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Spinal Cord Descending Tracts. Dr. Sama- ul - Haque. 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.
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Spinal CordDescending Tracts Dr. Sama-ul-Haque
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.
Descending Tracts • Bundles of nerve fibers in the white matter of the spinal cord descending from the supraspinal nerve centers. • Also called Upper Motor Neurons. • They are concerned with Motor Activity. • They originate from cerebral cortex and brain stem. • Lower Motor Neurons (Neurons which innervate the skeletal muscles). • Mainly present in the anterior gray column of the spinal cord.
Descending Tracts • First-order Neuron: • Cell body lies in the cerebral cortex • Second-order Neuron: • An internuncial neuron, situated in the anterior gray column of the spinal cord. • Third-order Neuron: • Axon of 2nd order neuron is short and synapse with 3rd order neuron,(Lower motor neuron), in the anterior gray column of the spinal cord. • Axon of 3rd order neuron innervates the skeletal muscle through anterior root and spinal nerve.
Descending TractsFirst-orderSecond-orderandThird-order Neurons
Descending Tracts • Corticospinal tract • Reticulospinal Tract • Tectospinal Tract • Rubrospinal Tract • Vestibulospinal Tract • Olivospinal Tract
Corticospinal Tracts • Fibers of corticospinal tract arise as Axons of pyramidal cells (situated in the 5th layer of cerebral cortex). • 1/3rd of the cells arise form Primary Motor cortex (Area 4). • 1/3rd of the cells arise form Secondary Motor cortex (Area 6). • 1/3rd of the cells arise form Parietal lobe (Area 3,1 and 2). • The descending fibers converge in corona radiata. • Then pass through posterior limb of the internal capsule and reaches to midbrain.
Corticospinal Tracts • On entering the Pons the tract is broken into many bundles by transverse pontocerebellar fibers. • In medulla oblongata the bundle of fibers grouped together anteriorly and form Pyramid(Pyramidal Tract). • Most of the fibers cross the midline at the decussation of Pyramids (Motor Decussation) and enter the lateral white column of spinal cord as Lateral corticospinal tract. • The lateral corticospinal tract lies in the whole length of spinal cord.
Corticospinal Tracts • The remaining fibers do not cross in the decussation and descend in anterior white column as of spinal cord as Anterior corticospinal tract. • These remaining fibers eventually cross the midline in the cervical and upper thoracic segments. • All the fibers terminate in the anterior gray column of the spinal cord by synapsing with internuncial or motor neurons. • Function: Concerned with Voluntary, discrete and Skilled movements (especially distal parts of the limbs).
Reticulospinal Tracts • Reticular Formation: • Group of scattered nerve cells and nerve fibers in midbrain, Pons and Medulla. • From Pons these neurons send uncrossed axons down into the anterior white column of the spinal cord and form Pontine (Medial) Reticulospinal Tract. • From medulla these neurons send crossed and uncrossed axons down into the lateral white column of the spinal cord and form Medullary (Lateral) Reticulospinal Tract.
Reticulospinal Tracts • Both sets of fibers enter in the anterior gray columns of the spinal cord. • Facilitate or inhibit the activity of Motor neurons. • Function: Facilitate or inhibit voluntary movements or reflex activity.