract Corticospinal tract Brain stem ms Spinal cord

Cortico-spinal tract

SC (AHC) 85%LCST 10% 4,6 312 15%VCST Internal capsule midbrain pons Corona radiata Internal capsule midbrain pons 85%LCST medulla 15%VCST SC (AHC) 10%

Corona radiata

Course of the tract 1- Corticospinal axons leave the cerebral hemispheres : Pyramidal cells (Betz 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.

Function Lesion: UMNL Above decussation: contralateral hemiplegia. Facilitation of flexor muscle tone Initiation of high skilled movements Lesion: UMNL Above decussation: contralateral hemiplegia. Below decussation: ipsilateral hemiplegia.

Signs of UMNL hypertonia. Hyper-reflexia. Spasticity. Babinski sign. Clonus. Loss of superficial abdominal reflexes & cremastric reflex.

Clonus

Babinski sign

Hyper-reflexia

Spasticity

Corticobulbar tract

4,6 Lower facial nucleus Cranial nerve.nuclei Internal capsule midbrain pons medulla Lower facial nucleus Cranial nerve.nuclei

Extrapyramidal system 2 from midbrain: Rubrospinal : from red nucleus to SC- facilitate the activity of flexor ms Tectospinal: from superior colliculus of midbrain to SC- mediate reflex movements in response to visual stimuli

Extrapyramidal system 2. 2 from vestibular nuclei: Medial and lateral nucleus. Input: from the labyrinthine system (inner ear) and from the cerebellum. Output: SC Function: balance by facilitate the activity of the extensor muscles

Extrapyramidal system 3. 2 from reticular formation: medial & lateral reticulospinal tracts. 4. 2 others: raphe spinal & descending autonomic fibers.

Descending autonomic fibers Origin: hypothalamus. Termination: sympathetic nucleus and sacral parasympathetic nucleus Lesion: above T1…Horner's syndrome

Horner's syndrome 1. Ptosis 2. Miosis 3. Flushing 4. Anhydrosis

The Main Descending Pathways to the Spinal Cord Function Origin Site of Crossover Destination Branches to Corticospinal tracts Rapid, skilled, voluntary movements, especially distal ends of limbs Primary motor cortex (area 4), secondary motor cortex (area 6), parietal lobe (areas 3, 1, and 2) Most cross at decussation of pyramids and descend as lateral corticospinal tracts; some continue as anterior corticospinal tracts and cross over at level of destination Internuncial neurons or alpha motor neurons Cerebral cortex, basal nuclei, red nucleus, olivary nuclei, reticular formation Reticulospinal tracts Inhibit or facilitate voluntary movement; hypothalamus controls sympathetic, para-sympathetic outflows Reticular formation Some cross at various levels Alpha & gamma motor neurons Multiple branches as they descend Tectospinal tract Reflex postural movements concerning sight Superior colliculus Soon after origin ? Rubrospinal tract Facilitates activity of flexor muscles and inhibits activity of extensor muscles Red nucleus Immediately Alpha and gamma motor neurons Vestibulospinal tract Facilitates activity of extensor inhibits flexor muscles Vestibular nuclei Uncrossed Olivospinal tract ?? Inferior olivary nuclei Cross in brainstem ? Alpha &gamma motor neurons — Descending autonomic fibers Control sympathetic and para-sympathetic systems Cerebral cortex, hypothalamus, amygdaloid complex, reticular formation   Sympathetic & parasympathetic outflows aNote that the corticospinal tracts are believed to control the prime mover muscles (especially the highly skilled movements), whereas the other descending tracts are important in controlling the simple basic movements. For simplicity, the internuncial neurons are omitted from this table.