DESCENDING TRACTS

Fiber Types A Fibers: Somatic, myelinated. Alpha (α): Largest, also referred to as Type I. Beta (β): Also referred to as Type II. Gamma (γ): Delta (δ): Smallest, referred to as Type IV.

Fiber Types B Fibers: Lightly myelinated. Preganglionic fibers of ANS. Unmyelinated. Found in somatic and autonomic systems. Also referred to as Type IV fibers.

Fiber Types Sensory fibers are either: A-α or A-β fibers: Conduction rate = 30-120 m/sec. A-δ fibers: Conduction rate = 4-30 m/sec. C fibers: Conduction rate is less than 2.5m/sec.

Fiber Types Nociceptors and thermoreceptors are related to C fibers or A-δ fibers.

Generalizations: Motor Paths Typical descending pathway consists of a series of two motor neurons: Upper motor neurons (UMNs) Lower motor neurons (LMNs) Does not take into consideration the association neurons between UMNs and LMNs

Upper Motor Neurons Are entirely within the CNS. Originate in: Cerebral cortex Cerebellum Brainstem Form descending tracts

Lower Motor Neurons Begin in CNS. From anterior horns of spinal cord. From brainstem cranial nerve nuclei. Made up of alpha motor neurons (A-α). Make up spinal and cranial nerves.

UMN Classification Classified according to where they synapse in the ventral horn: Medial activation system: Innervate postural and girdle muscles Lateral activation system: Associated with distally located muscles used for fine movements Nonspecific activating system: Facilitate local reflex arcs

Pyramidal System Characteristics: 75 – 85% Decussate in pyramids. Upper motor neurons: 75 – 85% Decussate in pyramids. Remainder decussate near synapse with lower motor neurons. Most synapse with association neurons in spinal cord central gray.

Pyramidal System Components: Corticospinal Tract Corticobulbar Tract

Corticospinal Tract Divisions Lateral corticospinal tract: Made up of corticospinal fibers that have crossed in medulla. Supply all levels of spinal cord. Anterior corticospinal tract: Made up of uncrossed corticospinal fibers that cross near level of synapse with LMNs. Supply neck and upper limbs.

Corticospinal Tract Pathway © 2006, Marshall Andersen

Corticospinal Tract (Tectum) © 2006, Marshall Andersen

Corticospinal Tract (Pons) © 2006, Marshall Andersen

Corticospinal Tract (Sup. Med.) © 2006, Marshall Andersen

Corticospinal Tract (Inf. Med.) © 2006, Marshall Andersen

Lateral Corticospinal Tract © 2006, Marshall Andersen

Anterior Corticospinal Tract © 2006, Marshall Andersen

Corticospinal Tract Functions Add speed and agility to conscious movements: Especially movements of hand. Provide a high degree of motor control: (i.e., movement of individual fingers)

Corticospinal Tract Lesions Reduced muscle tone Clumsiness Weakness Not complete paralysis Note: complete paralysis results if both pyramidal and extrapyramidal systems are involved (as is often the case).

Corticobulbar Tract Innervates the head Most fibers terminate in reticular formation near cranial nerve nuclei. Association neurons: Leave reticular formation and synapse in cranial nerve nuclei. Synapse with lower motor neurons.

Corticobulbar Tract (Sup. Tectum) © 2006, Marshall Andersen

Corticobulbar Tract (Inf. Tectum) © 2006, Marshall Andersen

Extrapyramidal System Includes descending motor tracts that do not pass through medullary pyramids or corticobulbar tracts. Includes: Rubrospinal tracts Vestibulospinal tracts Reticulospinal tracts

Rubrospinal Tract Begins in red nucleus. Decussates in midbrain. Descends in lateral funiculus (column). Function closely related to cerebellar function. Lesions: Impairment of distal arm and hand movement. Intention tremors (similar to cerebellar lesions)

Rubrospinal Tract: (Sup. Tectum) © 2006, Marshall Andersen

Rubrospinal Tract: (Inf. Tectum) © 2006, Marshall Andersen

Rubrospinal Tract: (Inf. Medulla) © 2006, Marshall Andersen

Rubrospinal Tract © 2006, Marshall Andersen

Vestibulospinal Tract Originates in vestibular nuclei: Receives major input from vestibular nerve: (CN VIII) Descends in anterior funiculus (column). Synapses with LMNs to extensor muscles: Primarily involved in maintenance of upright posture.

Vestibulospinal Tract © 2006, Marshall Andersen

Reticulospinal Tract Originates in various regions of reticular formation. Descends in anterior portion of lateral funiculus (column). Thought to mediate larger movements of trunk and limbs that do not require balance or fine movements of upper limbs.

Reticulospinal Tract © 2006, Marshall Andersen